Galileo’s New Telecommunications Strategy
Engineers and scientists involved with the Galileo mission to Jupiter have successfully devised several creative techniques to enable the NASA spacecraft to achieve the majority of its scientific objectives despite the failure of its main communications antenna to open as commanded.
Interrelated upgrades to Galileo's on board computer software and its ground-based communications hardware have been developed and tested by NASA's Jet Propulsion Laboratory (JPL) in response to what would have been a profound loss for the orbiter portion of the mission. (The spacecraft's Jupiter atmospheric probe mission can be executed without the new techniques, but the upgrades will enhance the orbiter's ability to reliably record and re-transmit the unique data to be collected by the probe as it descends on a parachute.) The new telecommunications strategy hinges on more effective use of the spacecraft's low-gain antenna, which is limited to a very low data rate compared to the main, high-gain antenna.
The switch to the low-gain antenna and its lower data rate means that far fewer data bits will be returned from Jupiter. However, new software on the spacecraft will increase Galileo's ability to edit and compress the large quantity of data collected by the spacecraft and then transmit it to Earth in a shorthand form. On Earth, new technology is being used to greatly sharpen the hearing of the telecommunications equipment that will be receiving Galileo's whisper of a signal from Jupiter. Together, these efforts should enable Galileo to fulfill at least 70 percent of its original scientific objectives.
The circuitous route Galileo has flown to reach Jupiter (one gravity-assist swingby of Venus and two of Earth) was necessitated by changes in U.S. space launch policy made after the Space Shuttle Challenger accident in 1986. A powerful upper stage called Centaur was to have been used to boost Galileo directly to Jupiter after its scheduled May 1986 launch aboard the Shuttle. After the Challenger accident, the Centaur was deemed too hazardous to carry on a human space flight vehicle. A less volatile, but also less powerful, upper stage was substituted for the Centaur; this change delayed Galileo's launch until October 1989. The Inertial Upper Stage lacked the power to send Galileo on a direct course to Jupiter, but mission designers devised a flight path that used the gravity fields of Venus and Earth to accelerate the spacecraft enough to reach Jupiter.
The High-Gain Antenna
The 4.8-meter (16-foot) wide, umbrella-like high-gain antenna is mounted at the top of the spacecraft. When unfurled, the antenna's hosiery-like wire mesh stretches over 18 umbrella ribs to form a large parabolic dish. Galileo was to have used this dish to radio its scientific data from Jupiter. This high-performance, X-band antenna was designed to transmit data back to Earth at rates of up to 134,400 bits of digital information per second (the equivalent of about one imaging frame each minute).
Galileo's original mission plan called for the high-gain antenna to open shortly after launch. For the Venus-Earth-Earth Gravity-Assist (VEEGA) trajec-tory mission, however, the heat-sensitive high-gain antenna had to be left closed and stowed behind a large sun shade to protect it during the spacecraft's passage through the inner solar system. During this portion of Galileo's journey, two small, heat-tolerant low-gain antennas provided the spacecraft's link to Earth. One of these S-band antennas, mounted on a boom, was added to the spacecraft expressly to bolster Galileo's telecommunications during the flight to Venus. The other primary low-gain antenna mounted to the top of the high-gain was destined to become the only means through which Galileo will be able to accomplish its mission.
On April 11, 1991, after Galileo had traveled far enough from the heat of the Sun, the spacecraft executed stored computer commands designed to unfurl the large high-gain antenna. But telemetry received minutes later at JPL showed that something went wrong. The motors had stalled and the antenna had only partially opened.
In a crash effort over the next several weeks, a team of more than 100 technical experts from JPL and industry analyzed Galileo's telemetry and conducted ground testing with an identical spare antenna. They deduced that the problem was most likely due to the sticking of a few antenna ribs, caused by friction between their standoff pins and sockets.
The excessive friction between the pins and sockets has been attributed to etching of the surfaces that occurred after the loss of a dry lubricant that had been bonded to the standoff pins during the antenna's manufacture in Florida. The antenna was originally shipped to JPL in Pasadena, CA, by truck in its own special shipping container. In December 1985, the antenna, again in its own shipping container, was sent by truck to NASA's Kennedy Space Center (KSC) in Florida to await launch. After Challenger, Galileo and its antenna had to be shipped back to JPL in late 1986. Finally, they were reshipped to KSC for integration and launch in 1989. The loss of lubricant is believed to have occurred due to vibration the antenna experienced during those cross-country truck trips.
Extensive analysis has shown that, in any case, the problem existed at launch and went undetected; it is not related to sending the spacecraft on the VEEGA trajectory or the resulting delay in antenna deployment.
Attempts to Free the Antenna
While diagnosis of the problem continued, the Galileo team sent a variety of commands intended to free the antenna. Most involved turning the space-craft toward and away from the Sun, in the hope that warming and cooling the apparatus would free the stuck hardware through thermal expansion and contraction. None of these attempts succeeded in releasing the ribs.
