Global Navigation Satellite Systems (GNSS) — Page 316, Lesson 306

Let me walk you through the Global Navigation Satellite Systems chapter. We're starting with the big picture — the three systems that make up GNSS, then the control segment, then the user segment. First, the three satellite constellations. NAVSTAR is the US system, GLONASS is the Russian system — that's the USSR entry in the table — and Galileo is the European Union system. Each has a different number of satellites in orbit. NAVSTAR and GLONASS each have 24 satellites, while Galileo has 30. The orbits differ too: NAVSTAR uses 6 orbital planes, GLONASS and Galileo each use 3. Now the orbit heights. NAVSTAR orbits at 20,180 kilometres, which is 10,898 nautical miles. GLONASS is at 19,099 kilometres, or 10,313 nautical miles. Galileo is highest at 23,222 kilometres, or 12,539 nautical miles. The orbital inclination — that's the tilt of the orbit relative to the equator — is 55 degrees for NAVSTAR, 65 degrees for GLONASS, and 56 degrees for Galileo. And the orbit time, the time for one complete revolution, is 11 hours 56 minutes for NAVSTAR, 11 hours 15 minutes for GLONASS, and 14 hours 8 minutes for Galileo. Now the frequencies, and this is where it gets important for you as a pilot. NAVSTAR broadcasts on L1 at 1575 MHz and L2 at 1227 MHz. GLONASS uses L1 at 1600 MHz and L2 at 1250 MHz. Galileo uses a different naming scheme: E1 spans 1559 to 1591 MHz, E5 spans 1164 to 1215 MHz, and E6 spans 1260 to 1300 MHz. Notice Galileo has three frequency bands, not two. The codes on those frequencies matter. On NAVSTAR, L1 carries both the P code and the C/A code, while L2 carries only the P code. GLONASS is the same: L1 has P and C/A, L2 has P only. The P code is the precise, military-grade code, and the C/A code — that's the coarse acquisition code — is the civilian one. Only the C/A code is available to civilian users, and that's why the use of two frequencies is so important, as shown in Figure 18.5. Finally, the geoid — that's the mathematical model of the Earth's shape used for position calculations. NAVSTAR uses WGS 84, GLONASS uses PZ 90, and Galileo uses ETRS 89. These are different reference frames, and that matters when you're combining data from multiple constellations. Now let's move to the control segment. The GPS control segment has three components: a Master Control Station, a Back-up Control Station, and 5 Monitoring Stations. The monitoring stations are located at Ascension, Diego Garcia, Hawaii, Kwajalein, and Colorado Springs — and the Master Control Station is at Colorado Springs, with the Back-up Control Station also there. There's also a ground antenna at each monitoring site. Here's the critical job of the monitoring stations. They check the satellites' internally computed position and clock time at least once every 12 hours. Now, the satellites calculate their position using Keplerian laws — that's the mathematics of orbital motion — and that calculation is precise. But the orbits are affected by the gravitational influences of the sun, the moon, and the planets, and also by solar radiation. So errors develop between the computed position and the actual position. When a monitoring station detects a positional error, it sends that correction to the satellite, and the satellite updates its knowledge of its position. But here's the subtle part about the clock. If an error is detected in the satellite's clock time, that error is also notified to the satellite — but the clocks cannot be adjusted. So instead, that clock error is included in the satellite's broadcast. The receiver on the ground then accounts for that broadcast error when it computes your position. That's a key distinction: position errors get corrected, clock errors get broadcast as data. Now the user segment. This is simply all the GPS receivers that use the space segment to determine position on, or close to, the surface of the Earth. These receivers can be stand-alone units or part of integrated systems. There are three types of receiver, and they differ in how they handle the satellites. Sequential receivers use one or two channels and scan the satellites sequentially — one after another — to determine the pseudo-ranges. Pseudo-range is the measured distance from the receiver to a satellite, and it's called "pseudo" because it includes the clock error we just talked about. Multiplex receivers may be single or twin channel, and they can move quickly between satellites to determine the pseudo-ranges. Because they switch fast, they have a faster time to first fix than sequential receivers — that's the time from power-on until you get a valid position. Multi-channel receivers monitor several satellites simultaneously to give instant positional information. These include the "all-in-view" receivers, which monitor all the satellites in view and select the best 4 to determine position. Because of their speed of operation, these are the preferred type for aviation. That's the key takeaway for you: in the cockpit, you're almost certainly using a multi-channel, all-in-view receiver. The figures on these pages show you the receiver's control unit, the initialization page where you set up the receiver, the position page showing your computed position, and the waypoint definitions page where you enter your route points. Those are the operational screens you'll interact with in the aircraft.