How is Voyager still talking after all these years?

Tech news outlets have recently been abuzz with stories about strange signals coming back from traveler 1. While the usual suspects drew the usual conclusions – aliens!! – in the absence of a firm explanation for the anomaly, some of us look to this event as an opportunity to marvel at the fact that the two Voyager spacecraft, now over 40 years old, are still in constant contact with us back to Earth, and this despite having traveled some 20 billion kilometers in one of the most hostile environments imaginable.

Like many NASA programs, Voyager far exceeded its original design goals and is still reporting useful scientific data to this day. But how is this possible? Radio technology from the 1970s made it to the twin space probes that allowed it to not only fulfill its primary mission of exploring the outer planets, but also let them go on an extended mission into interstellar space and still remain in two-way contact ? As it turns out, there’s nothing magical about Voyager’s radio – just solid engineering tempered with a healthy dash of redundancy and a bit of good luck over the years.

the big dish

For a program that in many ways defined the post-Apollo era of planetary exploration, Voyager was conceived surprisingly early. The mission’s complex profile had its origins in the “Planetary Grand Tour” concept of the mid-1960s, which was designed to take advantage of an alignment of the outer planets that would occur in the late 1970s. capable of reaching Jupiter, Saturn, Uranus and Neptune using only gravitational assistance after its initial push, before being launched on a course that would eventually take it into interstellar space.

The idea of ​​visiting all the outer planets was too appealing to pass up, and with the success of the Pioneer missions to Jupiter serving as dress rehearsals, the Voyager program was designed. Like all NASA programs, Voyager had certain primary mission objectives, a minimal set of planetary science experiments that project managers were reasonably sure they could perform. The Voyager spacecraft was designed to meet these core mission objectives, but planners also expected the vehicles to survive their final planetary encounters and provide valuable data as they traversed the void. And so the hardware, both on the spacecraft and on the ground, reflects that hope.

Voyager primary reflector being manufactured around 1975. The dish body is made of honeycomb aluminum and is covered with graphite impregnated epoxy laminated skins. The surface accuracy of the finished dish is 250 μm. Source: NASA/JPL

The most prominent physical feature of both the Deep Space Network (DSN) ground stations, which we have already covered in depth, and the Voyager spacecraft itself are their satellite dishes. While the scale might be different – ​​DSN sport telescopes up to 70 meters in diameter – the Voyager twins were launched with the largest dish that could fit in the fairing of the Titan IIIE launch vehicle.

Schematic of Voyager’s high gain antenna (HGA). Note the Cassegrain optics as well as the frequency selective subreflector which is transparent to the S band (2.3 GHz) but reflects the X band (8.4 GHz). Click to enlarge. Source: NASA/JPL

The primary reflector of the High Gain Antenna (HGA) on each Voyager spacecraft is a 3.7 meter diameter parabolic dish. The plate is made of honeycomb aluminum covered with a graphite-impregnated epoxy laminated skin. The reflector surface is finished to a high degree of smoothness, with a surface accuracy of 250 μm, required for use in both the S-band (2.3 GHz), used for uplink and downlink, and the X-band (8.4 GHz). GHz), which is downlink only.

Like their terrestrial counterparts in the DSN, the Voyager antennas are a Cassegrain reflector design, which uses a frequency selective subreflector (FSS) at the focus of the primary reflector. The sub-reflector focuses and corrects incoming X-band waves back to the center of the primary plate, where the X-band feed horn is located. This arrangement provides about 48 dBi of gain and a beam width of 0.5° in the X-band. The S-band arrangement is slightly different, with the power horn located inside the subreflector. The frequency selective nature of the subreflector material allows S-band signals to pass through it and illuminate the primary reflector directly. This gives about 36 dBi of gain in the S-band, with a beam width of 2.3°. There is also a low-gain S-band antenna with a more or less cardioid radiation pattern located on the Earth-facing side of the subreflector array, but which was only used for the first 80 days of the mission.

two is one

Three of the ten bays on each Voyager bus are dedicated to Radio Frequency Subsystem, or RFS, transmitters, receivers, amplifiers and modulators. As with all high-stakes space missions, redundancy is the name of the game – nearly every possible point of failure in RFS has some sort of backup, an engineering design decision that has proven mission savings in more than one instance in both spacecraft in the last 40 years.

