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How to calculate data rate of Voyager 1? mention's Voyager's data rate is currently 160 bits per second, and clicking on a recent entry in the Voyager DSN Tracking Schedule on the Voyager Mission Status page I also see

FDS MODE:              CR-5T
XB DATA RATE(MI):     160(32)

and I'll assume the 160 means bits per second.

@Hobbs' answer to How was magnetic tape decay prevented in Voyager 1? quotes the Voyager Backgrounder, October 1980 (PDF)1 which says that general science data is recorded and played back at 7200 bits per second:

The data-storage subsystem can record at two rates: TV pictures, general science and engineering at 115.2 kbps; general science and engineering at 7.2 kbps; and engineering only at 7.2 kbps (engineering is acquired at only 1,200 bps, but is formatted with filler to match the recorder input rate). The tape transport is belt-driven. Its 1/2 in. magnetic tape is 328m (1,076 ft.) long and is divided into eight tracks that are recorded sequentially one track at a time. Total recycleable storage capacity is about 536 million bits -- the equivalent of 100 TV pictures. Playback is at four speeds-- 57.6; 33.6; 21.6 and 7.2 kbps.

Question: If the Voyagers' lowest playback speed is 7200 bits per second, how does it transmit to Earth at only 160 bps? What system aboard Voyager (if any) performs this apparent data throttling/buffering/rate-adaptation?


1NASA-NEWS-RELEASE-80-160, P80-10172

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  • $\begingroup$ From a rough look at the capabilities of the computer system, it should be possible to buffer the data in memory. 10 words per second means a couple of thousand words of memory can buffer several minutes of communication. $\endgroup$ Jul 10 at 13:01
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    $\begingroup$ The DTR IS the buffer It could simply mean that the communication data rate cannot support playback anymore (hence shutting down of many scientific instruments). This is predictable, I would think (combined effect of inverse square law of distance and spacecrafts' power production decay). I guess that telemetry now just contains the general status of the spacecratfs. $\endgroup$
    – Ng Ph
    Jul 12 at 7:53
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Question: If the Voyagers' lowest playback speed is 7200 bits per second, how does it transmit to Earth at only 160 bps?

It doesn't. The 160 bps transmission is real-time data rather than playback data. The playback data is rather infrequent and was at 1400 bps. (It may be at 600 bps now.) From https://voyager.gsfc.nasa.gov/Library/DeepCommo_Chapter3--141029.pdf,

Two 6.6-hour digital tape recorder (DTR) playbacks are received per year per spacecraft, plus occasional special playbacks requested by scientists. These playbacks require a 70-m downlink station. Near the start of VIM (Voyager Interstellar Mission), playbacks were downlinked at 7200 bps. As the 7200-bps link margin approached exhaustion at the 70-m stations, 70-m/34-m HEF arrays were employed to gain a 0.8 dB performance improvement for approximately two more years of capability. When array link margin was exhausted, Voyager reduced the playback data rate to 1400 bps and returned to the use of 70-m stations standing alone. The mission will require arrays again if it is to capture 1400 bps playbacks downlinked from Voyager 1 beginning in 2007 and from Voyager 2 in 2011.

This of course raises the question: How do the Voyagers transmit playback data at 1400 bps if the lowest playback rate is 7200 bps? Note well: The linked document in the question was published in 1980.

The answer is that JPL has long realized that its computers and firmware need to be able to be reconfigured in flight. The lowest playback rate was 7200 bps in 1980, when that document was written.

As an extreme example of this reconfigurability, the rovers recently sent to Mars did not need to know how to rove until after they landed on Mars, and once they did land, they did not need to know how to perform entry, descent, and landing. Computer memory is a very precious thing in spaceflight. Shortly after landing, JPL wiped the programming related to EDL and added programming related to roving.

I can't find a specific reference that says when/how JPL updated the software/firmware for the Voyager digital tape recorders, but it obviously did happen given that the lowest rate was 7200 bps in 1980 and that the rate was reduced to 1400 bps in the 1990s and given that the playback rate may be further reduced to 600 bps.

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    $\begingroup$ mystery solved! $\endgroup$
    – uhoh
    Jul 10 at 15:10
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    $\begingroup$ The last playback from Voyager 1 I could find was on 2020/12/15 at 1400 kbps. voyager.jpl.nasa.gov/pdf/sfos2020pdf/20_12_17-21_01_03.sfos.pdf $\endgroup$
    – asdfex
    Jul 10 at 15:33
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    $\begingroup$ @asdfex i believe you want to lose that "k" in "1400 kbps". $\endgroup$
    – PcMan
    Jul 10 at 19:28
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    $\begingroup$ @PcMan Nah, I want to add a "." after the 1. $\endgroup$
    – asdfex
    Jul 11 at 9:14

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