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In this answer I discuss that the Voyager spacecraft can be commanded to use either high or low power to transmit to Earth on X-band.

Question: Are the Voyager spacecrafts' X-band Traveling Wave Tube Amplifiers (TWTAs) currently set to high or low transmit power? How often were they changed?


Background:

From DESCANSO Design and Performance Summary Series Article 4; Voyager Telecommunications by Roger Ludwig and Jim Taylor:

3.3.4 X-Band Exciters

The X-band exciter converts the frequency at the output of the S-band exciter to X-band to drive the X-band TWTA. Comparable to the S-band exciter function, the X-band exciter phase modulates the RF signal with the composite telemetry signal from the TMU and, if the X-band ranging channel is on, the ranging signal detected by the receiver.

3.3.5 X-Band Power Amplifiers

Only one X-TWTA can be powered at a time. Further, a control input from the CCS ensures that the X-TWTA is powered off when the X-exciter is off. As is the case for S-band, whether powered on or not, the X-TWTA power level is selected to either of two levels4 by CCS control input.

In October 1987, the Voyager 1 X-TWTA-2 failed, as annotated in Figure 3-1, legend item 3. The primary downlink was switched to X-TWTA-1. In November 1998, Voyager 2 switched from X-TWTA-2 to X-TWTA-1 [6].5 For both spacecraft, legend item 1 in the figure flags the changes from X-TWTA-2’s right hand circular polarized downlink to X-TWTA-1’s left hand circular polarized downlink.6

4The low-power and high-power RF levels to the HGA for the X-TWTA are 12 W and 18 W.

5The switch to the backup X-TWTA is in status report http://vraptor.jpl.nasa.gov/voyager/pressrel/vg981117.html

6The relationship between X-TWTA selection and the resultant polarization of the X-band downlink is described in the SXA section that follows.

Foot note #4 indicates that there is at least one method to decrease the transmit power of Voyager built into the system. A command from the ground can instruct the X-band High Gain Antenna (HGA) Exciters to change between 18 W and 12 W of transmit power delivered to the antenna. But what does that mean in terms of electrical power?

Figure 3.1 from Article 4; Voyager Telecommunications (click for larger), discussed further in the question Frequency responses of Voyager's S-band high gain antenna's feed-horn and receiver front end & IF?

Figure 3-1 from Article 4; Voyager Telecommunications

enter image description here

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Both are used:

  • low power for the 160 bps live telemetry: this is sent immediately, and not stored on board. This is done for at least 8 hours/day.
  • high power for the high-speed tape playback. This is done 6 times a year for Voyager 1, Voyager 2 tape operations were ended in 2002. For Voyager 1, tape recorder operations will end in the next few years.

Three times per week, Voyager 1 has 48 seconds of high rate (2.8 kbps) PWS data recorded onto the Digital Tape Recorder (DTR) for later playback. Voyager 1 has six playbacks per year. The playbacks require 70 meter and 34 meter DSN support for data capture.

The lowest playback speed supported by the tape recorder is 1.4 kbit/s.

From Descanso volume 4, page 14:

enter image description here

VIM = Voyager Interstellar Mission (which began in the 1990s).

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  • $\begingroup$ I "cruised" right on by that table without noticing, good find! $\endgroup$ – uhoh May 8 at 7:47
  • $\begingroup$ I'm still interested to find out if they are still being switched between high and low, or if they have settled on the low power setting and low data rate. Each time I check recently it's 160 bits/sec. If they've stopped using the high power setting then the answer to "Are the Voyager spacecrafts' X-band TWTAs currently set to high or low transmit power?" is "No." $\endgroup$ – uhoh May 8 at 7:54
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    $\begingroup$ Patience, grasshopper. High speed playback is done for one of them, a few times a year. Details later. $\endgroup$ – Hobbes May 8 at 7:55
  • $\begingroup$ Okay, I'll try, master Po. I keep grasping for that pebble... $\endgroup$ – uhoh May 8 at 7:56
  • $\begingroup$ Excellent, thank you! $\endgroup$ – uhoh May 12 at 0:00

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