Astrocast telemetry downlink specification


The telemetry protocols used for the telemetry downlink of Astrocast 0.1 are fully based on ECSS standards. These are the three applicable documents:

  • ECSS-E-ST-50-01C which defines the following coding steps that are applied to the telemetry downlink:
    • Reed-Solomon coding
    • Pseudo-randomizer
    • Frame synchronization
  • ECSS-E-ST-50-03C which defines the format of the telemetry transfer frames.
  • ECSS-E-70-41A which defines the format of the telemetry packets

Note that the telemetry downlink is only enabled for specific passes. It is not enabled most of the time, instead the beacon is transmitted.

The following image “TelemetryProcessing” shows the steps from channel symbols to packets.


Frequency: 437.150MHz (same as beacon)

Modulation: 2FSK, deviation ±4.8Khz (i.e. 437.1452MHz and 437.1548MHz)

Bitrate: 9600bps


Parameters applied to ECSS-E-ST-50-01C:

  • No convolutional coding
  • Reed-Solomon (223,255), E=16, I=5 (also known as CCSDS Reed-Solomon)
  • Attached Sync Marker (ASM) of 32 bits: 0x1ACFFC1D
  • Pseudo-Randomizer: used

Telemetry Transfer Frame

Here is the format of the Telemetry Transfer Frame and its header with values of fixed fields (fields and their usage are fully described in ECSS-E-ST-50-03C):

The Spacecraft ID is 1 for Astrocast 0.1, it will be 2 (0b0000000010) for Astrocast 0.2. It is by checking that field that it can be known which spacecraft transmitted the data.

Links to decoders:

14-FEB-2019 ARK