Galileo FAQ - Galileo's Antenna
![[Spacecraft showing Antenna and Probe]](images/hga_sc.gif)
What's the high-gain antenna (HGA)?
The umbrella-like high gain antenna is located at the top of the spacecraft, and is 4.8 meters (16 feet) in diameter. It was designed to transmit data back to earth at rates of up to 134,000 bits of information per second (the equivalent of about one television picture each minute). Compare this to a fast home modem, which only manages to send 14,400 bps!
The antenna, which is made of gold-plated metal mesh, was stowed behind a sun shield at launch, in order to avoid heat damage from the sun while the spacecraft flew "inside" the earth's orbit.
Why didn't the antenna open all the way up? Is JPL still trying to open it?
On April 11, 1991, the Galileo spacecraft began to deploy its high-gain antenna under computer-sequence control.
The antenna -- a 16-foot mesh paraboloid stretched over 18 umbrella-like ribs -- had been furled and hiding behind a small sunshade for the almost 18 months since launch, in which the spacecraft came closer to the sun than Earth and briefly closer even than Venus. Communications, including Venus and Earth-moon science data return, had been using the low-gain antennas.
Within minutes, Galileo's flight team, watching spacecraft telemetry 37 million miles away on Earth, could see that something was wrong: The motors had stalled, something had stuck, the antenna had opened only part way.
Within weeks, a tiger team had thoroughly analyzed the telemetry, begun ground testing and analysis, and presented its first report.
They attributed the problem to the sticking of a few antenna ribs due to friction between their standoff pins and their sockets. The first remedial action was taken -- turning the spacecraft to warm and expand the central tower, in hopes of freeing the stuck pins.
In addition to thermal cycling, the tiger team developed other ideas for loosening the stuck ribs: retracting the second low-gain antenna (on a pivoting boom), pulsing the antenna motors, and increasing the spacecraft spin rate to maximum 10 rpm (normally about 3 rpm).
After a nearly two-year campaign to try to free the stuck ribs there is no longer any significant prospect of deploying the HGA, though one last attempt will be made in March of 1996. The Project is proceeding to perform the Galileo Mission with the Low-Gain Antenna.
Also, see the Everything That You've Wanted to Know About Trying to Open the HGA, and Haven't Been Afraid to Ask.
For additional information, see "Galileo's antenna: the anomaly at 37 million miles," an article by Jim Wilson that appeared in JPL's newspaper on July 3, 1992, or Unfurling the HGA's Enigma in the August, 1991 Galileo Messenger.
What's that big black thing at the top of the spacecraft? Is it a mirror for the antenna?
That's one of the sunshades for the spacecraft, designed to protect the HGA from the sun's heat while the spacecraft was travelling inside of Earth's orbit.
The project briefly studied this option, but quickly determined that, given the short amount of time in which the relay satellite would need to be designed and launched, a relay would be prohibitively expensive.
There were also significant unresolved technical problems related to both orbital mechanics and telemetry. Engineers first proposed having a relay satellite that would follow Galileo closely, like a well-trained dog. But, the amount of propellant needed made the mission too expensive. Even keeping the relay satellite within a few million kilometers of the spacecraft throughout the entire two-year orbital tour would be a difficult (if not impossible) engineering problem, given the limited amount of propellant on board a relay satellite. Finally, the relay antenna itself would need to be comparable in size to Galileo's original high gain antenna, which would add to the expense and complexity of a relay mission.
What is the low-gain antenna, and where is it located?
The low-gain antenna is located on the tip of the cone (the "feed") sticking up over the High Gain Antenna. It broadcasts its signal over a wide cone (the half-angle of the cone is almost 120 degrees), which allows it to remain in communication with Earth even when it's not pointed directly at the Earth. This is unlike the high-gain antenna, which sends out a much more narrowly directed signal (with a half-angle of 1/6th of a degree). In both antennas, the same amount of power is being transmitted, but the low-gain spreads that power out over a much larger area of the sky than does the high gain. This is somewhat like the difference between a bare bulb and a spotlight, both operating at the same power. Since the ground antennas that are receiving Galileo's signal need to receive a certain amount of power in order to actually "hear" that signal, the low gain can't handle nearly as high a data rate as the high gain antenna--during Jupiter operations, the low gain's top data rate will be 160 bits per second, compared to the high gain's 134,400 bits per second.
There is a second low-gain antenna on the spacecraft--the long, thin antenna that hangs off of the short boom that holds the radioisotope thermoelectric generators--but it is stowed away permanently.
