Thales GeoSolutions

Thales GeoSolutions

Introduction to GPS

OBJECTIVES

By the end of this session you will be able to: List the 3 segments of the GPS system Explain how a range to a GPS satellite is obtained List the errors of the GPS system State the given accuracy of the GPS Standard

Positioning Service (SPS) Describe the characteristics of GPS signals Name & describe the three segments of the GPS

system

WHAT IS GPS?

Global Positioning System

GPS provides us with:

Worldwide, continuous, high accuracy, three dimensional position and velocity

Precise time transfer

GPS ACCURACY

Standard Positioning System (civilian)Single-frequency (L1) performance:

Position accuracy standard Global 95% horizontal error – 13 meters Global 95% vertical error – 22 meters

(L1) user performance given accuracy standard conditions Global 95% horizontal error – 33 meters Global 95% vertical error – 73 meters

Tests conducted June 2000

GPS ACCURACY

GPS single-FREQUECY (L1) all-in-view user performance:

Horizontal error statistics (95%) global average – 8.3 meters horizontal error (95%) worst site – 19.7 meters horizontal error

Vertical error statistics (95%) global average – 16.8 meters vertical error (95%) worst site – 44.0 meters vertical error


GPS ACCURACY

Cumulative global distribution of 95% accuracy performance:

95% of the earth had better then 16.4 meters 95% single-frequency horizontal performance

95% of the earth had better then 29 meters 95% single-frequency vertical performance


THE HISTORY OF GPS

1957 USSR LAUNCHES SPUTNIK 1964 USNSS TRANSIT SYSTEM OPERATIONAL 1973 US DOD NAVSTAR GPS DEVELOPMENT BEGINS 1985 GPS USED FOR OFFSHORE TASKS 1986 STARFIX DGPS SERVICE LAUNCHED 1989 SKYFIX GLOBAL DGPS SERVICE OPERATIONAL 1993 FULL GPS 2-D COVERAGE 1994 FULL GPS 3-D COVERAGE 1995-2000 GLONAS MERGE, INMARSAT NAV. 2000 SELECTIVE AVAILABILITY TURNED OFF

GPS NAVIGATION

ADVANTAGES OF GPS

For military users: User passive, jam resistant, selective access, low detectability, nuclear hardening, mil spec equipment. Receivers for all user classes - fighter aircraft, ships, tanks, bombers, soldiers, etc.

For civilian users: Low cost continuous positioning for general use Very high accuracy for industrial / commercial users.

NAVSTAR GPS CHARACTERISTICS 21 operational satellites & 3 active spares 12 HOURS, 20200km, NEAR CIRCULAR ORBITS Dual frequency: 1575.42 MHz & 1227.6 MHz TWO CODES:

C/A COARSE/ACQUISITION (civil users) P PRECISE (military & authorized users)

Accuracy of c/a code: (1.S.D.) 10-15m (no SA) 50m (S.A.Applied)

Carrier phase applications possible for high precision - stationary/low dynamic

GPS SEGMENTS

Control segment Monitor stations track all satellites Master control station injects updated orbit and clock data into satellites

Space segment 21 satellites plus 3 active spares in 6 orbit planes 12 hour 20,000 KILOMETRE orbits At least 4 satellites visible to any user at all times

User segment User set receive ranging signals from the satellites Compute position and velocity Many different types of receivers Civil and military users

THE GPS SEGMENTSG PS

G PS

G PS

G PS

GPS SYSTEM CONFIGURATION

GPS SEGMENTS

User segment User set receive ranging signals from the satellites Compute position and velocity Many different types of receivers Civil and military users

GPS CONTROL STATIONS

GPS CONSTELLATION

21 SATELLITES WITH 3 OPERATIONAL SPARES6 ORBITAL PLANES, 55 DEGREE INCLINATIONS

20,200 KILOMETRE, 12 HOUR ORBITS

THE GPS SATELLITE

THE GPS BLOCK 1 SATELLITE

GPS Block IIR SPACE VEHICLE

GPS SYSTEM - SPACE VEHICLES

GPS SATELLITE INSTALLATION

THE MS 750 GPS RECIEVER

GPS SIGNALS (Observables)

Signal L1 L2

Carrier Frequency 1575.42 1227.60

Wavelength 19 cm 24 cm

Codes C/A, P (P1) P (P2)

NAVDATA (ephemeris, clock corrections, etc)

GPS SIGNALS The p-code is clocked at 10.23 MHz and is 7 days long

(apparent wavelength = 30 Meters)

The c/a-code is clocked at 1.023 MHz and is 1ms long (apparent wavelength = 300 meters)

