GNSS

Navigation · Chapter 46 · DGCA CPL and ATPL

The GPS segments and signals, why four satellites make a fix, the errors and geometry, RAIM integrity and augmentation.

Remember

24 satellites, 6 planes, 55°, 20 180 km, ~12 h orbit; WGS-84.
Range = c × time (pseudo-range); 4 satellites: x, y, z and clock. Civil C/A on L1 1 575.42 MHz.
Ionosphere biggest error (two frequencies remove it); spread geometry = low DOP.
ICAO: ± 13 m horizontal, ± 22 m vertical. RAIM: 5 detect, 6 exclude.
GBAS: airfield station; SBAS: geostationary (EGNOS, WAAS, MSAS, GAGAN).

Satellites that tell the time

A clock in the sky • GNSS (global navigation satellite system): satellites that tell you where you are, anywhere on Earth • Each satellite carries a very exact clock and keeps saying the time and where it is • Your receiver times how long each signal took. Time × the speed of light = the range to that satellite
Timing it • Each satellite sends its own repeating code, a PRN code (pseudo-random noise): it names the satellite and carries the timing • The receiver makes its own copy of the code and slides it until the two match • How far it had to slide = the travel time
Whose are they? • GPS (USA), GLONASS (Russia), Galileo (Europe) and BeiDou (China) cover the whole Earth • India also has its own regional system, NavIC (7 satellites). Regional augmentation systems add corrections over one area: India’s is GAGAN
Measured from what? • Positions are worked out in x-y-z from the Earth’s centre, on a model Earth called WGS-84: an ellipsoid, a smoothly squashed ball • Real sea level (the geoid) is lumpier than that smooth model • So a GPS height is not a height above sea level: approach minima use the barometric altimeter

Space, control, user

Space • The space segment: 24 satellites in 6 orbit planes, each tilted 55° to the equator (its inclination), about 20 180 km up • Each orbit takes about 12 hours. From anywhere you see 5 to 8 of them • The orbits are drawn to scale: the Earth really is that small inside them
Signals • Every satellite sends on L1 (1 575.42 MHz) and L2 (1 227.6 MHz), each with its own code • The C/A code on L1 is free for everyone: the standard positioning service (SPS) • The more precise P code is for authorised (military) users: the precise positioning service (PPS)
Control • The control segment: a master station and monitor stations on the ground • They track every satellite, check its orbit and clock at least every 12 hours, and send up corrections for it to broadcast
User • The user segment: every receiver, in aircraft, ships and phones • Aircraft sets are multi-channel: one channel per satellite, tracking "all in view" at once • Older sets were sequential (one satellite after another) or multiplex (switching fast between them)
GLONASS and Galileo • Same idea, different orbits: GLONASS 24 satellites in 3 planes at 65°; Galileo 30 satellites at 56° • Each has its own model Earth: GLONASS uses PZ-90, GPS uses WGS-84