Iridium Toolkit

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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

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)
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
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.

  • iridium/toolkit.txt
  • Last modified: 2021/04/18 12:35
  • (external edit)