The c/a code is used to acquire a satellite

The c/a code provides the means to acquire the p-code

Under normal circumstances, the p-code is encrypted and referred to as the y-code

The navigation message can be decoded once either code is acquired and tracked

GPS SIGNALSC/A CODE

1023 bit binary sequence with a period of 1 millisecond

P CODE 2.34*1014 bit binary sequence with a period of 38

weeks

NAVIGATION MESSAGE Binary format of 1500 bits transmitted at 50hz

includes orbital, clock and ionospheric information

THE GPS NAVIGATION MESSAGE

EPHEMERIS TYPESPRECISE

Observed data from tracking stations available after approximately 10 days over the Internet

BROADCAST Available from the Navigation Message. Full

Kepler elements

ALMANAC Available from the Navigation Message. Reduced

Kepler elements, used for acquisition and predictions

THE C/A CODE AMBIGUITY

GPS ERROR SOURCES

Satellite clock offset from GPS time Satellite ephemeris Atmospheric delays (Ionosheric and

Tropospheric) Receiver clock offset from GPS time Receiver Multipath Receiver measurement noise Satellite geometry (-DOP) Selective Availability (SA) – Switched off May 2000

Thales GeoSolutions

Introduction to Differential GPS

OBJECTIVESBy the end of this session you will be able to: Explain why Differential corrections are needed Explain the operation of a differential correction

system Explain why time is critical in the application of

differential corrections Explain how dual-frequency DGPS operations

overcome Ionospheric delay Explain how SDGPS differs from DGPS Explain the techniques used in SDGPS & SkyFix-

XP

DIFFERENTIAL GPS Differential GPS is the use of a correction signal to improve

the accuracy of the Standard GPS DGPS utilizes GPS receivers at a fixed reference station and

at a mobile vehicle, vessel or user The GPS determined position of a reference station is

compared to it’s surveyed geodetic position from which a correction is derived

Some DGPS systems use the error in fix position, while others use individual satellite range errors to calculate the correction

The correction message is broadcast via a radio or satellite link

DIFFERENTIAL GPS PRINCIPLES

RTCM SC-104 MESSAGE TYPESRadio Technical Commission for Maritime Services

MESSAGE

TYPE NUMBER MESSAGE TYPE REMARKS

1* DIFFERENTIAL CORRECTION FINAL

2* DELTA DIFFERENTIAL CORRECTIONS FINAL

3* REFERENCE STATION PARAMETERS FINAL

4* SURVEYING TENTATIVE

5* CONSTELLATION HEALTH TENTATIVE

6 NULL FRAME FINAL

7* BEACON ALMANAC TENTATIVE

8* PSEUDOLITE ALMANAC TENTATIVE

9* HIGH RATE DIFFERENTIAL CORRECTIONS FINAL

10 P-CODE DIFFERENTIAL CORRECTIONS RESERVED

11 C/A CODE L1, L2 DELTA CORRECTIONS RESERVED

12 PSEUDOLITE STATION PARAMETERS NEW, RESERVED

13* GROUND TRANSMITTER PARAMETERS TENTATIVE

14 SURVEYING AUXILIARY MESSAGE NEW, RESERVED

15 IONOSPHERE (TROPOSPHERE) MESSAGE NEW, RESERVED

16 SPECIAL ASCII MESSAGE FINAL

17 EPHEMERIS ALMANAC NEW, RESERVED

18-59 UNDEFINED

60-63 DIFFERENTIAL LORAN-C MESSAGES NEW, RESERVED

* CHANGED SINCE 1987

CONSIDERATIONS FOR DGPS CORRECTIONS

Speed With SA on correction latency must be under 10 seconds

and with SA off must be under 50 seconds

Accuracy Must be free of errors

Range Must be capable of transmitting corrections from the

reference station to the work area

Distance Distance from reference station effects accuracy

INMARSAT COMMUNICATION SATELLITES

SKYFIX USES THE INMARSAT SYSTEM

CORRECTIONS ARE MONITORED 24/7

SKYFIX - NETWORK Operational since 1989

Global DGPS system using Inmarsat for broadcast

links

Also available over high-power SpotBeam links

World-wide network of >80 reference stations

Two control centers at Aberdeen and Singapore

provide 24 hour monitoring and quality control

Fully redundant equipment and links

SKYFIX CO-ORDINATIONS Existing control

GPS carrier phase data Broadcast Ephemeris Local/ITRF/DMA control used

Accuracy considered better than 2 meter using single reference station and better then 1-meter using multiple reference station network solution

SKYFIX CO-ORDINATION

CURRENT AND FUTURE CONTROL LOCAL GPS DATA REPROCESSED WITH IGS PRECISE EPHEMERIS (JPL) ADJUSTMENTS CONSTRAINED TO IGS EPOCH

