General Guide to Static and Rapid-Staticutdallas.edu/.../English/Static-Rapid_3_0en.pdf · General Guide to Static and Rapid-Static-3.0.0en 7 Overall planning for a GPS ... method.

Version 3.0English

50403020

General Guide to Static and Rapid-Static

GPS System 500

2 General Guide to Static and Rapid-Static-3.0.0en

Congratulations on your purchase of a new LeicaSystem GPS500.

System GPS500

3General Guide to Static and Rapid-Static-3.0.0en

Introduction

Overall planning for a GPS survey

Mission planning

Observation times and baseline lengths

Field observations

Importing the data to SKI-Pro

Deriving initial WGS 84 coordinates for one point

Processing Parameters

Baseline selection - Strategy for computation

Interpreting the Results

Inspecting the logfile and comparing results

Storing the results

Adjustment, Transformation and output of results

Notes on single-frequency Static and Rapid Static measurements

View of chapters

View of chapters

6

7

13

15

17

22

23

24

30

32

34

36

38

39


Contents

Contents

Introduction ................................................... 6

Overall planning for a GPS survey .............. 7Baseline length ........................................................... 7Temporary reference stations for Rapid Static surveys . 8Check the newly surveyed points ............................... 9Night versus day observations. Measuring long lines . 10Observation schedule - best times to observe ........... 10Consider the transformation to local coordinates ........11

Mission planning ......................................... 13GDOP - Geometric Dilution of Precision .................... 13Selecting good windows for successful GPS surveying 13

Observation times and baseline lengths... 15

Field observations....................................... 17Reference site ........................................................... 17

Need for one known point in WGS 84 ............................. 18Observing new points ................................................ 19

Use the Stop and Go Indicator as a guide ....................... 19Fill out a field sheet ................................................... 20

Importing the data to SKI-Pro..................... 22Checking and editing during data transfer .................. 22Backing up raw data and projects .............................. 22

Deriving initial WGS 84 coordinates forone point ...................................................... 23

Processing Parameters .............................. 24Cut-off angle ............................................................. 24Ephemeris ................................................................ 25Solution type ............................................................. 25Frequency................................................................. 26Fix ambiguities up to ................................................. 27Min. duration for float solution .................................... 27Tropospheric model .................................................. 28Ionospheric model ..................................................... 28Use stochastic modelling ........................................... 29

Baseline selection - Strategy forcomputation ................................................. 30


Interpreting the Results .............................. 32Resolving ambiguities ............................................... 32Float solutions ........................................................... 33

Inspecting the logfile and comparingresults........................................................... 34

Compare the logfile against the field sheets ............... 35Compare the results for double fixes.......................... 35

Storing the results ....................................... 36

Adjustment, Transformation and Outputof Results ..................................................... 38

Notes on single-frequency Static andRapid Static measurements........................ 39

Contents

Contents, continued


Introduction

Surveying with GPS has becomepopular due to the advantages ofaccuracy, speed, versatility andeconomy. The techniques employedare completely different however,from those of classical surveying.

Provided that certain basic rules arefollowed GPS surveying is relativelystraightforward and will producegood results. From a practical pointof view it is probably more importantto understand the basic rules forplanning, observing and computingGPS surveys rather than to have adetailed theoretical knowledge of theGlobal Positioning System.

This guide outlines how to carry outStatic and Rapid Static GPS surveysand emphasizes those points towhich particular care has to be paid.

Although this guide has been writtenspecifically for Leica GPS - System500 and System 300, much of theinformation is of a general natureand applicable to all GPS surveying.Further information may be found inthe various guidelines contained inthe System 500 or System 300documentation material.

Introduction

7 Overall planning for a GPS surveyGeneral Guide to Static and Rapid-Static-3.0.0en

Overall planning for a GPS survey

Rapid Static surveys feature shortobservation times. It is particularlyimportant for Rapid Static thationospheric disturbances are moreor less identical for both sites.

Thus, for all GPS surveying, and forRapid Static in particular, it is soundpractice to minimize baselinelengths.

Baseline length

A GPS receiver measures theincoming phase of the satellitesignals to millimeter precision.However, as the satellite signalspropagate through space to earththey pass through and are affectedby the atmosphere. The atmosphereconsists of the ionosphere and thetroposphere. Disturbances in theatmosphere cause a degradation inthe accuracy of observations.

GPS surveying is a differentialmethod. A baseline is observed andcomputed between two receivers.When the two receivers observe thesame set of satellites simultaneously,most of the atmospheric effectscancel out. The shorter the baselineis the more these effects will bereduced, as the more likely it is thatthe atmosphere through which thesignals pass to the two receivers willbe identical.

8Overall planning for a GPS survey General Guide to Static and Rapid-Static-3.0.0en

Temporary reference stations for Rapid Static surveys

In terms of productivity and accuracy,it is much more advantageous tomeasure short baselines (e.g. 5km)from several temporary referencestations rather than trying to measurelong baselines (e.g. 15 km) from onecentral point.

As observation time and accuracyare mainly a function of baselinelength, it is highly recommended thatbaseline lengths should be kept to aminimum.

Depending on the area and numberof points to be surveyed by GPS, youshould consider establishing one ormore temporary reference stations.

Baselines radiating from a temporaryreference station can be severalkilometers in length. Remember,however, that it is advantageous tominimize baseline lengths. The tableon page 16 provides a guide tobaseline lengths and observationtimes.


Depending on the accuracy required,the user should be prepared tocheck newly surveyed points. This isparticularly important if observationtimes have been cut to a minimumand recommendations regardingGDOP ignored.

For a completely independent check:

Occupy a point a second time in adifferent window. This ensures thatthe set-up, the satellite constellat-ion, and the atmosphericconditions are different.

