Global positioning system (gps)

Global Positioning System (GPS)

UNIT V

Global Positioning System or GPS, is a technology that can give your accurate position anywhere on earth (latitude/longitude).

It can be defined as “a satellite-based navigation system which records and displays location of an object on earth (in terms of coordinates, i.e. latitude and longitude) as well as height of a place above mean sea level.

A GPS device receives signals from satellites and these signal codes are processed & converted into values showing position, time and velocity (what we see on screen of GPS device). The instrument to which we generally call ‘GPS’ is basically a GPS receiver and it is a small part of a large system. A complete Global Positioning System consists of following three segments;

1. space segment,

2. control segment and

3. user segment.

Space segment consists of dedicated satellites for Global Positioning System and are referred as space vehicles (SVs). There are twenty four satellites in a nominal GPS constellation. Out of these twenty four satellites three are spare satellites which start operating if some of the functional twenty one satellites have some operational problem.

These satellites remain in six orbital planes (four in each) and are positioned such that five to eight of these SVs visible from any place on the earth at a given point of time.

Space segment

The control segment consists of tracking stations located around all over world. There are master control station and monitoring stations in control segment.

Control Segment

User segment of GPS is what we uses i.e. GPS receiver. It also includes the GPS user community. A GPS receiver consists of a screen for displaying information, buttons to operate it and antenna to receive signals from satellites. The antenna may be in-built in receiver instrument or it may be externally attached to it.

Signals from minimum four satellites are required for a GPS receiver to compute location (x, y & z) and time (t). GPS receivers are essential part of navigation system of air crafts and ships.

User segment

GPS Space Segment

GPS Orbits

Master control station

User Segment

User Segment

Function of Space Segment

• The Space Segment is designed to consist of satellites orbiting the earth at approximately 20200 km every 12 hours.

• Each GPS satellite has several very accurate atomic clocks on board. The clocks operate at a fundamental frequency of 10.23MHz. This is used to generate the signals that are broadcast from the satellite.

• The satellites broadcast two carrier waves constantly.

L1 carrier has two codes modulated upon it.

• The C/A Code or Coarse/Acquisition Code is modulated, has a length of one millisecond ; its chipping rate is 1.023MHz with corresponding wavelength of 300 mts.

L2 carrier : has just one code modulated upon it. The L2 P-code.

P-code or Precision Code. Has the frequency of 10.23MHz.this refers to the sequence of 10.23 million binary digits or chips per second. Frequency also referred as the chipping rate of p-code.P-code is extremely long and repeats only after 266 days. Portion of seven days each are assigned to the various satellites. As a consequence, all satellite can transmit on the same frequency and can be identified by their unique one week segment . This technique also called as the Code Division Multiple Access (CDMA)

Broad cast messages : for precise geodetic applications, third type of signal transmitted from the GPS satellite is broad message sent at a rather slow rate of 50 bits per sec and repeated every 30 sec. chip sequence of p- code and C/A code are separately combined with the stream of messages bit by binary addition i.e. same value for code gives 0 and different value gives 1,

• The Control Segment consists of one master control station, 5 monitor stations and 4 ground antennas distributed amongst 5 locations roughly on the earth's equator.

• The Control Segment tracks the GPS satellites, updates their orbiting position and calibrates and synchronizes their clocks.

• A further important function is to determine the orbit of each satellite and predict it’s path for the following 24 hours. This information is uploaded to each satellite and subsequently broadcast from it. This enables the GPS receiver to know where each satellite can be expected to be found.

Function of Control Segment

The most important tasks of the control segment are:

• Observing the movement of the satellites and computing orbital data

(ephemeris)

• Monitoring the satellite clock sand predicting their behavior

• Synchronizing on board satellite time

• Relaying precise orbital data received from satellites in communication

• Relaying the approximate orbital data of all satellites (almanac)

• Relaying further information, including satellite health, clock errors etc.

Function of Control Segment …..cont

User Segment

The User Segment comprises of anyone using a GPS receiver to receive the GPS signal and determine their position and/or time. Typical applications within the user segment are land navigation for hikers, vehicle location, surveying, marine navigation, aerial navigation, machine control etc.

Global Navigation Satellite Systems (GNSS) Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite

navigation systems (Sat NAV) that provide autonomous geo-spatial positioning with global

coverage. GNSS allows small electronic receivers to determine their location (longitude,

latitude, and altitude) to within a few meters using time signals transmitted along a line-

of-sight by radio from satellites. Receivers calculate the precise time as well as position,

which can be used as a reference for any purpose.

