Figure 1: Leland Langston, System Engineer, Voyager Data Storage Subsystem, Texas Instruments

Figure 2: Texas Instruments Voyager Engineering Team, 1975

Voyager can be said to have begun in 1965 as a Grand Tour, an extensive, if not grandiose, planetary mission planned in the midst of shrinking NASA and Federal budgets, at a time when NASA sought to define its mission in the post-Apollo era. This mission was estimated to cost over $1 billion when the entire NASA budget for 1972 was only slightly over $3 billion in 1972. It was truly a “big science” project. In response to social and political pressure on the Federal budget stemming largely from the Great Society programs and the Vietnam War, as well as the conservative fiscal policy of the Nixon administration, the Grand Tour mission was downsized. Congressional pressure, combined with internal competition from the recently approved Space Shuttle program, led to the decision to cancel the Grand Tour project in December 1971; it was replaced by a proposal to visit only two planets using a pair of Mariner-derived probes. The Mariner Jupiter-Saturn project was approved in early 1972, with an estimated cost of $360 million. The probes would be built by JPL, with the intention that they would last long enough to complete the original Grand Tour of the four giant planets, but be advertised as missions to only Jupiter and Saturn to reduce estimated total project costs. It was initially called the Mariner Jupiter-Saturn (MJS) mission.  In March 1977, just a few months before launch, NASA held a competition to rename the project; it then became known as Voyager. Thus was the auspicious beginning of the Voyager project. As a young engineer fresh off of the Apollo project and the Viking project, this was a “dream project” for me.  It is one of the highlights of my career.

Spacecraft Design Summary

Voyager carried twelve science instrument subsystems. These are:

1. "CRS – COSMIC RAY SUBSYSTEM".
2. "ISS NA – IMAGING SCIENCE SUBSYSTEM – NARROW ANGLE".
3. "ISS WA – IMAGING SCIENCE SUBSYSTEM – WIDE ANGLE".
4. "IRIS – INFRARED INTERFEROMETER SPECTROMETER AND RADIOMETER".
5. "LECP – LOW ENERGY CHARGED PARTICLE".
6.  "MAG – TRIAXIAL FLUXGATE MAGNETOMETER".
7. "PLS – PLASMA SCIENCE EXPERIMENT".
8. "PPS – PHOTOPOLARIMETER SUBSYSTEM".
9. "PRA – PLANETARY RADIO ASTRONOMY RECEIVER".
10. "PWS – PLASMA WAVE RECEIVER".
11. "RSS – RADIO SCIENCE SUBSYSTEM".
12. "UVS – ULTRAVIOLET SPECTROMETER".

These twelve instruments are arranged as shown in Figure 5.  In addition to the nine instruments shown on booms, three are contained in the 10-sided polygon bus housing electronics.  This bus also houses seven engineering subsystems including the data storage subsystem, the flight data subsystems, the telemetry modulation/demodulation subsystem, the attitude and Articulation Control Subsystem, the radio communications subsystem, and the power supply subsystem. The primary antenna is the 3.7 meter diameter high-gain antenna.  This antenna supports both S-band and X-band communications.

The physical characteristics of the spacecraft are summarized as follows. The mission module after injection weighed 825 kilograms (1,819 pounds) including a 117-kg (258-Ib.) science instrument payload. The propulsion module, with its large solid-propellant rocket motor, weighed 1,207 kg (2,660 lb.). Launch weight of the spacecraft was 2,066 kg (4,555 lb.).

Communications Subsystem Operation

Communications with the Voyagers is by radio link between Earth tracking stations and a dual-frequency radio system aboard the spacecraft. The uplink operates at S-band only, carrying commands and ranging signals from ground stations to one of a pair of redundant receivers. The downlink is transmitted from the spacecraft at either S-band or X-band frequencies.

The flight data subsystem, one of the three on-board computers controls the science instruments and formats all science and engineering data for telemetry to Earth. The telemetry modulation unit of the modulation demodulation subsystem feeds data to the downlink. The flight command unit of the modulation/demodulation subsystem routes ground commands received by the spacecraft.

Only one receiver is powered ON at any one time, with the redundant receiver at standby. The receiver operates continuously during the mission at about 2113 megahertz. (Different frequency ranges have been assigned to the radio-frequency subsystem of each spacecraft.) The receiver can be used with either the high-gain (dish) or low-gain (Omni) antenna. (Voyager 2's primary receiver failed on April 5, 1978, and the spacecraft is operating on its backup receiver.)

The S-band transmitter consists of two redundant exciters and two redundant RF power amplifiers, of which any combination is possible. Only one exciter-amplifier combination operates at any one time. Selection of the combination is by on-board failure-detection logic within the computer command subsystem, but with ground-command backup. The same arrangement of exciter/amplifier combinations makes up the X-band transmitting unit.