Close a traverse loop with abaseline from the last point to thestarting point.

Measure independent baselinesbetween points in networks.

A partial check can be obtained byusing two reference stations insteadof one. You will then have two fixesfor each point but each will be basedon the same roving-receiverobservations and set-up.

In all types of survey work it is soundpractice to cross check using inde-pendent measurements. In classicalsurvey you check for inaccurate orwrong control points, wronginstrument orientation, incorrectinstrument and target heights, etc.You close traverses and level loops,you fix points twice, you measurecheck distances! Depending on thejob and accuracy needed it is wellworthwhile applying the sameprinciples to GPS surveying.

One should be particularly carefulwith Rapid Static with shortobservation times. If the observationtime is too short, or the satellitegeometry (GDOP) is poor, or theionospheric disturbances are verysevere, it can happen that the post-processing software will resolveambiguities but the results mayexceed the quoted specifications.

Check the newly surveyed points


Observation schedule -best times to observe

For baselines up to about 20 km, youwill usually attempt to resolve theambiguities using the Rapid Staticalgorithm in the SKI-Pro post-processing software.

Under favourable conditions it ispossible to resolve ambiguities alsoon baselines longer than 20 km.

For baselines longer than 80 km, it isusually not advisable to resolveambiguities. In this case a differentpost-processing algorithm is used inSKI-Pro. This algorithm eliminatesionospheric influences to a largedegree but destroys the integernature of the ambiguities. Sufficientobservation time is needed to meetthe accuracy specifications.

Night versus day observations. Measuring long lines

Generally speaking, the longer thebaseline the longer one has toobserve.

The ionosphere is activated by solarradiation. Thus ionosphericdisturbance is much more severe byday than by night. As a result, thebaseline range for night observationswith Rapid Static can be roughlydouble that of day observations. Or,put another way, observation timesfor a baseline can often be halved atnight.

At the present time ionosphericactivity is increasing in an 11-yearcycle.

The table on page 16 provides aguide to baseline lengths andobservation times under the currentionospheric conditions.

When you inspect the satellitesummary and GDOP plots, you willusually see several good windows(see page 14) distributed through a24 hour period. You should try towork with Rapid Static during goodwindows, and plan your schedulecarefully.

It is impossible to plan GPSobservations to the minute. Ratherthan trying to squeeze the maximumnumber of points into a window bycutting observation times to the bareminimum, it is usually better tomeasure one point less and toobserve for a few minutes longer.Particularly for high-accuracy work, itpays to be conservative and not torisk poor results.


Consider the transformation to local coordinates

The common points should bespread evenly throughout the projectarea. For a correct computation of alltransformation parameters (shifts,rotations, scale), at least three - butpreferably four or more - points haveto be used.

Read the Online Help on Datum/Mapfor details on calculatingtransformations in SKI-Pro.

System 500 and System 300 provideaccurate relative positions of pointsthat are observed in a GPS networkand linked in post-processing. Thecoordinates are based on the WGS84 datum.

For most projects it will be necessaryto transform the WGS 84coordinates obtained from GPSsurvey into local grid coordinates, i.e.into grid coordinates on the localprojection based on the localellipsoid.

In order to be able to compute thistransformation, known points withlocal coordinates have to be includedin the GPS network. These commonpoints, with WGS 84 and localcoordinates, are used to determinethe transformation parameters and tocheck the consistency of the localsystem.


Temporary Reference Stations

In terms of productivity and accuracy, it is usuallypreferable to measure short baselines from severaltemporary reference stations rather than trying tomeasure long baselines from just one central point.

R-Temporary Reference Site

Example:

Establish 6 temporary reference stations using Static orRapid Static.

Check network of temporary reference stations usingdouble fixes or independent baselines.

Fix new points from temporary reference stationsusing Rapid-Static radial baselines.

Consider the need to check critical points.

Consider the transformation to local coordinates, continued

Overall Planning

Plan the campaign carefully

Consider the job, number of points, accuracy needed

Consider connection to existing control

Consider the transformation to local coordinates

Consider the best ways to observe and compute

For high accuracy, keep baselines as short aspossible

Use temporary reference stations- Consider the need for independent checks:- Occupying points twice in different windows- Closing traverse loops

Measuring independent baselines between points

Consider using two reference stations

Use good windows

Consider observing long lines at night

For high-accuracy work, try not to squeeze themaximum number of points into a window

13 Mission planningGeneral Guide to Static and Rapid-Static-3.0.0en

Mission planning

Selecting good windows forsuccessful GPS surveying

Poor windows should only be usedto bridge between two or more goodwindows when observing for longperiods of time, e.g. at referencestations and for long lines.

If there are obstructions near a point,use the sky plot to find out if thesignals from a satellite could beblocked. This could cause the GDOPto deteriorate. Check the GDOP byclicking the satellite "off" in theSatellite Availability component. Acareful reconnaissance of such sitesis well worthwhile.

GDOP - Geometric Dilution ofPrecision

The GDOP value helps you to judgethe geometry of the satelliteconstellation. A low GDOP indicatesgood geometry. A high GDOP tellsyou that the satellite constellation ispoor. The better (lower) the GDOPthe more likely it is that you willachieve good results.

Poor satellite geometry can becompared with the "danger circle" ina classical resection. If the geometryis poor, the solution in post-processing will be weak.

For Rapid Static you should observewhen the GDOP is less than or equalto 8. A GDOP of 5 or lower is ideal.

For successful, high-accuracy GPSsurveying it is advisable to take theobservations in good windows.Provided that you know the latitudeand longitude to about 1°, thesatellite summary, GDOP, elevation,and sky-plot panels in the SurveyDesign component of SKI-Pro willhelp you to select good windows inwhich to observe.

You should take particular care whenselecting windows for Rapid Staticobservations.

