Global Navigation Satellite Systems (GNSS) — Page 316, Lesson 309

Let's look at the GPS navigation message itself — the actual data stream that comes down from the satellites. This is the heart of how a receiver knows where it is and what time it is. The navigation message is structured as one frame, and that frame is made up of 5 sub-frames. Each sub-frame takes 6 seconds to transmit, so the whole frame of 5 sub-frames takes 30 seconds for the receiver to receive. So one full frame is 1500 bits, transmitted over 30 seconds. Now, what's in each sub-frame? Frame 1 contains the SV clock error — that's the satellite's clock correction data. Frames 2 and 3 contain the SV ephemeris data — that's the precise orbital information for that specific satellite, split into two parts. Frame 4 contains data on the ionospheric propagation model, GPS time, and its correlation with UTC — Coordinated Universal Time. The fifth frame is used to transmit the current SV constellation almanac data — that's the approximate orbital data for all the satellites in the constellation. Here's the key point about the almanac: a series of 25 frames is required to download the whole almanac. Since each frame takes 30 seconds, that's 25 times 30 seconds, which is 12.5 minutes to download the complete almanac. The almanac data is usually downloaded hourly and is valid from 4 hours to several months, depending on the type of receiver. Let me break down the format of each sub-frame. Each sub-frame is 300 bits and takes 6 seconds. Within each sub-frame, you have a TLM word — that's the Telemetry Word — and a HOW — that's the Handover Word. Each word is 30 bits: 24 data bits and 6 parity bits. The TLM word contains an 8-bit preamble, and the HOW contains a 17-bit time of week. So the structure is: TLM, HOW, then the data — whether that's clock correction, ephemeris, ionospheric data, or almanac — and then parity bits. Now, here's the clever part about how the receiver uses this. Because the orbits are mathematically defined, an almanac of their predicted positions can be and is maintained within the receivers. So when the receiver is switched on, provided it knows its position and time to a reasonable degree of accuracy, it will know which SVs — which Space Vehicles, the satellites — to expect, and it can commence position update immediately. But what if the almanac is corrupted, out of date, or lost? Or what if the receiver position or receiver clock time are significantly in error? Then it will not find the expected SVs, and it will download the almanac from the constellation. That download takes 12.5 minutes because the almanac fills 25 frames. Once the almanac is downloaded, the receiver will carry out a skysearch. This involves the receiver checking which SVs are above the horizon and selecting the 4 to give the most accurate fix, then commencing position fixing. That skysearch and selection takes at least a further 2.5 minutes. So the time to first fix — the total time from switch-on to a position fix — will be at least 15 minutes in that worst case: 12.5 minutes for the almanac download plus 2.5 minutes for the skysearch. But if there are no problems — if the almanac is valid and the receiver knows its position and time — then the first fix on initialization will be obtained within about 30 seconds. That's the fast, normal case. Finally, let me explain how the receiver actually measures the signal. The GPS receiver internally generates the PRN code — that's the Pseudo-Random Noise code, the unique code for each satellite — and compares the relative position of the two codes: the internally generated one and the received one. By comparing those, it determines the time interval between transmission and reception. That time interval, multiplied by the speed of light, gives the range to the satellite. So to tie it all together: the navigation message structure — 5 sub-frames per frame, 6 seconds per sub-frame, 30 seconds per frame — carries the clock error, ephemeris, ionospheric model, and almanac. The almanac is what lets a receiver start fast, but if it's missing or wrong, you pay the 12.5-minute download penalty plus the 2.5-minute skysearch, giving a 15-minute time to first fix. And the PRN code comparison is how the receiver measures the signal travel time.