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All (parsed) frames start with a header:
IRA: i-1472473197-t1 000001626 1627088000 97% 0.009 129 DL
| Field Number | Content | Example | Comment |
|---|---|---|---|
| 0 | Frame type | IRA: | |
| 1 | UNIX timestamp in seconds when the recording started | i-1472473197-t1 | The i- prefix and -t1 suffix a historical artefact… |
| 2 | Time in milliseconds inside the recording | 000001626 | |
| 3 | Frequency in Hertz | 1627088000 | Estimated center frequency |
| 4 | Confidence in percent | 97% | Confidence is estimated during demodulation by gr-iridium |
| 5 | Signal level | 0.009 | Arbitrary scale |
| 6 | Length in symbols | 129 | Excluding the 12 symbol sync word. One symbol equals two bits |
| 7 | Direction | DL | DL or UL |
Ring alerts are sent periodically by the satellite to every spot beam. Every 90 ms a new ring alert gets sent to another spot beam. There are 48 spot beams, so every 4.32 s a ring alert is sent to the same spot beam.
Example:
IRA: i-1472473197-t1 000002706 1627087872 97% 0.012 132 DL sat:81 beam:20 pos=(+46.64/-115.08) alt=791 RAI:48 ?00 bc_sb:07 PAGE(tmsi:0ca5b2e2 msc_id:01) PAGE(NONE)
| Field Number | Content | Example | Comment |
|---|---|---|---|
| 8 | Satellite number | sat:81 | 7 bit field |
| 9 | Spot beam number | beam:20 | |
| 10 | Satellite or spot beam position | pos=(+46.64/-115.08) | |
| 11 | Satellite or spot beam altitude | alt=791 | In km |
| 12 | Ring alert interval | RAI:48 | Always 48 in the live system |
| 13 | Unknown | ?00 | |
| 14 | Broadcast channel sub-band | bc_sb:07 | Brodcast channel for the spot beam is in this sub-band |
| 15- | TMSI pages | PAGE(tmsi:0ca5b2e2 msc_id:01) PAGE(NONE) | A NONE page signifies the end of the list of pages |
As there can be a different amount of pages, the size of a ring alert frame is not constant.
The ring alert contains two different, alternating, locations:
Locations with a high altitude (e.g. > 100 km) can be treated as as the location of the SV (Space Vehicle).
Locations with a low altitude (e.g. < 10 km) can be treated as the location where the SV thinks that the particular spot beam is hitting the earth’s surface. Note that this is not the location of a subscriber.