Global Navigation Satellite Systems (GNSS) — Page 307, Lesson 305

Right, let’s get into the heart of GNSS — the actual signals that make satellite navigation work. We’ve already set the scene with the constellation geometry, so now I want to walk you through what each satellite is actually transmitting, because that’s where the precision lives. Every GPS satellite broadcasts on two carrier frequencies. The L1 carrier is at 1575.42 MHz, and the L2 carrier is at 1227.6 MHz. Think of these as the radio highways that carry all the navigation data. On top of these carriers, the satellite modulates several codes and data streams, and each one has a very specific job. First, the C/A code — that stands for Coarse/Acquisition — and it runs at 1.023 MHz. This is the code that civilian users get. It’s the one your standard GPS receiver uses. Then there’s the P code, the Precise code, which runs at 10.23 MHz — exactly ten times the C/A rate. That higher rate is what gives the military much better ranging accuracy. The Nav/System Data is transmitted at 50 Hz — that’s the slow stream of information about the satellite’s position, clock, and health. Now, here’s a critical point about who gets what. Only the C/A code is available to civilian users. The P code is reserved for the US military, approved civilian users, and foreign military users — but only at the discretion of the US DOD, the Department of Defense. And when anti-spoofing measures are switched on, the P code is redesignated as the Y code. The Y code is encrypted, so it’s only usable by people who have the decryption algorithms. That’s a security layer you need to know about. Let’s talk about what these codes actually do. The PRN codes — Pseudo-Random Noise codes — serve two functions. First, they provide SV identification — each satellite has its own unique code, so the receiver knows which satellite it’s listening to. Second, they provide a timing function that lets the receiver measure the SV range — the distance to that satellite. That timing is the whole basis of the ranging measurement. Now, the Nav/System Data message — this is the payload of information the satellite broadcasts. It contains several key elements. The SV position — where the satellite is in space. The SV clock time — the satellite’s own clock reading. The SV clock error — how much that clock is off, so the receiver can correct for it. Information on ionospheric conditions — because the ionosphere delays the signal, and we need to know how much. And then supplementary information, which includes the almanac — that’s the orbital parameters for all the satellites, so the receiver knows where to look. Also SV health — but note, that’s P-code only. Plus the correlation of GPS time with UTC — so the receiver can give you civil time — and other command and control functions. This all feeds into the two services GPS provides. The Standard Positioning Service, or SPS, uses the C/A code — that’s the civilian service. The Precise Positioning Service, or PPS, uses both the C/A and P codes — that’s the military-grade service with the higher accuracy. Now, let me bring in GLONASS, the Russian system, because you need to know how it differs. GLONASS also has an operational constellation of 24 satellites, but they’re arranged in three orbital planes inclined at 65° to the equator. The orbital height is 10,313 NM, which is 19,099 km, giving an orbital period of 11 hours 15 minutes. Here’s the key contrast with GPS. In GPS, every satellite uses the same two frequencies but different codes. In GLONASS, it’s the opposite — the codes are the same for all satellites, but each satellite uses different frequencies. The L1 frequency is incremental from 1602 MHz, and the L2 frequency is incremental from 1246 MHz. So each GLONASS satellite is on its own slightly shifted frequency, and that’s how the receiver tells them apart. And just like GPS, GLONASS transmits both C/A and P codes — same structure, different frequency plan. That figure shows you the L1 and L2 signals, the codes riding on them, and the mixing process. The reason the two frequencies matter — why having L1 and L2 both available is so important — we’ll get into properly when we cover GNSS errors, because that’s where the ionospheric correction comes from. For now, just hold onto the structure: two carriers, two codes, one data stream, and two very different frequency plans between GPS and GLONASS.