PART 1: Galileo fact of the day
PART 2: ProbeSquash activity: Installment #8
PART 3: Reality arrives and play-acting the Jovian magnetosphere
PART 4: Pictures and figures describing the mission
PART 5: How do you do?
PART 6: Details of the Doppler Wind Experiment

To recap, we suggest that after each Probe Squash installment, you
and your students make a prediction about how long the Probe's
mission will last.

Installment #8: Radio link through the dense atmosphere

E.T., Phone the Orbiter: Communicating the Data Back Home

There's a well-known philosophical question that asks "if a tree falls in 
the forest, with no one around to hear it come crashing down, does it 
make a sound?" Probe engineers and scientists might update this to say 
"if the Probe collects all the data we've hoped for, but doesn't send that 
data anywhere, does it really matter if the Probe survives entry?" The 
Probe itself does not send data directly to Earth--all of the Probe's 
precious data must be transmitted by radio to the Orbiter waiting 
overhead.  If that radio link is never established, all of the Probe's 
science is lost forever. And if the radio link is disrupted, there might be 
"gaps" in the data that *is* returned to Earth.

Since the radio frequency link is so crucial to the success of the Probe 
Mission, it has a backup, or redundancy, built in: not one, but two parallel 
and simultaneous data "streams" are sent from the Probe to the Orbiter. 
On the other end, the Orbiter has two digital relay radio receivers that 
"acquire" (basically, tune into), track, and process the Probe data along 
with radio science and engineering data. The Probe's computer puts the 
data in the format needed for it to be transmitted to the Orbiter flying 
high above, which will then send the data back to Earth.

There are other problems that can also affect the Probe's relay link. For 
example:

*Keeping the Probe signal "in view": The relay antenna on board the 
Orbiter has to be repointed four times during the Probe relay so that the 
Probe's signal "hits" the antenna relatively near the center. This helps to 
keep the Probe's signal as loud and clear as possible, which means that 
we can keep collecting Probe data for as long as possible. Once the Probe 
has descended below the water clouds, there will be a pointing update 
every 10 minutes.

*Fading signal strength: As the Probe falls deeper into Jupiter's 
atmosphere and penetrates the clouds, the radio signal loses strength as 
the distance that the signal must travel increases, and as the radio signal 
is absorbed by the atmosphere. There is also a background noise coming 
from the high energy charged particles in Jupiter's radiation belts.

*The Probe's wild ride: Jupiter's winds and turbulence could cause the 
Probe to move about violently, making it harder to stay in lock (because 
of distortions to the signal).

Probe engineers have run through a worst possible case situation, which 
includes **dozens** of different factors that might cause problems with 
the radio link. Even then, it's estimated that there would still be a radio 
link until after the Probe descended below a pressure level of 10 bars 
(which would take no more than 42 minutes).


Technical Details:

After taking all of the above into consideration, each relay radio receiver 
has been tested and designed to meet the following requirements: It will 
acquire the Probe signal within 50 seconds, with an acquisition 
probability of 99.5 percent, and a probability of reading a false signal of 
only .001 percent. (That is, the odds are 99.5 in 100 that the Orbiter will 
tune into the Probe's signal within 50 seconds of when the Probe's 
transmission starts, and there is only a 1 in 100,000 chance that the 
antenna would mistakenly pick up the wrong signal.) The minimum 
signal strength at acquisition is 31 dB-Hz. The system is required to track 
a signal strength as low as 26 dB-Hz. If either receiver loses the Probe 
signal because it's not quite tuned in (rather like an FM radio station 
drifting in and out on your car stereo while you're driving in the 
mountains) , the software is built into the system which allows the signal 
to be reacquired within 60 seconds. The unit also measures Probe signal 
strength and Doppler rates for radio science use (wind and atmosphere 
absorption measurements).