Further engineering analysis and testing suggested that "hammering" the antenna deployment motors -- turning them on and off repeatedly -- might deliver the force needed to free the stuck pins and open the antenna. After more than 13,000 hammerings between December 1992 and January 1993, engineering telemetry from the spacecraft showed that additional deployment force had been generated, but it had not freed the ribs. Other approaches were tried, such as spinning the spacecraft up to its fastest rotation rate of 10 rpm and hammering the motors again, but these efforts also failed to free the antenna.
Project engineers believe the state of the antenna has been as well-defined as long-distance telemetry and laboratory tests will allow. After the years-long campaign to try to free the stuck hardware, the project has determined there is no longer any significant prospect of the antenna being deployed.
Nevertheless, one last attempt will be made in March 1996, after the orbiter's main engine is fired to raise Galileo's orbit around Jupiter. This "perijove raise maneuver" will deliver the largest acceleration the spacecraft will have experienced since launch, and it follows three other mildly jarring events: the release of the atmospheric probe, the orbiter deflection maneuver that follows probe release, and the Jupiter orbit insertion engine firing. It is possible, but extremely unlikely, that these shocks could jar the stuck ribs enough to free the antenna. This will be the last attempt to open the antenna before radioing the new software to the spacecraft to inaugurate the advanced data com-pression techniques designed specifically for use with the low-gain antenna.
The Low-Gain Antenna
The difference between Galileo sending its data to Earth using the high-gain antenna and the low-gain is like the difference between the concentrated light from a spotlight versus the light emitted diffusely from a bare bulb. If unfurled, the high-gain would transmit data back to Deep Space Network (DSN) collecting antennas in a narrowly focused beam. The low-gain antenna transmits in a comparatively unfocused broadcast, and only a tiny fraction of the signal actually reaches DSN receivers. Because the received signal is 10,000 times fainter, data must be sent at a lower rate to ensure that the contents are clearly understood.
Forced to make do with the low-gain antenna, the Galileo project has developed solutions on the spacecraft and on the ground to assure a rich harvest of data from the orbiter.
After arriving in orbit on December 7, 1995, Galileo will perform its tour of Jupiter as originally planned, including 10 close encounters with the giant gas planet's major moons over the 23-month orbital mission. The spacecraft will also return nearly uninterrupted streams of data about the complex charged particles and powerful magnetic field surrounding Jupiter. Importantly, Galileo will still return more than 1,500 images of Jupiter and its moons. The technology innovations developed for the low-gain antenna operations will also expedite the return of 100 percent of the atmospheric probe data and provide a backup technique to guarantee the capture of the most important subset of data from the probe.
New Software on the Spacecraft
Key to the success of the mission are two sets of new flight software. The first set, called Phase 1, began operating in March 1995 and was designed expressly to partially back-up and ensure receipt of the most important data collected from the atmo-spheric probe. Once the critical scientific data from the probe is safely returned to Earth, a second set of new software will be radioed and loaded onto the spacecraft in March 1996.
This Phase 2 software will provide programs to shrink the voluminous science data the Galileo orbiter will collect during its two-year mission, while retaining the scientifically important infor-mation, and return that data at the lower data rate. The first use of the Phase 2 software will be to return data collected during the orbiter's final approach and arrival at Jupiter, including the close encounters with the moons Europa and Io, observations of the atmospheric probe's Jupiter entry site prior to entry, and detailed profiles of the planet's inner magnetosphere.
Without any new enhancements, the low-gain antenna's data transmission rate at Jupiter would be limited to only 8-16 bits per second (bps), compared to the high-gain's 134,400 bps. However, the innovative Phase 2 software changes, when coupled with hardware and software adaptations at Earth-based receiving stations, will increase the data rate from Jupiter by as much as 10 times, to 160 bps. The data compression methods will allow retention of the most interesting and scientifically valuable information, while minimizing or eliminating less valuable data (such as the dark background of space) before transmission. Two different methods of data compression will be used. In both methods, the data are compressed onboard the spacecraft before being transmitted to Earth.
The first method, called "lossless" compression, allows the data to be reformatted back to their original state once on the ground. This technique is routinely used in personal computer modems to increase their effective transmission rates. The second compression method is called "lossy," a term used to describe the dissipation of electrical energy, but which in this case refers to the loss of some original data through mathematical approximations used to abbreviate the total amount of data to be sent to the ground. Lossy compression will be used to shrink imaging and plasma wave data down to as little as 1/80th of its original volume.
Customizing Receivers on Earth
S-Band telecommunication was once the standard for space missions, and several S-band perfor-mance-enhancing capabilities were implemented at DSN tracking stations in the 1980s. For Galileo and its S-band low-gain antenna, these capabilities are being restored at the Canberra, Australia, 70-meter antenna. Because Australia is in the southern hemisphere and Jupiter is in the southern sky during Galileo's tour, the Canberra complex will receive most of Galileo's data.
Another critical, ongoing DSN upgrade will be the addition of so-called Block V receivers at the tracking stations. These receivers, which are being installed for multi-mission use, will allow all of Galileo's signal power to be dedicated to the data stream by suppressing the traditional carrier signal, thus allowing use of higher data rates.
