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| Subframe | 1 word | 2 words | 3-10 words |
|---|---|---|---|
| 1 | TLM | How | Clock correction, GPS week, Satellite Health, etc. |
| 2 | TLM | How | Ephemeris |
| 3 | TLM | How | Ephemeris |
| 4 | TLM | How | Ionosphere, PRN 25-32 Satellite Almanac and Health, UTC, etc. (Subframe 4 contains 25 subcommutated pages) |
| 5 | TLM | How | PRN 1-24 Satellite Almanac and Health, etc. (Subframe 5 contains 25 subcommutated pages) |
Each of these five subframes begins with the same two words: the telemetry word (TLM) and the handover word (HOW). Unlike nearly everything else in the NAV message, these two words are generated by the satellite itself. As shown in the column headed Seconds of the Week at Midnight on that Day in Table 1.1, GPS time restarts each Sunday at midnight (0:00 o’clock). These data contain the time since last restart of GPS time on the previous Sunday, 0:00 o’clock.
The TLM is the first word in each subframe. It indicates the status of uploading from the Control Segment while it is in progress and contains information about the age of the ephemeris data. It also has a constant unchanging 8-bit preamble of 10001011, and a string helps the receiver reliably find the beginning of each subframe.
The HOW provides the receiver information on the time of the GPS week (TOW) and the number of the subframe, among other things. For example, the HOW’s Z count (an internally derived 1.5 second epoch) tells the receiver exactly where the satellite stands in the generation of positioning codes.
The telemetry word indicates the status of uploading the control segment, if it's in process or not. This allows your receiver to know that.
Also, it allows you to know the beginning of each word from the data string. The handover word is useful in a couple of ways, but probably most importantly to tell your receiver where the satellite is in its broadcast of the codes. There are several codes in GPS. We will talk more about those.