At the time that the receiver was built, it was truly a unique design: 
among other features, the receiver can do "comm on the move," meaning 
it can respond to rapid environmental changes in the radio link. 
Considering that the Probe's frequency is going to change dramatically as 
a result of tremendous changes in the way that the Probe and Orbiter are 
oriented, and also the velocity between them, it's absolutely necessary 
for the Orbiter's receiver to be able to respond to these changes and keep 
the signal locked up!

When we started this activity, you were told that Probe relay would last 
no longer than 75 minutes. In the next installment, we'll learn why that's 
so. 

Part 1
 It must have been November 9 when I was coming to work and 
somebody had out a sign on the door saying 28 days until Jupiter 
Arrival! I really felt a moment of epiphany when I saw that sign. I've 
been working for the project for 9 years (with a leave of absence in 
there somewhere to go back to school), and we had been working and 
waiting for this exciting moment all that time. I remembered the 
endless boring meetings, especially at the beginning when I didn't 
understand the technical aspect of what was going on and it was all 
rather distant. I remembered when I had been in school studying 
science and asking questions about the evolution of the moons of 
Jupiter, "why did this happen, why did that happen," and being told 
"we don't really know the answers but the Galilieo mission will be 
the time when we will really find out." And here we are, less than 30 
days from entering our prime mission! It was a bit like moving into a 
house that you've planned on buying for years and years. I finally felt 
that, omigod, this is really going to happen!

Part 2
 Jo and I agreed to help Annette, one of the PR coordinators, provide 
a demonstration of the Jovian magnetosphere to kids on November 9. 
We were very busy and stressed out keeping up with the tape 
recorder anomaly, assisting with the redesign of the mission if the 
tape had failed, worrying about the details of what sort of science 
we were going to get out of it, and keeping up with the progress of 
the design of the sequences that were underway, and I remember 
being very tired and irritable and thinking that the only reason I was 
going to do this was to help Jo. Jo and I sat in the conference room 
the day before and made ourselves laugh trying to practice the 
routine we were going to go through the next day with the kids, 
talking kiddie talk to an empty room. Jo has a kid and I don't, so it 
was easy for her. Then she went and wimped out on me the next day 
and I had to do it myself! [EDITOR'S NOTE: editor Jo feels compelled 
to step in here and point out that she was busy playing the role of 
"Jupiter" at a planetary press conference for 200 4th-7th graders, 
and that since the press conference ended up starting an hour behind 
schedule, she was unable to assist the author :-)]  But it turned out 
to be fun to play with the kids, and I needed a good laugh by that 
time.

Part 3
I gave a talk to the Advisory Council for Women on November 20, 
with Carol, Leslie, and Rosaly. We were supposed to talk about what 
we expected to learn from the mission, and I was trying to make it a 
little fun and perk up the audience and demonstrated how my 
instrument worked by spinning around with my mouth open. I think 
the audience did enjoy it, but now certain people mimic me by 
spinning around with their mouths open when they pass me in the 
hallway.

November 30, 1995
This week is a mad rush of getting a zillion and one little details 
ready for arrival day (what a time for my office phone to be on the 
fritz!). So, even though I usually leave work early enough for the 
babysitter to leave by 4:30, my husband is getting home early these 
days so that I can work a few hours later.  I've got a computer and a 
modem at home, but my 2.5 year old daughter assumes that if I'm 
turning on the computer it's so that *she* can play games on it.

Today, I'm checking on some of the pictures and figures that we're 
going to be putting out on the World Wide Web and on NASA TV. For 
example, there will be a "bird's eye" view that looks down on 
Jupiter's north pole, showing the current position of the orbiter and 
the probe as they move in towards Jupiter. There's quite a bit of 
discussion about the color scheme (certain color combinations don't 
work well on video), making me feel like I'm some sort of 
interplanetary interior decorator. Richard's also got a new spiffy 
spacecraft model so that instead of watching a blob moving along, 
we actually see a miniature Galileo orbiter and probe. Admittedly, 
the spacecraft isn't to scale--it's almost as large as Jupiter!--but 
this compromise between artistic license and total accuracy is 
worth it. 

