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

Let's start with the navigation message itself, because that's the heart of how a GPS receiver knows where it is and what time it is. The navigation message is structured as a single frame, and that frame is made up of five sub-frames. Each sub-frame takes exactly 6 seconds to transmit. So, if you multiply 6 seconds by 5 sub-frames, the whole frame takes 30 seconds for the receiver to receive. That's the basic timing structure you need to hold onto. Now, each sub-frame has a specific job. Sub-frame 1 carries the satellite clock error data. Sub-frames 2 and 3 carry the satellite's ephemeris data — that's the precise orbital information for that specific satellite. Sub-frame 4 carries data on the ionospheric propagation model, plus GPS time and its correlation with UTC, which is Coordinated Universal Time. The fifth sub-frame is used to transmit the current satellite constellation almanac data — that's the approximate orbital data for all satellites in the constellation. Here's a key point: a single frame only gives you part of the almanac. You need a series of 25 frames to download the whole almanac. Since each frame takes 30 seconds, 25 frames take 12.5 minutes. That's a critical number. The almanac data is usually downloaded hourly, and it stays valid from 4 hours up to several months, depending on the type of receiver. Let me also walk you through the internal structure of a sub-frame, because the format matters. Each sub-frame is 300 bits and takes 6 seconds. A full data frame is 1500 bits and takes 30 seconds. Within each sub-frame, you have words. Each word is 30 bits — 24 bits of data and 6 bits of parity. The first word is the TLM, the telemetry word, which contains an 8-bit preamble. The second word is the HOW, the handover word, which contains a 17-bit time of week. So, each sub-frame starts with TLM and HOW, then the data for that sub-frame, and ends with parity bits. Now, here's where the almanac becomes practically important. Because the orbits are mathematically defined, the receiver can maintain a predicted almanac of satellite positions internally. So, when you switch the receiver on, if it knows its position and time to a reasonable degree of accuracy, it already knows which satellites to expect. It can commence a position update immediately. But if the almanac is corrupted, out of date, or lost — or if the receiver's position or clock time are significantly in error — it won't find the expected satellites. In that case, it has to download the almanac from the constellation. That download takes 12.5 minutes because it fills 25 frames. Once the almanac is downloaded, the receiver performs a skysearch. That means it checks which satellites are above the horizon and selects the 4 that will give the most accurate fix. Then it commences position fixing. That skysearch and selection takes at least a further 2.5 minutes. So, the time to first fix, in the worst case, is at least 15 minutes — 12.5 for the almanac plus 2.5 for the skysearch. If there are no problems, though, the first fix on initialization will be obtained within about 30 seconds. Finally, let's look at how the receiver actually measures distance. The GPS receiver internally generates the PRN code — that's the Pseudo-Random Noise code unique to each satellite. It compares the relative position of the two codes — the one it generated and the one it received from the satellite — to determine the time interval between transmission and reception. That time interval, multiplied by the speed of light, gives the range to the satellite. That's the fundamental principle of operation. So, to tie it together: the navigation message structure, the almanac download process, and the PRN code comparison are the three pillars of how a GPS receiver gets its first fix.