One S-band and both X-band amplifiers employ traveling wave tubes. The second S-band unit is a solid state amplifier. The S-band transmitter is capable of operating at 9.4 watts or at 28.3 watts when switched to high power and can radiate from both antennas. X-band power output is 12 watts and 21.3 watts. X-band uses only the high-gain antenna. (S-band and X-band never operate at power simultaneously.)

When no uplink signal is being received, the transmitted S-band frequency of about 2295 MHz and X-band frequency of 8418 MHz originate in the S-band exciter's auxiliary oscillator or in a separate ultra-stable oscillator (one-way tracking). With the receiver phase-locked to an uplink signal, the receiver provides the frequency source for both transmitters (two-way tracking). The radio system can also operate with the receiver locked to an uplink signal while the downlink carrier frequencies are determined by the on-board oscillators (two-way non-coherent tracking). Only the 64-m (210-ft.) antenna stations of the Deep Space Network can receive the downlink X-band signal.

The high-gain antenna, with its 3.66-m-diameter (12 ft.) parabolic reflector, provides a highly directional beam. The low- gain antenna provides essentially uniform coverage in the direction of Earth. After the first 80 days of the mission, all communications -- both S-band and X-band -- have been via the high-gain antenna.

Power

Because of the extreme distances from the sun, and hence low light intensity, the Voyager spacecraft required a different primary power source.  Voyager was one of the first spacecraft to use Radioisotope Thermoelectric Generators (RTGs) as the primary power source. The power source was the Pu238 isotope of Plutonium with a half-life of 87 years.  Three RTG modules produced approximately  2,400 watts thermal, but the efficiency of the thermocouples is very low, and only about 160 watts of electricity was produced by each module to yield less than 500 watts at mission start.  Because the Plutonium decays to produce half power after 87 years, the power output has steadily decreased to only about 300 watts after 40 years.  The voltage from the RTGs is DC and varies over time.  Hence power conditioning was used to convert the output to a 2400 Hz square wave tightly regulated voltage which was distributed to each electronics package. Each electronic subsystem then converted this voltage to the voltages required by the subsystem.

Downlink Telemetry

Data telemetered from the spacecraft consists of engineering and science measurements prepared for transmission by the flight data subsystems, telemetry modulation unit and data storage subsystem.
The encoded information includes voltages, pressures, temperatures, television pictures and other values measured by the spacecraft telemetry sensors and science instruments. See Figures 7 and 8 for some images received from Voyager.

Because spacecraft maneuvers during encounters prevented the antenna from being continuously pointed toward earth, as well as the rate at which data was collected during the planet and moon encounters was too high for real-time transmission, this data was stored on a digital tape recorder for later play-back at a much lower rate.  The data storage subsystem consisted of a digital tape recorder and associated electronics.  The tape recorder consisted of a belt driven mechanism powered by a servo-controlled brushless DC motor. (This was one of the first designs using a brushless DC motor.) The tape was a little over 1000 feet in length and stored data using Miller encoding to yield approximately 6700 bits/inch packing density.  The recorder had nine parallel tracks--eight data tracks and one tachometer track that was used for servo control of the brushless DC motor. The tape could store approximately 650 Mb (80 MB) of data (although only 536 Mb of data storage capacity was deemed reliable over the mission lifetime). Data was typically recorded at 115 kbps, and could be played back in non-real time at data rates of 57.6; 33.6; 21.6 and 7.2 kbps with the lower rates used at the greater distances (e.g., from Neptune). These are the actual information rates; the information was encoded for error correction using a rate 1/2, constraint length 7 convolution code before transmission.  (The difficult task of decoding was done on the ground using a Viterbi decoder.)

The received signal-to-noise (Eb/No) ratio was near the noise threshold when the spacecraft was near Neptune, even with a system noise temperature of 25oK. The rate 1/2 error correction code resulted in a Bit Error Rate of approximately 10-5.  The link budget calculations (approximate) are shown in Table 1.

The success of Voyager is measured in terms of the data returned as well as its record for longevity. The Voyager 1 spacecraft entered interstellar space in August 2012, 35 years after launch.  Quite remarkable for a system designed for a four-year mission in the ‘70’s when the state of the art for integrated circuits was MSI with the highest level of integration being a 4-bit counter, and the imaging device being a Vidicon imaging tube!  And like the bunny in the battery commercial, “It keeps going and going and going”.

Figure 6: Photograph of one of the Multilayer PCBs for Voyager

Figure 7: Montage of the four outer planets taken by Voyager

Figure 8: Montage of the four gas giants and some of their moons photographed by Voyager