The geometrical situation that you describe with occurs when the Sun lies directly between Jupiter and Earth (scientists refer to this as "conjunction"). The Sun is a strong source of electromagnetic activity, and it wreaks havoc with the spacecraft's radio signal, essentially reducing the spacecraft's data rate to Earth to 0 for the two and a half weeks centered around conjunction. Mission planners and telemetry engineers define this problem area as occurring when the Sun-Earth-Galileo angle is less than 7 degrees (see figure below); a relatively "quiet" Sun can mean that data can be successfully returned at angles as small as 3-5 degrees.
Galileo's two conjunction periods will run from December 11-28, 1995,
and January 11-28, 1997. Conjunction lowers the amount of data that can
be returned to Earth. However, Galileo still has roughly two years in which
to investigate the Jovian system, so not being able to return data for 18 days
out of those two years is not a serious difficulty.
Following the first in-flight system software update, Galileo's top data rate rose to 16 bits per second. This will be boosted following the final update to the system software in May of 1996 to 160 bits per second. This isn't to say that Galileo will always be communicating at such a high bit rate: the actual achieveable data rate depends on a variety of factors, including the Earth-spacecraft distance, how close Galileo and the Sun appear to be in the sky when seen from Earth, and the geometry between the spacecraft and the Deep Space Network antennas that are tracking Galileo. Most of Galileo's data will be sent to Earth using a downlink rate of 80 bits per second (the average value, over the entire two-year orbital mission, is closer to 50 bits per second, but this includes periods when the Sun is directly between Earth and Jupiter, and when the data rate consequently plummets).
Keep in mind that a great deal of the science data will be compressed before being sent down to Earth, boosting the effective data rate significantly.
Without the HGA, how long does it take to return a single image?
At maximum possible downlink rate, with no compression or editing being used, it will take just under 9 hours to return a single full-size image. All images will be compressed or edited by at least a factor of 2 and will be returned in far less time (typically 1 to 2 hours).
An average of 2-3 images per day will be returned starting in late June of 1996.
The Telecommunications Strategy Fact Sheet says
that
there will be performance-enhancing capabilities added at the Canberra Deep
Space Network 70-m antenna.
Why aren't ultracones (which significantly lower
the system noise, and hence boost the performance by roughly 20-25%) being
installed at the Goldstone and Madrid sites as well?
Ultracones are "listen only", that is, they receive the radio signal from
Galileo but can't transmit a signal to it. Several times a week during
Galileo's mission operations it is necessary to send commands to the spacecraft
or collect what's called "two-way doppler" data, which allows the navigators to
determine where the spacecraft has been and where it is going. Both of those
activities are incompatible with using an ultracone, and so they are usually
planned to be conducted at Goldstone or Madrid. Since Goldstone and Madrid havemuch shorter view periods of Galileo than Canberra, and since the ultracone
would have to be bypassed frequently to allow for commanding and doppler
collection, it was felt that the cost of installing ultracones at those sites
outweighed the benefits.
There appears to be little hope that the ribs might come free, but, if they
did, there would be a noticeable change in the spacecraft's wobble angle. The
spacecraft's attitude control system detects a component of the wobble angle and
sends it back to Earth. Analysts monitoring the spacecraft telemetry would see
the change associated with rib release. Several steps would have to be
completed before the HGA could be used, however. First, the antenna motors
would have to be commanded to unfurl the antenna. Second, the spacecraft would
have to be turned to point the narrow beam of the HGA at the Earth. This would
allow testing of the HGA's performance. Third, this measured performance would
have to be analyzed and subsequently incorporated into ground software to use in
future sequence planning. Lastly, the already-built sequences would have to be
updated to take advantage of the capability. Such a process would certainly
take weeks to months to complete. Clearly a high priority use of any HGA
capability would be to increase the science downlink rate.
Not all, but several are. Mars Pathfinder, Mars Global Surveyor, and Pluto
Express have all expressed interest in the telemetry subsystem being developed
for Galileo. The ultracone is usable by any spacecraft which transmits in S-
band. It is also fair to say that Galileo's emphasis on data compression and
encoding has spurred new research and development which is leading to the
incorporation of sophisticated data compression schemes on projects under
development.
Galileo's computers are not fast enough to compress data at the rate which
was to be transmitted through the HGA! It is important to remember that at the
time Galileo was being developed, spacecraft designers were using fairly
state-of-the-art computers, but those computers are extremely slow by
contemporary standards. With the advent of increasingly faster and more
densely packed microprocessors, such computationally intensive processes are
becoming practical for spacecraft to perform and are being incorporated into
future spacecraft designs.
Galileo puts out about 20 watts of power, slightly less than the power of a
refrigerator lightbulb. By the time it reaches the DSN antennas on Earth, a 70
Meter antenna is able to scoop up only about one part in 10 to the 20th watt, in
other words .00000000000000000001 watt. But it's enough to do great science.
Return to Project Galileo Homepage
Return to Galileo Frequently Asked Questions
Submit your own question