Chapter 3. Global Positioning System

1. Chapter 3. Global Positioning System

2. Introduction • 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.

4. Space segment • This constellation provides the user with between five and eight SVs visible from any point on the earth

7. Control segment: a system of tracking stations located around the world. • 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: uploads ephemeris and clock data to the SVs. The SVs then send subsets of the orbital ephemeris data to GPS receivers over radio signals.

10. User segment: the GPS receivers and the user community • GPS receivers convert SV signals into position, velocity, and time estimates. • 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.

12. GPS Positioning Services (the Federal Radionavigation Plan) • Precise Positioning Service (PPS) • Authorized users with cryptographic equipment and keys and specially equipped receivers • Predictable Accuracy • 22 meter Horizontal accuracy • 27.7 meter vertical accuracy • 200 nanosecond time (UTC) accuracy

13. Standard Positioning Service (SPS) • Civil users worldwide use the SPS without charge or restrictions • SPS Predictable Accuracy • 100 meter horizontal accuracy • 156 meter vertical accuracy • 340 nanoseconds time accuracy

14. GPS Satellite Signals • SVs transmitting two microwave carrier signals: • L1 frequency (1575.42 MHz) carrying navigation message and the SPS code • L2 frequency (1227.60 MHz) used to measure the ionospheric delay by PPS equipped receivers.

15. Three binary codes shift the L1 and/or L2 carrier phase • The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. A repeating 1 MHz Pseudo Random Noise (PRN) Code (repeating every 1023 bits or one millisecond). For civil SPS. • The P-Code (Precise) modulates both the L1 and L2 carrier phases. Seven days 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code (requires a classified AS Module) • The Navigation Message, also modulates the L1-C/A code signal. A 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters.

17. GPS navigation data • The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. • A data bit frame consists of 1500 bits divided into five 300-bit subframes. • A data frame is transmitted every thirty seconds.

19. Code Phase Tracking • The GPS receiver produces replicas of the C/A and/or P (Y)-Code, with some form of a C/A code generator • The C/A code generator produces a different 1023 chip sequence for each phase tap setting

21. The receiver slides a replica of the code in time until there is correlation with the SV code.

23. The receiver PRN code start position at the time of full correlation is the time of arrival (TOA) of the SV PRN at receiver. • This TOA is a measure of the range to SV, offset by the amount to which the receiver clock is offset from GPS time. • This TOA is called the pseudo-range

26. Pseudo-Range Navigation • Position is determined from multiple pseudo-range measurements at a single measurement epoch. • Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates.

27. Time is used to correct the offset in the receiver clock, allowing the use of an inexpensive receiver clock. • SV Position in XYZ is computed from four SV pseudo-ranges and the clock correction and ephemeris data. • Receiver position is computed from the SV positions, the measured pseudo-ranges (corrected for SV clock offsets, ionospheric delays, and relativistic effects), and a receiver position estimate.

33. Pseudo-Range Navigation • Position is determined from multiple pseudo-range measurements at a single measurement epoch. • Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates.

46. Summary • geographic information is information about places on the earth's surface • geographic information technologies include global positioning systems (GPS), remote sensing and geographic information systems. • geographic information systems are both computer systems and software • GIS can have many different manifestations • GIS is used for a great variety of applications • geographic information science is the science behind GIS technology

47. Questions 1. Will GPS technology really make much difference to most GIS applications? 2. What GIS applications can make the best use of GPS technology? Which application will be affected the least?