A suitable observation window forRapid Static must have four or moresatellites, with GDOP < or = 8, abovea cut-off angle of 15° at both thereference and roving receiver.

14Mission planning General Guide to Static and Rapid-Static-3.0.0en

Selecting good windows for successful GPS surveying, continued

Selecting Good Windows

Window for Rapid Static:

4 or more satellites above 15° cut-off angle.

GDOP < or = 8.

Whenever possible:

5 or more satellites.

GDOP < or = 5.

Satellites above 20°.

Always:

Use sky plot to check for obstructions.

Recompute GDOP if a satellite is obstructed.

Be wary if 2 out of 4 or 5 satellites are low (<20°).

Example:

Good window - GDOP low and stable Poor window - GDOP high

Avoid observing during this "spike"

15 Observation times and baseline lengthsGeneral Guide to Static and Rapid-Static-3.0.0en

Observation times and baseline lengthsUnless one is extremely restrictive, it is impossible to quote observationtimes that can be fully guaranteed. The following table provides a guide. It isbased on tests in mid-latitudes under the current levels of ionosphericdisturbance with a dual frequency Sensor.

Ionospheric activity is currently increasing to a high level in an 11-year cycle.As the activity increases it can be expected that observation times have tobe increased or baseline lengths reduced. Ionospheric activity is also afunction of position on the earth's surface. The influence is usually less inmid latitudes than in polar and equatorial regions.

Note that signals from low-elevation satellites are more affected byatmospheric disturbance than those from high satellites. For RapidStatic observations, it can be worth increasing the observation timesif two out of four or five satellites are low ( say < 20°).

The observation time required for anaccurate result in post-processingdepends on several factors: baselinelength, number of satellites, satellitegeometry (GDOP), ionosphere.

As you will only take Rapid Staticobservations when there are four ormore satellites with GDOP < 8, therequired observation time is mainly afunction of the baseline length andionospheric disturbance.

Ionospheric disturbance varies withtime and position on the earth'ssurface. As ionospheric disturbanceis much lower at night, night-observation times for Rapid Staticcan often be halved, or the baselinerange doubled. Thus it can beadvantageous to measure baselinesfrom about 20 km to 30 km at night.

16Observation times and baseline lengths General Guide to Static and Rapid-Static-3.0.0en

Observation times and baseline lengths, continuedTimes and Baseline Lengths

Observation time depends upon:

• Baseline length• Number of satellites• Satellite geometry (GDOP)• Ionosphere

Ionospheric disturbance varies with time, day/night, month, year, positionon earth's surface.

The table provides an approximate guide to baseline lengths andobservation times for mid latitudes under the current levels of ionosphericactivity when using a dual frequency Sensor.

Obs. No. sats. Baseline Approximate observation Method GDOP <= 8 Length time

By day By night

Rapid 4 or more Up to 5 km 5 to 10 mins 5 min Static 4 or more 5 to 10 km 10 to 20 mins 5 to 10 mins

5 or more 10 to 15 km Over 20 mins 5 to 20 mins

Static 4 or more 15 to 30 km 1 to 2 hours 1 hour4 or more Over 30 km 2 to 3 hours 2 hours

17 Field observationsGeneral Guide to Static and Rapid-Static-3.0.0en

Note that the referencereceiver does not have to beset up on a known point. It isfar better to establishtemporary reference stationsat sites that fulfill therequirements listed abovethan to set up the referencereceiver on known points thatare not suitable for GPSobservations.

For computing the transformationfrom WGS 84 to the local system,known points with local coordinateshave to be included in the GPSnetwork. These points do not have tobe used as reference stations. Theycan be measured with the rovingreceiver.

Field observations

Reference site

GPS surveying is a differentialtechnique with baselines being"observed" and computed from thereference to the rover. As manybaselines will often be measuredfrom the same reference station, thechoice and reliability of referencestations are of particular importance.

Sites for reference stations shouldbe chosen for their suitability for GPSobservations. A good site shouldhave the following characteristics:

• No obstructions above the 15° cut-off angle.

• No reflecting surfaces that couldcause multipath.

• Safe, away from traffic andpassers-by. Possible to leave thereceiver unattended.

• No powerful transmitters (radio,TV antennas, etc.) in the vicinity.

The results for all roving points willdepend on the performance of thereference receiver! Thus thereference receiver must operatereliably:

• Power supply must be ensured.Use a fully-charged battery.Consider connecting two batteries.When possible, consider atransformer connected to themains.

• Check that there is ample capacityleft in the memory device forstoring all observations.

• Double-check the antenna heightand offset.

• Make sure that the configurationparameters (observation type,recording rate etc.) are correctlyset and match those of the rovingreceiver.

18Field observations General Guide to Static and Rapid-Static-3.0.0en

Need for one known point in WGS 84

The computation of a baseline inGPS-processing requires that thecoordinates of one point (reference)are held fixed. The coordinates of theother point (rover) are computedrelative to the "fixed" point.

In order to avoid that the results areinfluenced by systematic errors, thecoordinates for the "fixed" point haveto be known to within about 20meters in the WGS 84 coordinatesystem. Whenever possible, theWGS 84 coordinates for the "fixed"point should be known to withinabout 10 meters otherwise scaleerrors of about 1 to 2 ppm will beintroduced.

This means that for any precise GPSsurvey the absolute coordinates ofone site in the network have to beknown in WGS 84 to about 10meters. WGS 84 coordinates for onesite will often be available or can beeasily derived as explained on page23.

If WGS 84 coordinates for one siteare not known or cannot be derived,the Single Point Positioncomputation in SKI-Pro can be used.Remember, however, that SelectiveAvailability (SA) may be switched on.The only way to overcome SA is toobserve for sufficient time for theeffects of SA to be averaged out inthe Single Point Positioncomputation.

