GPS Reference Stations and Networks An introductory guide

GPS Reference Stations and NetworksAn introductory guide

Contents

1. Introduction: aim of this book2. What is a Continuously Operating Reference

Station (CORS)? What is a network?

3. What are GPS reference stations used for?3.1 Geodetic control for surveying, engineering,

mapping, cadastre, etc.3.2 Monitoring the Earth's crust, natural and

man-made structures3.3 Machine guidance3.4 GIS data collection3.5 Endless possibilities

4. What does a GPS reference station or a network of stations have to do?

5. Selecting suitable sites6. Open view of the sky7. Setting up a reference station on a pillar 8. Setting up a reference station on a building9. Setting up a reference station on a man-made

structure

10. GPS receivers, GPS antennas, cables10.1 GPS receivers10.2 GPS antennas10.3 Antenna cables

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11. Power supply11.1 Power supply at the receivers11.2 Power supply for the computer running the

reference station software

12. GPS reference station software and control computer (server)

12.1 Reference station software and control computer (server)

12.2 Reference station software: calculation of RTK/DGPS data

12.3 Reference station software: continuous network analysis and calculation of correction parameters for enhanced RTK

12.4 Reference station software: calculation of the positions of the antennas

13. RTK and DGPS data 13.1 RTK and DGPS data: output from the receivers13.2 RTK and DGPS data: computed and output at

the server

14. The need for reliable communication15. Communication between the server and the

receivers15.1 Dial-up links (open when required)15.2 Permanently open links (continuously on)

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16. Communication: transmission of RTK and DGPS data

16.1 Transmission directly from the receivers16.2 Distribution (transmission) from a control center16.3 Radio versus phone: advantages and

disadvantages

17. Using the Internet, or other IP-based methods, for communication between the server and the receivers

18. Using the Internet to distribute and access RTK and DGPS data

18.1 Equipment needed by RTK and GIS rovers18.2 Distribution from a single stand-alone reference

station18.3 Distribution from a network control center18.4 Summary of the various ways by which

RTK/DGPS data can be distributed

19. NTRIP - Networked Transport of RTCM via Internet Protocol

20. Continuous network analysis and calculation of network correction parameters for enhanced RTK

20.1 Accuracy and range of RTK: the influence of distance dependent errors

20.2 Network analysis and computation of network correction parameters

20.3 Network data in RTCM V3.0 format and network correction parameters are transmitted to the rover and the rover applies them.

20.4 Network correction parameters are applied at the server and the server transmits “corrected” RTK data to the rover.

20.5 Comparison of the two methods20.6 Improved RTK performance

21. Using an FTP server to distribute RINEX data

22. Processing baselines between stations to check antenna positions and monitor movements of natural and man-made structures

22.1 Processing the baselines between the stations of a network

22.2 Checking the positions of the antennas22.3 Monitoring movements of the Earth's crust, and of

natural and man-made structures

23. Determining the WGS84 coordinates of the reference stations

24. Where should the stations be positioned? What should be the distances between the stations?

25. Charging for products such as RINEX and RTK/DGPS data

26. Some examples of stand-alone reference stationsand networks

26.1 Stand-alone reference station for a small area such as an opencast mine, construction site, or local community

26.2 A simple network of a few stations providing complete RTK coverage for a relatively small well developed area.

26.3 A more complex network providing complete RTK coverage for a relatively large highly developed area.

26.4 A network of a few stations providing RINEX data and limited RTK/DGPS for a very large undeveloped area

26.5 A combined approach: networks of stations providing RTK/DGPS coverage for highly developed areas, widely spaced stations providing RINEX data throughout undeveloped areas.

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27. Meteorological and tilt sensors27.1 Meteorological sensors27.2 Tilt sensors

28. Protection against lightning28.1 Lightning conductor28.2 Lightning surge arrestor

29. Points to take into account when deciding what type of stations and/or networks are needed

30. Useful links

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1. Introduction: aim of this book

Although the benefits of ContinuouslyOperating GPS Reference Stations -often referred to by the acronym CORS- are generally well recognized today,there is still considerable uncertaintywithin many organizations andamongst many would-be users as tothe best way to establish referencestations and provide appropriate levelsof services for the GPS user community.

This book offers practical advice onhow to set up and run both individualGPS reference stations and networksof stations and to provide the servicesthat are required.

As budgets are sometimes limited,and as reference stations and networks can vary considerably incomplexity and, therefore, the investment needed, emphasis is placed on efficient, cost-effective solutions.

1

For general background information on

GPS, please see the book “GPS Basics”

713282 by Leica Geosystems.

2

A continuously operating GPS reference station - or permanent reference station as it is often called - comprises a GPS receiver and antenna set up in a stable mannerat a safe location with a reliable powersupply. The receiver operates continuously, logging raw data, perhaps also streaming (continuouslyoutputting) raw data, and often outputting RTK and DGPS data fortransmission to RTK, GIS and GPS navigation devices. The receiver is usually controlled by a computer thatcan be located remotely if necessary.The PC will usually download data files

1 2 3 n

at regular intervals and pass them toan FTP server for access by the GPSuser community.

One or more single reference stationssupplying GPS services to users in theimmediate surrounding areas may beall that is required by some organizations. Other authorities,however, may need to establish networks of reference stations - perhaps 5, 10, 20, 50, or even morestations - to provide complete GPSservices over entire regions and evencountries. A single server (computer) running a GPS reference station

software and communicating by telephone, LAN, WAN or Internet cancontrol all the stations in the network(PCs are not required at the receivers).

This brief introduction illustrates thatreference stations and networks canvary considerably in extent and complexity. Organizations that are studying the establishment of reference stations should consider carefully what the stations will be usedfor, what services they will have to provide, and what will be the appropriate levels of sophistication and cost.

2. What is a Continuously Operating Reference Station (CORS)? What is a network?

3

3. What are GPS reference stations used for?

The first reference stations, in the dayswhen GPS was in its infancy, were setup along coastlines to transmit DGPScorrections to improve the accuracy ofship navigation.

Today, with the widespread acceptance of high-precision GPS measurement techniques, GPS reference stations are being established all over the world in ever increasing numbers. They are used tomonitor the Earth's crust, to providegeodetic control, to support surveying, engineering, GIS data collection, machine control and precise positioning, as well as to monitor natural and man-made structures. GPS reference stations provide the control needed for a wide variety of applications.

3.1 Geodetic control for surveying, engineering, mapping, cadastre etc.

A network of continuously operatingGPS reference stations is more efficient

Such a network can be of almost anysize. Whilst one or two stand-alonereference stations may be all that isrequired for a local area, town, municipality, opencast mine or engineering site, a multi-station network will usually be needed to provide full GPS service coverage for alarge county, region or entire country.

than a traditional triangulation and traverse network. The stations can beset up at convenient locations in areaswhere they are needed (rather thanon remote hilltops). Network geometryis not as critical as with traditionalnetworks, and the accuracy is higherand more consistent. Users set uptheir field receivers in the areas inwhich they are working, downloadreference station data via the Internet,and compute their positions. The sta-tions can also transmit RTK and DGPSdata for direct use by RTK and GISfield rover equipment.

3. What are GPS reference stations used for? (continued)

In regions where earthquakes are likely to occur, along major fault lines,and in areas of volcanic activity, networks of suitably positioned GPSreference stations are often used tomonitor movements of the Earth'scrust.

A central computer with a reference station software controls the receivers, downloads data, and computes the network to determinethe positions of the antennas.Movements can be analyzed.

Similar networks, though usually on a smaller scale, are used to monitorthe positions and movements of natural and man-made structures suchas glaciers, landslides, dams, bridges,buildings, towers, offshore oilrigs etc.

3.2 Monitoring the Earth's crust, natural and man-made structures

4

5

3.5 Endless possibilities

GPS reference stations and networkscan be used in many ways for manyapplications.

Stations and networks can be set upand configured for just one particularapplication and one user group. Orthey can be designed to be multi-functional to support a wide range ofapplications and a multitude of users.

A single reference station may be perfectly sufficient for a small locality.A multi-station, multi-purpose networkwill often be preferred for an entireregion. The permutations are endless.

One or more permanent reference stations will be needed whenever GPSsurveying or monitoring has to be carried out repeatedly over the samearea for a long period of time.

3.4 GIS data collection

Water, electricity, gas and telephoneauthorities, cadastral offices, municipalities and other similar organizations often operateGeographical Information Systems.The GIS database shows the locationof property boundaries, infrastructure,and assets such as water pipes,hydrants, power lines, gas pipes, telephone lines etc.

RTK and DGPS measurement techniques are widely used for updating the database, surveying new features, and re-surveying existing features toimprove the accuracy of the data.

GPS reference stations provide thecontrol and support needed by RTKand GIS rover equipment.

3.3 Machine guidance

On large construction sites and inopencast mines, work can be carriedout faster, with higher accuracy andusing less material when machines areequipped with automatic guidance andheight-control systems. Bulldozers,graders, scrapers, drilling machinesetc. can be positioned and steered tocentimeter-accuracy with RTK. Theheight of the blade can be controlledautomatically.

When fields are large, agriculturalmachines for sowing seed, distributingfertilizer, harvesting, spreading insecticide etc. operate more efficiently and cost effectively whenguided by RTK or DGPS.

GPS reference stations provide thecontrol and support needed for machine guidance.

3. What are GPS reference stations used for? (continued)

6

GPS reference stations and networks support all types of users and applications

7

4. What does a GPS reference station or a network of stations have to do?

The GPS receivers at reference stationsrun continuously. The raw code andphase measurement data are usuallylogged internally in files of specifiedlength. Depending on the application,the file length can be set to any required value from a few minutes toseveral hours or even to a full day.