Finally, starting early in the orbital tour, the 70-meter and two 34-meter DSN antennas at Canberra will be arrayed to receive Galileo's signal concur-rently, with the received signals electronically combined. The arraying technique allows more of the spacecraft's weak signal to be captured, thereby enabling a higher data rate, which translates into the receipt of more data. In addition, other arraying is planned: the 64-meter Parkes Radio Telescope in Australia will be arrayed with the Canberra antennas, as will the 70-meter DSN antenna in Goldstone, California, when its view of Galileo overlaps with Canberra's.
Science Saved and Science Lost
Very few of Galileo's original measurement objectives have had to be completely abandoned as a result of the high-gain antenna problem. For the most part, science investigations on the spacecraft have adapted to the lower data rates using a variety of techniques, depending on the nature of the experiment. The new software and DSN receiver hardware will increase the information content of the data that will be returned by at least 100 times more than what would have been possible otherwise.
The onboard data processing made possible by the Phase 2 software will allow the spacecraft to store and transmit nearly continuous observations of the Jovian magnetosphere and extensive spectral measurements of the planet and its satellites in the infrared, visible, and ultraviolet, including more than 1,500 high-resolution images.
While tens of thousands of images would be required for large-scale movies of Jupiter's atmospheric dynamics, the hundreds of images allocated to atmospheric imaging will allow in-depth study of several individual features in the clouds of Jupiter. Cooperative observations with Hubble Space Telescope investigators and ground-based observers has long been planned as part of the Galileo mission to provide information on the global state of Jupiter's atmosphere.
Like a tourist allotted one roll of film per city, the Galileo team will select its observations carefully at each encounter to ensure the maximum amount of new and interesting scientific information is returned. The imaging campaign will focus on the planet and the four large Galilean moons, but it will also cover the four inner minor satellites and Jupiter's rings. Eleven close satellite encounters will be conducted: one Io flyby (on approach), three of Europa, three of Callisto, and four of Ganymede. Five additional mid-range encounters (from closer than 80,000 kilometers, or about 50,000 miles) will be conducted with these moons.
For the orbiter portion of the mission, it is useful to realize that Galileo, with its sophisticated instru-ments, closer satellite flybys, and long duration in Jovian orbit, was specifically designed to answer many of the questions that the Pioneer and Voyager spacecraft were unable to answer. None of those characteristics have been affected by the loss of the high-gain antenna: only the total volume of data has been reduced.
As a result, when Galileo examines a class of phenomena, fewer samples of that class can be studied, and often, the spectral or temporal resolution will be reduced to lessen the total volume of data. The resulting information, however, will nevertheless provide unique insight into the Jupiter system.
Some specific impacts from the loss of the high-gain antenna include: elimination of color global imaging of Jupiter once per orbit; elimination of global studies of Jupiter's atmospheric dynamics such as storms, clouds, and latitudinal bands (efforts to image atmospheric features, including the Great Red Spot, are still planned, however); a reduction in the spectral and spatial coverage of the moons, which provided context for study of high-resolution observations of their key features; and reduction of much of the so-called fields and particles microphysics (requiring high temporal- and spectral-frequency sampling of the environment by all instruments) during the cruise portion of each orbit. Most of the fields and particles microphysics, however, will be retained during the satellite encounters.
Galileo has already returned a wealth of surprising new information from the "targets of opportunity" it has observed on the way to Jupiter. Two first-ever asteroid encounters yielded close-up images of the asteroids Gaspra and Ida, and the extraordinary discovery of a moon (later named Dactyl) orbiting Ida.
Galileo was in a unique position to observe the remarkable impact of Comet Shoemaker-Levy 9 as its shattered remains slammed into Jupiter in July 1994. Galileo's observations of SL-9 provided key information regarding the initial impact, including the duration, size, and temperature of the fireball, and also of the splash-back phase, when the material flung into space by the impact fell back into the planet's atmosphere.
Important scientific observations were also made by Galileo of Venus, Earth, and the Moon during those respective flybys. These events served as very effective demonstrations of the orbiter's ability to make excellent scientific observations with all its instruments. Furthermore, the Ida/Dactyl and SL-9 observations were returned to Earth at data rates substantially lower than those that will be available with the new Phase 2 capabilities.
Highlights of Jupiter Science to be Returned Via Galileo’s Low-Gain Antenna:
100 percent of probe data
Nearly continuous, real-time survey of Jovian magnetosphere for two years
Approximately 1,500 images of the four Galilean satellites, four inner minor satellites, Jupiter and its rings
Eleven very close encounters: Io (1), Europa (3), Callisto (3),
and Ganymede (4)
Five Voyager-class (less than 80,000 km) encounters with Galilean satellites
In the face of adversity, the Galileo project team has succeeded in developing extraordinary means to ensure that the mission's most important science can be obtained. Beginning in May 1996, Galileo's data "pipeline" should be providing a constant flow of new information, including an average of two to three images per day, through the end of the mission.
Galileo's scientific instruments represent the most capable payload of experiments ever sent to another planet. The data they will return promises to revolutionize our understanding of the Jovian system and reveal important clues that it holds regarding the formation and evolution of our solar system.
For more information on the Galileo mission, please contact:
Galileo Public Information
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109