Differential GPS (DGPS)

SAJIKRISHNAN. K Scientific Officer

NATMO

GPS – Course 7

09 April 2012

HISTORY OF GPSDGPSERROR POSSIBILITIESDOMAINS OF DGPSCALCULATION & POSITION ESTIMATIONCOMPONENTS OF DGPSSURVEY METHODS IN DGPS

TOPICS OF DISCUSSION

IMPORTANT DATES IN THE DEVELOPMENT OF GPS

GLOBAL POSITIONING SYSTEM IS DEVELOPED AND FUNDED BYUS GOVERNMENT AND MANAGED BY DEPARTMENT OF DEFENCE.

1973 - DECISION TO DEVELOP A SATELLITE NAVIGATION SYSTEM FOR MILITARY1974 - 1979 CONDUCTED SYSTEM TESTS BY US AIR FORCE AND NAVY1977- FIRST RECEIVER TEST WAS CONDUCTED WITHOUT PLACING THE SATELLITE IN THE ORBIT. SIGNALS RECEIVED FROM PSEUDO –SATELLITES.1978 - 85 A TOTAL OF 11 BLOCK I SATELLITES WERE LAUNCHED.1979 - DECISION TO EXPAND GPS WITH 18 SATELLITES IN SPACE.

1980 - 1982 FINANCIAL CRISIS OCCURS WHEN THE SPONSERSQUESTIONED THE USEFULNESS OF THE SYSTEM.

1988 - NUMBER OF SATELLITES WERE INCREASED TO 24.

1983 - CIVILIAN USE OF GPS WAS ALLOWED AFTER SOVIET UNION SHOT DOWN KOREAN AIRPLANE THAT GET LOST OVER SOVIET TERRITORY.

1986 - GPS PROGRAMME SUFFERED A SET BACK DUE TO THE ACCIDENT OF CHALLENGER WHICH WAS SUPPOSED TO CARRY BLOCK II SATELLITES TO THE ORBIT. THEN DELTA ROCKETS WERE USED FOR THE PURPOSE.

1989 – FIRST BLOCK II SATELLITES WERE INSTALLED AND ACTIVATED

1990 – 1991 TEMPORAL DEACTIVATION OF SA DURING GULF WAR

1993 – INITIAL OPERATIONAL CAPABILITY (IOC) WAS ANNOUNCED AND DECIDED WORLDWIDE CIVILIAN USE FREE OF COST.

1994 – LAST BLOCK II SATELLITES COMPLETE THE SATELLITE CONSTALLATION

1995 – FULL OPERATIONAL CAPABILITY WAS ANNONCED

2000 – FINAL DEACTIVATION OF SA TO GIVE POSITIONAL ACCURACY OF20m FROM 100m.

2005 – LAUNCHING OF THE IIRM GPS SATELLITE THAT SUPPORTS THE NEW MILITARY M SIGNAL AND THE SECOND CIVIL SIGNAL L2C.

•The current system became fully operational June 26, 1993 whenthe 24th satellite was lunched.

Control StationsSchriever Air Force Base (formerly Falcon AFB) in Colorado.HawaiiAscension IslandsDiego GraciaKwajalein

Newly added Control Stations after 2005

Washington DCEnglandEquadorArgentinia BahrainAustralia

What's the Differential?What's the Differential?

Until 2000, civilian users had to contend Until 2000, civilian users had to contend with Selective Availability (SA). The with Selective Availability (SA). The DoDDoDintentionally introduced random timing intentionally introduced random timing errors in satellite signals to limit the errors in satellite signals to limit the effectiveness of GPS and its potential effectiveness of GPS and its potential misuse by adversaries of the United misuse by adversaries of the United States. These timing errors could affect States. These timing errors could affect the accuracy of readings by as much as the accuracy of readings by as much as 100 meters.100 meters.

With SA removed, a single GPS With SA removed, a single GPS receiver from any manufacturer can receiver from any manufacturer can achieve accuracies of approximately 10 achieve accuracies of approximately 10 meters. To achieve the accuracies meters. To achieve the accuracies needed for quality GIS records from one needed for quality GIS records from one to two meters up to a few centimeters to two meters up to a few centimeters requires differential correction of the requires differential correction of the data. The majority of data collected data. The majority of data collected using GPS for GIS is differentially using GPS for GIS is differentially corrected to improve accuracy.corrected to improve accuracy.

