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A transponder amplifies, (often) frequency-shifts, and rebroadcasts a received signal. Old-style telephone and TV distribution satellites in GEO used simple transponders to rebroadcast the uplinked signals from Earth without manipulating the data. Some were even of the "bent-pipe" variety (they didn't frequency shift, just amplify the signal and turn it around towards Earth.

A coherent transponder (in the context of modern spaceflight navigation and planetary science) uses a phased locked loop (or similar) to generate a downlink carrier frequency that is locked to the uplink carrie in some integer ratio, for example the Voyager S to S and X to S-band ratios are 240/221 and 880/221, respectively. That way the down-link is coherent with the uplink, and comparing the returning frequency to the uplinked frequency gives extremely precise relative radial velocity measurements.

Deep space spacecraft have these so that their distances (from delay time) and velocities (from Doppler shift) can be measured on Earth and their positions calculated and tracked.

Answer(s) to How will “InSight's onboard communications gear perform a radio-science experiment to shed further light on Mars' innards?” (Space.com) make it clear that InSight will have a coherent transponder for precision velocity and distance measurements, and the paper InSight coordinates determination from direct-to-Earth radio-tracking and Mars topography model confirms this, and Signatures of the Martian rotation parameters in the Doppler and range observables as well as Mars dynamics from Earth-based tracking of the Mars Pathfinder lander make it clear that previous Mars landers likely also had coherent transponders for precise measurements of Mars' nutation.

See also How will InSight's RISE antennas end up pointed in the right direction?

See also Spaceflight 101's InSight Instrument Overview

Question: While I'd like to ask "How many coherent radio transponders have been placed on solar system bodies?" but that might be too much of a list, so my Reduced Question is instead just "How many solar system bodies have had coherent radio transponders?"

Deep space spacecraft that are moving in space do not count. For this question, it's just coherent transponders that sit or have sat on solar system bodies. Rovers would count because they can optionally not rove for a while. Links directly with DSN, and links with other spacecraft that also link with Earth on a different pair of frequencies are both okay.

Example, RISE on InSight Mars lander. From mars.nasa.gov's Mars InSight Mission's Surface Operations (cropped)

enter image description here

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    $\begingroup$ bent-pipe is also coherent $\endgroup$
    – pericynthion
    Nov 23, 2018 at 16:46
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    $\begingroup$ @pericynthion yes indeed. While transponders (often) frequency-shift, the bent-pipe variety doesn't and so would be a perfect (and mathematically trivial) example. That's a good thing to point out, so I've added the caveat in the context of modern spaceflight navigation and planetary science. $\endgroup$
    – uhoh
    Nov 23, 2018 at 17:10
  • $\begingroup$ @uhoh arguably Odyssey's "bent-pipe" mode frequency shifts (though it actually decodes incoming UHF and "immediately" transmits X band, described here & here) $\endgroup$
    – BrendanLuke15
    Jan 12, 2022 at 15:53
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    $\begingroup$ @uhoh I'm sceptical you can get enough isolation to do bent pipe without changing frequency. Maybe with separate antennas? $\endgroup$
    – Roger Wood
    Jan 12, 2022 at 19:52
  • $\begingroup$ @RogerWood I don't know how to parse your comment; yes it seems obvious it will be extremely difficult to receive and retransmit the same signal on the same antenna; there are recirculators and duplexers and I see isolations of 50 dB to 90 dB for some single-antenna radar applications but I don't know what the early communications satellite bent pipe systems used and doubt enough isolation could be obtained for deep space transponders. $\endgroup$
    – uhoh
    Jan 12, 2022 at 20:16

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How many solar system bodies have had coherent radio transponders?

To communicate with other spacecraft or ground systems at Earth, one needs a transponder or transceiver. So I will show an abbreviated list of objects where landers successfully sent data to either an orbiting spacecraft or directly to Earth. However, the caveat is that I didn't dig into whether the probe specifically had a coherent transponder or a transceiver. Note that a transceiver is an independent electronics unit that houses both a transmitter and receiver. A transponder, the transmitting carrier frequency is derived from the received signal, which is an efficient way to do Doppler ranging etc. without extra power consuming signals.

The following solar system bodies have had at least one artificial, robotic system touch down on its surface (in working order, even if only shortly):

  • Earth's moon: NASA's Apollo missions and Surveyor program, Soviet Luna program, China's Chang'e 3, Chang'e 4, and Chang'e 5 missions, and India's Chandrayaan-2 (though it lost contact with Earth shortly before landing so perhaps it doesn't count).
  • Venus: Venera program
  • Mars: Three countries have had successful landers: the United States, Russia (Soviet Union), ESA (several European countries), and China. India and the UAE both have successful orbiters. Several attempts have been made to land on the Martian moons, but have failed too.
  • Titan: The Huygens probe landed on Titan in 2005 and sent data for ~90 minutes.
  • 433 Eros: The NEAR mission successfully made the first small solar system body landing in 2001 (apparently this was not planned).
  • 25143 Itokawa: The JAXA Hayabusa probe was a mixed success but did land in 2005.
  • 67P/Churyumov–Gerasimenko: The ESA Rosetta spacecraft landed on this comet in 2014.
  • 101955 Bennu: OSIRIS-REx touched down in October 2020 to collect a sample for return to Earth.
  • 162173 Ryugu: The Hayabusa 2 landed in February 2019 and returned samples to Earth in December 2020.

There are several future planned missions that include landing elements, but the landers are often part of the descope list (i.e., the parts that are cut during the initial design and build phase of a mission).

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