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The Voyager program involved sending two space probes, Voyager 1 an...
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Vovager
Firsts
Mission Telecommunication
Edward
C.
Posner
Lawrence
L.
Rauch
Boyd
D.
Madsen
1
HIS
ARTICLE TELLS ABOUT
THE
COMMUNICATIONS
firsts of the National Aeronautics and Space Administration’s
(NASA’s) Voyager mission. This dual-spacecraft mission to
Jupiter. Saturn. Uranus, Neptune, and their moons and rings
was launched in 1977 and completed its planetary phase some
I7
years later with the Voyager
2
Neptune encounter in August
1989 (see Figure
I).
Although the spacecraft hardware repre-
sents early 1970s technology (see Figure
2),
the absolutely out-
standing system design included computer control of almost
all spacecraft ar?d instrument functions. This provided the
flexibility for recovering from a wide range of malfunctions.
The potential was also created to effectively almost redesign
the spacecraft and its instrument data systems in flight via
uplinked software (a first) to take advantage of post-launch-
developed technology (such as image compression) and also of
new requirements developed later in the mission. This was es-
pecially valuable because the original mission commitment
did not include Uranus
or
Neptune.
Half of the Voyager telecommunication system
is
located
on Earth (downlink receiver and uplink transmitter). For the
better part of a decade and a half after the spacecraft hardware
design was frozen, the technology of the ground system could
continue to develop to meet the needs associated with steadily
decreasing received signal strength and also to improve naviga-
tion techniques in the face of the increasing round-trip light
time to the spacecraft. Significant parts of the ground-system
technology in use for the Uranus and Neptune encounters were
simply unavailable when the spacecraft was launched. The
ground system is embodied in the Deep Space Network (DSN),
which the Jet Propulsion Laboratory (JPL) develops and oper-
ates for NASA’s Office of Space Operations. JPL designed and
built the two Voyager spacecraft for NASA’s Office of Space
Science and Applications.
This technology flexibility in both the Voyager spacecraft
and ground systems enabled a mission which, along its way to
becoming a truly remarkable success, established many firsts
associated with its telecommunication system. Ofcourse no as-
This article represents the results
of
one phase of research carried out
at the Jet Propulsion Laboratory, California Institute of Technology,
sponsored
by
the National Aeronautics and Space Administration.
22
September
1990
-
IEEE
Communications Magazine
Fig.
1.
Blue Neptune. Color image ofNeptune taken by Voyager
2
when
it was
7
inillion
kin
(4.4
million miles) from Neptune’s surface. The
Great Dark Spot is visible in the center, accompanied by white
high-altitude clouds. Two color.filters (green and orange) were used, and
about
2
inillion hits were sent,for each ,filter, totalling almost
4
million
hits.
pect of a deep-space mission would be possible without tele-
communication, but by “telecommunication firsts” we mean
first-time accomplishments specifically relating to
or
interact-
ing with the telecommunication system.
Radio
Telemetry
The two Voyager spacecraft were not the first to send imag-
es from Jupiter and Saturn, nor the first to leave the
solar
system-these records belong to the Pioneer
10
and
1
1
space-
0
163-6804/90/0009-0022 $01
.OO
a
1990
IEEE
craft, built by TRW for the Pioneer project at NASA’s Ames
Research Center. However, the Voyager spacecraft, built and
managed by JPL for NASA, have much larger downlink data-
rate capability at all distances due to their 20 W transmitters
and larger 3.66 m antennas (a deep-space first) with X-band
downlink frequency (a deep-space first) providing an antenna
gain of 48.2 dB. The resulting largest ever Effective Isotropic
Radiated Power (EIRP) of 1.32 MW from deep space permit-
ted the transmission of almost
80,000
high-resolution images
during the mission; the current runner-up is the Mars Viking
dual orbitedlander mission
(1
976 encounter) with almost
60,000 images. Actually, the Voyager cameras had about
97,000 shutter activations-some exposures were not trans-
mitted and others were combined to form color images.
The Voyager two-spacecraft mission also holds the records
for the most planets visited (4), most bodies imaged (58, count-
ing the four ring sets around the four target planets, as well as
Earth and its moon), and the most data bits (about 200 Gb)
transmitted from deep space over the life of a mission. This last
record, however, is expected to be bested by the end of the
Magellan prime mission, a radar mapper of Venus which went
into Venus orbit on August
IO,
1990.
The Voyager Uranus (January 1986) and Neptune encoun-
ters established at those times the records for the most distant
image transmission ever, 2.75 billion miles from Neptune. The
data rate of 2 1,600 b/s from Neptune established a record for
the largest distance-normalized data rate, 4.2E23 (b/s
x
km2).
For example, at synchronous Earth satellite altitude this would
give a data rate of greater than
1
E 14 b/s. Because of the wide
range of distances for the Voyager mission, the spacecraft
made use of the largest array of data rates of any deep-space
mission (and probably of any space mission, but this is hard to
check). In addition to the 2 1,600 b/s rate, rates from 40 b/s to
1
15,200
b/s were used by the Voyagers at various times, for a
total of
28
telemetry rates.
