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

We're starting a brand-new topic now: Global Navigation Satellite Systems, GNSS. This is the umbrella term for the satellite constellations that give us worldwide position. Let me walk you through the three systems and then the ground and user segments. First, the big picture. There are three constellations in this comparison. NAVSTAR from the USA, GLONASS from the USSR, and Galileo from the EU. 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 lower at 19,099 kilometres, 10,313 nautical miles. Galileo is the highest at 23,222 kilometres, 12,539 nautical miles. The inclination to the equator also varies. NAVSTAR is inclined 55 degrees, GLONASS 65 degrees, and Galileo 56 degrees. Orbit time, the time for one full 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 important. 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 E1 from 1559 to 1591 MHz, E5 from 1164 to 1215 MHz, and E6 from 1260 to 1300 MHz. The codes carried on these frequencies also differ. NAVSTAR carries P and C/A codes on L1, and P code on L2. GLONASS also carries P and C/A on L1, and P on L2. The geoid, the reference model of the Earth's shape, is WGS 84 for NAVSTAR, PZ 90 for GLONASS, and ETRS 89 for Galileo. This is the comparison table you see in Figure 18.6. Now let's move to the Control Segment. This is the ground infrastructure that keeps the satellites accurate. The GPS control segment comprises a Master Control Station, a Back-up Control Station, and 5 Monitoring Stations. The monitoring stations are located at Ascension, Diego Garcia, Hawaii, Colorado Springs, and Kwajalein. The monitoring stations check the satellites' internally computed position and clock time at least once every 12 hours. Here's the key mechanism. The calculation of position using Keplerian laws is precise, but the satellite orbits are affected by gravitational influences from the sun, moon, and planets, and also by solar radiation. So errors between the computed position and the actual position occur. When a positional error is detected by the ground station, it's sent to the satellite so the satellite can update its knowledge of its position. Similarly, if an error is detected in the satellite clock time, this is notified to the satellite. But here's the crucial limitation: the clocks cannot be adjusted. So this clock error is included in the satellite broadcast. That means the error is transmitted as part of the signal, and the receiver must account for it. Finally, the User Segment. This is all the GPS receivers using the space segment to determine position on and close to the surface of the Earth. These receivers may be stand-alone or part of integrated systems. There are several types of receiver. Sequential receivers use one or two channels and scan the satellites sequentially to determine the pseudo-ranges. Pseudo-range is the measured distance from the receiver to the satellite, which includes clock errors — that's why it's "pseudo." Multiplex receivers may be single or twin channel. They can move quickly between satellites to determine the pseudo-ranges, and hence have a faster time to first fix than sequential receivers. Time to first fix is how long it takes to get an initial position. Multi-channel receivers monitor several satellites simultaneously to give instant positional information. These include 'all-in-view' receivers, which monitor all the satellites in view and select the best 4 to determine position. Because of the speed of operation, these are the preferred type for aviation. So to tie it together: the space segment provides the signals, the control segment keeps the satellites honest, and the user segment — the receivers — turn those signals into your position. For aviation, you want the multi-channel all-in-view receiver because it's fastest.