Reference station software running ona computer - let us call it a server -controls the receivers and downloadsthe data files at regular intervals. Ifrequired, the raw data can also bestreamed continuously, second bysecond, from the receivers to the server. The reference station softwarerunning on the server converts thedata to RINEX (Receiver INdependentEXchange format) and produces compressed RINEX files. The RINEXfiles are pushed to an FTP server foreasy Web access by the GPS usercommunity and are also archived forsafekeeping.

A server running reference station software can control a single receiverat a stand-alone reference station oran entire network comprised of manyreceivers. In case of a single stand-alone station, the computer will oftenbe connected directly to the receiver. In case of a multi-station network, theserver will usually be at a control centerand connected to the receivers by telephone, LAN, WAN or Internet (PCsare not required at the receivers).

Once set up and configured, the stations and network will run fullyautomatically. However, system supervisors can log in, inspect thereceivers and the network and makeany changes that are necessary.

A main requirement today is to provide the data needed by real-timesurvey and GIS rover equipment. Thereceivers at the reference stations canoutput data in standard RTCM formatsand in other proprietary formats (Leica,

CMR, CMR+) for transmission to anduse by RTK and GIS field rover receivers. Transmission can be directly from the stations or via othersuitable locations. Communication fortransmission of RTK and DGPS datawill usually be by radio, high-speedwireless (GSM, GPRS, CDMA etc) oreven by the Internet.

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5. Selecting suitable sites

• The need for an open view of the sky

• No objects in the vicinity that could cause multipath

• No transmitters in the area that could cause interference

• What the stations will be used for• How to provide a stable mount for

the antenna?

• How to provide reliable power and communication?

• How to house and protect the equipment?

• How to ensure security against vandals and passers-by?

• Accessibility for inspection and service

• Cost

When selecting sites for continuously operating GPS reference stations carefulconsideration must be given to the following:

If it is required to monitor any movements within a network, it is alsopossible to arrange for the positions of the antennas to be computed automatically at regularintervals.

The functionality and complexity of anindividual station or a network of stations will depend on a number offactors, including:

• The applications for which the station or network is to be used for

• The services that have to be provided

• The number and type of users that have to be supported

• The most suitable methods of communication

• The available infrastructure• The costs of running the station or

the network• The possibility to charge for

services and data• The available budget

4. (continued)

9

Receivers at reference stations will usually be set to track satellites downto 10° above the horizon (10° cut-offangle). For some applications it mayeven be required to track satellitesdown to the horizon, i.e. to 0°elevation.

Obstructions can lead to loss of satellite signals and may also causemultipath (reflected signals). Multipathcan have a negative influence on thequality of the data and, therefore,accuracy.

For these reasons sites should beselected where there will be noobstructions above 10° above the horizon of the antenna. This is particularly important for reference stations that will form part of a highaccuracy geodetic network. The best,of course, is if there will be noobstructions at all above the antennahorizon.

One way to make fully certain that asite is absolutely perfect for a geodetic reference station is to set up

a receiver and antenna, log data forseveral days, and then analyze thedata using the TEQC tool from UNAVCO (see 30. Useful links).

6. Open view of the sky

✓

✘No

OK

10

7. Setting up a reference station on a pillar

Well-constructed pillars with solidfoundations, ideally bedrock, will usually be preferred for stations thatare to be used for monitoring movements of the earth.

Pillars are also sometimes used at stations that form part of a geodeticcontrol network (set ups on buildingsare often more practical however).

If pillars are to be used the followingshould be considered carefully:• How to provide reliable power and

communication?• Where to place the receiver, power

supply and communication device?• Security

Pillars can be of concrete or metal.Note that, in hot climates, very tall pillars could be liable to slight diurnaldeformations caused by the heat ofthe Sun.

If the pillar is suitably designed, it maybe possible to place the GPS receiver,

power supply and communicationdevice inside the pillar. Otherwisethese items will have to be located ina suitable container, shed or nearbybuilding. Depending on the climate,ventilation or even air conditioningmay be necessary.

If the equipment can be placed in anearby building, the required powerand telephone connections will oftenbe available. If the equipment has tobe placed in the pillar or in a container, special power and telephone lines may have to be arranged, which can increase costssignificantly.

At remote sites in sunny climates, it is possible to arrange a bank of batteries with a solar panel chargingsystem to power the GPS receiver anda communication device, such as asatellite phone or mobile phone.

Unless the reference station can beset up within a secure area, a highfence may be needed to protect theequipment from vandals and passers-by.

Reference stations using pillars needcareful planning especially as therequired infrastructure can lead tohigh costs. Examples of pillars can befound on the UNAVCO web site (see30. Useful links).

✘No

11

8. Setting up a reference station on a building

There are significant advantages insetting up reference stations on buildings, as power and telephoneconnections are usually available andthe equipment should be relativelysafe.

Buildings are usually suitable for reference stations that form part of ageodetic control network, that supportsurveying and engineering applications, and that transmit RTKand DGPS data to RTK and GIS roverunits.

Although it is usually advantageous toplace the GPS antenna as high as possible, towers and thin, tall, skyscraper-type buildings are oftennot suitable as they may sway instrong winds. In hot climates, very tallstructures may also deform (bend)slightly due to expansion caused by the Sun.

Flat-roofed buildings of mediumheight with a relatively large base will

normally be very stable and suitablefor reference stations.

It is usually easy to arrange a stablemount for the GPS antenna on theroof of a building. There are variousways in which this can be done, forinstance: using a metal plate with a5/8 inch screw fixed firmly in position,building a small pillar, attaching a pipeor console to a wall, etc.

The GPS receiver, power supply andcommunication equipment (and possibly even a PC) can often be

✓OK

located in a room below the roofwhere power and telephone connections are available.

In most cases infrastructure costs canbe kept reasonably low when settingup a reference station on a building. It should also be easy to ensure that only authorized persons have access to the equipment.

12

9. Setting up a reference station on a man-made-structure

Networks of GPS reference stationscan be used very effectively for monitoring deformations and movements of man-made structuressuch as dams, bridges, buildings, offshore oilrigs etc.

If the need is to identify slow, long-term movements, data can belogged in the receivers, downloadedat regular intervals to a central control computer, and then processed automatically to obtain the positionsof the antennas.

In order to monitor rapid, short-termmovements and even vibrations, theraw data have to be streamed continuously at a high rate from thereceivers to the control computer.Software running on the computerprocesses the baselines between thestations continuously.

Movements and deformations can beidentified.

The type of structure and the movements to be monitored willdetermine where and how the GPSantennas have to be set up. The availability of power and telephoneconnections has to be consideredwhen positioning the GPS receivers,power supply and communicationdevices.

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10. GPS receivers, GPS antennas, cables

10.1 GPS receivers

Modern, universal, dual-frequencyreceivers, such as those of the LeicaSystem 1200 and System 500 series,are by far the most suitable for reference stations as they produce alltypes of measurement data (L1, L2,code, phase), generate all requiredoutputs (RTK, DGPS, NMEA) and support every kind of application.

In order to be able to provide therequired services to different users atthe same time in an optimum manner,it is best if the receiver can log dataat high rates, continuously stream rawdata, and also output RTK and DGPSdata in all commonly used formats(RTCM, Leica, CMR, CMR+).

Simultaneous data logging at two different rates to two different filescan also be required for some applications.

The receivers used at reference stations should have sufficient portsfor:

• Connecting to a control computer running the reference station software

• Streaming raw data to a control computer if this is a requirement

• Attaching communication devices for transmitting RTK and DGPS data

• Connecting primary and backup power supplies

• Connecting peripheral devices such as meteorological and tilt sensors

Single-frequency receivers are limitedin performance and not suitable foruse at the multi-purpose referencestations that are often required today.They can be used, however, at stations that simply transmit DGPSdata to GIS rovers and navigation receivers. Although it is possible touse single-frequency receivers withinsmall, short-baseline networks thatare used for monitoring movements,dual-frequency receivers will usually bepreferred.

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10.2 GPS antennas

If reference stations form part of anational, first-order, geodetic-controlnetwork, IGS type choke-ring antennas fitted with Dorne & Margolinelements will often be mandatory. Anyreference station that forms part ofthe global International GPS Service(IGS) network will also usually requirea choke-ring antenna. This type ofantenna has very high phase-centerstability, suppresses multipath tonegligible levels, and helps to ensurethat measurement data will be of thehighest possible quality.

For single, stand-alone reference stations and for reference stations innetworks that are used mainly to support surveying and engineeringand to provide data to RTK and GISrovers, standard, compact geodeticantennas are often suitable.

Compact geodetic antennas providegood quality data that are sufficient formost users and applications, and theyare significantly less expensive thanchoke-ring antennas.

10.3 Antenna cables

A standard 10m cable will often be sufficient to connect the antenna tothe receiver. If the receiver has to belocated further away from the antenna, a longer cable will be needed.

Antenna cables up to 100m or moreare obtainable, but the longer thecable the thicker it has to be to minimize loss of signal and the heavier and more unwieldy it becomes. Extra-long cables also costmuch more than standard cables.

The best is to use cables that are asshort as possible. Cables longer than30m should rarely be needed.

10. GPS receivers, GPS antennas, cables (continued)

Choke-ring antenna

Compact geodetic antenna

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11.1 Power supply at the receivers

A GPS reference station needs a reliable, continuous power supply. An AC to DC converter connected tothe mains (line) will normally be usedfor powering the receiver and anyother ancillary equipment such ascommunication devices.