There are also going to be pretty much true-to-life pictures showing 
what Jupiter would look like if you were traveling just a little 
behind the orbiter.  These pictures are absolutely spectacular 
looking--they really make you feel like you're out in space! These 
images are being generated by the people in the DIAL (the Digital 
Imaging Animation Laboratory). DIAL is your typical state-of-the-
art recording studio, full of high-powered computer workstations--
each with a bar of soap perched on top (the workstations are named 
after brands of soap-- including Dial). In fact, there are *so* many 
workstations that there are brands of soap that I've never seen 
before. We spent a good hour talking about how to best present the 
pictures--for example, do we want Jupiter's night side to be 
completely black (completely realistic, but not interesting to look 
at), or do we want to just have it slightly darkened? After some 
jokes about how the lines showing the orbits of Jupiter's moon 
currently make it look like Jupiter is infested with space worms, I 
get to go home--but the DIAL crew will be pulling an all-nighter.

While these pictures can be created before arrival day, the Jupiter 
Orbit Insertion burn data will be live, or in "real time." Even though 
these graphs (showing how the spacecraft's speed changes as the 
burn takes place) don't look as interesting as the pictures, there is 
something about seeing real data coming in that is tremendously 
exciting. When the probe was released back in July, people watched a 
similar type of display, waiting to see a sudden drop in the graph 
that would show us that the probe and orbiter had pushed away from 
each other. You wouldn't think that people would get worked up 
watching a graph--but seeing real evidence that the spacecraft is 
*actually performing* some very critical activity is a huge relief. 
So we've been "practicing" the burn with simulated data fed to us 
from the Navigation team.  

Jo Pitesky

Nov. 30 1995
Well, here we are a week from JOI. How long a trip it's been!
I started working for Galileo in late 1983, and felt I was a
latecomer at that point. After the 1989 launch, and the extended
solar system cruise was developed to take us to Jupiter, it
seemed we would never get there! Now, the suspense is very
strong: will we get in orbit properly? Will the Probe do its
thing and send us the first up-close and personal view of a
giant planet? If those two activities go well, the sound you
hear will be cheers and sighs of relief from Pasadena.

November 22,  1995

What is the Doppler Wind Experiment?

I have been asked this question many times, and, as we get closer to 
Jupiter, the question is popping up more frequently. So I thought this 
might be a good forum for a brief introduction. My involvement with 
the Doppler Wind Experiment started in the early 1980's when I 
began working at NASA Ames Research Center. Jim Pollack of Ames 
had responsibility for the Doppler Wind Experiment when I joined 
him in 1981. At the time the experiment was nothing more than a 
few hand written equations that Jim had scribbled down. I remember 
studying and struggling to understand these early attempts at a wind 
retrieval method. Neither Jim nor I could make heads or tails of 
these equations and, to this day, I don't understand (and Jim could 
never remember) where these equations came from or where they 
were leading. In 1984 Jim gave overall responsibility for the Doppler 
Wind Experiment to me. Although his participation from this time on 
was as an advisor, until he passed away in June of 1994 Jim 
maintained a deep interest in the progress of the experiment. The 
current acceptance of the Galileo Doppler Wind Experiment as an 
important, viable probe science investigation is in large part due to 
Jim Pollack.

The Doppler Wind Experiment is really quite straightforward in 
concept, if not in application. If you release a helium balloon on a 
windy day, you can get an idea of wind speeds by watching the drift 
of the balloon as it rises in the sky. In the same way, we can infer 
Jupiter's wind speeds by monitoring the motion of the probe as it 
descends on parachute deeper and deeper into atmosphere. Unlike a 
balloon drifting upwards in our atmosphere, however, we can't watch 
the probe with our eyes (or with the orbiter's "eyes"). We can, 
however, "see" its motion by using something called the Doppler 
Effect.

