The data link for the Ames baseline probe as applied to the MJU spacecraft specifically with an entry at Uranus is analyzed. A frequency analysis, a trajectory analysis, and a discussion of the effects on the spacecraft design by the data link are presented. The possibilities of a two-way link are considered

Parsons, P.

English

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DATA LINK RELAY DESIGN
Paul Parsons _ N 75
Jet Propulsion Laboratory
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MR. PAUL PARSONS: We have analyzed the data link for the
Ames baseline probe as applied to the MJU spacecraft specifi-
cally with an entry at Uranus. I am going to cover four gen-
eral areas. I will have a few introductory remarks and discuss
a bit about the link, look at the effects on the spacecraft and,
then, just briefly, touch on the aspects of the two-way link.
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We have been studying effects on the link design and what
happens to the spacecraft; and, as I said, we are looking at the
effects of a two-way link. I will get into the reasons for
that in just a moment.
The first thing to look at in this link design is the
Frequency Aanlysis. (Figure 7-20). There is a relatively small
choise in frequency. You can have UHF or perhaps L-Band. S-
Band is conceivable, but it doesn't have very many advantages.
We noted that the atmospheric absorption increases wi_
frequency. The receiver and planet noise increase with fre-
quency. In most cases the planet noise decreases with fre-
quency, or at least levels off, but at Uranus it increases
slightly.
We noted that the baseline probe is designed to operate
at 400 MegaHertz and we are concerned here with a couple of
things: partially, the transmitter, but mainly the antenna
pattern. The antenna pattern from this probe is basically
that of an open-end _ve guide coming back along the longi-
tudinal axis. And the lower frequencies make it a bit easier
to get a wider beam width. We will see in a few minutes a
wide beam width pattern from the probe is very important.
VII-31
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The last major aspect we examined is the Viking orbiter
receiver, which is now being built, and is to operate at about
398 MegaHertz. One of the advantages of using this receiver is
that all of the EMI work has been done. We know where the in-
terference frequencies will fall, and they will not interfere
with the other receiver or with the science; at least the
science on the Viking orbiter.
The next major area to get into is the trajectory. There
are several parameters here that are of major importance.
(Figure 7-21).
The first is the range and shown on the figure in megameters
or thousands of kilometers. The first column is the RU, the
periapsis distance in Uranus radii. R I is the range from the
spacecraft to the probe at the entrance into the atmosphere. RF
is the range from the spacecraft to the probe at the termination
of transmission.
Notice that at a periapsis of two radii, the range varies
from about 95 megameters down to about 38. The 95 megameters
correspond to about 184 db path loss at UHF, and you can see
that there is about a 5 db change in path loss, reduction in
path loss throughout the life of the probe.
We also looked at the case of i.i radii, which is perhaps
better from a celestial mechanics view point. They get closer
to the planet and perhaps a little more sensitivity to some of
the J factors in the expansion of the gravity field, but the
range is quite short there. The disadvantage of that and the
reason I did not show it is there is such a range of cone
angles on the spacecraft that we should be very hard pressed to
follow it with the antenna.
The second factor in trajectory parameters is the track on
the spacecraft. This is the track that the probe would trace out as
VII-33
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it enters the atmosphere. Figure 7-21 has this listed in cone
and clock. For those of you who are not familiar with this sys-
tem, it is a coordinate system on the spacecraft in which two
coordinates describe the entire sphere. Zero degrees cone would
be pointed at Earth, and right at encounter the planet would be
about 90 degrees cone. Prior to that, it would be close to 180.
Clock is measured from the South celestial pole, or Canopus,
clockwise, looking at Earth. So you can see that for the two !)
entry, we are looking just a little below horizontal. If it
were over 270, it would be horizontal looking off toward the
right; it would be 7 degrees below that and at the end of this
would be a 252, which would mean we had moved up a bit.
The cone angle starts about 150, which is near the antisolar
point, and goes to just a little bit on the sun side of the
90-degree point.
It is interesting to note that the latter portion of the
entry is closer to what might be considered the equator of the
spacecraft, if you consider the cone the pole. And this has
quite an effect on the antenna pattern that we would develop.
If we were to go at 1.1 R u, we would wind up with a final
cone angle of about 50 degrees. That would be on the other side
of the 12-foot antenna which would make it a little difficult
for the relay antenna to follow it in.
The most important difference here in these flyby periapses is
the angle from the probe axis. Now I have said this probe antenna