A UPS (Uninterruptible Power Supply)unit will provide backup power for alimited time should the mains (line)supply fail. UPS units are available invarious types and sizes (capacity). The larger the capacity, the longer theUPS will power the equipment.

A UPS of appropriate size will almostcertainly be needed at a reference station where the mains (line) poweris unreliable. However, if the powersupply at the mains is very reliableand unlikely to fail, it may be decidedthat a UPS is not necessary.

When studying whether a UPS isrequired, it is worth considering whether the reference station is partof a network of stations or whether it is a stand-alone station.

If one station in a network of stationsshould stop operating very occasionally for a short period of timedue to power failure, users of fieldreceivers, RTK and GIS rovers may beable to obtain the data and servicesthat they require from other nearbystations and continue to work; thusthe occasional short power failure at one station may not be too critical.

If the station is a single, stand-alonestation (not part of a network) - i.e.the only station supplying GPS services to the surrounding area - apower failure will result in all users offield equipment having to stop work.

When evaluating whether to installUPS units and what capacity would beneeded, it is also important to

consider the level of reliability that isdemanded. Users that have to pay fordata and services are unlikely to tolerate failures; in this case UPS unitswill usually be required.

If stations form part of a monitoring network and if failure could lead todanger or even loss of life, UPS unitsof appropriate size will probably beobligatory.

Note that modern GPS receivers usedfor reference stations will restart automatically when power is restored.

It may sometimes be necessary toestablish a reference station in anarea where there is no power. Atremote sites in sunny climates, it ispossible to arrange a bank of batteries with a solar panel chargingsystem to power the GPS receiver anda communication device such as asatellite phone or mobile phone.

11. Power supply

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11.2 Power supply for the computer running the reference station software

It is advisable to consider installing aUPS (Uninterruptible Power Supply)unit to provide backup power for thecomputer running the reference station software. The UPS will bridgeshort power cuts and will also enablethe software to shut down safelybefore power fails completely.

A UPS of appropriate size may well bemandatory if the computer controls anentire network of receivers. At astand-alone reference station, a UPS(if required) should be able to providebackup power for both the receiverand the computer.

If power fails at the computer but notat the reference stations, the receivers will continue to operate normally, log data, and transmit RTKand DGPS data. As soon as power isrestored, the computer will reboot and

the software should restart automatically and download the datafiles from the receivers. Thus powerfailure may lead to a delay in down-loading data but may not result in aloss of data.

11. Power supply (continued)

UPS

AC to DC Converter

A CAC

AC

DC

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Although it is possible to run a single,stand-alone reference station - perhaps to support a mine, construc-tion site or community - without a computer running reference stationsoftware, such a reference station willhave severe disadvantages and is unlikely to be more than a temporarysolution. Such a station may be sufficient if the only requirement is totransmit RTK and DGPS data to RTKand GIS rovers. However, if raw datahas to be archived and made available for users, the receiver tracking will have to be stopped - perhaps every evening - and the logged data downloaded manually.

Even for a single, stand-alone reference station, it is always advantageous if the receiver can runcontinuously, the data can be downloaded fully automatically atregular intervals, and the entire operation can be controlled andinspected as required by a supervisor.

12. GPS reference station software and control computer (server)

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GPS Reference Stations

Dial-up modem

Serial

Radio

TCP / IP

RTK / DGPS

RTK / DGPS

19

configurations, parameters and operating modes, upload new firmware to the receivers etc.

Once configured and started, a reference station or a network of stations controlled by software running on a server will operate fullyautomatically. Well-designed reference stations and networks areextremely powerful yet, once set up,they are very easy to use.

continuously to the server instead oflogging data or even stream raw dataat the same time as they are loggingdata.

Reference station software running onthe server checks the raw data forcompleteness, compresses the rawdata, converts it to RINEX, compactsRINEX, archives raw data and RINEXfiles, and pushes raw data and RINEXfiles to an FTP server for easy accessby the GPS user community. The software also monitors the operationof the receivers, the quality of thedata, the communication links, thefunctioning of the entire network, andgenerates messages and reports asnecessary.

System supervisors have full controlover the receivers at the stations andthe entire network. They can log in tothe server, perhaps even from remotelocations, inspect the operation of thereceivers and the network, start andstop the various operations, change

12.1 Reference station software and control computer (server)

Reference station software running ona computer - let us call it a server -can control a single receiver at astand-alone station or all the receivers at all stations in a network.In case of a single stand-alone station, the receiver will often be connected directly to the computer. In case of a multi-station network, theserver will usually be at a control center and connected to the receiversby telephone, LAN, WAN or Internet(PCs are not required at the receivers).

GPS receivers at reference stations runcontinuously. The raw measurementdata are usually logged internally inthe receivers in files of the requiredlength. Reference station softwarerunning on the server controls thereceivers and downloads the data filesautomatically at regular intervals.Receivers can also stream raw data

12. GPS reference station software and control computer (server) (continued)

12.2 Reference station software: calculation of RTK/DGPS data

Although the receivers can output RTKand DGPS data directly in standardRTCM formats and in proprietary formats (Leica, CMR, CMR+) for immediate transmission from the stations to RTK and GIS field rovers,some organizations prefer to have the distribution of RTK and DGPS datacontrolled from one central location.

If permanently open communicationlinks are available, raw data can be

20

12.3 Reference station software: continuous network analysis and calculation of correction parameters for enhanced RTK

The maximum range from a referencestation at which a standard RTK rovercan operate successfully (i.e. resolveambiguities) is usually quoted asabout 30km. This assumes favourableatmospheric conditions and that therover is receiving standard RTK datafrom the station. The limitation inrange is due to the effects of distantdependent errors relating to satelliteorbits, ionospheric delays and tropospheric delays.

Using a network of several referencestations, it is possible to model thesedistance dependent errors and to correct for them.

The raw data have to be streamed continuously from the receivers to the server. Network analysis software running on the server continuouslyanalyzes the “state” of the networkdata and models the distance dependent errors. Correction parameters are computed continuously.

If the RTK rover receivers are equipped with the appropriate algorithms, these correction parameters can be transmitted to therovers and the rovers apply them.Alternatively, the correction parameters can be applied at the server and corrected RTK data can betransmitted to the rovers. In bothcases, improved RTK at longer rangescan be achieved. See 20 for full information.

12. GPS reference station software and control computer (server) (continued)

streamed continuously from the receivers to the server. A softwarecomponent running on the servercomputes the required RTK/DGPS datacontinuously in the required format - standard RTCM format or proprietary (Leica, CMR, CMR+) format - and outputs it immediatelyfor distribution to RTK and GIS rovers.

12.4 Reference station software: calculation of the positions of the antennas

Networks of GPS reference stationsare often used for monitoring movements of the Earth's crust, particularly along fault lines and inareas of volcanic activity. Small networks can be used to monitor thepositions and movements of naturaland man-made structures such asdams, bridges, buildings, oilrigs, landslides, glaciers etc.

A software component running on theserver can compute the baselines ofthe network continuously or at regular intervals to determine thepositions (coordinates) of the antennas.

Depending on the applications andthe requirements, slow, long-termmovements, rapid, short-term movements, and even vibrations canbe investigated.

21

13. RTK and DGPS data

13.1 RTK and DGPS data: output from the receivers

The receivers at reference stations canbe configured to output RTK and/orDGPS data continuously in standardRTCM V2.1/2.2/2.3/3.0 formats and/orin Leica, CMR and CMR+ proprietaryformats.

The data can be transmitted from one or more ports in one or more formats.

Radios and phones can be used totransmit RTK and DGPS data directlyfrom the receivers; see 16.1. TheInternet can also be used as explained in 18.2.

13.2 RTK and DGPS data: computed and output atthe server

Some organizations prefer to have thedistribution of RTK and DGPS data managed from one central location, rather than transmitting directly from the receivers. This allows better control over user access and facilitatescharging for data if this is a requirement.

Using permanently open communication links, raw data can bestreamed continuously from the receivers to the server. Software running on the server computes therequired RTK/DGPS data continuously instandard RTCM V2.1/2.2/2.3/3.0 formats and/or in Leica, CMR and CMR+proprietary formats. The data are output in the required format(s) and transmitted to RTK and GIS rovers.

Radios, phones and even the Internet can be used to transmit the RTK/DGPSdata. For more details see 16.2 and 18.3.

RTK/DGPS

RTK/D

GPS

22

Reliable communication is vital for the efficient operation of GPS reference stations and networks ofstations.• The reference station software

running on the server has to control the receivers and download the data.

• RTK and DGPS data have to be transmitted for use by RTK and GIS rover receivers.

Communication technology and information technology are evolvingvery quickly. The availability of thevarious techniques - or at least thefeasibility of using them - can alsovary considerably from country tocountry.

When selecting the most suitablemethod for communication betweenthe server and the receivers, variousfactors should be considered carefully, including:

• What the station or network of stations is to be used for.

• The communication technologies that are available locally and for which reliable support is available.

• The cost of the communication equipment and the installation costs.

• The running costs.• The service and support costs.

When making a decision on the mostsuitable means for transmitting RTKand DGPS data to RTK and GIS rovers,the following should also be takeninto account:

• The number of RTK and/or GIS rovers that the station or network has to support.

• The range at which RTK rovers have to operate.

• The communication equipment that will be needed by RTK and GIS rovers.

• The cost of this equipment and the running costs.

From the above it will be obvious thatthere is no standard solution. What isbest for a station or network in onecountry may not be appropriate for astation or network in another country.

The following chapters outline thevarious communication methods thatmay be considered. The final decisions as to which methods are themost suitable should only be takenafter consulting carefully with communication and information companies and specialists that understand the requirements and that are thoroughly familiar with the local market.

