The Cassini-Huygens concept began in 1982, and it resulted from a working group that was comprised of the European Science Foundation and the National Academy of Sciences. The group was actively seeking opportunities to be involved in joint missions to space. Earlier tensions that existed between NASA and the European Union were reversed in 1988 when the two engaged in a joint mission. The mechanism that was used in the achievement of this mission was the Cassini-Huygens concept, and both ESA and NASA cooperated in a bid to survive budget cuts because the United States had the obligation of matching ESA commitments (SEBOK, 2017).
Both ESA and NASA were in approval of the Cassini-Huygens program, and the program commenced under a traditional type of management. NASA was tasked with building the Cassini orbiter which became the most complex and the largest space probe that was unmanned. Conversely, the European Space Agency built the Huygens lander (SEBOK, 2017).
The division of labor between NASA and ESA almost caused the Titan survey part of the mission to fail. Cassini which was expected to carry out different scientific surveys of the planetary system of Saturn was supposed to pass on transmissions to NASAs Deep Space Network from Huygens. However, there lacked proper management of the interface between the orbiter and the lander. The assumptions made about how the lander/orbiter system would behave after separation were also erroneous and this almost doomed the part of the mission that involved the Titan exploration (Perrot & Giordani, 1998).
Concept Development Stage
The function of the part of the Titan survey in the Cassini-Huygens mission was to showcase that the Huygens lander was going to separate from the Cassini orbiter and begin its one-way descent into the Titans atmosphere for two and a half hours. Data would be sent back to the orbiter by its modest transmitter. The modest transmitter would consequently relay the data back to earth (Schilling, 2005). The radio link between spacecraft thus became a poorly characterized single point of failure (SPOF).
The Italian vendor who built the system, Alenia Spazio Spa, failed to take into consideration the Doppler shift, which is approximately 38kHz, that would occur once the separation between the Huygens and the Cassini occurred and the Cassini began its descent. The communications protocol involved a phase of key-shifting that was binary. Varying the outgoing carrier waves phase would bring a transmission system that represented either 0s or 1s (SEBOK, 2017). The recovery of the bits requires precision in timing.
There was the appropriate configuration of the receiver in a bid to provide compensation for the carriers Doppler shift, but it would be a carrier that is phase-modulated. Thus, the communication systems would not be able to decode the landers data, and they would then pass on to NASA scrambled information (Khurana, 2006). The data would then be completely unrecoverable because of the failure mechanism.
Though both the Huygens and the Cassini had received testing before the launch, none of the tests carried out reflected accurately the Doppler shift that would occur at this point in the mission. Budget constraints caused a high fidelity radio test that was full scale not to be conducted, and this test would have disassembly as well as recertification of the probes.
Correction of this problem before the launch of the spacecraft would have been trivial since it only needed a minor firmware upgrade. However, any corrective action would have been limited and very expensive once the spacecraft was on its way to Saturn (SEBOK, 2017).
Once the mission began, the probe moved along its trajectory to Saturn and its moons. The European Space Agency ground operations manager, Claudia Sollazzo, did not feel okay about the communications system that was untested. He gave Boris Smeds, who was an engineer with telemetry and radio experience the task of coming up with a method that could be used to test the communications system with the use of a signal generated from earth (Geake & Mill, 1998).
The engineer developed test protocols that used an exact duplicate of Huygens and JPL (jet propulsion laboratory) ground stations. Simulated telemetry would consequently be broadcast to Cassini from the earth and then relayed back (Wendel, 2017). The test signal would vary in Doppler shift and power level in a bid to reflect the parameters accurately to be anticipated during the descent of the Huygens into the Titans atmosphere and exercise the communications link fully.
Even though Smeds faced opposition from people who felt that his test plans were not necessary, he received support from the Huygens project scientist, Jean-Pierre Lebreton, and Sollazo. Smeds was able to carry out the test at least two years after the launch of the mission.
The broadcast test signal was received by Cassini, from where it was retransmitted to the DSN site, and consequently passed on to the European Space Agencys (ESOC) European Space Operation Centre in Germany (Israel et al., 1999).
It was mandatory for the testing to be conducted when the orbiter was in the right relative position in the sky; it was more than two hundred and fifty thousand miles away and had a signal round trip of almost one hour. After the test began, it immediately exposed a major challenge; the stream of data was corrupted, and the failures did not correlate with the levels of power that the signal had (Lebreton & Matson, 2007). No clear cause was identified during the first two days of testing.
Many science teams competed for time to be able to communicate with the probe even though it was far from its destination. Smeds identified that the Doppler shifts that were induced in the simulated signals were the root cause of the issues. His test plan, however, failed to include unshifted telemetry. He then modified his test plan, and the planned tests were shortened by sixty percent, and this enabled him to recover enough time to inject into the test protocols an unshifted signal (Khurana, 2006).
Similar degradation was not suffered by the unshifted signal, but engineers did not accept the problem diagnosis. There was follow up testing using different equipment and probe mockups that ultimately showed ESA that an issue existed which took many months (Khurana, 2009).
ESA told NASA about the latent failure of a proper link of communication between Huygens and Cassini. It was identified that Alenia Spazio had reused the features of the timing of a communication system that was used on Earth-orbiting satellites. This was wrong because they these did not compensate for the magnitude of the Doppler shifts. Moreover, since NASA was considered to be a competitor, complete communications module specifications were not shared with JPL (Lavvas, Coustenis & Vardavas, 2008). The implementation of the protocols of communication was in the firmware of the system which was trivial if corrected before launch but it was impossible to correct after the launch.
A Huygens Recovery Taskforce which had a team of forty men was created in a bid to research on potential courses of action that could aid in mitigating the problems. It was determined after analysis that modification would not aid in preventing degradation. Ultimately, the team proposed that the trajectory of the Cassini should be changed in a bid to reduce the Doppler shift (Perrot & Giordani, 1998). The efficacy of this remedy was verified through multiple studies, and eventually, the mission was able to finish the Titan survey successfully.
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Israel, G., Cabane, M., Coll, P., Coscia, D., Raulin, F., & Niemann, H. (1999). The Cassini-Huygens ACP experiment and exobiological implications. Advances in Space Research, 23(2), 319-331. http://dx.doi.org/10.1016/s0273-1177(99)00053-8
Khurana, K. (2006). Saturn: Cassini/Huygens arrival and system science. Advances in Space Research, 38(4), 763. http://dx.doi.org/10.1016/j.asr.2006.09.023
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Lebreton, J., & Matson, D. (2007). The Cassini-Huygens Mission (Part I). Space Research Today, 169, 11-19. http://dx.doi.org/10.1016/s1752-9298(07)80035-1
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SEBOK. (2017). How Lack of Information Sharing Jeopardized the NASA/ESA Cassini/Huygens Mission to Saturn. SEBOK Guide to the Systems Engineering Body of Knowledge. Retrieved from http://sebokwiki.org/wiki/Guide_to_the_Systems_Engineering_Body_of_Knowledge_(SEBoK)
Wendel, J. (2017). Saturn Unveiled: Ten Notable Findings from Cassini-Huygens. Eos. http://dx.doi.org/10.1029/2017eo077957
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