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So the satellite was rebroadcasting the signal at a frequency where it could interfere with GSM? Does the satellite just rebroadcast whatever it receives, verbatim?

Block up-converter (BUC) at uplink site (in this case, likely part of a VSAT terminal or Satellite Internet "antenna") takes L-Band (950-1525mhz) input via cable, converts it to C-band uplink (5850-6425mhz) frequency, and transmits to satellite. Satellite then rebroadcasts verbatim at C-band downlink frequency (3400-4200mhz). Except that one of the inputs didn't have a cable or filter attached so it picked up a GSM signal at 936-960mhz. This caused interference on the satellite.

For further context, the reason for this slightly convoluted setup is that normal coaxial cables are really lossy at the high frequencies used to communicate with satellites, so most systems use lower frequency bands on the cable runs and convert the signals by a fixed frequency offset at the actual antenna. It just so happens that the block of frequencies used on the cable runs is similar to common mobile phone frequencies.

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