14. The need for reliable communication

23

15. Communication between the server and the receivers

The reference station software runningon the server (computer) has to control the receivers and downloadthe internally logged data files.

For some applications it may benecessary to stream the raw data continuously (second by second) from the receivers to the server instead of downloading the loggeddata files. Some organizations mayeven prefer to have both streameddata and downloaded files.

For controlling the receivers anddownloading logged data files, thecommunication links can be eitherdial-up (i.e. opened when required) orpermanently open (i.e. continuouslyon). Dial-up links will usually be preferred, however, as the costs aregenerally lower than for permanentlyopen links.

If the raw data have to be streamedcontinuously, permanently open communication links are essential.

Note that the costs of permanentlyopen links - in particular the runningcosts - are usually higher than thoseof dial-up links.

15.1 Dial-up links (open when required)

If standard telephones are available atthe reference station sites, standardlandlines and telephone modems canbe used to connect the receivers tothe server. The reference station software running on the server willdial the receivers and download the logged files automatically at presettimes.

For a small network, one telephonemodem at the server should be sufficient. Such a network should berelatively easy to set up and shouldnot be too expensive.

Mobile phone modems (GSM, CDMA,TDMA, GPRS etc) can be used if standard telephones are not availableat the reference station sites. The phones have to be powered and permanently switched on. Runningcosts (call charges) will often be higher than with standard landline.

Another solution could be to use theInternet. See 17 for more details.

logged data files

Dial when required

24

15. Communication between the server and the receivers (continued)

15.2 Permanently open links (continuously on)

If the raw data have to be streamedcontinuously from the receivers to theserver, the communication links between the receivers and the serverhave to be permanently open.

Standard landline telephone modemsand mobile phone modems can beused, however the running costs (callcharges) will often tend to be high. It is also worth investigating the possibility of using leased lines, asthese will usually provide very high reliability.

Note that a port and telephonemodem will be needed at the serverfor each reference station from whichdata is to be streamed. Thus a 10-station network will require 10 permanently open lines, a modem ateach of the 10 receivers, and 10modems at the server. Another possibility is to use a telephone

access router at the server. Note thatthe costs of this hardware can add up!

The trend today is to try to make useof the Internet to achieve permanentlyopen links for streamed data betweenthe receivers and the server. Runningcosts with the Internet should be significantly lower than with telephone connections. See 17 for more details.

Permanently open link

Streamed raw data

25

16. Communication: transmission of RTK and DGPS data

The GPS receivers used at referencestations can output RTK and DGPSdata from one or more ports in standard RTCM formats and/or in proprietary formats. See 13.1. Thedata can be transmitted directly fromthe receivers.

If raw data are streamed continuouslyfrom the receivers to the serverthrough permanently open links, software running on the server cancompute and output RTK and DGPSdata in standard RTCM formats and/orin proprietary formats. See 13.2.

16.1 Transmission directly from the receivers

One way to transmit RTK and DGPSdata directly from the receivers is touse radio modems. If two radiomodems are attached to a receiver,they have to transmit at different frequencies (use different channels).RTK and GIS rover receivers have to beequipped with compatible radiomodems. The advantages are that any

number of RTK and GIS rover receivers can receive the data andthat the users of rover receivers donot incur telephone charges.

Another common solution is to attacha telephone modem - landline or evena mobile - to the reference station receiver. RTK and GIS roverreceivers with suitable mobile telephone modems dial in to obtainthe required RTK and DGPS data. Atelephone connection usually permitslonger RTK and DGPS range than aradio modem, but the rovers incurtelephone charges and only one roverat a time can dial in to one telephoneat a reference station.

If a suitable telephone access routeror a bank of telephone modems isattached to the reference stationreceiver, several rovers (perhaps 5 or10 or even more) can dial in to thestation at the same time. The morerovers the router or bank of modemshas to support, the more it is likely tocost.

Transmission by both radio and tele-phone is possible if a radio modem isattached to one port of the receiverand a telephone modem (or bank ofmodems or router) is attached toanother port.

Another solution could be to use theInternet. See 18 for more details.

RTK / DGPS RTK / DGPS

Radio modemTelephone modem (or banks of phone modems or telephone access router)

26

16. Communication: transmission of RTK and DGPS data (continued)

16.2 Distribution (transmission) from a control center

Some organizations prefer to distribute the RTK/DGPS data fromone central location rather than trans-mitting directly from the receivers.

The raw data have to be streamedcontinuously from the receivers to theserver thorough permanently opencommunication links as outlined in15.2. Software running on the server computes the requiredRTK/DGPS data in standard RTCM formats and/or in proprietary formatsfor each reference station.

The RTK/DGPS data for a specific reference station will be output via adedicated port, thus the number ofRTK/DGPS output ports at the servershould equal the number of referencestations. Radios, phones or even theInternet can then be used to distribute the RTK/DGPS data.

If radios are used, the RTK/DGPS data for different reference stationshave to be transmitted on different frequencies (use different channels) inorder to prevent interference. As therange of radio modem transmission isusually limited, radio transmission stations or repeaters may be neededto provide full coverage for a networkarea.

If phones are used, a unique phonenumber will be needed for each

reference station for which RTK/DGPSdata is available. Telephone accessrouters or banks of modems will beneeded in order to ensure that several rovers can access theRTK/DGPS data from the same station at the same time.

It is possible to have a single tele-phone number for all reference stations if the rover receivers that areequipped with mobile phones cansend their positions to the server.

RTK / DGPS

RTK / DGPS

Banks of phone modemsor telephone access router

Server computesRTK / DGPS data

Radios: different frequenciesfor different stations

27

The rover phones the control centerand sends its position coordinates inNMEA format. A software componentrunning on the server decides whichreference station is closest to therover. A telephone access router transmits the RTK/DGPS data for thisstation to the rover. The router has tobe of sufficient size to allow simultaneous multi-user access to the RTK/DGPS data from all stations.

It is also possible to distribute theRTK/DGPS data from the control center using the Internet. See 18 formore information.

16.3 Radio versus phone: advantages and disadvantages

Radio modemsThe advantages of using radiomodems are that any number of RTKand GIS rovers can benefit from theservice and that there are no charges.

The disadvantage is that the range atwhich RTK and GIS rovers can operateis usually less than if phones areused.

• The higher the output power of the transmitting radio, the greater will be the range at which rovers can operate. Output power is restricted in many countries, however.

• The higher the radio antenna can be set up at the transmitting station,the greater will be the range at which rovers can operate.

• Use of high quality antennas at the transmitting radios and at the rover receivers will also improve the range.

• Transmission from UHF radio modems is “semi” line-of-sight. Obstructions, particularly at longer ranges, can cause loss of reception at the rover.

• Interference on adjacent frequenciesmay also sometimes lead to poor reception at the rover.

PhonesThe advantage of using phones is thatthe range at which rovers can operateis usually not limited by the communication means. The connection is also very reliable with no interference and little loss of reception due to obstructions. A disadvantage is that the rovers incur call charges.

A telephone access router or a bank of telephone modems will be required if several rovers have to accessthe RTK/DGPS data from the same reference station at the same time.Depending on the number of users ithas to support, this equipment can bequite costly.

Radio and phoneNote that it is possible to transmitRTK/DGPS data directly from the receivers or from a control center byboth radio and phone. Users of roverunits can then use the service thatthey prefer.

16. Communication: transmission of RTK and DGPS data (continued)

28

Modem

Server

IP Ports

INTERNET

There is a rapidly growing interesttoday in using IP-based methods forcommunication between the receivers and the server running thereference station software, and alsofor distribution of RTK and DGPS data.IP-based communication can be LAN,WAN, WLAN, Internet, Intranet, andRadio IP etc.

For simplicity, this document refers tothe Internet to represent all IP-basedmethods.

The main attraction of using theInternet for communication betweenthe server and the receivers is that itis usually possible to reduce runningcosts (charges). If raw data have to bestreamed continuously from the receivers to the server as outlined in15.2, the running costs with theInternet will usually be much lowerthan with telephone connections.

17. Using the Internet, or other IP-based methods, for communication between the server and the receivers

29

A possible disadvantage of using theInternet could be that the reliabilityand quality of standard Internet connections may not be quite as highas with standard telephone connections.

Internet connections of very high reliability can normally be guaranteedif dedicated (leased) lines or MPLS(MultiProtocol Label Switching) areused between the server and thereceivers. However, the cost is likely tobe higher than the cost of standardInternet connections.

The costs and reliability of the variousoptions can usually be clarified bydiscussing carefully with the localTelecom Company and Internet ServiceProvider.

In order to access the Internet, thereceiver at a reference station willrequire a modem, a Com Server orEthernet port, and a static IP address.The modem could be a telephone,cable, or broadband/ADSL modem.

The server will require a suitablemodem and one IP port for each reference station from which data willbe streamed (i.e. 10 IP ports are needed if data is to be streamed from 10 stations).

The modem could be a telephone,cable, or broadband/ADSL modem.However, if the server has to receivecontinuously streamed raw data simultaneously from several reference stations - which will oftenbe the case - the best would be a broadband/ADSL or cable modem witha suitably large bandwidth.

17. Using the Internet, or other IP-based methods, for communicationbetween the server and the receivers (continued)

The reference station software running on the server operates inexactly the same way with Internetconnections as with standard telephone connections.

30

18.2 Distribution from a single stand-alone reference station

At a stand-alone reference station, thereceiver will often be connected directly to the server (computer) running the reference station soft-ware. Raw data has to be streamedcontinuously from the receiver to theserver.