Differential GPS (DGPS) is a relatively Differential GPS (DGPS) is a relatively simple technique to improve positional simple technique to improve positional accuracy and integrity. This technique was accuracy and integrity. This technique was developed in the early 1980s, and it is developed in the early 1980s, and it is widely used in various forms.widely used in various forms.

DGPS is a method of improving the accuracy of your receiver by adding a local reference station to augment the information available from the satellites. It also improves the integrity of the whole GPS system by identifying certain errors.

•Differential GPS uses one unit at a known location and a rover.

–The stationary unit compares its calculated GPS location with the actual location and computes the error.

–The rover data is adjusted for the error.

The underlying premise of differential GPS The underlying premise of differential GPS (DGPS) is that any two receivers that are (DGPS) is that any two receivers that are relatively close together will experience similar relatively close together will experience similar atmospheric errors.atmospheric errors.

DGPS requires that a GPS receiver be set up DGPS requires that a GPS receiver be set up on a precisely known location. This GPS on a precisely known location. This GPS receiver is the base or reference station.receiver is the base or reference station.

The base station receiver calculates its position The base station receiver calculates its position based on satellite signals and compares this based on satellite signals and compares this location to the known location. location to the known location.

The difference is applied to the GPS data recorded The difference is applied to the GPS data recorded by the second GPS receiver, which is known as by the second GPS receiver, which is known as the roving receiver. The corrected information can the roving receiver. The corrected information can be applied to data from the roving receiver in real be applied to data from the roving receiver in real time in the field using radio signals or through time in the field using radio signals or through postpost--processing after data capture using special processing after data capture using special processing software.processing software.

Differential correction techniques are used to Differential correction techniques are used to enhance the quality of location data gathered enhance the quality of location data gathered using global positioning system (GPS) receivers.using global positioning system (GPS) receivers.

Differential correction can be applied in realDifferential correction can be applied in real--time directly in the field or when posttime directly in the field or when post--processing processing data in the office. Although both methods are data in the office. Although both methods are based on the same underlying principles, each based on the same underlying principles, each accesses different data sources and achieves accesses different data sources and achieves different levels of accuracy. Combining both different levels of accuracy. Combining both methods provides flexibility during data methods provides flexibility during data collection and improves data integrity.collection and improves data integrity.

RealReal--Time DGPSTime DGPS

RealReal--time DGPS occurs when the base time DGPS occurs when the base station calculates and broadcasts station calculates and broadcasts corrections for each satellite as it receives corrections for each satellite as it receives the data. The correction is received by the the data. The correction is received by the roving receiver via a radio signal roving receiver via a radio signal As a result, the position displayed and As a result, the position displayed and logged to the data file of the roving GPS logged to the data file of the roving GPS receiver is a differentially corrected receiver is a differentially corrected position.position.

Satellite Differential ServicesSatellite Differential Services

Another method for obtaining realAnother method for obtaining real--time time differential correction data in the field is by differential correction data in the field is by using geostationary satellites. This system using geostationary satellites. This system obtains corrections from more than one obtains corrections from more than one reference station, sends the information to reference station, sends the information to a geostationary satellite for verification. a geostationary satellite for verification. The verified information is sent to the The verified information is sent to the roving GPS receiver to ensure it obtains roving GPS receiver to ensure it obtains GPS positions in real time.GPS positions in real time.

Errors in GPSErrors in GPS

Error possibilities in GPSError possibilities in GPSThe receiver is not synchronized with theThe receiver is not synchronized with theatomic clock in the satelliteatomic clock in the satelliteThe estimate of the position of the satelliteThe estimate of the position of the satelliteSpeed of light is only constant in vacuumSpeed of light is only constant in vacuum”Multi path errors” : Ghost signals from”Multi path errors” : Ghost signals fromreflected radio wavesreflected radio waves”Selective availability (SA)” :Added noise from ”Selective availability (SA)” :Added noise from department of defense department of defense Not free sight to many enough satellitesNot free sight to many enough satellitesNoise in the receiverNoise in the receiver

Error Value

Ionosphere 4.0 meters

Clock 2.1 meters

Ephemeris 2.1 meters

Troposphere 0.7 meters

Receiver 0.5 meters

Multipath 1.0 meter

Total 10.4 meters

SOURCES OF ERROR IN GPS

GPS ErrorsGPS Errors

NoiseNoise

BiasesBiases

BlunderBlunder

Clock Clock

Errors addressed by Errors addressed by dgpsdgps

Eliminates or reduces clock errors, path errors, Eliminates or reduces clock errors, path errors, ephemeris errors and ephemeris errors and ionosphericionospheric effects effects

