A Critical Design Flaw Resolved
Long after launch, a few persistent engineers discovered that the communication equipment on Cassini had a potentially fatal design flaw, which would have caused the loss of all data transmitted by the Huygens probe.
Since Huygens was too small to transmit directly to Earth, it was designed to transmit the telemetry data obtained while descending through Titan's atmosphere to Cassini by radio, which would in turn relay it to Earth using its large 4-meter diameter main antenna. Some engineers, most notably ESA Darmstadt employees Claudio Sollazzo and Boris Smeds, felt uneasy about the fact that, in their opinion, this feature had not been tested before launch under sufficiently realistic conditions. Smeds managed, with some difficulty, to convince superiors to perform additional tests while Cassini was in flight. In early 2000, he sent simulated telemetry data at varying power and Doppler shift levels from Earth to Cassini. It turned out that Cassini was unable to relay the data correctly.
The reason: under the original flight plan, when Huygens was to descend to Titan, it would have accelerated relative to Cassini, causing the Doppler shift of its signal to vary. Consequently, the hardware of Cassini's receiver was designed to be able to receive over a range of shifted frequencies. However, the firmware failed to take into account that the Doppler shift would have changed not only the carrier frequency, but also the timing of the payload bits, coded by phase-shift keying at 8192 bits per second.
Reprogramming the firmware was impossible, and as a solution the trajectory had to be changed. Huygens detached a month later than originally planned (December 2004 instead of November) and approached Titan in such a way that its transmissions traveled perpendicular to its direction of motion relative to Cassini, greatly reducing the Doppler shift.
The trajectory change overcame the design flaw for the most part, and data transmission succeeded, although the information from one of the two radio channels was lost due to an unrelated error.
The trajectory change was not the only mitigation to the Doppler shift problem, and software patches were uplinked to several instruments on the probe from the Deutsche Aerospace facility in Darmstadt to further reduce the risk of data loss.
Read more about this topic: Huygens (spacecraft)
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