The reference receiver will usuallyobserve for several hours as therover moves from point to point. Insuch a case, the Single Point Positi-on for the reference receivercomputed in SKI-Pro should berelatively free from the effects of SA.If a Single Point Position is computedfrom only a few minutes ofobservations, the effects of SelectiveAvailability will not be averaged out.The result could be wrong by 100mor more due to SA.

When computing the Single PointPosition for the starting point of anetwork, always compute for a sitefor which you have several hours ofobservations. The resulting WGS 84coordinates should then be correctwithin about 10 meters.

The minimum observation for thecomputation of a reliable Single PointPosition is probably about 2 to 3hours with four or more satellites andgood GDOP. The longer theobservation time, the better theSingle Point Position will be.

19 Field observationsGeneral Guide to Static and Rapid-Static-3.0.0en

Observing new points Use the Stop and Go Indicator as a guide

As the Stop and Go Indicator canonly monitor the roving receiver itcan only provide an estimate for therequired measuring time. It shouldbe used only as a guide.

The operator of the roving receivershould also pay attention to certainpoints. This is particularly importantfor Rapid Static surveys with shortmeasuring times.

• Make sure that the configurationparameters (e.g. recording rateetc.) are correctly set and matchthose of the reference receiver.

• Check the antenna height andoffset.

• Watch the GDOP when observingfor only a short time at a point.

• For 5 to 10mm + 1 ppm accuracywith Rapid Static, only takemeasurements withGDOP < or = 8.

The Stop and Go Indicator on thesensor provides the roving-receiveroperator with an approximate guideto measuring times for Rapid Staticobservations with four or moresatellites and GDOP less than orequal to 8. It estimates whensufficient observations should havebeen taken for successful post-processing (ambiguity resolution) tobe possible.

At the present time estimates arecalculated for three baseline ranges,0 to 5 km and 5 to 15 km and above15 km. The estimates are basedapproximately on the currentsituation for GPS observations in midlatitudes and assume that thereference and roving receiver aretracking the same satellites.

20Field observations General Guide to Static and Rapid-Static-3.0.0en

Fill out a field sheet

Reference Stations

No obstructions above 15° cut-off angle.

No reflecting surfaces (multipath).

Safe, can leave equipment unattended.

No transmitters in vicinity.

Reliable power supply.

Ample memory capacity.

Correct configuration parameters (e.g. recording rate).

Check antenna height and offset.

Does not have to be a known point.

It is better to establish temporary reference stations atgood sites rather than at unsuitable known points.

For precise GPS surveying, WGS 84 coordinates for onepoint have to be known to about 10 meters.

Roving Receiver

15° cut-off angle.

Obstructions should not block signals.

No reflecting surfaces (multipath).

No transmitters in vicinity.

Fully-charged battery.

Sufficient memory capacity.

Correct configuration parameters (e.g. data-recordingrate).

Check antenna height and offset.

Observe in good windows.

Watch the GDOP <= 8.

Use Stop and Go Indicator as a guide.

Fill out a field sheet.

As with all survey work, it is well worthwhile filling out afield sheet for each site when taking GPS observations.Field sheets facilitate checking and editing at the data-processing stage.


Fill out a field sheet, continued

Practical Hints

Tribrachs: check the bubble and optical plummet.

Level and center the tribrach and tripod correctly.

Check the height reading and antenna offset.

An error in height affects the entire solution!

Use a radio to maintain contact between referenceand rover.

Consider orienting the antennas for the most precisework.

Field Sheet

Point Id.: Date:

Receiver Serial No.: Operator:

Memory card No.:

Type of set up:

Height reading:

Time started tracking:

Time stopped tracking:

Number of epochs:

Number of satellites:

GDOP:

Navigation position: Lat. Long. Height

Notes:

Field observations



Checking and editing duringdata transferData can be transferred to SKI-Prodirectly via a PC-card slot, or via acard reader, from the controller(System 300) or receiver (System500), or from a disk with backed-upraw data. During data transfer, theoperator has the opportunity tocheck and edit certain data. It isparticularly advisable to check thefollowing:

• Point identification: Checkspelling, upper and lower caseletters, spaces etc.

• Make sure that points that havebeen observed twice have thesame point identification. Makesure that different points in thesame project have different pointidentifications.

• Height reading: Compare with fieldsheets.

Note that some of the abovesite-related parameters canbe changed in somecomponents of SKI-Pro.However, the affectedbaselines have then to berecomputed.

Backing up raw data andprojectsAfter reading in a data set alwaysmake a back-up on either a disketteor on the hard disk. You can thenerase and reuse the memory cardbut you still have the raw data. Whenbacking up data from severalmemory cards, it is advisable tocreate a directory for each card.

After importing all the data related tothe project it is often worthwhilemaking a backup of the wholedirectory where the project is locatedbefore starting to process the data.



Deriving initial WGS 84 coordinates for one pointAs explained on page 18, thecomputation of a baseline requiresthat the coordinates of one point areheld fixed. The coordinates of theother point are computed relative tothe "fixed" point.

For any precise GPS survey theabsolute coordinates of ONE site inthe network have to be known inWGS 84 to about 10 meters. WGS84 coordinates for one site will oftenbe available or can be easily derived.

Using SKI-Pro it is easy to convertthe grid coordinates of a known pointto geodetic or Cartesian coordinateson the local ellipsoid. If theapproximate shifts between the localdatum and WGS 84 are known,WGS 84 coordinates to well withinthe required accuracy can bederived. The local Survey Depart-ment or University will usually beable to provide approximatetransformation parameters.

As explained on page 17, thereference receiver does not have tobe on a known point. If the referencereceiver was on a new (unknown)point and a known point wasobserved with the roving receiver,simply compute the first baselinefrom the known point (rover) to theunknown point (reference) in order toobtain and store the required initialWGS 84 coordinates for thereference receiver.

