A2.2NP1 Environmental Practical 1

1. A2.2NP1Environmental Practical 1 TOPIC 2 THE GLOBAL POSITIONING SYSTEM

2. Acknowledgement • This lecture has been adapted from the Geographers’ Craft website www.colorado.edu/geography/gcraft/notes/gps/gps_f.html • authored by P.H.Dana

3. WHAT IS GPS?

4. GPS is a Satellite Navigation System

5. Basic ideas • GPS is funded by and controlled by the U. S. Department of Defense (DOD). It is made freely available for civilian use. • GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. • Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.

7. There are three ‘segments’ • The space segment • The control segment • The user segment

8. The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space. • The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. • The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. • There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. • This constellation provides the user with between five and eight SVs visible from any point on the earth.

13. The Control Segment consists of a system of tracking stations located around the world. • These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites. • The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite. The Master Control station (in Colorado) uploads ephemeris and clock data to the SVs. • The SVs then send subsets of the orbital ephemeris data to GPS receivers over radio signals.

15. The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. • Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. • Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals. • Precise positioning is possible using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers. Surveying, geodetic control, and plate tectonic studies are examples.

17. Standard Positioning Service (SPS)

18. Accuracy of the SPS • 22 metre horizontal accuracy • 28 metre vertical accuracy • 200 nanosecond time accuracy • Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. • The SPS accuracy is no longer degraded by the DOD by the use of Selective Availability.

19. Selective Availability was removed on 1st May 2000. This resulted in a 10x improvement in the average error

20. Accuracies • These GPS accuracy figures are 95% accuracies, and express the value of two standard deviations of radial error from the actual antenna position to an ensemble of position estimates made under specified satellite elevation angle (five degrees) and PDOP (less than six) conditions. • For horizontal accuracy 95% is the equivalent of 2drms (two-distance root-mean-squared), or twice the radial error standard deviation. For vertical and time errors 95% is the value of two-standard deviations of vertical error or time error. • Root-mean-square (RMS) error is the value of one standard deviation (68%) of the error in one, two or three dimensions.

21. GPS Satellite Signals

22. GPS Signals • The SVs transmit two microwave carrier signals. • The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. • The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers.

24. GPS Data

25. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. • The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits (one millisecond). • There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. • The C/A code that modulates the L1 carrier is the basis for the civil SPS.

26. The Navigation Message also modulates the L1-C/A code signal. • The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters. • The Navigation Message consists of: • Data marking the time of transmission of each subframe at the time they are transmitted by the SV. • Orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. • Almanacs of approximate orbital data parameters for all SVs.

27. Clock data parameters describe the SV clock and its relationship to GPS time. • Ephemeris data parameters describe SV orbits for short sections of the satellite orbits. The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set. • The ten-parameter almanacs describe SV orbits over extended periods of time (useful for months in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes (at least). • Each complete SV data set includes an ionospheric model that is used in the receiver to approximates the phase delay through the ionosphere at any location and time.

28. Pseudo-Range Positioning

29. Pseudo-Ranging • The core method of GPS position fixing relies on distance ranging to four or more satellites • The distance is determined by a comparison of the PRN signal transmitted from the satellite with a PRN signal generated within the receiver, using the same algorithm • There will be a time shift between the two signal sequences due to the distance to the satellite. This time shift allows this pseudo-range to be calculated, if the velocity of the signal is known.

30. Pseudo-Ranging • The basic procedure is thus: • The receiver acquires the first satellite and downloads the time and emphemeris data. This gives the approximate position of the satellite in space. • It then generates a PRN and matches it to the PRN from the satellite. • This data provides the first pseudo-range.

31. Pseudo-Ranging • The receiver then downloads the almanac and locates the next nearest satellite and repeats the operation. • This is again repeated for the next satellite and so on until at least four satellites have been acquired. • The receiver then uses the four pseudo-ranges and the four time measurements to calculate its own 3D position and its own clock error. • This position is converted to the required local coordinate system (eg National Grid + OD).

33. GPS Error Sources

34. Errors • The GPS signal is subject to a number of possible errors. • These can be classed as • Noise • Velocity of propagation • Geometrical Dilution of precision

35. Noise • Noise errors are the combined effect of PRN code noise (around 1 metre) and noise within the receiver noise (around 1 metre). • SV clock errors uncorrected by Control Segment can result in one metre errors. • Ephemeris data errors: 1 metre

36. Propagation Errors • Tropospheric delays: 1 metre. Changes in temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters. • Unmodelled ionosphere delays: 10 metres. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten metre un-modelled residual. • Multipath: 0.5 metres. Multipaths are 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.

38. Geometrical Dilution of Precision • Noise and bias errors may together result in typical ranging errors of around fifteen meters for each satellite used in the position solution. • GPS ranging errors are magnified by the range vector differences between the receiver and the SVs. The volume of the shape described by the unit-vectors from the receiver to the SVs used in a position fix is inversely proportional to GDOP.

39. GDOP • GDOP is computed from the geometric relationships between the receiver position and the positions of the satellites the receiver is using for navigation. • Good GDOP results when angles from receiver to SVs are different. • Poor GDOP results when angles from receiver to the set of SVs used are similar.

43. Differential GPS (DGPS) Techniques

44. Differential GPS • GPS is not necessarily highly accurate. Traditional survey methods will give 100 times better accuracy unless the GPS high precision codes are used. These are not available for normal civilian use. • To overcome this problem the method of differential GPS has been developed.

45. Differential GPS • The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. • A reference receiver, or base station, computes corrections for each satellite signal. • Differential position accuracies of 1-10 metres are possible with DGPS based on C/A code SPS signals.

46. Differential GPS • Because individual pseudo-ranges must be corrected prior to the formation of a navigation solution, DGPS implementations require software in the reference receiver that can track all SVs in view and form individual pseudo-range corrections for each SV.

47. Differential GPS • Differential corrections may be used in real-time or later, with post-processing techniques. • Real-time corrections can be transmitted by radio link. • Corrections can be recorded for post processing. Many public and private agencies record DGPS corrections for distribution by electronic means.

50. THE END