Iridium Toolkit

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Frame Format

Frame Header

All (parsed) frames start with a header:

IRA: i-1472473197-t1 000001626 1627088000  97%   0.009 129 DL
Field NumberContentExampleComment
0Frame typeIRA:
1UNIX timestamp in seconds when the recording startedi-1472473197-t1 The i- prefix and -t1 suffix a historical artefact…
2Time in milliseconds inside the recording000001626
3Frequency in Hertz1627088000Estimated center frequency
4Confidence in percent97%Confidence is estimated during demodulation by gr-iridium
5Signal level0.009Arbitrary scale
6Length in symbols129Excluding the 12 symbol sync word. One symbol equals two bits
7DirectionDLDL 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.

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 NumberContentExampleComment
8Satellite numbersat:817 bit field
9Spot beam numberbeam:20
10Satellite or spot beam positionpos=(+46.64/-115.08)
11Satellite or spot beam altitudealt=791 In km
12Ring alert intervalRAI:48 Always 48 in the live system
13Unknown?00
14Broadcast channel sub-bandbc_sb:07Brodcast channel for the spot beam is in this sub-band
15-TMSI pagesPAGE(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.

Location Information

The ring alert contains two different, alternating, locations:

SV Location

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.