If good initial WGS 84 coordinatesfor the reference site are not knownor cannot be derived as explained inthe last two paragraphs, the SinglePoint Position computation in SKI-Pro can be used. When using theSingle Point Position computationalways compute for a site for whichthere are several hours ofobservations. The effects ofSelective Availability should thenaverage out and the resulting WGS84 coordinates should be correct towithin the required 10 meters.

See section "Need for one knownpoint in WGS 84" on page 18 forfurther details.

Always keep in mind that poor initialcoordinates for the referencereceiver will affect the baselinecomputation and can lead to resultsoutside the quoted specifications.

Deriving initial WGS 84 coordinates

24Processing Parameters General Guide to Static and Rapid-Static-3.0.0en

Processing ParametersIn the vast majority of cases, thedefault settings for GPS-processingcan be accepted and never need tobe changed by the operator. Onsome rare occasions the operatormay need to modify one or more ofthe data processing parameters. Themost common ones are describedbelow.

Cut-off angle

It is common practice in GPSsurveying to set a 10° cut-off angle inthe receiver. 15° is the systemdefault value in data processing.Avoid cut-off angles below 10° ifprecise results are to be obtained.

Although you can increase the cut-off angle you should be cautiouswhen doing so. If the cut-off angle fordata processing is set higher than inthe receiver some observations willnot be used for the baselinecomputation and you may "lose" asatellite. It could happen that onlythree satellites would be used in thecomputation instead of four. Youcannot expect a reliable answer withonly three satellites.

It can sometimes be advantageous,however, to increase the cut-offangle to about 20° in case of adisturbed ionosphere and providedthat sufficient satellites above 20°with good GDOP have beenobserved (use the SatelliteAvailability component in SKI-Pro tocheck the GDOP).

You may sometimes find that abaseline result is outsidespecifications even though fivesatellites have been observed. If oneof the satellites never rises aboveabout 20° the observations to thissatellite may be badly affected by theionosphere. Raising the cut-off angleand computing with only four high-elevation satellites can sometimesproduce a better result.

25 Processing ParametersGeneral Guide to Static and Rapid-Static-3.0.0en

Ephemeris

SKI-Pro uses the broadcastephemeris recorded in the receiver.This is standard practice throughoutthe world for all routine GPSsurveying. For standard GPS surveywork there is little to be gained byusing precise ephemeris.

Solution type

For precise GPS surveying younormally accept the system defaultsetting "Automatic". If available Codeand Phase observations will then beused for computation.

It should make little differencewhether you select "Automatic" or"Phase" only. The results should bemore or less identical.

Use"Code" for the rapid calculationof baselines when high accuracy isnot required, for instance inexploration or offshore work. If onlycode observations are processed theaccuracy cannot be better thanabout 0.3m in position.

Selecting "Float" enforces thatambiguities are not resolved.Depending on the frequency you canprocess an L1 float, an L2 float, anL1+L2 float or an L3 float solution.

Selecting an L3 float solution isuseful when you process longbaselines and have long observationtimes.Note that if a baseline is longer thanthe value set under "Fix ambiguitiesup to" a float solution will becomputed automatically.


Frequency

SKI-Pro automatically selects thebest frequency or combination offrequencies for the final solution.Thus, there is little point inprocessing with anything but"Automatic". If dual-frequency data isavailable both frequencies willtypically be used.

Because signal delay through theionosphere is different for the L1 andL2 frequency a linear combination ofthe two frequencies, whicheliminates the influence of theionosphere can be calculated.However, this so-called L3 solutionalso destroys the integer nature ofthe ambiguities. A float solution iscomputed instead while theambiguities remain unfixed. For verylong baselines (e.g. longer than 80km) it is not critical to have a floatsolution (instead of ambiguitiesfixed). The L3 float solution isaccurate enough according to thesystem specifications provided thatthe observation time is long enough.

If L1 and L2 ambiguities can beresolved previously a secondprocessing run can be startedintroducing the fixed L1 and L2integer ambiguities into theionospheric-free linear combination.Ionospheric disturbances areeliminated while fixed ambiguitiesare used. This strategy is preferablyused if ambiguities can be resolvedbut the ionospheric influence issignificant (e.g. with baselines longerthan 15 km).

With short baselines, though, usingthe ionospheric-free linearcombination would increase thenoise with little benefit. A standardL1+L2 solution is best used then.

Selecting "Automatic" makes SKI-Pro use an L3 solution if dual-frequency data is available and thebaseline is longer than 15 km. Ifambiguities can be resolved theseare introduced into the ionospheric-free solution.

If ambiguities cannot be resolved,the result will be an L3 float solution.If the baseline is shorter than 15 kmL1+L2 will be processed.

Selecting "L1+L2" will force thecomputation to use both frequenciesL1 and L2 without a second iono-freeprocessing run independent of thebaseline length.

Selecting "Iono free (L3)" makes thesystem compute an L3 solutionindependent of the baseline length.


Fix ambiguities up to

This value defines the maximumdistance of a baseline for which thesystem should try to resolveambiguities. The system defaultvalue is 80 km. Although you can setthe limitation to a higher value, youshould take care when doing so.Certainly there is no point in settingthe value unrealistically high. Forbaselines above the limit a floatsolution will be computed.

The frequency used for computationdepends on the selected "Frequency"parameter. If "Automatic" has beenselected an L3 solution will becomputed for baselines longer than15 km. For long baselines (withtypically long observation times!) it isnot critical to have an L3 floatsolution (instead of ambiguitiesfixed). The L3 float solution will beaccurate enough according to thesystem specifications.