Till date we have following GNSS launched by different country;

1. NAVSTAR Global Positioning System: by United States

2. GLONASS by Russian

Planning to launched

1. European Union's Galileo positioning; scheduled to be operational in 2014

2. Beidou navigation system; by The People's Republic of China by 2015-2017.

NAVSTAR GPS

Navigation Satellite Timing and Ranging Global Positioning System

It is a satellite based radio navigation system providing precise three dimensional position,

navigation and time information to suitably equipped users everywhere on a continuous

basis.

GPS has been under development in the USA since 1973. It is primary a military system with

limited asses to civilian users. GPS satellite network is operated by the U.S. Air Force to

provide highly accurate navigation information to military forces around the world. The

network is also being used by a growing number of commercial products

The NAVSTAR Global Positioning System is managed by the NAVSTAR GPS Joint Program

Office at the Space and Missile Systems Center, Los Angeles Air Force Base, California.

Navstar continues to perform as the world’s premier positioning and navigation systems.

Endeavors such as mapping, aerial refueling, geodetic surveying, and search and

rescue operations have all benefited greatly from GPS’s accuracy. . 

"GLObal NAvigation Satellite System" is a radio-based satellite navigation system,

developed by the former Soviet Union and now operated for the Russian government by the

Russian Space Forces. It is an alternative and complementary to the United States' Global

Positioning System (GPS), the Chinese Compass navigation system, and the planned

Galileo positioning system of the European Union (EU).

GLONASS

GLONASS was developed to provide real-time position and velocity determination,

initially for use by the Soviet military for navigation and ballistic missile targeting. It was the

Soviet Union's second generation satellite navigation system, improving on the TSIKLON

system which required one to two hours of signal processing to calculate a location with high

accuracy. By contrast, once a GLONASS receiver is tracking the satellite signals, a position

fix is available instantly. It is stated that at peak efficiency the system's standard positioning

and timing service provide horizontal positioning accuracy within 57–70 meters, vertical

positioning within 70 meters, velocity vector measuring within 15 cm/s, and time transfer

within 1 µs (all within 99.7% probability).

Galileo is a global navigation satellite system (GNSS) currently being built by the

European Union (EU) and European Space Agency (ESA). This project is an

alternative and complementary to the U.S. Global Positioning System (GPS) and

the Russian GLONASS.

European Union's Galileo positioning

Beidou Navigation System

It is a project by China to develop an independent satellite navigation system.

The current Beidou-1 system (made up of 4 satellites) is experimental and has

limited coverage and application. However, China has planned to develop a

global satellite navigation system consisting of 35 satellites (known as Compass

or Beidou-2).

Positioning Methods Using GPSThere are several different methods for obtaining a position using GPS. The method used depends on the accuracy required by the user and the type of GPS receiver available. Broadly speaking, the techniques can be broken down into three basic classes:

– Autonomous Navigation (or Point Positioning):• Position of a stationary or moving point is determined with respect to a

well defined coordinate usually by three coordinate values by using a single GPS receiver, by observation to four or more satellite.

– Differential Phase positioning (Relative Positioning):• The coordinate of an unknown point is determined with respect to a

known point. In other words relative positioning aims at determination of the vector between two points by observation to four or more satellites by two receives placed at two points simultaneously.

– Differentially corrected positioning: • It is a system in which differences between observed and computed

coordinates or ranges known as differential correction at a particular known point called reference station are transmitted to users (GPS receives called rovers at other points) to improve the accuracy of the user’s receiver position

TriangulationGPS receivers calculate the position of objects in two dimensional or three

dimensional space using a mathematical process called trilaterlation or

Triangulation.

Trilateration can be either two dimensional or three dimensional.

Two dimensionalTrilateration

Locate a place which is

1). 300 KM from City A

2). 200 KM from City B

3). 400 KM from City C

Two dimensionalTrilateration

Effect of Errors

Foghorn 3D3+e3

Foghorn 1

D+e1

Foghorn 2

D2+e2

Estimated Location Area of Ship

Three dimensionalTrilateration

The accuracy with which a position can be determined using GPS in navigation mode

depends, on the one hand, on the accuracy of the individual pseudo-range measurements,

and on the other, on the geometrical configuration of the satellites used. This is expressed

in a scalar quantity, which in navigation literature is termed DOP (Dilution of Precision).

There are several DOP designations in current use:

• GDOP: Geometrical DOP (position in 3-Dspace, incl. time deviation in the solution)

• PDOP: Positional DOP (position in 3-Dspace)

• HDOP: Horizontal DOP (position on a plane)

• VDOP: Vertical DOP (height only)

Dilution of Precision

Basic of GPS Signaling Process

Transit Time

The signals transmitted by the satellites take approx. 67 milliseconds to reach a receiver.

As the signals travel at the speed of light, their transit time depends on the distance

between the satellites and the user.

Four different signals are generated in the receiver having the same structure as those

received from the 4 satellites. By synchronising the signals generated in the receiver with

those from the satellites, the four satellite signal time shifts Δt are measured as a timing

mark as shown in figure. The measured time shifts Δt of all 4 satellite Signals are used to

determine signal transit time.