Software running on the server computes the required RTK/DGPS datain the required format (standard RTCM

18.1 Equipment needed by RTK and GIS rovers

RTK/DGPS data can also be distributed using the Internet. In order to access the Internet andobtain the required data, RTK and GIS rover receivers have to be equipped with Internet capable devices such as GPRS or CDMA phone modems.

18. Using the Internet to distribute and access RTK and DGPS data

formats or proprietary formats; see12.2). The server needs one IP portfor output of the RTK/DGPS data tothe Internet.

RTK and GIS rovers, equipped withInternet capable devices, access the IPport and obtain the requiredRTK/DGPS data. Multiplexing softwarerunning on the server allows severalrovers to access the IP port and theRTK/DGPS data at the same time.

Server with one IP portand multiplexing software

Rovers with Internet capable devices

INTERNETStreamed

raw dataRTK / DGPS

RTK / DGPS

31

18.3 Distribution from a network control center

In case of a multi-station network, the server will be at a control center.Raw data have to be streamed continuously from the receivers to theserver. The connection between theserver and the receivers can be via theInternet as explained in 17. Softwarerunning on the server computes therequired RTK/DGPS data in the required format (standard RTCM formats or proprietary formats; see12.2) for each reference station.

The computed RTK/DGPS data can beoutput via the Internet. The serverneeds one IP port for each referencestation for which RTK/DGPS data areto be output (i.e. if RTK/DGPS data areto be output for 10 stations, 10 IPports are needed).

RTK and GIS rovers, equipped withInternet capable devices, access the IPports and obtain the RTK/DGPS datafor the stations. Multiplexing software

running on the server allows severalrovers to access the RTK/DGPS datafrom the same the IP port at the sametime.

It is possible to have a single IP portfor all reference stations if the roverreceivers that are equipped withInternet capable devices can sendtheir positions to the server.

18. Using the Internet to distribute and access RTK and DGPS data (continued)

The rover accesses the server via the IP port and sends its position coordinates in NMEA format. A software component running on theserver decides which reference station is closest to the rover. TheRTK/DGPS data for this station arethen transmitted to the rover.Multiplexing software running on theserver allows several rovers to accessthe server via the same IP port at thesame time.

Server with IP portsand multiplexing software

Streamed raw data

INTERNET INTERNET

RTK / DGPS

RTK / DGPS

32

18.4 Summary of the various ways by which RTK/DGPS data can be distributed

Using radio modems (see 16)

Any number of rovers can access thedata and the rovers do not incur running costs (charges). Limitedrange, obstructions and interferencecan be drawbacks.

Using phones (see 16)

The connections are reliable and thereare few restrictions to range. However,rovers incur calling charges. Routers orbanks of modems are needed in orderthat several rovers can access thedata from the same station at thesame time.

18. Using the Internet to distribute and access RTK and DGPS data (continued)

Using the Internet

The running costs (call charges) forrovers should be less than with mobile phones, but the reliability ofthe connections may not always bequite as high. There are few restrictions to range at which connections are possible and multi-user access can be arranged.

In order to compare the use of theInternet with the use of mobile phones, the best is to clarify all costand reliability issues very carefully withthe local Telecom Company andInternet Service Provider.

INTERNET

33

19. NTRIP - Networked Transport of RTCM via Internet Protocol

NTRIP is a new protocol, developed bythe Federal Agency for Cartographyand Geodesy of Germany, for streaming Global Navigation SatelliteSystem (GNSS) data over the Internet.NTRIP has become an RTCM standard.

NTRIP provides certain advantages fora reference station software serverwhen distributing RTK/DGPS data toRTK and GIS rovers. NTRIP can be usedfor distributing data in any format, forexample in standard RTCMV2.1/2.2/2.3/3.0 formats and/or inLeica, CMR and CMR+ proprietary formats.

With NTRIP, all communication anddata passes through a single IP port at the server. RTK and DGPS roverscan only obtain data if they are authorized, thus the network operatorhas full control over access to thedata. This facilitates billing (chargingfor data) if this is a requirement.

An RTK or DGPS rover that accessesthe server IP port can request datafrom a so-called mount point orrequest a list of the available mountpoints.

For example the mount points couldbe the sources for the RTK and/orDGPS data from the various referencestations, the source for the networkdata in RTCM V3.0 format and network correction parameters (see20.3), the source for network corrected RTK data (see 20.4), etc.

If the requested mount point is available, the data from this sourceare transmitted to the rover. If therequested mount point is not available, the rover has to select another mount point (source of data)from the list.

• At 1km: 10mm +1mm = 11mm rms• At 10km: 10mm +10mm = 20mm rms• At 30km: 10mm + 30mm = 40mm rms

If RTK measurements could be carried out in an absolutely perfect environment, there would be no ppm accuracy component and no restriction to range. Unfortunately, however, the environment is never perfect and it also changes continuously.

Satellite orbits

Troposphere

Ionosphere

34

20. Continuous network analysis and calculation of networkcorrection parameters for enhanced RTK

20.1 Accuracy and range of RTK: the influence of distance dependent errors

When receiving standard RTK datafrom a reference station, an RTK rovercan usually operate successfully (i.e.resolve ambiguities) at ranges up toabout 30km in reasonably favourable conditions. In some regionsof the World, where conditions forRTK are often exceptionally good, ranges of about 40km or more may be achieved at times. In other areas,especially during the afternoon and inperiods of high ionospheric activity,the maximum range for RTK may bereduced significantly.

The accuracy of RTK is normally quoted as about 10mm + 1ppm rootmean square. Thus the positionalaccuracies that can be expected areapproximately as follows:

Various influences, particularly thoserelating to ionospheric delays, tropospheric delays and satelliteorbits, lead to distance dependenterrors and restrict the range at whichthe rover can resolve the ambiguities.It follows that if the distant dependent errors within the networkcan be modelled with a reasonabledegree of success and appropriatecorrections can be computed and applied, the accuracy and range ofRTK can be improved.

35

20. Continuous network analysis and calculation of network correctionparameters for enhanced RTK (continued)

20.2 Network analysis and computation of network correction parameters

The raw data have to be streamedcontinuously from the receivers at thenetwork reference stations to the server through permanently opencommunication links. Use of theInternet will generally be preferred, as the running costs should be muchlower than with telephone connections (see 17 and 15.2).

A network analysis component of thereference station software processesthe incoming data continuously inorder to analyze the state of the environment within the network. It models the distance dependenterrors and computes network correction parameters. The entire process runs continuously and automatically.

There are two ways in which the network analysis and network corrections parameters can be used toimprove the performance of RTK:

• The network data in RTCM V3.0 format and the network correction parameters can be transmitted to the rover and the rover applies them.

• The network correction parameters can be applied at the server and theserver transmits “corrected” RTK data to the rover.

Note that if a network has a very largenumber of stations, it will be split intocells (subsets of the network).

36

20. Continuous network analysis and calculation of network correctionparameters for enhanced RTK (continued)

20.3 Network data in RTCM V3.0 format and network correction parameters are transmitted to the rover and the rover applies them.

The network analysis software running on the server produces therequired RTCM V3.0 data for the net-work. This RTCM V3.0 data includesthe network correction parameters.

In order to be able to use the data,RTK rover receivers have to be RTCMV3.0 compatible. They have to understand the RTCM V3.0 format andthey have to be able to apply the network correction parameters.

The RTK rovers operating in the network area receive the RTCM V3.0 network data with the network correction parameters. This data contains all of the required information for the entire network, i.e. for all stations in the network.

Depending on how RTK processing isimplemented in a particular roverreceiver, the rover can now processthe baseline from one reference station (probably from the closest station), or perhaps even the

baselines from several stations, resolve the ambiguities and deriveposition coordinates. The results arelargely free of the distance dependenterrors described in 20.1.

RTCM V3.0 for entire network

with network correction parameters

One-way communication

All RTK rovers receive the data stream

Network corrections applied at the rovers

Server

37

The accuracy and range are superiorto those of standard RTK.

As the RTCM V3.0 data contains theinformation for the entire network(i.e. for all stations), all RTK roversoperating within the network canreceive exactly the same information.Thus only a single RTCM V3.0 datastream has to be output by the server.

If a network has a very large numberof stations, it will be split into cells(subsets of the network).

Radios, phones or the Internet can beused to distribute the RTCM V3.0 datato RTK rovers. The communicationneeds to be one-way only, i.e. serverto rover. The rover does not sendinformation to the server.

If radios are used, the RTCM V3.0 network data stream will probablyhave to be redistributed via repeaterstations or transmission stations inorder to ensure full coverage over thenetwork area.

If phones are used, all rovers shouldbe able to dial a single number for the RTCM V3.0 network data stream. A suitable router will be needed toensure simultaneous multiple useraccess.

If the Internet is used, all roversshould be able to access the same IPaddress for the RTCM V3.0 networkdata stream. Multiplexing softwarerunning on the server will allow simultaneous multiple user access.

20.4 Network correction parameters are applied at the server and the server transmits “corrected” RTK data to the rover.

The network analysis software running on the server analyzes the network and computes the network correction parameters for the network as explained in 20.2. With this method,however, the network correction parameters are applied at the server and not at the rover.

In order to obtain the required data, the RTK rover has to transmit its position in an NMEA message to the server. Using this position information,the network analysis software identifieswhich reference station is closest to therover.

The software then applies the network correction parameters to themeasurements from this reference stationin such a way that a baseline computa-tion between the reference station andthe rover should be free of the distancedependent errors described in 20.1.

20. Continuous network analysis and calculation of network correctionparameters for enhanced RTK (continued)

A suitable telephone access router will be needed to ensure simultaneous multiple user access.

