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