Transit time

GPS Signals/ Satellite SignalsThe signals from a GPS satellite are fundamentally driven by an atomic clocks. The fundamental frequency is 10.23 Mhz.

Satellite Distance measured from the multiplication of time difference and speed of light it is called as pseudorange.

CTT)epseudorangor(cetanDis S

Where, C is the speed of light.

The following information (navigation message ) is transmitted by the satellite at a rate of 50 bits per second;

• Satellite time and synchronisation signals

• Precise orbital data (ephemeris)

• Time correction information to determine the exact satellite time

• Approximate orbital data for all satellites (almanac)

• Correction signals to calculate signal transit time

• Data on the ionosphere

• Information on satellite health

The time required to transmit all this information is 12.5 minutes. By using the navigation message the receiver is able to determine the transmission time of each satellite signal and the exact position of the satellite at the time of transmission.

Each of the 28 satellites transmits a unique signature assigned to it. This signature consists of an apparent random sequence (Pseudo Random Noise Code, PRN) of 1023 zero sandones.

Function of Satellite signals

Information is encoded in the form of binary bits on the carrier signals by a process known as phase modulation.

The binary digits 0 and 1 are actually represented by multiplying the electrical signals by either +1 or -1.

There are three types of code on the carrier signals:

1. The C/A code (i.e. course acquisition code)

2. The P code (i.e. precise code)

3. The Navigation Message

It is a unique Gold code on each satellite, which is a pseudorandom sequence of bits with a

repeating sequence length of 1023. C/A bit transitions occur at 1.023 Mhz. Since, the

fundamental frequency in the satellite is 10.23 Mhz, so this represents one transition every

10 cycles. At this rate of bit transitions, the full sequence of 1023 bits is transmitted in 1 ms.

Therefore, the sequence repeats 1000 times per second. The chip length (distance between

bit transitions) is 293 m. Therefore, the sequence repeats every 300 km.

C/A code

The P code is generated from a combination of two different registers (i.e. linear feed back

register), in such a way that it repeats every 266.4 days. Each 7 day section is assigned a

“PRN code.” Satellites are often identified by their PRN number; the user should beware that

any given satellite can have its PRN code changed. Therefore, PRN codes should not be

used in place of Satellite Vehicle Numbers (SVN) when talking about particular satellites.

There are 38 possible PRN codes; given that there are 24 nominal satellites, some PRN

codes are left unused. The PRN sequence is reset at Saturday midnight, defining the start of

“GPS week.”

P code

The carrier can transmit the P code at 10.23 Mbps, with a chip length of 29.3

meters. Again, the basic information is the satellite clock time or transmission, which is

identical to the C/A information, except that it has ten times the resolution.

The Navigation Message can be found on the L1 channel, being transmitted at a

very slow rate of 50 bps. It is a 1500 bit sequence, and therefore takes 30

seconds to transmit. The Navigation Message includes information on the

Broadcast Ephemeris (satellite orbital parameters), satellite clock corrections,

almanac data (a crude ephemeris for all satellites), ionosphere information, and

satellite health status.

The Navigation Message

Satellite Signal Generation (Block Diagram)

Modulated Satellite Signals

Differential GPS (DGPS)A horizontal accuracy of approx. 20m is probably not sufficient for every situation. In order to

determine the movement of concrete dams down to the nearest millimetre, a greater degree

of accuracy is required.

In GPS it can be with a reference receiver which will always be used in addition to the user

receiver. This is located at an accurately measured reference point (i.e. the co-ordinates are

known). By continually comparing the user receiver with the reference receiver, many errors

can be eliminated. This is because a difference in measurement arises, which is known as

Differential GPS (DGPS).

Principles:

• DGPS based on the measurement of signal transit time (achievable accuracy approx. 1m)

• DGPS based on the phase measurement of the carrier signal (achievable accuracy

approx. 1cm)

Depending upon the purpose of the accuracy, we have different DGPS;

• Local area DGPS

• Regional area DGPS

• Wide area DGPS

Working Principal of DGPS

GPS Surveying Techniques

+ Static survey

+ Rapid static survey

+ Stop-and-go survey

+ Continuous kinematics survey

+ Real-time kinematic (RTK) survey

GPS Static Survey Static GPS surveying typically uses a network or multiple baseline approach for positioning.

It may consist of multiple receivers, multiple baselines, multiple observational redundancies

and multiple sessions. A least squares adjustment of the observations is required. This

method provides the highest accuracy achievable and requires the longest observation

times – from less than an hour to five hours or longer.