If the Internet is used, all rovers willaccess the same IP address in order toobtain “corrected” RTK data. The software ensures that each roverreceives the RTK data stream that itrequires. Multiplexing software running on the server permits simultaneous multiple user access.

Each rover receives its own individual corrected data streamNetwork correction parameters

applied at the server

Rover transmits position to server

Corrected RTK data transmitted to rover

Two way communication

38

“Corrected” RTK data in the formatsRTCM V2.3/3.0 and Leica, can then beoutput for transmission to the rover.The RTK rover receives the data andprocesses the baseline from the reference station, resolves the ambiguities, and derives positioncoordinates.

As the RTK data received by the roverare already “corrected”, the accuracyand range are superior to those ofstandard RTK.

Note that as the “corrected” RTK datacan be transmitted in RTCM V2.3 andLeica formats, rover receivers do nothave to be RTCM V3.0 compatible.

As the RTK rover has first to send itsposition to the server, two-way communication is needed. Thus phones and the Internet can be usedbut not radios.

As the “corrected” RTK data transmitted to the rover depends onthe position of the rover, all rovingreceivers operating within the network area will receive differentindividual data steams.

If phones are used, all rovers will diala single number in order to obtain“corrected” RTK data. The softwareensures that each rover receives theRTK data stream that it requires.

20. Continuous network analysis and calculation of network correctionparameters for enhanced RTK (continued)

39

20.5 Comparison of the two methods

With the first method described in20.3, the rover receivers have to beRTCM V3.0 compatible in order to beable to use the transmitted data.Radios as well as phones and theInternet can be used. The softwareneeded at the server will be somewhat less complex than for thesecond method described in 20.4.

With the second method described in20.4, the RTK data can be output inany required format. Rover receiversdo not need to be RTCM V3.0 compatible. Phones and the Internetcan be used but not radios. The software needed at the server will beslightly more complex than for thefirst method described in 20.3.

20.6 Improved RTK performance

Both of the methods described abovewill improve the accuracy, reliabilityand range of RTK.

• Accuracy will be improved, as the ppm accuracy component - i.e. the distance dependent component - will be reduced significantly.

• Thus RTK accuracy will be much more uniform at different distances.

• The reliability of RTK position fixes (ambiguity resolution) will be improved, particularly under difficult ionospheric conditions and at longer ranges.

• The time needed for RTK position fixes will be reduced when operating at long ranges and under difficult ionospheric conditions.

• The maximum range will be improved.

As the range of RTK is increased, reference stations can be further apart.Thus fewer stations are needed to provide RTK coverage for a given area.

Note that the reference stations shouldbe reasonably evenly distributed throughout the area in order that thesoftware can effectively analyze the state of the environment within the network and compute the appropriatenetwork correction parameters.

20. Continuous network analysis and calculation of network correctionparameters for enhanced RTK (continued)

Standard RTK

RTK with network correction parametersLonger rangeHigher accuracy

40

RINEX (Receiver INdependentEXhange) is a standard data formatthat is universally accepted by the GPSsurveying community and that can beused by all commercially available postprocessing software.

The reference station software running on the server downloads theraw data files logged in the receivers, converts the data to RINEX, and produces RINEX files of the requiredlength (e.g. 1 hour, 2 hours, 3 hoursetc.). The software can also createRINEX files from the raw data that arestreamed continuously from the receivers to the server.

Using RINEX files of reference stationdata, users of GPS field equipmentcan post process baselines from reference stations and determine positions to very high accuracy. Postprocessing is often used to determinehigh order geodetic control points.

There is also almost no practical rangelimit to post processing. With sufficient data, very long baselines canbe processed successfully.

In order to make RINEX data readily available for the GPS user community, the reference station software can transfer the RINEX filesautomatically to an FTP server. FTPstands for File Transfer Protocol and isused to facilitate the distribution offiles via the Internet.

There is a range of FTP server software available and your IT support personnel will certainly beable to suggest a suitable product.

FTP server software can usually be configured for public and/or restrictedaccess. It can be arranged that anyone can obtain data or - which willoften be preferable - that users haveto be registered and enter passwordsin order to access the RINEX files.

Although the reference station serversoftware and the FTP server softwarecould run on the same computer, theneed for a firewall to protect the reference station server will usuallymean that it is preferable to use separate computers.

21. Using an FTP server to distribute RINEX data

Server running reference station software

FTP ServerFirewall Internet GPS Usercommunity

INTERNET

RINEX files

Raw data files

22.2 Checking the positions of the antennas

Although antennas that are set up ina stable manner at safe locationsshould not move, some organizationsthat are responsible for reference stations still prefer to be assured atregular intervals that the antennashave not been disturbed.

41

22.1 Processing the baselines between the stations of a network

Baselines between the reference stations of a network can be computedin real time or by post processing.

For real time processing, raw datahave to be streamed continuously from the receivers to the server running the reference station software.A software component computes thebaselines between the reference stations continuously in real time asthe data are received. The computationprocess resolves the ambiguities continuously and is similar to the realtime processing in an RTK receiver. Theaccuracy that can be expected and thelength of baselines that can be processed are also essentially the sameas for RTK receivers.

For post processing, data files loggedin the receivers and downloaded atregular intervals are normally used.

22. Processing baselines between stations to check antennapositions and monitor movements of natural and man-made structures

The reference station software passesthe files to a post processing softwarethat can be configured to process thebaselines automatically as the files arereceived. With sufficient data - i.e.from sufficiently long observation periods - post processing can successfully compute baselines ofalmost any length and achieve higheraccuracy than real-time processing.

Coordinate results can be output forfurther analysis.

42

The positions (coordinates) of theantennas can be determined by realtime processing or by post processing as explained in 22.1.

As the requirement is usually one ofperiodic quality control, it is often sufficient to post process the baselines between stations once aday, or once a week, or perhaps evenonce a month. Depending on the software, processing can often beorganized to run automatically at therequired time intervals.

22.3 Monitoring movements of theEarth's crust, and of natural and man-made structures

Networks of GPS reference stationsprovide the most effective method formeasuring movements between tectonic plates, in earthquake zones,along major fault lines, and in areas ofhigh volcanic activity.

Small networks with short baselinesare often very suitable for monitoring

the positions and movements of man-made structures such as dams,bridges, buildings, oilrigs etc.

If rapid, short-term movements, andeven vibrations, have to be detectedand monitored, real time processingas described in 22.1 will usually beneeded. Real time processing produces a continuous stream of independent position fixes (coordinates). Movements can beidentified and analyzed.

• Real time processing uses raw data that are streamed continuously through permanently open communication links. Running costs for permanently open links can be high.

• Post processing can use data files that are logged internally in the receivers and communication links that need to be opened only when files have to be downloaded. Thus running costs may be lower thanwith permanently open links.

If the movements that have to bemonitored are known to be slow andto occur over a relatively long periodof time, it may be more economicaland more suitable to use post processing.

Using data files of appropriate lengths,baselines between reference stationscan be processed at required timeintervals, perhaps every 12 hours, 24hours or even 7 days. If required, theresults could be passed to an adjust-ment package for automatic leastsquares network adjustments.

Post processing will certainly be preferred if the baselines betweenstations are very long. Networks withstations that are far apart and thatcover very large areas are often usedfor monitoring the movements of tectonic plates.

22. Processing baselines between stations to check antenna positionsand monitor movements of natural and man-made structures (continued)

43

GPS baseline processing - whether inpost processing or in real time mode -solves a baseline between a knownreference and an unknown rover position. The baseline computation isalways carried out in the WGS84 coordinate system. Thus WGS84 coordinates of the rover are determined relative to the WGS84coordinates of the reference. Anytransformation to local coordinates iscarried out afterwards.

For successful baseline processing, theWGS84 coordinates of the referenceshould be known accurately, to atleast about + or - 10m. As the purpose of reference stations is toserve as the basis for determining the positions of rover receivers, it followsthat accurate WGS84 coordinates areneeded for the reference stations.

There is a global network of over 300IGS (International GPS Service) stations. The coordinates of these stations are given in an InternationalTerrestrial Reference Frame (ITRF). For simplicity one can say that theITRF coordinates of the IGS stationsare just about the most accurateWGS84 coordinates available.

23. Determining the WGS84 coordinates of the reference stations

44

Many countries already have a national network of GPS reference stations. Such a network will usuallyhave been connected to the IGS network and the stations will haveaccurate WGS84 coordinates.

When establishing a new stand-alonereference station or a new network ofstations, it will usually be preferable toconnect the new station or the newnetwork to the national network.Contact the national survey organization for information on thenational network and how to obtaindata from the national stations.

If there is no suitable national network, the best is to connect thenew station or the new network ofstations to the IGS network.• For general information on the IGS

network go to the IGS web site (see 30. Useful links).

• For information on IGS stations and how to obtain RINEX data files consult the SOPAC web site (see 30. Useful links).

• Note that post processing software may provide links to the closest IGS stations and facilitate the download of RINEX data files.

To determine the WGS84 coordinates ofa new stand-alone station or the“master” station of a new network, proceed as follows:

• Select one station of the new network as the “master”.

• At the master log data (1 minute rate is sufficient) for at least 24 hours, and possibly for up to 7 days.

• Download RINEX data from the closest IGS station or from the closest national station.

• Process the baseline from the IGS station, or from the national station, to the master using 24 hours of data. If data have been logged for several days, process the baseline for different 24-hour periods and take the mean.

• Perhaps process the baselines to the master from an additional one or two IGS, or national, stations. Take the mean.

• This will provide very accurate WGS84 coordinates for the master and link the master to the IGS, or national, network.