Idea: The errors are almost the same for two Idea: The errors are almost the same for two receivers close to each other receivers close to each other

•• Place a fixed receiver on a well defined location.Place a fixed receiver on a well defined location.•• Compute the error in the positionCompute the error in the position•• Estimate from the satellitesEstimate from the satellites•• Calculate backwards to find the time errorCalculate backwards to find the time error•• Broadcast it by radio to other receiverBroadcast it by radio to other receiver

Noise ErrorNoise Error

Noise errors are the combined effect of code Noise errors are the combined effect of code noise (around 1 meter) and noise within the noise (around 1 meter) and noise within the receiver (around 1 meter).receiver (around 1 meter).

Selective Availability (SA)Selective Availability (SA)SA is the intentional degradation of the SPS SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by signals by a time varying bias. SA is controlled by the DOD to limit accuracy for nonthe DOD to limit accuracy for non--U. S. military U. S. military and government users.and government users.Selective availability is turned off.Selective availability is turned off.

Ephemeris data errors: 1 meterEphemeris data errors: 1 meterSatellite orbits are constantly changing. Any error Satellite orbits are constantly changing. Any error in satellite position will result in an error for the in satellite position will result in an error for the receiver position.receiver position.

BIAS ERROR

SV clock errors uncorrected by Control SV clock errors uncorrected by Control Segment can result in one meter error.Segment can result in one meter error.

TroposphericTropospheric delays: 1 meter.delays: 1 meter.

The troposphere is the lower part The troposphere is the lower part (ground level to from 8 to 13 km) of the (ground level to from 8 to 13 km) of the atmosphere that experiences the atmosphere that experiences the changes in temperature, pressure, and changes in temperature, pressure, and humidity associated with weather humidity associated with weather changes. changes.

Complex models ofComplex models of tropospherictropospheric delay delay require estimates or measurements of require estimates or measurements of these parameters.these parameters.

Bias Error--cont.•Unmodeled ionosphere delays: 10 meters.

–The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual.

•Multipath: 0.5 meters.–Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite.

BlunderBlunder

Blunders can result in errors of hundred of Blunders can result in errors of hundred of kilometers.kilometers.

Control segment mistakes due to computer or human Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of error can cause errors from one meter to hundreds of kilometers.kilometers.

User mistakes, including incorrect geodetic User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to datum selection, can cause errors from 1 to hundreds of meters.hundreds of meters.Receiver errors from software or hardware Receiver errors from software or hardware failures can cause blunder errors of any size.failures can cause blunder errors of any size.

Differential GPS of USCGDifferential GPS of USCG

�� Maritime Differential GPS (DGPS)�� Maritime Differential GPS (DGPS)�� Managed by the U.S. Coast Guard (USCG)�� Managed by the U.S. Coast Guard (USCG)�� Employs ground stations along the coasts�� Employs ground stations along the coasts

with known fixed locations.with known fixed locations.�� Corrections are transmitted from ground �� Corrections are transmitted from ground

stations at low frequencies (200stations at low frequencies (200--500kHz).500kHz).�� Requires an additional Differential Beacon �� Requires an additional Differential Beacon

Receiver (DBR) and an additional antenna.Receiver (DBR) and an additional antenna.�� Accuracy is a function of the distance from �� Accuracy is a function of the distance from

the the ground station.ground station.

WAASWAAS

Wide Area Augmentation System (WAAS)Wide Area Augmentation System (WAAS)�� Managed by the FAA�� Managed by the FAA�� Communicates with several ground stations.�� Communicates with several ground stations.�� Provides atmospheric corrections.�� Provides atmospheric corrections.�� Early warning of GPS failures.�� Early warning of GPS failures.�� Same frequency as GPS�� Same frequency as GPS�� Higher data rate 250 Hz�� Higher data rate 250 Hz�� Satellites are in geostationary orbits.�� Satellites are in geostationary orbits.

Wide Area Augmentation SystemWide Area Augmentation SystemGeostationary WAAS satellites

GPS Constellation

WAAS Control Station (West Coast)

WAAS Control Station (East Coast)Local Area System (LAAS)

Calculating a Position��

• Measure distance to satellites.• Obtain satellite positions.• Perform triangulation calculations.

(Trilateration)• Adjust local clock bias.

