Global Navigation Satellite Systems (GNSS) — Page 307, Lesson 304

Let’s start with the big picture. The Global Navigation Satellite System, or GNSS, is the umbrella term for satellite-based navigation, and the system we’re focusing on here is GPS — the Global Positioning System, run by the United States. I want to walk you through the Space Segment, which is the part of the system that lives in orbit. This is the foundation for everything else, so we’ll take it piece by piece. The operational constellation for GPS is specified as comprising 24 SVs. SV stands for Space Vehicle — that’s the official term for a GPS satellite. So the design standard is 24 satellites. Right now, the USA actually has 31 SVs providing a navigational service, so there are more in orbit than the minimum specification, which gives redundancy. Now, the orbits. These satellites orbit at an average height of 10,898 nautical miles, which is 20,180 kilometres. The orbital period is 12 hours — so each satellite completes one full orbit around the Earth every half a day. The orbital planes are inclined at 55° to the equator, and they are equally spaced around the equator. That even spacing is what ensures global coverage. Because of that geometry, an observer on or close to the surface of the Earth will have between five and eight SVs in view at any one time, and crucially, those satellites will be at least 5° above the horizon. That 5° figure is important, and I’ll come back to it in a moment. Each SV carries 3 or 4 atomic clocks, of caesium or rubidium standard, with an accuracy of 1 nanosecond. A nanosecond is one billionth of a second. These clocks are the heartbeat of the system — GPS works by measuring the time it takes a signal to travel from the satellite to you, and if the clock were off by even a tiny fraction, the position error would be enormous. That’s why the clocks are atomic and that accurate. Now, that 5° elevation figure I mentioned. An SV will be masked — and masking means it is not selected for navigation use — if its elevation is less than 5° above the horizon. So even if a satellite is physically in view, if it’s sitting low in the sky, below that 5° threshold, the receiver will ignore it. Low-elevation satellites have signals that travel through more atmosphere, which introduces more error, so the system simply excludes them. Let me show you the constellation layout. Now, what do these satellites actually broadcast? Each SV broadcasts pseudo-random noise codes — PRN codes — of one millisecond duration, on two frequencies in the UHF band, plus a NAV and SYSTEM data message. Each SV has its own unique code. That uniqueness is how the receiver tells which satellite a signal came from — it’s like each satellite has its own signature. Let’s break down the frequencies. There’s the L1 frequency, which is 1575.42 MHz. On L1, the satellite transmits the coarse acquisition code, abbreviated C/A code. That C/A code is repeated every millisecond, with a modulation of 1.023 MHz. Also on L1 is the precision code, the P code, which has a modulation of 10.23 MHz and repeats every seven days. And L1 also carries the navigation and system data message, which is transmitted at 50 Hz. That navigation and system data message is used by both the P and C/A codes — so it’s shared between them. Then there’s the L2 frequency, which is 1227.6 MHz. L2 transmits the P code. And here’s the key point: the second frequency is used to determine ionospheric delays. The ionosphere is a layer of the atmosphere that slows radio signals, and the amount of delay depends on the frequency. By comparing how the signal arrives on two different frequencies, the receiver can calculate and correct for that ionospheric error. That’s why having two frequencies matters so much. Let me show you that point. Finally, there’s the L3 frequency, which is 1381.05 MHz. L3 has been allocated as a second frequency for non-authorized users, and its use is the same as the L2 frequency. So for civilian, non-authorized users, L3 gives them a second frequency to do that same ionospheric delay correction that military users get from L2. So to tie it all together: the Space Segment is 24 to 31 satellites in 12-hour orbits, inclined at 55°, each carrying atomic clocks accurate to a nanosecond, broadcasting unique PRN codes on L1, L2, and L3. The receiver masks anything below 5° elevation, and the dual-frequency capability is what lets the system correct for ionospheric delay. That’s the foundation of how GPS knows where you are.