The WGS84 coordinates of the stationsof the new network have to be fiducially accurate, i.e. the network hasto have a very high degree of relativeaccuracy between the stations. Theeasiest is to proceed as follows:

• At all stations log data (1 minute rate is sufficient) for at least 24 hours, and possibly for several days.

• Using the WGS84 coordinates of the master as the starting point, process all the baselines of the network using 24 hours of data. Possibly repeat for another one or two 24-hour periods. Adjust the network if adjustment software is available.

• This will provide WGS84 coordinates for all stations. The coordinates will have a high degree of relative accuracy.

23. Determining the WGS84 coordinates of the reference stations (continued)

45

If the prime purpose of a network is tomonitor a natural or man-made structure - e.g. a fault line, volcano,dam, bridge etc. - the positions of the stations will be determined largely bythe structure that has to be monitoredand the movements that are expected.

If a network is to be a geodetic control network that has to supply services to the GPS surveying community, careful thought should begiven to the positioning of the stations and the separation distancesbetween the stations. Consider the following:

• The extent of the area to be covered.• The areas of high development and

high population.• The areas of low development and

low population.• The services that have to be

provided:Perhaps only RINEX data for post processing.Perhaps also RTK data.Perhaps also DGPS data.

• The number of GPS rover receivers operating in the area and that will use the services.

• The available budget.

GPS reference stations should be setup where they are needed most andwhere they can provide services to thelargest number of users. This will usually mean establishing more stations in highly developed areas andless stations in under developedareas.

For relatively small highly developedareas, such as in the regions of majorconurbations where many RTK roversare operating, GPS reference stationswill usually have to transmit RTK data.The range limit at which an RTK rovercan operate successfully (i.e. resolveambiguities in real time) will have tobe taken into account when determining the separation distancebetween stations.

Depending on the requirement, therange of RTK that is typically achievedin the area, whether standard RTKdata is transmitted, or whether thereference station software providesnetwork correction parameters forenhanced RTK (see 20), one couldimagine that the separation distancebetween the reference stations couldbe between about 30km and 70km.

In very large countries, with immenseopen spaces of undeveloped land andwhere few RTK rovers are operating, itwill usually be pointless and uneconomical to try to achieve RTKcoverage. A suitable approach couldbe to set up reference stations only atthe major population centers andthose areas where there are major development projects. If sufficient RTKrovers are being used, these stations could transmit RTK data toprovide RTK coverage within about a20km to 30km radius.

24. Where should the stations be positioned? What should bethe distances between the stations?

46

A surveyor carrying out a project at along distance from the reference stations would have to set up a temporary field reference, log data fora sufficient period of time, downloadthe RINEX files from the reference stations, and post process to obtainan accurate position.

The temporary reference station wouldthen transmit RTK data and providecoverage for the project area.

Using several hours of data, post processing will provide good results(5mm + 0.5ppm or better) with baselines of 100km, 200km, 300kmand even longer. Thus, if necessary,for very large undeveloped areas, thedistances between GPS reference stations could even be up to severalhundred kilometres.

24. (continued)

Reference stations and networksrequire significant investments.Running costs, particularly for networks, have also to be considered.

Some organizations establish stand-alone reference stations or networkspurely for their own use.

Other organizations provide productsand services for the GPS user community: they allow access to RINEXfiles and possibly raw data files on anFTP server, they distribute RTK andDGPS data, and they may even provide transformation parameters inorder that RTK and GIS rovers caneasily transform WGS84 values intothe local coordinate system.

If users of GPS rover receivers are ableto obtain the required data easily andreliably from permanent reference stations, they derive benefits from the services and do notneed to invest in additional receiversfor use as temporary field references.

Many organizations that operate reference stations and networks areinterested in recovering at least partof their investment and covering theirrunning costs. They would like to charge for the data and the servicesthat they provide.

25. Charging for products such as RINEXand RTK/DGPS data

47

If users have to be registered andenter passwords to access an FTP server and obtain RINEX files and possibly raw data files, it should berelatively easy to generate a log ofwho has logged in and obtained data.Charges could then be made fordownloaded data. It may also be possible to ask for a registration fee.

If RTK/DGPS data are distributed byradio, it is almost impossible to arrange a charging system. A solutioncould be that the authority runningthe reference stations sells or rentsthe required radio modems to users ofRTK and GIS equipment.

It is easier to create a charging systemif users of RTK and GIS rovers obtainthe required data by telephone orInternet.

If RTK and GIS rovers use mobile tele-phone modems to obtain RTK/DGPSdata, it may be possible to arrangewith the telephone company that partof the charges are credited to theauthority operating the reference stations.

Another possibility would be that RTK and GIS rovers, with mobile telephone modems or Internet IPaddresses, would have to registertheir telephone numbers or IP addresses with the authority operating the reference stations. Only telephone and Internet connections from registered deviceswould be able to obtain data.Software at the control server couldcreate a log showing the durationduring which each registered device(i.e. user) had obtained data. Charging could then be arranged.

Another possibility would be thatusers of RTK and GIS rovers, withmobile telephone or Internet connections, would have to enterpasswords. Only those rovers able to authenticate themselves (i.e. make connections with registered passwords) would be able to obtain data. Software at thecontrol server could create a log showing the duration during whicheach password (i.e. user) had obtained data. Charging could then be arranged.

25. Charging for products such as RINEX and RTK/DGPS data (continued)

48

From the preceding chapters it will beobvious that reference stations andnetworks can be set up and configu-red in many different ways, for manydifferent applications, and with manydifferent levels of complexity. Thesolutions will vary according to therequirements, the areas to be covered, the services to be provided,and the available budgets.

The following examples are intendedonly as general guides and providesuggestions for possible solutions.They should not be taken as completerecommendations.

26.1 Stand-alone reference stationfor a small area such as an opencast mine, construction site, or local community

GPS survey work on construction sitesand in opencast mines is usually carried out with RTK. Thus a singlestand-alone reference station transmitting RTK data by radio directlyfrom the receiver may be all that is

required. In most cases it will be preferable to have reference station software running on a PC to controlthe receiver, and to download andarchive logged data.

A single stand-alone reference stationmay also be perfectly sufficient for asmall community. Radio and/or phonecould be used to transmit RTK/DGPSdata. A router or bank of phonemodems would be needed in orderthat several users could phone in atthe same time.

Reference station software running on a PC will control the receiver,

download and archive data. With a PC connected, it is also possible to distribute RTK/DGPS data to a numberof users at the same time via theInternet.

For civil engineering and constructionsites where very high accuracy isrequired, for example for a bridge, itmay be desirable to set up a secondindependent reference station in orderthat RTK rovers can obtain two independent position fixes at criticalpoints. A single PC (server) runningthe reference station software couldcontrol the two reference stationreceivers; this would then become atwo-station network.

26. Some examples of stand-alone reference stations and networks

about 30 kmRTK coverage RTK coverage

Such a network would be relativelysimple and easy to set up and, asthere would be no permanently openlines, the running costs should be low.

49

26.2 A simple network of a few stations providing complete RTK coverage for a relatively small well developed area.

In developed areas with high populations, such as around metropolitan areas, most GPS surveywork will be carried out with RTK.Thus the main requirement will be toprovide complete RTK coverage.

Purely as an example, imagine thatRTK/DGPS services have to be provided throughout an area of about60km x 60km. A relatively simple solution would be to have 4 or 5 reference stations spaced about 30kmapart. This would provide good RTKcoverage throughout the area.

Dial-up lines could connect the stations to a central server runningthe reference station software. The software would control the receivers,download logged data files, and pushRINEX files to an FTP server.

RTK/DGPS data could be transmitteddirectly from the receivers by radioand/or phone. The radios at the stations would have to be set to different channels (frequencies). Arouter or bank of phone modemswould be needed at each station inorder that several users could dial in at the same time.

26. Some examples of stand-alone reference stations and networks (continued)

abou

t 60

km

about 60 km

50

26.3 A more complex networkproviding complete RTK coverage for a relatively largehighly developed area.

If the area for which RTK coverage isrequired is relatively large, there aretwo possibilities:i) As outlined in 26.2 but with morestations. However, this will meanincreased costs for equipment andinfrastructure.ii) Place the stations further apart anduse software for continuous networkanalysis and calculation of networkcorrection parameters. This will allowRTK at longer ranges (see 20.).

The following example is based onpossibility ii), i.e. using software forcontinuous network analysis and calculation of network correction parameters.

Imagine that RTK/DGPS services haveto be provided throughout an area ofabout 200km x 200km. With about 9

reference stations and a separationdistance between stations of about70km, RTK rovers will usually be within about 35km of a station and the maximum distance from a stationshould not be more than about 50 km.

For continuous network analysis andcalculation of network correction parameters, raw data have to be streamed continuously from the receivers to a central server running the reference station software.

Permanently open communicationlinks between the receivers and theserver are required. The best is to usethe Internet, as running costs (charges) will be lower than with permanently open telephone lines.

The software running on the serverwould control the receivers, download logged data files, and push RINEX files to an FTP server.

26. Some examples of stand-alone reference stations and networks (continued)

abou

t 20

0 km

about 200 kmNet

wor

k ar

ea

–

RTK

cove

rage

Continuous networkanalysis

Fewer stationsneeded

51

In order to increase the range atwhich RTK rovers can operate successfully, the software would also:• Transmit the network data in RTCM

V3.0 format and the network correction parameters to RTK rover receivers. Transmission could be by radio, phone or Internet.

• And/or• Apply the network correction

parameters at the server and transmit “corrected” RTK data to therover receivers. Transmission could be by phone or Internet.

See section 20 for full details.

As the distribution of RTK/DGPS dataand logged data is all controlled froma central server, it should also be possible to set up a system for charging for data if this is a requirement.

