A radio-telescope network strategically positioned around the globe and organized only six months before it was needed, coupled with a high-speed disk recording and playback systems at ground stations, saved a crucial space experiment that was more than eight years in the making. On October 15, 1997, the NASA Cassini orbiter and the European Space Agency’s (ESA) Huygens atmospheric probe were launched toward Saturn and its largest moon, Titan, on an eight-year journey covering more that 2 billion miles.

On January 14, 2005, after separating from the Cassini spacecraft three weeks earlier, the Huygens probe began its descent to Titan’s surface as the first probe to land on the surface of a moon of another planet. The Doppler Wind Experiment, conceived by NASA’s Jet Propulsion Laboratory (JPL), was designed to create an accurate profile of Titan’s winds along the probe’s descent trajectory as it dropped for 2 1/2 hours through the thick atmosphere of Titan.

Aerodynamic fins on the Huygens probe generated a slow controlled spin as the probe descended beneath a parachute. Winds in the atmosphere affected the horizontal speed of the probe’s descent; the component of the motion in the direction of the Earth produced a slight Doppler shift in Huygen’s radio frequency of the signal as received on Earth, allowing that velocity component to be measured.

Critical additional measurements were conducted using Very Long Baseline Interferometry (VLBI), which used a global array of radio telescopes that simultaneously recorded the Huygens signal, as well as radio signals from a quasar nearby in the sky. When these recordings were cross-correlated and analyzed, the position of the Huygens probe on the two-dimensional plane of the sky was determined to within a few meters at the position of Titan.

Combining the Doppler measurements and the VLBI measurements allowed scientists to reconstruct an extremely accurate three-dimensional record of the motion and position of the probe during its descent to the surface. From this information, they were able to deduce the speed and direction of the winds at varying altitudes, concluding that Huygens encountered winds as high as 250 MPH during some parts of its descent.

Originally, the experiment called for measuring the Doppler shift in the probe’s signal frequency both by the Cassini mother spacecraft and by ground-based radio telescopes in the U.S., Australia, Japan and China. Cassini was best positioned to gain information on the east-west component of the winds, and the ground-based telescopes were positioned to determine the north-south wind component.

Unfortunately, the Cassini wind data were lost due to an onboard configuration problem that prevented the re-transmission of the Huygens signal to the Earth with its high-gain antenna. Had the VLBI data not been available, only the single component of the probe’s motion from the Doppler-shift data would have been available, considerably compromising the value of the experiment.

It was the good fortune of forward-thinking scientists to conceive the VLBI experiment as a backup plan just six months before the Titan encounter. Under the leadership of Dr. Leonid Gurvits of the Joint Institute for VLBI in Europe (JIVE), a global network of radio telescopes was created, nearly all of which recorded the Titan and quasar signals onto Mark 5 high-speed, digital data recorders developed by MIT Haystack Observatory in collaboration with Longmont, Colorado-based Conduant Corporation.

Conduant’s StreamStor direct-to-disk technology replaced MIT’s earlier-generation magnetic tape systems that were unreliable, expensive to maintain and cumbersome to manage. MIT realized that the cost of disk storage was dropping rapidly and would soon fall below that of the special-purpose tapes being used at the time, concluding that the next generation of the VLBI data systems should be based on commercial off-the-shelf components (COTS) using standard IDE hard disk drives. Thus was born the Mark 5, which is now the global workhorse VLBI data acquisition system with more than 100 units deployed to more than 15 countries. StreamStor technology forms the foundation of the data capture process in these Mark 5 systems, the same ones that captured the faint signals from the Huygens experiment.

Mark 5 (Figure 1) is the first high data rate VLBI data system based on magnetic disk technology capable of sustained recording and playback at rates in excess of 1 Gbit/s. This PC-based disk recording system flawlessly acquired the weak signals from the radio telescopes and allowed scientists to measure the probe’s movements through Titan’s dense atmosphere. And once again, embedded controllers, often the unseen workhorses in scientific research and industrial applications, had their “night in the moonlight” as they recorded the barely perceptible radio signals emanating from Huygens high above the Titan moon.