1992.5 CO-ORDINATES

ACCURACY CONSIDERED TO BE BETTER THAN 5cm

SKYFIX - REFERENCE STATIONS Dual equipment, redundancy of communications Typically 12 channel Trimble 4000 DS receivers with

RS 4000 software High level of remote access and control capabilities

(change of parameters, initialization of logging etc) Both sets of RTCM data returned to control center Multipath audit every 6 months Coordinated relative to ITRF91 - WGS84 (G730) 24 hour local support at each station

BASIC REFERENCE STATION LAYOUT

GPSSatellites

VSAT Satellites

80+ Reference Stations• Dual-Frequency Stations• Single Frequency• Generate, process and transmit messages

2 MCC facilities • Aberdeen and Singapore• 2 remote MCC facilities at Perth and Reston

• Monitor and Control• Archive• Maintain and Plan• Manage external entities• Interface

LES facilities• Uplink SkyFix messages• Including SkyFix Premier Messages

19 inches rack

RIMS A

RIMS A

X25NETWORK

19 inches rack

RIMS A

High Power andLow Power

satellite links

MultiFix IIIMultiFix III

THALES SKYFIX INFRASTRUCTURE

THE IONOSPHERE – A SUMMARY

Two distinct problems:

1. Differential Ionospheric delay errors Propagate into the navigation solution causing

position biases

2. Scintillation effects Loss of DGPS corrections from satellite DGPS

links Intermittent tracking of GPS satellites

SOLAR DISTURBANCE AND THE IONOSPHERE

Reference StationUser

GPS Satellite

Large ionospheric delay

Small ionospheric delay

WHERE: AREAS AFFECTEDOperational ‘Hot Spots’

ScintillationSevere Disturbance

Severe Disturbance

Severe Disturbance

Geomagnetic Boundaries

GLOBAL IONOSPHERIC TEC MAP

SOLAR ACTIVITY CYCLE

0

40

80

120

160

200

Jan 97

Jan-98

Jan-99

Jan-00

Jan-01

Jan-02

Jan-03

Jan-04

Jan-05

Time

S U N S P O T #

Present Day

SKYFIX PREMIER

Introduced to mitigate ionospheric disturbances Use dual-frequency GPS reference stations SkyFix premier messages (type 55’s) Dual-frequency GPS user setup Thales’ MultiFix III or MultiFix IV software Calculated Iono-free DGPS positions: unbiased

and consistent performance around the clock

SKYFIX XP SDGPSWhat is SkyFix XP? SDGPS – Satellite Differential GPS

High Accuracy 10cm – Horizontal (1- 68%) 15cm - Vertical (1- 68%)

Global Coverage from a single set of corrections No Station Range Restrictions – seamless worldwide

coverage

Available on all SkyFix Beams (excluding local SPOT services)

Compatible with all existing SkyFix Hardware

SKYFIX XP SDGPS – HOW DOES IT WORK?

Global Network of stations used to track all satellites simultaneously

Each individual error source on each satellite is identified and uniquely corrected for

All satellite corrections are combined into a single global correction message

Ionospheric delay and other local error sources are measured using a dual frequency GPS receiver and MultiFix 4 software.

The user software corrects the standard GPS pseudorange based on the global XP corrections and the local ionospheric observations

“Error Free” GPS position calculation formed

Tropospheric Delay

Ionospheric Delay

SKYFIX XP – WHAT ARE THE ERRORS?

Satellite Clocks

Orbits

Multipath

SKYFIX XP – CORRECTING THE ERRORS

OrbitsBy tracking each satellite throughout its orbit a

precise orbit correction can be generated. Regularly updated in case of satellite re-tasking

(deliberate movement of SV within the orbit)

ClocksThe global network of reference stations is also used

to generate a highly accurate clock corrections

SKYFIX XP – CORRECTING THE ERRORS

IonosphereDual Frequency GPS observations used to correct

for ionospheric errorTroposphere

Tropospheric modeling and GPS delay measurements used to correct for Tropospheric delay

MultipathHandled using the Strobe Edge correlator technology

in the Thales ZX-Sensor Dual Freq. GPS receiver

SKYFIX XP - SDGPS vs. DPGS

24 Hour Plots for Norwich, England

SKYFIX XP HARDWARE

Compatible with all Existing SkyFix Hardware Mk.5 SkyFix Decoder 90938 SkyFix Decoder 2403 SkyFix Decoder MiniDome Inmarsat Taps SPOT beam antennas

