This was an extremely serious bug in NASA/ESA's Cassini-Huygens probe, in the S-band link between Huygens (landing on Saturn's moon Titan) and Cassini (acting as radio relay).
It was a timing bug. There'd be a very high relative velocity between Cassini and Huygens, creating a significant (~2e-5) Doppler shift in the link. This shifted the frequency of the 2 GHz carrier (by 38 kHz). Likewise, it shifted the symbol rate of the 16 kbps bit stream (by 0.3 bps). The second effect was overlooked. On the demodulating end (Cassini), the bit-synchronizer expected the nominal bit rate, not the Doppler-shifted bit rate. Since its bandwidth was narrower than the 0.3 bps Doppler shift, it was unable to recognize frame syncs; this was proven in experiments post-launch. The parameter that set the bitrate was stored in non-modifiable firmware.
As it was when launched, Huygens would be unable to return any instrument data. For some context, this was the only probe that's ever visited Titan, at a cost of about $400 million.
The workaround
[spoiler]
The workaround was a major change in the orbit trajectory of Cassini (a $3 billion probe). Details aside, it set up an orbit geometry with this feature: at the time Huygens was descending in Titan's atmosphere, Cassini would be flying at a ~90° angle to their separation. The relative velocity was still 20,000 kph, but tangential velocity doesn't contribute to Doppler shift.
http://spectrum.ieee.org/aerospace/space-flight/titan-callin...
http://descanso.jpl.nasa.gov/seminars/abstracts/viewgraphs/H...
This was an extremely serious bug in NASA/ESA's Cassini-Huygens probe, in the S-band link between Huygens (landing on Saturn's moon Titan) and Cassini (acting as radio relay).
It was a timing bug. There'd be a very high relative velocity between Cassini and Huygens, creating a significant (~2e-5) Doppler shift in the link. This shifted the frequency of the 2 GHz carrier (by 38 kHz). Likewise, it shifted the symbol rate of the 16 kbps bit stream (by 0.3 bps). The second effect was overlooked. On the demodulating end (Cassini), the bit-synchronizer expected the nominal bit rate, not the Doppler-shifted bit rate. Since its bandwidth was narrower than the 0.3 bps Doppler shift, it was unable to recognize frame syncs; this was proven in experiments post-launch. The parameter that set the bitrate was stored in non-modifiable firmware.
As it was when launched, Huygens would be unable to return any instrument data. For some context, this was the only probe that's ever visited Titan, at a cost of about $400 million.
The workaround
[spoiler]
The workaround was a major change in the orbit trajectory of Cassini (a $3 billion probe). Details aside, it set up an orbit geometry with this feature: at the time Huygens was descending in Titan's atmosphere, Cassini would be flying at a ~90° angle to their separation. The relative velocity was still 20,000 kph, but tangential velocity doesn't contribute to Doppler shift.