Such a network would be more complex than that outlined in 26.2 but it would require fewer stations for the area to be covered.

26.4 A network of a few stations providing RINEX data and limited RTK/DGPS for a very large undeveloped area

Some parts of the world have lowpopulations and/or are undeveloped.As in such regions there will be fewRTK and DGPS rovers operating, it willusually be uneconomical to attempt toachieve RTK coverage. A few referencestations that are widely separated maybe quite sufficient.

As an example, imagine that a geodetic control network is neededfor a region of about 1000km x1000km where there are only a fewsmall centers of population.

The most economical solution wouldbe to set up a reference station ateach of the important population centers. If there is sufficient demand,these stations could transmitRTK/DGPS data by radio and/or phoneto provide local RTK coverage (e.g.within a radius of about 30km).

26. Some examples of stand-alone reference stations and networks (continued)

About 1000 km

About

100

0 km

52

26. Some examples of stand-alone reference stations and networks (continued)

With large distances (perhaps up toseveral hundred kilometres) separatingthe reference stations, most of theregion would have no RTK coverage at all.

Although, if necessary, the referencestations could be single stand-alonestations, the best would be to connectthe receivers by dial-up lines to a central server running the referencestation software. The software wouldcontrol the receivers, download logged data files, and push RINEX filesto an FTP server.

A survey crew operating in a remotearea would have to set up a tempo-rary field reference station, log datafor a sufficient period (perhaps several hours), download the RINEXfiles from one or more reference stations, and post process the baselines from these stations. Thiswould provide position coordinateswith an accuracy of a few centimetresthat could then be used as the starting point for the new survey.

26.5 A combined approach: net-works of stations providing RTK/DGPS coverage for highly developed areas, widely spaced stations providing RINEX data throughout undeveloped areas.

Some countries or regions have one or more large, highly developed conurbations, in which millions ofpeople live and work, separated byvast areas of thinly populated land.

Whereas RTK/DGPS services will beessential for the developed zones itwill often not be economical to provide RTK coverage for the thinlypopulated areas.

A network of widely spaced referencestations supplying RINEX data as outlined in 26.4 will usually be sufficient for the less developed areas with low population.

Networks of more closely spaced stations providing full RTK coveragewill be needed for the highly developed zones surrounding themajor cities.

Depending on the areas to be covered, such networks could be asdescribed in 26.2 (without continuous network analysis and calculation of network correction parameters) or as described in 26.3(with continuous network analysis and calculation of network correction parameters).

A single server could handle the combined network.

In some cases, however, it may bepreferable to have separate servers foreach of the smaller “RTK” networksand for the larger “RINEX” network.The smaller “RTK” networks will ofcourse be linked to and form part ofthe larger “RINEX” network.

53

27. Meteorological and tilt sensors

27.1 Meteorological sensors

The delay in the GPS signals as theypass through the atmosphere provides valuable information formeteorologists. Some organizationsthat operate GPS reference stationssupply data to meteorological authorities. The data is used to assistweather forecasting and studying climatic change.

For such applications it is usuallyrequired to connect a meteorologicalsensor to the GPS receiver at the reference station. The meteorologicalsensor delivers temperature, humidity and pressure data, which arerecorded together with the GPS data.

The reference station software running on the server downloads thedata and provides the meteorologyand GPS data in RINEX format.

27.2 Tilt sensors

If the GPS antenna at a reference station is set up on a high column orpillar, some organizations prefer toattach a tilt sensor to the column orpillar. The tilt sensor is connected tothe GPS receiver.

A twin-axis tilt sensor will measure tiltin two directions. The data are recorded in the receiver together withthe GPS data. The reference station

software running on the server down-loads the data and provides the tiltdata in RINEX format. The tilt data canthen be analyzed in order to monitorany inclination of the column or pillar.

When studying whether to install a tiltsensor, it is worth considering the following:

•The higher the pillar or column, the more likely it is to tilt.

• High metal pillars or columns may undergo daily deformation due to heating by the Sun.

• A low well-constructed pillar with a good foundation is unlikely to tilt.

• If any tilt occurs it will often be slowand over a long period of time.

• Movement of the antenna can be determined to a high degree of accuracy by post processing the baselines between the stations as outlined in 22.2.

• Checking the positions of the antennas by post processing will often be all that is required by many organisations.

Met sensor

Tilt sensor

54

28. Protection against lightning

If a reference station is in an exposedarea and in a region where thunderstorms are likely to occur, it is advisable to protect the equipmentagainst lightning. Protection can beprovided by a lightning conductorand/or by a lightning surge arrestor.

28.1 Lightning conductor

A tall building will usually have a lightning conductor. This usually comprises one or more metal rodsconnected by a thick copper strip, thelower end of which is firmly embedded in the ground below.Specialized firms install lightning conductors for buildings.

A lightning conductor can be placedclose to or around the GPS antenna.In order not to obstruct the satellitesignals, the rod or rods should not betoo high. One solution is to have foursmall rods placed around the antennain such a way that they do not riseabove 10° above the horizon of theantenna.

If the antenna is on a building, thelightning protector should be connected to the lightning protectionsystem of the building and thus to theearth (this should be done by a specialist).

If the antenna is on a pillar, the light-ning protector will need a copper stripthat is correctly earthed. Alwaysemploy or consult a specialist.

55

28. Protection against lightning (continued)

28.2 Lightning surge arrestor

A surge arrestor is a small device thathas to be placed within the antennacable between the GPS antenna andthe receiver. It has also to be correctlyearthed.

If the antenna is on a building, thesurge arrestor should be connected tothe lightning protection system of thebuilding and thus to the earth (thisshould be done by a specialist). If theantenna is on a pillar, the lightningsurge arrestor has to be connected toa copper strip that is correctly earthed. Always employ or consult aspecialist.

The surge arrestor protects the receiver and any equipment connected to the receiver (e.g. a computer) from electrical surges caused by lightning strikes. But theGPS antenna is NOT protected.

There are several products on themarket and they are not expensive.

The manufacturers will advise whichproducts are the most suitable.

Employ or consult a specialist to ensure that the connection to earth iscarried out correctly.

Many organizations consider that it issufficient to install only a lightningsurge arrestor and not a lightning conductor.

56

only the stations and services that arereally needed. Afterwards, as therequirements increase, the number ofusers grows and additional funds areavailable, new stations and featurescan be added and the services thatare provided can be improved andenlarged.

The initial investment is never lost.

• What is required?• What applications have to be

supported?• What will the stations and/or net-

work be used for?• What is needed today?• What will be needed in future?• The area to be covered• Where to establish the stations?• What can be the separation distance

between stations?• The need for suitable sites with an

open view of the sky• Power, communication, security• The infrastructure that is readily

available and can be used• The new infrastructure that has to

be added• The type of users that have to be

supported• The number of users that have to

be supported• Is it sufficient simply to log data and

provide RINEX files?• Is it also required to transmit

RTK/DGPS data?

29. Points to take into account when deciding what type ofstations and/or networks are needed

• The most suitable methods of communication between the receivers and the server

• The most suitable methods for distributing RTK/DGPS data

• The most suitable communication for distributing RTK/DGPS data

• The cost of establishing the stations and/or network

• The cost of running the stations and/or network

• The running costs for RTK and GIS rovers

• Computing the baselines between stations to check the positions of the antennas

• The budget that is available today• The budget that will be available in

future• Charging for services and data

GPS reference stations and networksare readily scalable. They can be easilyenhanced and upgraded as require-ments change and the number ofusers increases. Thus, initially, it willoften be quite sufficient to establish

57

Leica Geosystems AGhttp://www.leica-geosystems.comFor further information on GPS referencestations and networks

UNAVCOhttp://www.unavco.org/facility/software/preprocessing/preprocessing.htmlFor TEQC tool.

IGS (International GPS Service)http://igscb.jpl.nasa.gov/For general information on the IGS network

EUREF (RINEX data)http://www.epncb.oma.be/_dataproducts/datacentres/index.htmlFor information on the UREF network.

SOPAChttp://sopac.ucsd.edu/cgi-bin/dbShowArraySitesMap.cgiFor information on IGS stations and howto obtain RINEX data files

30. Useful links

NGShttp://www.ngs.noaa.gov/CORS/instructions2/ For information on RINEX.

RINEXReceiver Independent Exchange Formathttp://igscb.jpl.nasa.gov/igscb/data/format/rinex210.txt

RTCM http://www.rtcm.orgFor information on RTCM formats.

ITRFInternational Terrestrial ReferenceFrame http://lareg.ensg.ign.fr/ITRF/

NTRIPhttp://igs.ifag.de/NTRIP.htm

NMEANational Marine ElectronicsAssociationhttp://www.nmea.org

58

Leica GRX1200 Product brochureut labore et.

Leica GPS SpiderProduct brochureut labore et.

Leica GPS1200 Product brochure

Leica SmartStationTotal Stationwith integrated GPS

LeicaSmartStationProduct brochure

Leica Geosystems AGSwitzerland

www.leica-geosystems.com

Whether providing corrections from just a single reference station, or an extensiverange of services from a nationwide RTK network – innovative reference station solutions from Leica Geosystems offer tailor-made yet scalable systems, designed for minimum operator interaction whilst providing maximum user benefit. In full compliance with international standards, Leica's proven and reliable solutions arebased on the latest technology.

Precision, value, and service from Leica Geosystems. When it has to be right.

Illustrations, descriptions and technical specifications are not binding and may change. Printed in Switzerland – Copyright Leica Geosystems AG, Heerbrugg,Switzerland, 2005. 747352en – IV.05 – RVA