Min. duration for float solution

This parameter defines the minimumtime for which SKI-Pro allows thecomputation of a float solution forstatic intervals. For short observationtimes float solutions may not beaccurate enough and a simple codesolution may be preferable. Thedefault setting of 300 sec. makesSKI-Pro switch to a code-onlysolution in case the ambiguitiescannot be resolved for observationperiods which are shorter than 300sec.

For baselines up to the givenlimitation value, L1 and L2measurements are introduced asindividual observations into thecomputation. The Lambda searchdeveloped by Prof. Teunissen andhis co-workers at the TU Delft isused as an efficient approach to findpossible candidate sets of integerambiguities. Various statisticaldecision criteria are used to verifythe correctness of the ambiguityresolution.


Tropospheric model

It does not make much differencewhether you select the Hopfield orthe Saastamoinen or the Essen andFroome model, but you shouldnever work with "No troposphere".You cannot expect to achieve goodresults if no tropospheric model isused.

Ionospheric model

This parameter defines which modelis used to reduce the impact of theionosphere. This is of specialimportance if you try to resolveambiguities.

The default parameter is"Automatic", which automaticallyselects the best possible choice. Ifsufficient observation time isavailable on the reference, this willbe the "Computed model". Otherwisethe "Klobuchar model" will be takenprovided that almanac data isavailable. Typically there is no needto change the default.

The Computed model can also beselected manually. It is computedusing differences in the L1 and L2signal as received on the ground atthe sensor. The advantage of usingthis model is that it is calculatedaccording to conditions prevalent atthe time and position ofmeasurement.

At least 45 minutes of data isrequired for a Computed model to beused.

The Klobuchar model reflects the 11-year cycle of solar activity particularlywell. To use this model can beadvantageous during the time of highsolar activity. The Klobuchar modelshould only be selected ifobservation data from Leicareceivers is being processed, sincethis kind of data contains thenecessary almanac files.

The "Standard" model is based onan empirical ionospheric behaviourand is a function of the hour angle ofthe sun. When the Standard model ischosen corrections are applied to allphase observations. The correctionsdepend on the hour angle of the sunat the time of measurement and theelevation of the satellites.


Use stochastic modelling

Select this option if you want tomodel the ionosphere additionally bycalculating the ionospheric impact foreach epoch. Stochastic modellingsupports ambiguity resolution onmedium and longer baselines whenyou suspect the ionosphere to bequite active.

You should, however, be careful withshort baselines since bad data –e.g.data influenced by multipath orobstructions- may be misinterpretedas being influenced by ionosphericnoise.

It is recommended to leave thedefault value for the "Min. distance"set to 8 km. With shorter baselinesthe ionospheric influence is smallerand stochastic modelling is notnecessary.

It is advisable to leave theIonospheric activity option set to"Automatic".

SKI-Pro will then, depending on thebaseline length, automatically set thelevel by which the changing of theionospheric activity from epoch toepoch is modelled.

You may set the Ionospheric activityparameter manually to Low, Mediumor High, if you have reliableindications on the currentionospheric activity.

30Baseline selection - Strategy for computation General Guide to Static and Rapid-Static-3.0.0en

Baseline selection - Strategy for computationBefore starting the GPS-processingyou should consider carefully howthe network can best be computed.Points to be considered are:

• Obtaining good initial WGS 84coordinates for one point.

• Connections to existing control.

• Computing the coordinates oftemporary reference stations.

• Rapid static measurements fromtemporary reference stations.

• Long lines.

• Short lines.

If more than one temporary-reference station has been used, this"network" of temporary-referencestations should be computed first.This may also involve the connectionto existing control points. Select andcompute line by line, inspect theresults, and store the coordinates oftemporary reference stations if thebaseline computations are in order.

It is highly advisable to check thecoordinates for each temporary-reference station using double fixesor other means, as all radial rovingpoints depend on temporary-reference stations.

Once the "network" of temporary-reference stations has beencomputed, all remaining baselines -i.e. the radial baselines from thetemporary-reference stations toroving-receiver points - can becomputed.

If baselines of greatly differinglengths have to be computed, it canbe worthwhile making two or morebaseline selections and computationruns. In this way you can select andcompute batches of baselines whichfall into the same category ofparameter sets.

Try to avoid mixing baselines oftotally different lengths in the samecomputation run. And avoid mixingshort-observation "Rapid-Static"baselines with long-observation"Static" baselines.

31 Baseline selection - Strategy for computationGeneral Guide to Static and Rapid-Static-3.0.0en

Data Import and Computation

Check and edit during data transfer:

Point identification

Height reading and antenna offset

WGS 84 coordinates of initial point

Back up raw data and project

Baseline selection - Strategy for computation, continued

Consider the following carefully:

• How best to compute the network

• The need for good WGS 84 coordinates for one point

• Connection to existing control

• The need to transform to local coordinates

• Computation of network of temporary referencestations

• Computation of new points from temporary referencestations

• Long lines

• Short lines

• GPS-processing parameters

32Interpreting the Results General Guide to Static and Rapid-Static-3.0.0en

Interpreting the ResultsBefore starting a processing run youWhen interpreting the results, youhave to distinguish betweenbaselines for which ambiguityresolution has been attempted andbaselines for which a float solutionhas been requested.

For baselines up to the limitationvalue (“Fix ambiguities up to”),ambiguity resolution is alwaysattempted unless the Solution typehas been set to Float or Code. Whenusing the Rapid Static technique withshort observation times, ambiguityresolution is always necessary ifgood results are to be achieved.

For long baselines with longobservation times a so-called L3float solution (a linear combination ofL1 and L2 measurements) can beused. This eliminates the ionosphericeffects, but destroys the integernature of the ambiguities. Accuracyspecifications can only be met withsufficient observation times.