pattern has a maximum on the longitudinal axis and falls off fairly
slowly, and at 50 degrees I believe it is down to about 0 dB.
We see on Figure 7-21 that the two R U case starts out at about
15 degrees which is very good, and winds up at about 46 as a final
VII-35
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angle from the axis, which is not too bad. The 3.5 case starts
out at about 34 and winds up at 42. In neither of these two
cases is the change anything like monotonic. It gets down to a
minimum of about 8 degrees in one case, and I believe 12 in the
other. It does not exceed 46 for the 2 RU case or 42 for the
3.5 Ru .
Because of this variation, we do want to keep the antenna
beam width as wide as possible; and also this would accommodate any
oscillations that will occur in the spacecraft due to the dynam-
ics of entry. 1,
Figure 7-22 covers the dispersion of the probe. It is easy
to get shot down on the subject of dispersions, because there
are so many different factors entering it. In this case, we
have assumed that the Uranus ephemeris has been improved tO be
more in line with the knowledge of the ephemeris of Jupiter and
Saturn. Right now the ephemeris is more unknown or known to a
lesser degree. If we have ho live with the ephemeris as it
stands now, I am afraid our dispersion would be much worse and
we would have to revise our analysis.
The entry dispersion analysis I have done so far assumes
that the only error is in entry angle. We have assumed a nominal
40 degree entry angle, and we have looked at the difference in
parameters that you get with a 30 and a 50 degree entry angle.
As you might expect, the 30 and 50 degree entry angles move most
of these parameters in opposite directions.
The range will vary by a maximum of five megameters from the
nominal case of 40 degrees entry, which would amount to approxi-
mately 0.5 db, path loss, which is negligible. It will move the
probe trace on the spacecraft by a maximum of three degrees,
which is a small amount. However, it can affect the probe axis
angle by ten degrees. The angle off the axis can get up to
around 55 degrees or so. At this angle, we have not only reached
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a region of decreased gain, we have reached a region of some
lobing in the pattern. This will, obviously, give you some
scintillation of the received signal, something we would rather
avoid.
That pretty well covers what we have done on the trajectory
analysis. I would like to go into the spacecraft design, Figure
7-23.
The required view region comes directly from the probe trace
on the spacecraft, and we see that it covers a region of roughly
30 degrees by 80 degrees. Now that 80 degrees is in cone. This
is a fairly narrow trace going along what we consider the 270
degree longitude line. The required gain is about 6 db. Most
of this is concentrated at the initial portion of the pattern,
which is around 150 degrees cone.
The receiver we see is a modification of the Viking orbiter
receiver to include AFC because of the requirements of tracking
the dynamics of the frequency as required by the low data rates.
In detection, of course, we see a detector, some sort of symbol
synchronizer, and we see probably a decoder being built into
the spacecraft. The probe would have convolutional encoding and
we would expect that we would decode that and send just the bits
down rather than the entire symbols.
I would like to just touch very briefly on the two-way con-
siderations (Figure 7-24) and show a block diagram (Figure 7-25).
The reason for thinking about two-way is that it could provide
Doppler data if we could find some way of breaking this off out
of the receiver, that could give some scientific data and per-
haps something about the atmosphere on entry.
The problems are two phase locked loops cascaded and you
are going to have some noise, additional noise that you would
not have normally. The real big problem is in acquisition, and
VII-38
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there is some problem in tracking and re-acquiring.
On the block diagram, Figure 7-25, we have the normal link
with the spacecraft, the ground transmitter out through a phase
lock loop, a multiplier, the down link receiver, and the Doppler
extractor.
On the probe we have to have a different multiplier out to
the second antenna to the probe. The probe would lock up to the
received signal, then another offset - transmitting a slightly
offset signal back to the spacecraft. The spacecraft would now
have to lock up to this signal from the probe and then there would
be a Doppler extraction and this would have to be read out and
sent down on a telemetry link.
You see we have complicated the relay link greatly. Instead of
simple transmitter on the probe, we now have a transponder that
has to lock to the signal from the spacecraft, and instead of
a simple receiver on the spacecraft, we now have to have another
phase lock receiver.
We are quite concerned about the two-way acquisition as-
pects of this. Thank you.
MR. GRANT: The next speaker is Mr. Carl Hinrichs, senior
engineer at the McDonnell-Douglas Corporation. Mr. Hinrichs
will report on a digital receiver simulation study recently con-
cluded at MDAC.
VII-42


Data link relay design

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