THE PARKES GALILEO TRACKS
If I have seen further than others, it is because I have stood on the shoulders of giants - Isaac Newton referring to Galileo
The Parkes radio telescope tracking the Galileo spacecraft around the Planet Jupiter. Venus can be seen setting to the left of the dish - 6 November 1997.
Soon after the Galileo spacecraft was launched in October 1989, the spacecraft's main communications antenna failed to unfurl properly. Unlike on previous spacecraft, the Galileo high-gain antenna was not a rigid structure. Instead, it was designed to unfurl, like an umbrella, once the spacecraft was safely en route to Jupiter. Unfortunately, the straps holding the folded antenna in place failed to respond to commands, and despite several ingenious attempts to open the antenna, it remained stuck in a partly unfurled state. This rendered the antenna, designed to transmit data at rates of up to 134,000 bits per second (bps), useless.
Fortunately, a second, small antenna on the spacecraft was brought into operation to transmit the precious scientific data. This is a small rod-type, low-gain antenna that transmits at very low power in all directions, so that any signal reaching the Earth is extremely weak - weaker even than the static in the radio receivers on Earth. The difference between Galileo's sending its data to Earth using the high-gain compared to the low-gain antenna is like the difference between the concentrated light from a spotlight versus the light emitted diffusely from a bare bulb.
The passband from the Parkes Galileo receiver. The weak signal is lost in the static on the plateau region of the band.
This effort salvaged not only the mission from near disaster, but also salvaged 70% of the science.
The secret of the Galileo mission's success is due to the linking, or arraying, of various tracking antennas around the world. This network originally cost NASA US$30.5 million to establish, and will be used, in one way or another, for all future planetary missions.
Essentially, the arraying involved linking the 64-metre antenna at Parkes with the 70-metre antennas of NASA's Deep Space Tracking Network at Goldstone in California, Tidbinbilla near Canberra, and Madrid in Spain.
The Full Spectrum Resolvers (FSR) were used to extract the signal from the static, and then pass the data to Tidbinbilla for combining.
With the handing over of one station to another, the data rate had to be constantly adjusted to accommodate the various antennas tracking the spacecraft.
At times, the spacecraft was as much as 48 light minutes from the Earth, that is, it took the radio signal 48 minutes to travel from the vicinity of Jupiter to the Earth. The spacecraft was pre-programmed to alter the rate at which it transmitted its data to Earth by the correct amount, and at just the right moment, for it to be received on Earth when the various stations of the tracking network came into view.
It was imperative that each of the antennas was locked on and tracking the spacecraft at its scheduled time. Any loss of tracking time, by any of the dishes in the array, meant that the data were lost. Since the gathered data were always first recorded on the spacecraft's onboard video tape, any lost data that the JPL scientists considered important were re-scheduled for later playback. You may have seen some Galileo images with black stripes cutting across them. This indicates that during the transmission, one or several of the antennas was off source, but the data loss was not deemed serious enough to warrant re-transmitting the missing frames.
It was soon after the antenna problem was diagnosed that the Australia Telescope National Facility (ATNF) was contacted by NASA/JPL to ask that the Parkes observatory be allowed to assist in recovering the data from Galileo.
The new aerial cabin being installed. The old cabin is in the foreground resembling a lunar module.
As well as the new aerial cabin, the ATNF built a new low-noise S-band receiver specifically for picking up Galileo's weak signals. Radio astronomers can also use it for many other diverse astronomical observations.
The construction of the new aerial cabin was completed in early 1996, and the first test track was made in March. The contracted tracking ran from 28 October 1996 to 6 November 1997 for about nine hours each day.
THE TRACKING DUTIES
The daily tracks were done by three principal Galileo operators: Dr Ian Stewart, Mr Russell Pace and Mr John Sarkissian with occasional help from other observatory staff.
As the seasons changed, the starting times of the daily nine-hour tracks slowly migrated through the day. Over the year, the tracks shifted from being wholly in daylight to wholly at night. The night tracks often saw us alone at the telescope quietly going about our tasks. During the early morning drive from the telescope to the town, operators often had to dodge kangaroos and rabbits along the way, while Jupiter rose, beckoningly.
Some of the Galileo operators, at the end of the last Parkes Galileo track - 6 November 1997.
The most serious problem happened in early October 1997, when a critical component of the observatory's hydrogen maser clock failed. This clock is extremely accurate and was used to time the arrival of the Galileo signals so they could be correctly combined with the signals from the other antennas. Fortunately for us, the Mars Pathfinder probe was experiencing communications problems at the time so that all Galileo tracking duties were suspended briefly, not just ours. (In fact, proper contact was never re-established with the Mars Pathfinder probe.) This break in tracking gave us enough time to repair the clock, and no Galileo tracking time was lost as a result.
Another problem came from an unexpected source. The telescope tracking control system uses a directed laser beam to lock the telescope on to the desired position in the sky. On balmy summer evenings, moths from the surrounding fields often infiltrated the room housing the laser, and fly across the beam, causing the telescope to temporarily lose lock on the tracked position. This was only a minor irritant and many solutions were proposed; the most effective weapon turned out to be a rolled-up newspaper. It's odd how such a little thing as a moth can disrupt very sophisticated technology, and on a global scale.
On all previous planetary missions, the data rate from the spacecraft had been high enough to have the data processed in real time and displayed within minutes of reception. With Galileo, however, the low bit-rate meant that it often took weeks for the information to be fully received and processed. And so we saw the magnificent images of the Jovian system when everyone else did - when they were put on the Internet; there was no privileged position for us in that respect.
Contrary to the experience on other missions, the encounter periods, when the spacecraft made close passes of the Jovian moons, were never occasions of frantic and critical activity at the telescope. Because of the low bit-rate, the information gathered during these fly-bys was recorded on the onboard video tapes and played back at leisure in the weeks following the encounter. The unique and important bits of information were downlinked first, followed by the less critical information. Sometimes, if not all the data were downlinked before the next encounter, they were written over. Such was the way of Galileo.
The Parkes Galileo team developed an excellent working relationship with the various tracking teams at Tidbinbilla and JPL. We were often able to solve problems before they arose and posed a threat to the data acquisition.
Celebrating the end of the Galileo tracks. Parkes Observatory staff - 14 November 1997.
The spacecraft will perform for a further two years in the extended "Galileo Europa Mission" (GEM). It will gather new and vital information about the Jovian system, in particular the moon Europa. Eight close fly-bys of the moon are scheduled, followed by four fly-bys of the second-largest Jovian moon, Callisto. In a grand final, Galileo will pass through the Jovian radiation belts and encounter Io once or twice more.
Though Parkes will not be involved in this extended mission, we all wish the NASA/JPL Galileo team continued success and good fortune. We look forward to seeing the sweeping images of the awesome Jovian vistas.
It has been a privilege for the observatory to share in the acquisition of so much new knowledge of the Jovian system. With its participation in this mission, the observatory has continued its proud record of contributing to the success of some of the most significant and historic space missions.
John M. Sarkissian.
Co-ordinator of Galileo Tracking Operations
Parkes Radio Observatory,
Australia Telescope National Facility.