[chris (OP)]

This evening a question on the radio programme provoked an interesting question. We were debating the rate of data transmission back to Earth from probes like Voyager? And how does this differ for more modern instruments like Gaia?

[Janus]

Gaia transmits at a rate of ~7.5 mb/sec.  Voyager, during its passage of Jupiter transmitted at 115.2 kb/sec, but is down to 160 b/sec now.   A lot of this has to do with the distance the signal has to travel and how it weakens with distance.   Gaia is only ~1.7 million km away vs the several  100 million km distance of Voyager when passing Jupiter and its 19 billion km distance now.  As the signal gets weaker, noise becomes a larger and larger problem.  Its the noise to signal ratio which requires the bit rate to decrease in order to make it possible to extract the information being carried.

[syhprum]

I understand that one of the Voyager spacecraft had an AFC problem with its receiver and very careful adjustment of the transmitted frequency was needed to establish communication is this the one with which communication is still possible

I found a detailed report of the receiver problem on voyager 2 but had a copy and paste failure so I can only summarise it
a capacitor in the AFC system of the receiver has failed and it will no longer compensate for the Doppler shift so a change of 100Hz in the transmitted frequency has to be made

[evan_au]

New Horizons could transmit at 2,000 bits per second when it was at Pluto.
When it was at Jupiter (much closer to Earth), it could transmit at 38,000 bits per second.

One of the advantages of modern high-capacity data storage on space probes is that New Horizons could collect lots of data and images during the couple of hours it was near Pluto+Charon, and then transmit it back to Earth over the subsequent year. So the actual data transmission speed is not quite so critical.

NASA has a protocol for transmitting files from its space probes, so that if part of a file is lost, the Earth receivers can report that back to the space probe, which retransmits the missing data.

See: http://pluto.jhuapl.edu/Mission/Spacecraft/Data-Collection.php

[evan_au]

The maximum speed of a communications channel is given by the Shannon limit, defined in the 1940s.
 C =  B log₂(1+S/N)
Where:
- C is the channel capacity, in bits per second
- B is the bandwidth of the channel, in Hertz
- S is the Signal power
- N is the Noise power
- log₂ is the logarithm, to base 2

- Signal power S drops with distance, due to the inverse square law.
- Noise Power N includes the noise of space, and the noise in the cryogenically-cooled receivers on Earth. This is pretty much the same between Voyager and New Horizons - in fact, they use the same network of 70m dishes in NASA's Deep-Space Network.

Comparing Voyager 1 & 2 (launched in 1977) with New Horizons (launched in 2006):
- Transmit Speed at Jupiter: V: 115 kilobits per second; NH: 38 kilobits per second; mainly due to bigger dish on V.
- Transmitter Dish: V: 3.7m; NH: 2.1m; a bigger dish allows a narrower beamwidth (for the same wavelength)
- Transmit Wavelength: V: 3.6cm; NH: 3.6cm
- Transmitter Beamwidth: V: 0.5°;  NH: 1° (a narrower beam concentrates more power onto Earth)
- Transmit Power: V: 22W; NH:2x12W
- Distance from Sun: V1: 141 AU; V2: 116 AU; NH: 40 AU (in Jan 2018)
- Electrical power source: V:470W;  NH: 245W (at launch; radioactive output declines steadily with time)
- Onboard Data Storage: V: 64 kiloBytes; NH: 8,000,000 kiloBytes (=8 GB)

In practice, no communications channel actually reaches the Shannon Limit. By transmitting extra error-correcting codes, satellites can come close this this limit. More recent satellites have more powerful onboard processing, which allows more sophisticated error-correcting codes, and are edging closer to this fundamental limit.

But with lots of onboard storage in more recent satellites, transmission speed is not so critical.

See: https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem
https://en.wikipedia.org/wiki/New_Horizons#Telecommunications_and_data_handling
https://en.wikipedia.org/wiki/Voyager_1#Communication_system

PS: I often fly on commercial jets just to the north or south of the NASA Deep Space Network at Tidbinbilla - close enough to see the white dishes pointing in various directions. I wonder how often these commercial flights have caused loss of a bit or two from some satellite far out in space...

[evan_au]

I often fly on commercial jets just to the north or south of the NASA Deep Space Network at Tidbinbilla

I was sitting on the right side of the plane today, and took this snap out the passenger window on my smartphone.
It's rather blurry from 10km altitude, with no zoom on the smartphone..
Despite the scattered cloud, you can see the cluster of white dishes, pointing in different directions.

[petelamana]

Real time telemetry for Voyager, according to NASA.gov, is at a rate of 160 bits per second, using a 34 meter DSN (Deep Space Network) radio telescope. Once every 6 months, the bitrate goes up for a “data dump” of sorts.