A variation of the static survey is the fast-static method (also called rapid-static by some

manufacturers of GPS equipment). This will allow shorter occupation times (i.e., 8 to 20+

minutes) than static positioning and may use a radial baseline technique, network

technique, or a combination of the two. Baseline lengths may not exceed 10 kilometers for

L1 only receivers and 20 kilometers for L1/L2 receivers

Typically, the occupation time is a minimum of 8 minutes for baseline up to 20 km and a minimum of 12 minutes for baselines up to 30 km. Fast Static requires a least squares adjustment or other multiple baseline statistical analysis capable of producing a weighted mean average of the observations. More than one base station will be used to provide redundancy for each vector.

Static survey

+ stable platforms or pillars

+ Long distances (10 km to thousands of kilometres)

+ Long occupation time (hours to days)

+ Control surveys

+ Simultaneous recording at several stations

+ Observation rates varying from 5 to 30 seconds

+ Reducing multipathing effects

+ Post-processing required

Rapid static survey

+ shorter distances (up to 10 km)

+ shorter occupation time (10 minutes)

+ densification of control networks

+ Observation rates varying from

a second to a few seconds

+ Post-processing required

+ 2 reference receivers required Referencereceiver 1

Referencereceiver 2

1

2

3

4

Stop-and-go survey

+ distances less than 1 km

+ 1 minute occupation time

+ observation rates of seconds

+ initialisation required

+ repeat initialisation when less

than 4 satellites are being tracked

Reference receiverinitialisation

Continuous kinematic survey

+ initialisation required

+ non-stop occupation

+ observation rates of 1 second

Real-time kinematic (RTK) Survey

receiverreceiver

radio

radio

antennaantenna

Initialisation Methods

+ Static survey

static survey between any two points (usually short baseline) is performed

with

sufficient measurements. Specific details are in equipment documentation.

A B

+ Known baseline

survey is performed between any two

points whose coordinates are

previously determined. Usually one

epoch is sufficient. Only ambiguities

are estimated with constraining the

position vector.

Initialisation Methods

+ Antenna swap Step 1: Reference & rover receivers are located over well defined marks,

collecting simultaneous observations for a period of 1 minute (A)

Step 2: Reference & rover receivers are swapped without changing the

tripods, collecting observations for a period of 1 minute (B)

Step 3: Reference & rover receivers are swapped again to return back to

their original locations, for a period of 1 minute (C)

In general, the first two steps are sufficient to resolve the integer ambiguities.

However, the third step is recommended for a further check.

Reference Rover Rover Reference Reference Rover

A B C

Initialisation Methods

+ On the fly

+ the first three methods require the receivers to be stationary

+ there are restrictions in some applications, such as aerial photogrammetry

where camera positions are determined with GPS. It is not possible to stop

the aircraft to perform the above initialisation techniques.

+ The “on the fly” method resolves the integer ambiguities while the

receiver is moving.

+ 5 satellites with good geometry are required, 6 or more are preferred.

+ Dual frequency receivers are required.

+ Ambiguity resolution in 5 minutes, 2 minutes with 6 or 7 satellites.

+ Specific details given in the equipment documentation.

GPS Surveying Techniques

1. Autonomous

2. Static

3. Fast Static

4. Post Processed Kinematic

5. Real Time Kinematic (RTK)

6. Continuous Kinematic

7. Airborne GPS

8. Networked RTK (VRS)

9. Code based data collection

Energy Transfer Mechanisms

Conduction: molecule to molecule within a substance

Convection (and advection): mass movement of a fluid

Radiation: absorption of electromagnetic waves

Image Interpretation Keys

Image Interpretation KeysImage interpretation key is simply a reference material designed to permit rapid and accurate identification of objects or features represented on aerial images. A key usually consists of two parts.

a). Collection of annotated or captioned images or stereo-gramsb). Graphic and/or world description

These materials are organized in a systematic manner that permits retrieval of desired images by, e.g. data, season, region, subject

Elements of photo interpretation: are following eight elements;

1. shape (depends on the object outline) 2. size (relative to one an other)

3. tone (brightness-hue, color) 4. texture (smooth or coarse) 5. shadow (helps to determine height) 6. site (location helps

recognition)7. association (features that are normally found near object)8. pattern

Image Interpretation Strategies

Image interpretation strategy can be defined as a disciplined procedure that enables the interpreter to relate geographic patterns on the ground to their appearance on the image.

We have following five categories of image interpretation strategies given below;

1. Field Observations – Identification in the field by observation, photography, GPS, etc.

2. Direct Recognition – Direct recognition intuitiveness Inference –

3. Interpretation by Inference - based on knowledge and possible surrogates or proxy

4. Probabilistic Interpretation – Utilizing addition data in the form of overlays to reveal relationships

5. Photomorphic Regions – Areas of relatively uniform tone and texture

let’s go

for the

last

unit !!!

Take Care.