All (parsed) frames start with a header:
IRA: i-1472473197-t1 000001626 1627088000 97% 0.009 129 DL
|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|
IRA: Ring Alert
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.
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)
|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|
|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).
Spot Beam Location
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.