TYPICAL SKYFIX XP INSTALLATION

Ships Inmarsat and/or MiniDome and/or SPOT Antenna

SkyFix DecoderDual Frequency GPS Rx

or

MultiFix 4 PC

Dual Frequency GPs Antenna

or

SKYFIX XP + SKYFIX PREMIER COMBINED INSTALLATION

LOW POWER BEAM

Ships Inmarsat or MiniDome

Dual Frequency GPS Rx

MultiFix 3 PC

Dual Frequency GPs AntennaHIGH POWER BEAM

SkyFix SPOT antenna

MultiFix 4 PC

SkyFix Decoders – RTCM + SkyFix XP

SkyFix (Premier)SkyFixXP

SKYFIX XP MultiFix 4 Software

Based on MultiFix 3 Full UKOOA based QC

Statistic testing and displays

SkyFix XP,Premier and Standard DGPS calculations available

Automatic switching between calculations for optimum solution

SUMMARY OF DGPS SERVICESStandalone DGPS (or single reference DGPS)

Using a single reference station to provide corrections to a user’s GPS receiver

Maximum effective range from reference station is approximately 2000km

Accuracy is range, constellation, and system dependent – approximately 2-3 meters of horizontal error within 1000km

SUMMARY OF DGPS SERVICESMultiple reference DGPS (and/or network adjusted)

Uses a network adjusted, multiple reference station correction and user system for a more accurate and reliable position

Maximum effective range from reference station is approximately 2000km

Accuracy is range, constellation, and system dependent – approximately 1 meter of horizontal error within 2000km

SUMMARY OF DGPS SERVICESHigh Precision DGPS or SDGPS

Corrects clock & orbit of each satelliteUser software & hardware corrects for Ionosphere,

Troposphere, Multipath, receiver noise and Earth Tides

Reference stations are actually tracking stations observing satellite orbit and clock

No range limitations – Global correctionAccuracy is constellation dependent – approximately

10 centimeters of horizontal error - Globally

Thales GeoSolutions

Introduction to GPS & DGPSTest Session

QUESTIONS1. What is the stated 2-sigma (95%) position

accuracy standard of the single-frequency (L1) service as of June 2000?

2. What are the 2-sigma (95%) horizontal & vertical global averages for L1 all-in-view user performance?

3. What are the 2-sigma (95%) horizontal & vertical global averages for L1 all-in-view user performance for 95% of the earth’s surface?

4. State the L1 GPS frequency?5. State the L2 GPS frequency?

QUESTIONS6. How many active GPS satellites are in orbit?7. GPS satellites orbit the earth in how many orbital

planes?8. What is the minimal number of GPS satellites that

must be in view above the GPS receiver’s elevation mask in order to calculate a 3D position?

9. What is the wavelength of the L1 signal?10. What is the C/A code used for?11. When can the navigation message be decoded?12. What information is contained in the navigation

message?

QUESTIONS

13. What are the three types of ephemeris information?

14. What are the GPS error sources?

15. What is differential GPS?

16. What does RTCM stand for?

17. Name two methods used to calculate corrections?

18. What is the approximate maximum effective range from a reference station that a user can be for the corrections to be valid?

QUESTIONS19. Name the two most common methods of receiving

corrections? 20. What are the two types of ionospheric effects that

plague DGPS operations? 21. Where are ionospheric delay and scintillation

most often experienced?22. About how long is the solar activity cycle?23. What does SDGPS stand for?24. What are three types of DGPS services? 25. What horizontal accuracies can be obtained

utilizing SDGPS?

ANSWERS

1. 13 meters horizontal & 22 meters vertical

2. 8.3 meters horizontal & 16.8 meters vertical

3. 16.4 meters horizontal & 29 meters vertical

4. 1575.42 MHz

5. 1227.6 MHz

6. 21 operational and 3 active spares

7. 6

8. 4

9. 19 centimeters

ANSWERS

10. Acquire GPS satellites11. After the C/A code is acquired and tracked12. Orbital, Clock and Ionospheric information13. Precise, Broadcast, and Almanac14. Satellite clock, Satellite orbit, Ionospheric delay,

Tropospheric delay, Receiver clock, Multipath, Receiver noise, and satellite geometry

15. The use of a correction signal to improve the accuracy of a standard GPS

16. Radio Technical Commission for Maritime Services

ANSWERS

17. Calculate the error in the fix position or individual satellite ranges

18. 2000Km19. Satellite and radio broadcast20. Ionospheric delay errors and Scintillation21. Along the Magnetic Equator (or in Equatorial

regions)22. 11 years23. Satellite Differential GPS (or Satellite-corrected

Differential GPS)

ANSWERS

24. Standalone or Single Reference DGPS, Multiple Reference and/or Network Adjusted DGPS, and SDGPS

25. 10 centimeters horizontal

Documents

Thales GeoSolutions