The Doppler Effect was first described by the Austrian physicist 
Christian Johann Doppler in 1842 while he was writing about the 
colors of binary stars  (systems where there are two stars that 
orbit about each other). As a source of radiation, either sound or 
light, is moving towards or away from an observer, the apparent 
frequency of the radiation is shifted. If the source is approaching, 
then the shift is towards higher frequencies. If the source is 
receding the shift is towards lower frequencies. 

Most people are familiar with the Doppler Effect from hearing the 
apparent change in pitch of a siren from a passing fire engine, or 
horn from a passing train. Astronomers depend on the Doppler Effect 
to measure the speeds of stars, galaxies, and quasars, looking at 
how the color of visible light from these objects gets shifted. This 
shift is often called a "redshift," since the Doppler Effect moves 
visible light towards the red end of the spectrum if an object is 
moving away from us (if something is moving towards us, the visible 
light is "blueshifted").

When the Galileo probe encounters winds, its speed will change. This 
will be reflected by an apparent change in the probe radio signal 
frequency due to the Doppler effect. By making accurate 
measurements of the probe signal frequency the probe motion can be 
determined and, assuming the probe motion follows the changing 
winds, the wind speed at the probe location throughout descent can 
be calculated. If the winds blow the probe towards the orbiter, the 
Doppler effect will cause the received frequency to increase. If the 
wind causes the probe to move away from the orbiter, the Doppler 
effect causes the received frequency to decrease.

In concept, this is not a new idea. Similar methods have been used to 
measure the winds on Venus. However, Doppler wind measurements 
at Venus are different in significant ways. Venus is a nearby planet, 
the same size as the Earth, that spins very slowly. In addition, the 
Venus probes were tracked from the Earth, not from another 
spacecraft. Each of these factors complicates the Jupiter wind 
measurements. First, Jupiter is very large and spins very fast. The 
probe, while descending on parachute, moves with the spinning 
planet. This spinning motion can cause a Doppler shift of as much as 
10,000 Hertz, or Hz. If our knowledge of the probe location is in 
error by as little as 0.5 degrees in longitude (in the east-west 
direction), the Doppler shift due to the spinning planet can be 
several hundred Hz more or less than we expect.  Keep in mind that 
the winds are expected to cause a Doppler Shift of somewhere 
between 10-90 Hz, so if there's even a small uncertainty in the 
probe location, the wind measurements could get swamped by errors 
from not knowing the accurate Probe position, causing major 
problems. If Jupiter was not as large, or did not spin as fast, the 
effect of small errors in the probe location would not be nearly as 
large.

Another difficulty arises from the geometry of the probe, orbiter, 
and spinning planet. While the probe is sending data to the orbiter, 
the orbiter is almost directly above the probe. Since we are 
measuring horizontal winds, the winds do not cause the probe to 
approach or recede from the orbiter very much. If we were directly 
"in front of" or directly "in back of" the orbiter (so the winds would 
be blowing the probe straight towards us or away from us) then we 
could measure the Doppler shift much more easily. Instead, we only 
see a small part of the shift: for example, a wind of 100 meters per 
second (216 mph) will cause the distance between the probe and 
orbiter to decrease by only about 5 meters per second, resulting in a 
Doppler frequency (due to the winds) of only about 25 Hz.

It should be kept in mind, of course, that what we are *really* 
measuring is not the winds, but the motion of the probe. We expect, 
however, that the probe motion will trace the winds fairly closely. 
But how good an assumption is this? Al Seiff, who is in charge of 
the Galileo Atmospheric Structure Instrument, showed that the time 
it takes for the probe to "catch up" to the winds is about equal to the 
probe descent speed divided by the acceleration of gravity. Up high in 
the atmosphere, where the air is thin, the probe descends very 
rapidly since the parachute does not provide much "slowing" in thin 
air. If the probe is dropping at 100 meters per second, and the 
acceleration of gravity is about 23 meters per second per second, 
the probe response time is about 4 seconds. And, in 4 seconds at 100 
m/s, the probe will drop through 400 meters. As a result we should 
be able to see changes in the wind that occur across altitudes of 
about 1/2 kilometer high in the atmosphere. As the probe descends 
and the atmosphere gets thicker, the parachute works better and the 
probe speed of descent decreases. Towards the end of the mission 
the descent speed may be 25 meters per second, in which case the 
probe will respond to wind changes in about one second. At 25 
meters per second the probe will descend through 25 meters of 
atmosphere in one second. Late in the mission we can therefore 
expect to see wind variations that occur over altitude ranges as 
small as about 25 to 50 meters.