Resolving ambiguities

For baselines up to about 20-30 kmambiguity resolution should alwaysbe attempted if good results are tobe achieved. Under favorableconditions ambiguity resolution canalso be successful on longerbaselines.

For baselines up to the limitationvalue, SKI-Pro searches for allpossible combinations ofambiguities. Rigorous statisticaltechniques are used to determinethe “most probably correct” solutionand the “second most probablycorrect” solution. These two “mostprobable” solutions are thencompared and if the probability thatthe first solution is much more likelyto be correct than the secondsolution then the first solution istaken as the correct answer.

Immediately after having completedthe ambiguity search routine andhaving computed the most likelyambiguities with one set of GPSobservations, SKI-Pro repeats thewhole ambiguity search routine usinga different set of GPS observations.This results in a second set ofambiguities.

The ambiguities computed from thissecond search routine are thencompared with the ambiguitiescomputed from the first ambiguitysearch. If the two sets of ambiguitiesare identical, then the ambiguitiesare considered to be correct. In orderto ensure the highest possiblereliability the ambiguity searchroutine is continually repeated for theentire observation interval.

Note, however, that very severeionospheric conditions, multipath orother disturbances can causesystematic biases in the phaseobservations.

33 Interpreting the ResultsGeneral Guide to Static and Rapid-Static-3.0.0en

Resolving ambiguities, continued

If the ambiguities are not resolved inRapid Static (short observationtimes) it is difficult to give anindication of accuracy. However, togive you a rough guideline, you canmultiply the sigma values for eachestimated coordinate by 10 in orderto obtain an approximate estimate ofthe accuracy of the baselinecomputation.

Note that for baselines upto 20-30 km it shouldnormally be possible toresolve the ambiguitiesprovided that sufficientobservations have beentaken (see page 15 for aguide to baseline lengthsand observation times). Ifthe ambiguities are notresolved check the logfile(see next page).

This can result in discrepancies inthe computed ambiguity sets and theambiguities will then not be declaredresolved. Under such conditionsenough observation time with goodsatellite geometry is necessary toallow a successful ambiguityresolution.

If the guidelines for baseline lengths,observation windows, number ofsatellites, GDOP and observationtimes are followed (combinedperhaps with your own experience),the results of baselines for which theambiguities are resolved should bewithin the system specifications.

If insufficient observations weretaken or the satellite constellationwas poor, SKI-Pro will not be able toresolve the ambiguities. If theambiguities are not resolved it ismost unlikely that the systemspecifications will be achieved.

Float solutions

For baselines above the limitationvalue (system default = 80 km), SKI-Pro eliminates the ionosphericeffects but does not attempt toresolve ambiguities. This can also beenforced by selecting Solution typeFloat and Frequency Iono-free (L3).

The result will always show"Ambiguities not resolved“(Ambiguity status = no) in thesecases. The accuracy specificationswill however be met, if sufficientobservation time was taken.

Note that there is usuallyno benefit in trying toresolve ambiguities forlines longer than 80km.

34Inspecting the logfile and comparing results General Guide to Static and Rapid-Static-3.0.0en

Inspecting the logfile and comparing resultsCheck, if there are parts inside thestatic interval, for which noambiguities could be fixed.

• Cycle Slip Statistics: Cycle slipsare discontinuities in the integerambiguities resulting from signalinterruptions. Normally, they arerepaired and reported in thelogfile. Check if there is a highnumber of cycle slips for a specificsatellite.

SKI-Pro also allows you to inspectthe DOP values and the residuals ofthe computation. DOP values reflectthe satellite geometry, whereasresiduals give an indication of themeasurement noise contained in thedata. To inspect DOP values orresiduals the correspondingcheckboxes have to be ticked in theGPS-Processing Parameters/ Exten-ded Output page before acomputation run is started.

Please refer to the Online Help fordetailed information.

The minimum and maximum DOPvalues are also listed in the ResultsManager, which gives you a quickoverview on whether a baseline hasbeen computed with poor satellitegeometry.

If ambiguity resolution has not beensuccessful with short observationtimes, the results will not be withinthe specifications. Consider a se-cond processing run with usingsatellite windows, deselecting aspecific satellite or modifying theparameters. However you shouldtake care to that you retain enoughobservation time. Only change yoursettings, if the logfile gives a reasonto do so.

For baselines up to the limitationvalue ambiguity resolution asdescribed in the previous chapteris always attempted.

When you look at the logfile, you willfind a summary of the computationresults for each baseline. You shouldcheck the following:

• Observation Statistics: Check thenumber of satellites: there shouldalways be at least four. The logfilelists the number of observationsused and the number ofobservations rejected for eachsatellite. Check if there is anunusual high number of rejectionsfor a specific satellite.

• Ambiguity Statistics: The totalnumber of independent ambiguityfixes for the whole observationinterval is shown.

35 Inspecting the logfile and comparing resultsGeneral Guide to Static and Rapid-Static-3.0.0en

Compare the results fordouble fixes

If a point was observed twice indifferent windows or two referencereceivers were operatingsimultaneously, you should comparethe resulting coordinates.

For baselines above the limitationvalue (system default = 80 km) or ifan L3 Float solution has beenselected manually, SKI-Proeliminates the ionospheric effects butdoes not attempt to resolveambiguities.

When inspecting the logfile checkthe number of satellites observed.

The results will show "Ambiguitiesnot resolved" (Ambiguity status =no). However, these results meet thesystem specifications provided thatsufficient observations have beentaken.

If baselines of greatly differinglengths have to be computed, it isadvisable to make two or morecomputation runs. In this way youcan select and compute batches ofbaselines which fall into the samecategory of processing parametersets.