On the uplink side, each Voyager has two S-band double-conversion superhet receivers. In April 1978, just a year before its scheduled encounter with Jupiter, the main S-band receiver in Voyager 2 was turned off by fault protection algorithms on the spacecraft which failed to pick up any commands from Earth for a long time. The backup receiver was turned on, but it was found to have a bad capacitor in the phase-lock loop circuit intended to adjust for Doppler shift changes in frequency due primarily to the Earth’s motion. Mission controllers ordered the spacecraft to return to the primary receiver, but this failed again, leaving Voyager 2 without any form of being commanded from the ground.

Fortunately, the failsafe routines turned the backup receiver back on after a week of no communication, but this left the controllers stuck. To continue the mission, they needed to find a way to use the unstable backup receiver to command the spacecraft. They created a complex scheme where the DSN controllers guess what the uplink frequency will be based on the predicted Doppler shift. The problem is, thanks to the bad capacitor, the signal needs to be within 100 Hz of the receiver’s blocking frequency, and that frequency changes with the temperature of the receiver, about 400 Hz per degree. This means controllers need to run tests twice a week to determine the current lock frequency and also allow the spacecraft to thermally stabilize for three days after uplinking any commands that might change the spacecraft’s temperature.

double downlinks

Ken Shirriff

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An Apollo-era TWTA, similar to the S-band and X-band power amplifiers used on Voyager. Source: Ken Shirriff

On the transmission side, X-band and S-band transmitters use separate exciters and amplifiers, and again multiples of each for redundancy. Although the downlink is primarily through the X-band transmitter, either of the two S-band exciters can be fed to either of the two different power amplifiers. A solid state amplifier (SSA) provides a selectable 6W or 15W power output to the feedhorn, while a separate traveling wave tube amplifier (TWTA) provides 6.5W or 19W. Dual X-band exciters , which uses the S-band exciters as a frequency reference, use one of two dedicated TWTAs, each of which can send either 12W or 18W to the high gain antenna.

Redundancy built into the downlink side of the radio system would play an important role in saving the main mission on both spacecraft. In October 1987, traveler 1 suffered a failure in one of the X-band TWTAs. Just over a year later, Voyager 2 went through the same problem. Both spacecraft were able to switch to the other TWTA, allowing traveler 1 to send back the famous “Family Portrait” of the Solar system, including the Pale Blue Dot image of Earth, and to Voyager 2 to send data back from its flyby of Neptune in 1989.

Slower and slower

The radio systems on the Voyager systems were primarily designed to support planetary flybys and therefore were optimized to transmit as much scientific data as possible back to the DSN. The approaches of each of the outer planets meant that each spacecraft accelerated dramatically during flybys, right at the moment of maximum data output from the ten onboard science instruments. To avoid bottlenecks, each Voyager included a digital tape recorder (DTR), which was essentially a sophisticated 8-track tape deck, to store scientific data for later downlink.

In addition, the increasing distance to each Voyager has dramatically decreased the bandwidth available for downlinking scientific data. When the spacecraft made its first flybys of Jupiter, data was transmitted at a relatively animated speed of 115,200 bits per second. Now, with each spacecraft approaching a light-day away, the data comes in at just 160 bps. Uplink commands are even slower, a mere 16 bps, and are launched across the space of the DSN’s 70 meter satellite dishes using 18 kW of power. The uplink path loss over the current distance of 23 billion kilometers to traveler 1 exceeds 200 dB; on the downlink side, DSN telescopes have to dig a signal that has faded to attowatt (10-18 W) reach.

That the radio systems of traveler 1 and Voyager 2 worked while still on the main part of their planetary mission is a technical achievement worth celebrating. The fact that both spacecraft are still communicating, despite the challenges of four decades in space and multiple system failures, is almost a miracle.

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