Table of Contents Frame Format Frame Header IRA: Ring Alert Location Information SV Location Spot Beam Location Iridium Toolkit On GitHub 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. iridium/toolkit.txt Last modified: 2021/04/18 12:35(external edit)