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

Let’s start with the Space Segment of GPS, because that’s the hardware up in orbit that makes the whole system work. The operational constellation — the full, working set of satellites — is specified as 24 SVs. SV stands for Space Vehicle, which is just the professional name for a GPS satellite. Right now the USA actually has 31 SVs providing a navigational service, so the constellation is running above its minimum. Now the geometry. These satellites orbit at an average height of 10,898 nautical miles, which is 20,180 kilometres. Their orbital period is 12 hours — so each satellite goes around the Earth twice a day. The orbital planes are inclined at 55° to the equator, and they’re equally spaced around the equator. That spacing is deliberate: it’s arranged so that an observer on or close to the Earth’s surface will always have between five and eight SVs in view, and crucially, those satellites will be at least 5° above the horizon. That 5° elevation is a hard rule. An SV is masked — meaning it’s not selected for navigation use — if its elevation is less than 5° above the horizon. So even if a satellite is physically up there, if it’s too low on the sky, the receiver ignores it. 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 — that’s the timing precision the whole system depends on, because GPS positioning is fundamentally a timing measurement. Now, what do these satellites actually broadcast? Each SV transmits pseudo-random noise codes — PRN codes — of one millisecond duration, on two frequencies in the UHF band, plus a NAV and SYSTEM data message. Every satellite has its own unique PRN code, which is how the receiver tells which satellite is which. Let’s go through the frequencies. The L1 frequency is 1575.42 MHz. On L1, the satellite transmits the coarse acquisition code — the C/A code — repeated every millisecond, with a modulation of 1.023 MHz. It also carries the precision code, the P code, with a modulation of 10.23 MHz, which repeats every seven days. And L1 carries the navigation and system data message at 50 Hz. That data message is used by both the P and C/A codes — so it’s shared. The L2 frequency is 1227.6 MHz, and it transmits the P code. The key point here is that the second frequency is used to determine ionospheric delays. That’s the correction for signal slowing as it passes through the ionosphere — having two frequencies lets the receiver measure and remove that error. Finally, the L3 frequency is 1381.05 MHz. It’s been allocated as a second frequency for non-authorized users, and its use is the same as the L2 frequency. So L3 gives civilian users a second frequency for ionospheric delay correction, just like L2 does for authorized users. So to tie it together: the constellation geometry guarantees you always see enough satellites above the 5° mask angle, each satellite carries ultra-precise atomic clocks, and each broadcasts its unique PRN code on multiple frequencies — L1 for the C/A and P codes plus the data message, L2 for the P code and ionospheric correction, and L3 as the civilian second frequency. That’s the space segment.