Compare the logfile againstthe field sheets

If the results are not as good as youwould expect, it can be wellworthwhile comparing theinformation in the logfile with that inthe field sheets. Check if the numberof satellites used in the baselinecomputation is the same as thatnoted in the field sheets. Rememberto check the reference station as wellas the rover. If the number of thesatellites is not the same, the GDOPvalues could be higher than youexpected. Check the actual GDOPfor the satellites used in thecomputation using the SatelliteAvailability component of SKI-Pro.

36Storing the results General Guide to Static and Rapid-Static-3.0.0en

Storing the results

After inspecting the summary ofresults and the logfile, store theresults that meet your accuracyrequirements.

The coordinates are averaged(weighted mean) if more than onesolution for a point is stored. Forinstance if you store the coordinatesfor point A from one baseline solutionand then you compute and store thecoordinates for point A again fromanother baseline solution, the storedcoordinates will be updated to theweighted mean values from the twosolutions. The weighted mean istaken provided the coordinatesagree in both height and position towithin the "Limits for AutomaticCoordinate Averaging" set in SKI-Pro(default = 0.075m).

It follows that you should exercise acertain amount of care when storingpoints that have been fixed in morethan one baseline computation.Compare the results before storing.

37 Storing the resultsGeneral Guide to Static and Rapid-Static-3.0.0en

Storing the results, continued

• Baselines above the limitation value(default = 80 km):

L3 solution, ambiguity resolution is not attempted.

Results should meet the specifications providedsufficient observations are taken.

Long lines need long observation times.

• Inspect double fixes, independent baselines etc.

• Store results that meet accuracy requirements.

• Coordinates averaged if more than one result isstored.

Interpreting and Storing the Results

• For Rapid Static baselines with short observationtimes ambiguity resolution must be successful if high-accuracy results are to be obtained.

• For longer lines with longer observation times the L3solution without ambiguity resolution will normally beused.

• Baselines up to the limitation value (default = 80 km):

Ambiguity resolution is always attempted.

Ambiguities resolved (Ambiguity status = yes):

SKI-Pro has found the most probable solution.

Results should normally meet the specifications.

Ambiguities not resolved (Ambiguity status = no):

A Float solution is presented.

The Result is outside the specifications unlessyou have long observation times.

Inspect the logfile and consider recomputing withmodified settings.


After the observations have beencomputed, you may wish to adjustthe results if multiple observations topoints exist. This provides the bestestimates for the position of thepoints. Refer to the SKI-Pro OnlineHelp on "Adjustment" for furtherdetails.

The results of the baselinecomputations are coordinates in theWGS 84 system. Using a"Coordinate System" in SKI-Pro,these coordinates can betransformed into coordinates in anylocal datum or grid system. Refer tothe SKI-Pro Online Help for furtherdetails.

The final coordinates can then beexported in various formats using theASCII Export functionality of SKI-Pro.Refer to the SKI-Pro Online Help forfurther details.

Adjustment, Transformation and Output of Results

Adjustment, Transformation and Output of Results


When measuring with the SR510(System 500) or SR9400 / SR261(System 300) there are severalpoints that should be noted in orderthat the measurements aresuccessful and good results can beobtained.

Only observation windows with aminimum of 5 satellites above 15°and a good GDOP (< 8) should beused.

The minimum observation time inStatic or Rapid Static should neverbe less than 15 minutes.

As a rule of thumb the baselineobservation time should be 5minutes per kilometre of the baselinelength with a minimum time of 15minutes.

Notes on single-frequency Static and Rapid Static measurementsRecommended (minimum)observation times:

Baseline length Observation time

1 km 15 min.

2 km 15 min.

3 km 15 min.

4 km 20 min.

5 km 25 min.

6 km 30 min.

7 km 35 min.

8 km 40 min.

9 km 45 min.

10 km 50 min.

> 10 km > 60 min.

A Rapid Static observation canusually be considered to besuccessful if SKI-Pro can resolve theambiguities. Providing an estimate ofthe required observation time ismore difficult for single frequencyreceivers than for dual frequencyequipment as considerably lessinformation is available for the postprocessing software. Never the less,the above table should serve as aguide.

By default, SKI-Pro will not attemptto resolve ambiguities if less than 5minutes of (rapid) static, single-frequency data is available. This isdone in order to avoid unreliableresults. Once the ambiguities areresolved correctly the length of thebaseline will normally be accurate toabout 5 - 10 mm plus 2 ppm.

Notes on single-frequency/Rapid Static measurements


If the highest possible accuracyshould be achieved it isrecommended to orient the antennasin a common direction.

On long baselines above 10 km theaccuracy which can be achieved withsingle frequency Sensors is inferiorto which can be achieved with dualfrequency Sensors due toionospheric effects which cannot beeliminated with single frequencydata. Users who have previouslyworked with dual frequencyequipment should be aware of thisfact.

Notes on single-frequency Static and Rapid Static measurements, continued

Notes on single-frequency/Rapid Static measurements


TQM

Leica Geosystems AG, Heerbrugg,Switzerland, has been certified as beingequipped with a quality system whichmeets the International Standards ofQuality Management and QualitySystems (ISO standard 9001) and Envi-ronmental Management Systems (ISOstandard 14001).

Total Quality Management-Our commitment to total customersatisfaction

Ask your local Leica Geosystems agentfor more information about our TQMprogram

TQM

Leica Geosystems AGCH-9435 Heerbrugg

(Switzerland)Phone +41 71 727 31 31

Fax +41 71 727 46 73www.leica-geosystems.com

Printed in Switzerland - Copyright LeicaGeosystems AG, Heerbrugg, Switzerland 2003Original text

712168-3.0.0en

General Guide to Static and Rapid-Staticutdallas.edu/.../English/Static-Rapid_3_0en.pdf · General Guide to Static and Rapid-Static-3.0.0en 7 Overall planning for a GPS ... method.

Documents