Other firsts for the Voyager spacecraft radio hardware in-
clude the dual X-band&-band
(8.5
GHz/2 GHz) antenna feed
(see Figure 3) with the former providing left- as well as right-
hand circular polarization to rely on polarization isolation in-
stead of on less reliable antenna switches for the two X-band
transmitters. There was also the first use of RF channel selec-
tion by a spacecraft (the X-band channels numbered 14 and
18), and the first use of modulation index selection, including
the possibility of fully suppressed carrier. This was used not for
telemetry but in connection with “delta Very Long Baseline
Interferometry (VLBI)” for radio navigation’ (see Figure 4),
another first.
The spacecraft transponder provided the first use of the
two-way non-coherent mode and the subcarriers were
selectable. There were two power amplifiers for X-band and
two for S-band with one of the latter two being a solid-state am-
plifier (a deep-space first). The spacecraft transponder also in-
cluded the most stable oscillator (2 parts in lE12 over
100
s,
aptly named the Ultra-Stable Oscillator-USO) ever yet used
in deep space, and at the time the best in space (GPS cesium de-
vices are now an order of magnitude better), as well as the first
application of a Surface Acoustic Wave (SAW) filter in deep
space, used in the transponder multiplier chain.
‘The direction of a spacecraft with respect to the baseline between
two DSN antennas is determined by measuring the time difference be-
tween the two one-way paths from the spacecraft to the antennas.
To
do this, the spacecraft transmits a wideband signal (in this case the
telernetering signal was used). The signals from the two antennas are
cross-correlated. The “delta”
in
“delta VLBI” refers
to
the procedure
whereby the system is calibrated in real time by alternately observing
both the spacecraft and some directionally nearby quasar that is part of
a quasar “grid.” The grid was very accurately determined over
a
long
period of time by ordinary VLBI. This navigation application of VLBI
is
often called “delta Differenced One-way Ranging (DOR).”
The Earth-based part of the Voyager telecommunication
system also achieved many firsts. The three 70-m DSN anten-
nas have the lowest ever system noise temperature of an opera-
tional X-band (8.5 GHz) receiving system for space
or
any
other X-band communication-20.9 Kat 90 degrees elevation
and 25.5
K
at 30 degrees elevation (in clear dry weather). The
same antennas provided the first operational use of hydropho-
bic coating on feedhorn covers to mitigate weather-dependent
microwave system noise increase during rain. Also, for the
Voyager mission, the DSN made the most advanced and deli-
cate use of multisite weather probability estimates to improve
weather-dependent X-band performance during rain. The
X-band arraying of the National Science Foundation/National
Radio Astronomy Observatory’s (NSFINRAO’s) Very Large
Array (VLA) in New Mexico with the 70 m and 34 m antennas
at the Goldstone Complex for the Neptune encounter involved
the first operational space use of High Electron Mobility Tran-
sistor (HEMT) amplifiers; these were at each ofthe 27 VLA an-
tennas.
This arraying with the VLA (see Figure
5)
established a
number of records: the most antennas (29) ever arrayed any-
where at once (27 VLA plus 2 at Goldstone); the largest fully-
steerable equivalent aperture
(1
5
1
m) ever used for a commu-
nications link (the overall record belongs to the Cornell-
Arecibo 300-m antenna used for the S-band International
Cometary Explorer in 1985, but the Arecibo antenna is not
fully steerable).
Also, the VLA arraying was the longest (aperture separa-
tion) array- 1,200 miles-ever used for communications, or,
in real time, for anything else. The prior record was Canberra/
Parkes for Voyager Uranus, 200 miles via ground microwave
link; Parkes, a 64-m antenna, is operated by the Australian
Commonwealth Scientific and Industrial Research Organiza-
tion (CSIRO). Finally, this was the first arraying for telemetry
via satellite (real-time VLA to Goldstone; see Figure 6).
The 70 m antenna at the DSN Goldstone complex had the
highest power operational coherent uplink for spacecraft-up
to 400 kW Continuous Wave (CW) at S-band. During the mis-
sion, this was used with a margin of
5
dB. That was the highest
EIRP communications transmission ever, about 200 GW.
The Voyager downlink EIRP of 1.32 MW from the Neptune
distance of 4.42E9 km gives a power flux density of 5.38E-2
1
W/m2 at Earth receiving stations. At the data rate of 2 1,600
ds,
this is an energy per bit flux density of 2.49E-25 (J/b)/(m2) at
the receiving stations-far smaller than ever before used any-
where for operational radio
or
any other communication.
Early in the mission, one of the Voyager 2 radio receivers
failed completely. The other had a capacitor short circuit in the
filter of the carrier phase-locked loop which very greatly re-
duced the lock-in frequency range. This greatly reduced lock-in
range was much smaller than the loop oscillator’s drift due to
such things as spacecraft temperature change. Yet it was possi-
ble to maintain the full command function of the impaired re-
ceiver by creating the first continuous frequency-pro-
grammable uplink, derived originally from DSN planetary
radar. The programmed frequency was obtained from a model
for the spacecraft frequency drift.
Coding
During the entire mission, the telemetry downlink channel
code was the NASA Standard constraint-length 7, rate 1/2
convolutional code using a real-time hardware Viterbi decoder
(first space-based short-constraint-length convolutional code
Viterbi decoded) at each receiving station. For the Voyager Ju-
piter and Saturn encounters, uncompressed image data was
sent directly over the convolutionally coded channel. This pro-
vided the required bit error probably of 5E-3 at a zero-margin
signal-to-noise ratio E,/N, of 2.34 dB, the lowest anywhere
ever, as was hinted in the previous section. Other science data
September
1990
-
IEEE
Communications Magazine
23