Finally, a question that cannot be ignored - why do we care? Why is 
it important to make the wind measurements? The simplest answer 
is that the winds are a key element to a complete understanding of 
Jupiter's weather. Just as knowledge of the composition of the 
atmosphere, the atmospheric pressures and temperatures, the 
location, composition, and densities of the clouds, the existence and 
location of lightning, and the overall atmospheric energy structure 
(including where solar energy is absorbed) is necessary for us to 
completely understand Jupiter's atmosphere, we need to know the 
dynamics, or motions, of the atmosphere.

But above and beyond this somewhat generic answer, the variation in 
the winds with altitude will yield clues to the source of energy that 
drives the circulation of Jupiter's atmosphere. On Earth the energy 
that powers the winds is ultimately derived from the Sun. The 
Earth's atmosphere, land, and oceans receive more solar energy near 
the equator than at the poles. It is this difference in heating (along 
with the somewhat more subtle effects of differences in the ability 
of oceans and land masses to retain heat, the effect of the Earth's 
rotation, topography, etc.) that is ultimately responsible for the 
Earth's winds. But Jupiter is further away from the Sun; it only gets 
1/25th as much solar energy as does the Earth. And in planets even 
further away from the Sun--Saturn, Uranus, and Neptune--  the 
winds are also seen to be very large at the cloudtops. We therefore 
suspect that the energy causing the winds might come from a 
different source. 

If the winds on Jupiter depend on energy input from the Sun, we'd 
expect that the winds would vanish a short distance beneath the 
cloud tops. If a more exotic source of energy supports the wind, such 
as latent heat released at the bottom of the water clouds, or 
internal energy from the center of Jupiter, the wind profile would be 
expected to fall off much more slowly. If energy originating in the 
center of Jupiter is supporting the winds, wind speed could still be 
as high as 70 meters per second (about 150 miles per hour) at the 
end of the probe mission.

As a final comment in this altogether too long journal, consider 
again the Earth's weather. The winds provide a good example - even 
on familiar Earth, the mystery of the winds is not completely solved. 
If we could somehow place the Earth in a laboratory, and experiment 
with different properties of the Earth that might affect the winds, 
we might decide to try making Earth spin faster, or slower, or take 
away the surface or oceans, or move the Earth closer to the sun, or 
farther from the sun. In each case, we could examine how the winds 
change and begin to understand how different factors on Earth affect 
the winds. Unfortunately, although I have not tried, my guess is that 
NASA would hesitate at funding experiments to move the Earth 
closer to the sun, spin it up, or remove its oceans. But consider the 
other planets in our solar system - Venus, a heavily clouded earth-
sized, slowly rotating planet closer to the sun. Mars, a (virtually) 
cloudless planet smaller than the Earth with a thin atmosphere, no 
oceans, and somewhat farther from the sun. Jupiter, Saturn, Uranus, 
and Neptune - planets that are much larger than Earth, much more 
distant from the sun, no solid surface, and spinning very rapidly. 
Titan, the largest moon of Saturn - very far from the sun, smaller 
than the Earth, slowly spinning, with a very cold atmosphere similar 
in composition to the Earth (primarily nitrogen) and, perhaps (but by 
no means certain) some small seas of liquid methane/ethane. By 
studying the weather on these other bodies, we can begin to decipher 
some of the mysteries of our own atmosphere. In a very real sense 
the planets are therefore our laboratory for studying the Earth.