How is Voyager talking after so many years?

Technical news channels have been buzzing with stories recently coming back from weird signals. Voyager d. Although normal suspects jump to normal conclusions – Aliens !! – In the absence of a convincing explanation for the inconsistency, some of us saw it as an opportunity to marvel at the fact that two Voyager spacecraft, now over 40 years old, are still in constant contact with us back to Earth, and it is about 20 billion kilometers in the most unfavorable environment Despite being covered.

Like many NASA programs, Voyager has far exceeded its original design goals, and is still reporting useful science data. But how is this possible? Did radio technology from the 1970s make it into a pair of space exploration that not only allowed them to fulfill their initial mission of exploring the outer planets, but also allowed them to go on an extended mission into interstellar space and still have two-way communication? As it turns out, there’s nothing magical about Voyager’s radio – just a healthy dash of solid engineering redundancy and lots of good luck over the years.

Large dish

For a program that in many ways defined the post-Apollo era of planetary exploration, Voyager was surprisingly first conceived. The origins of the complex mission profile date back to the “Planetary Grand Tour” concept of the mid-1960s, which was planned to take advantage of an alignment of the outer planets that would occur in the late 1970s. When launched at the right time, a probe will be able to reach Jupiter, Saturn, Uranus and Neptune after its initial propelled boost using only gravitational assistance, before embarking on a course that will eventually take it into interstellar space.

The idea of ​​visiting all the outer planets was very tempting to pass and with the success of the Pioneer mission on Jupiter serving as a dress rehearsal, the Voyager program was designed. Like all NASA programs, some of Voyager’s initial missions were aimed at a minimal set of planetary science tests that project managers could reasonably be sure they could achieve. The Voyager spacecraft was designed to meet the goals of this original mission, but planners also hoped that the spacecraft would survive after facing their final planet and provide valuable data as it transcends emptiness. And so the hardware, both in space and on the ground, reflects that hope.

Voyager’s initial reflector is being built, around 1975. The core of the dish is made of beehive aluminum and covered with graphite-embedded epoxy laminate skin. The surface accuracy of the finished dish is 250 μm. Source: NASA / JPL

The Deep Space Network (DSN) is the most prominent physical feature of both ground stations, which we have already covered in depth, and the Voyager spacecraft itself is their parabolic dish antenna. Although the scale may vary – DSN sports telescopes span up to 70 meters – the Voyager Twins were launched with the largest meal to match the firing of a Titan IIIE launch vehicle.

Voyager High-gain antenna (HGA) designed. Notice the Kasgrain optics, as well as the frequency-selective subflexor that reflects transparent from the S-band (2.3-GHz) but reflects the X-band (8.4-GHz). Click to enlarge. Source: NASA / JPL

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

Like their earth-bound counterparts in DSN, Voyager antennas have a casegrain reflector design that uses a frequency selective subreflector (FSS) to focus the primary reflector. The subflexor focuses and adjusts the incoming X-band waves back to the center of the primary plate, where the X-band feed horn is located. This format gives the X-band a gain of about 48 dBi and a beam width of 0.5. The S-band configuration is slightly different, the feed horn is located inside the subflexor. The frequency-selected nature of the subflexor material allows S-band signals to pass through it and illuminate the primary reflector directly. This gives a gain of about 36 dBi in the S-band, with a beam width of 2.3. The sub-reflector assembly has a low-gain S-band antenna with a low-to-high cardiovade radiation pattern located on the earth-facing side, but it was only used for the first 80 days of the mission.

Two one

Each of the Voyager bus’s ten bays is dedicated to three radio frequency subsystems or RFS transmitters, receivers, amplifiers and modulators. Like all high-risk space missions, redundancy is the name of the game – almost every possible single point of failure in RFS has some kind of backup, an engineering design decision that has proven mission-saving in multiple instances in both cases. Spacecraft for the last 40 years.

On the uplink side, each Voyager has two S-band double-conversion superheat receivers. In April 1978, just a year before its scheduled encounter with Jupiter, the primary S-band receiver Voyager 2 The spacecraft was shut down by fault-protection algorithms that failed to take any commands from Earth for an extended period of time. The backup receiver was turned on, but it found a bad capacitor in the phase-locked loop circuit that was primarily intended to adjust for a Doppler-shift change in frequency due to the movement of the earth. The mission controllers instructed the spacecraft to return to the primary receiver, but it failed again and left. Voyager 2 Without any way to be ordered from the ground.

Fortunately, fault-protection routines restart the backup receiver after a week of not communicating, but this puts the controllers in a jam. To continue the mission, they need to find a way to use the Wanky Backup Receiver to command the spacecraft. They came up with a complex scheme where DSN controllers guessed what would happen based on the uplink frequency predicted Doppler shift. The problem is, thanks to the bad capacitor, the signal needs to be within 100 Hz of the receiver’s lock frequency, and that frequency varies with the temperature of the receiver by about 400 Hz per degree. This means that regulators need to be tested twice a week to determine the current lock frequency, and allow the spacecraft to remain thermally stable for three days after uplinking any commands that could change the spacecraft’s temperature.

Double downlink

Like the Apollo-era TWTA, the S-band and X-band power amp used in Voyager. Source: Ken Sharif

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

The redundancy created on the downlink side of the radio system will play a role in preserving the initial missions of both spacecraft. In October 1987, Voyager d X-band failed in one of the TWTAs. A little over a year later, Voyager 2 Experience the same problem. Both spacecraft were able to switch to another TWTA with permission Voyager d To return the famous “Family Portrait” of the solar system with a picture of the pale blue dot of the earth and for Voyager 2 1989 to return data from Neptune’s flyby.

Slow and slow

Voyager system radio systems were originally designed to support planetary flybies and were therefore optimized to stream as much science data as possible on DSN. Moving closer to each of the outer planets means that each spacecraft accelerated dramatically during the flybis, at the very moment of producing the most data from the ten science instruments on board. To avoid bottlenecks, each Voyager incorporates a digital tape recorder (DTR), which was originally a fancy 8-track tape deck, to buffer science data for subsequent downlinks.

Also, the increasing distance of each Voyager has greatly reduced the bandwidth available for downlinking science data. When the spacecraft made their first flybus on Jupiter, the data streamed relatively 115,200 bits per second. Now, with each spacecraft approaching a full light-day, data drips at just 160 bps. Uplinked commands are slower, using a mere 16 bps, and 18 kilowatts of power, exploding across space from DSN’s 70-meter dish antenna. The damage to the uplink path is from the current distance of 23 billion kilometers Voyager d Exceeds 200 dB; Towards the downlink, DSN telescopes need to emit a signal that has faded to atowatt (10).-18 W) range.

That radio system Voyager d And Voyager 2 They worked while on the core of their planetary mission which is a technological achievement to celebrate. Despite four decades of challenges in space and multiple system failures, both spacecraft are still communicating, which is almost a miracle.

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