Measuring Distance

Distance• �� Distance = Velocity * Time• �� Velocity is that of a radio wave.• �� Time is the travel time of the signal.• �� Measure the travel time.• �� Receiver generates the same codes as the

satellite (PRN codes).• �� Measure delay between incoming codes and

self generated codes.• �� D = Speed of light * measured delay.

Obtain Satellite positions.

• Orbital data (Ephemeris) is embedded in the satellite data message.

• Ephemeris data contains parameters thatdescribe the elliptical path of the satellite.

• Receiver uses this data to calculate theposition of the satellite. (X,Y,Z)

Perform triangulation calculations.

Triangulation in 2D• If location of point A is known, and the

distance to point A is known, desired position lies somewhere on a circle.

• Could be anywhere along circle A

Triangulation in 2D

• �� Distance to two points are known.• �� Desired position is in one of two locations.

• �� Distance to three points are known.

• �� Position is known!

Triangulation in 3D

• �� Distance to two points is known.

��• Measure distance to satellites.• �� Use pseudo ranges• �� Obtain satellite positions.• �� Decoded ephemeris from satellite message.• Perform triangulation calculations.• �� Need at least 3 satellites for triangulation.

• �� Adjust local clock bias to find position.

• �� Need 4th satellite to adjust bias.

• Calculating a Position Review

SURVEY METHODS IN DGPS

S1

S4

Master Station

S2

S3

Schematic diagram of Rapid Static Method

Master station

S3

S4

S5

Schematic diagram of Traverse method

S1

S2

Master station

S2S3

S1

Trilateration method

DATA DISPLAY IN GPS

Once the GPS receiver has located its position it is usually displayed in one of two common formats: – Latitude and longitude

– Universal transverse mercator (UTM).

LATITUDES AND LONGITUDES

Latitudes and longitudes are angles.Both use the center of the earth as the vertex, and both utilize the equator, but they use a different zero reference.

LATITUDE

Latitude gives the location of a place on the Earth north or south of the Equator. Latitude is an angular measurement in degrees (marked with °) ranging from 0° at the Equator to 90° at the poles (90° N for the North Pole or 90° S for the South Pole)

The earth’s circumference is approximately 24,859.82 miles around the poles. So

miles 69 latitude of degreeEach ≈

Latitude cont.

Longitude

Longitude describes the location of a place on earth east or west of a north-south line called the Prime Meridian.

–Longitude is given as an angular measurement ranging from 0° at the Prime Meridian to +180° eastward and −180° westward.

–In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian or zero point of longitude.

Longiude cont.

Longitude--cont.

Each degree of longitude ≈ 69 miles

The circumference of the earth at the equator is approximately 24,901.55 miles. So

A longitude of 134o west would be 9,246 west of the prime meridian.

Longitude--cont.• There is an important

difference between latitude and longitude.

• The circumference of the earth declines as the latitude increase away from the equator.

• This means the miles per degree of longitude changes with the latitude.

• This makes determining the distance between two points identified by longitude more difficult.

COMPONENTS OF DGPSCOMPONENTS OF DGPS

MASTER RECEIVER AND ROVERSMASTER RECEIVER AND ROVERSMASTER RECEIVER IS KEPT AT MASTER RECEIVER IS KEPT AT KNOWN POINT.KNOWN POINT.RELATIVE POSITION OF ROVERS ARE RELATIVE POSITION OF ROVERS ARE FIXED WITH RESPECT TO THE FIXED WITH RESPECT TO THE MASTER RECEIVERMASTER RECEIVER

The main components of GPS receivers are:

Antenna with pre-amplifierSensor to sense the dataMemory and display panelKeyboardPrecision oscillator (clock)-quartzPower supply – Ni-Cd - 12v batteryComputer with supporting software

for data download and processing.

Antenna detects the electromagnetic waves arriving form satellite, converts the wave energy into electric current, amplify the signal strength and hands the signal over to receive electronics.

Several antennas are availableMonopole or dipoleHelixSpiral helixMicrostripChoke ring

S1

S4

Master Station

S2

S3

Schematic diagram of Rapid Static Method

Master station

S3

S4

S5

Schematic diagram of Traverse method

S1

S2

Master station

S2S3

S1

Trilateration method

GPS

Different Hardware available

Trimble dual frequency 5000 series

Ashtech single frequencyLeica (Garmin)Links point

GPS

Software available for downloading and processing the data

ESRI Arc PadLinks PointGPS PathfinderThales NavigationTrimble Data TransferTrimble Geomatics Office