Rescuing the Huygens Mission From Fiasco

On January 14, 2005, the Huygens probe, which forms part of the joint NASA/ESA/ASI deep-space mission Cassini-Huygens, accomplished its spectacular descent through the atmosphere of Titan, Saturn's largest moon. This mission success, however, became possible only after the rescue of the endeavor from an implementation flaw that was discovered in 2000 during an in-orbit test of the Huygens relay-link receiver. The problem threatened to cripple the entire $350 million probe mission. This paper presents the model of the faulty implementation of a data transition tracking loop. This model was the driver for the trajectory redesign leading to the revised relay-link geometry of the Huygens mission. The recovery eventually allowed for the data retrieval from the probe during its descent and after the landing on Titan, despite the tracking deficiency in the symbol synchronizer. The approach taken to identify and solve the communication problem is also an illustrative tutorial example of synchronization theory applied to save an entire space mission from disaster.