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The JPL News article How NASA Will Know When InSight Touches Down is interesting and worth a read. It says that during the atmospheric entry, descent and landing, the Mars InSight probe will be transmitting simple radio tones to be received on Earth. The presence, strength, and mostly the Doppler shift of these transmissions can be used to monitor aspects of the trajectory and provide information on any issues if they arise.

Question: Why will it be Green Bank and Max Planck listening, and not NASA's Deep Space Network?

As a reference point I'll note that radio telescopes have been used to "listen in" on Mars atmospheric entries before: Was the time of Schiaparelli's landing chosen specifically so the Giant Meter Wave Radiotelescope could listen?

From the JPL News article How NASA Will Know When InSight Touches Down

Radio Telescopes

As the InSight lander descends into Mars' atmosphere, it will broadcast simple radio signals called "tones" back to Earth. Engineers will be tuning in from two locations: the National Science Foundation's Green Bank Observatory in Green Bank, West Virginia and the Max Planck Institute for Radio Astronomy's facility at Effelsberg, Germany. Their results will be relayed to Mission Control at JPL.

These tones don't reveal much information, but radio engineers can interpret them to track key events during InSight's entry, descent and landing (EDL). For example, when InSight deploys its parachute, a shift in velocity changes the frequency of the signal. This is caused by what's called the Doppler effect, which is the same thing that occurs when you hear a siren change in pitch as an ambulance goes by. Looking for signals like these will allow the team to know how InSight's EDL is progressing.

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    $\begingroup$ My guess (without doing any research) is that it's because the signal will be very weak. It's my understanding that landers usually communicate with an orbiter (MRO or Maven typically), which relays the signal to Earth. The orbiters have more powerful transmitters designed to work across interplanetary space. Maybe when InSight is landing, none of these orbiters are in an optimal position for relaying values. For whatever reason, they want to be listening to InSight itself, which likely has a much weaker transmitter. So they want to use a really big, sensitive telescope to be able to hear it. $\endgroup$
    – Phiteros
    Commented Nov 17, 2018 at 4:16
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    $\begingroup$ @Phiteros DSN picks up not only a tone, but encoded data from the Voyagers, who's signals are pretty darn weak, circa -180 dBW (-150 dBm). You might be right, and it might be related to the nature of the antenna that is exposed during atmospheric entry being small and having very low gain. A simple link budget calculation might be helpful here. $\endgroup$
    – uhoh
    Commented Nov 17, 2018 at 4:21
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    $\begingroup$ The Voyager satellites have the advantage of a much bigger transmitting dish though. InSight's transmitters are really small by comparison. Also, I found this: "As with previous Mars landers and rovers, the InSight mission relies on Mars-orbiting spacecraft to relay data from the spacecraft to the antennas of the Deep Space Network." (down at the very bottom) $\endgroup$
    – Phiteros
    Commented Nov 17, 2018 at 4:44
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    $\begingroup$ "The first time, it will communicate with a tone beacon that the radio telescopes will try to detect. The second time, it will send a "beep" from its more powerful X-band antenna, which should now be pointed at Earth. This beep includes slightly more information and is only heard if the spacecraft is in a healthy, functioning state. If NASA's Deep Space Network picks up this beep, it's a good sign that InSight survived landing." Another thing I hadn't thought of is that during EDL, InSight won't have its antennas deployed, thus making the signal even weaker. $\endgroup$
    – Phiteros
    Commented Nov 17, 2018 at 4:50
  • $\begingroup$ @Phiteros yep, those 48 dB come in handy. ;-) $\endgroup$
    – uhoh
    Commented Nov 17, 2018 at 4:50

2 Answers 2

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InSight used an UHF transmitter to transmit a tone beacon during EDL:

At the top of each of InSight’s legs is a trigger sensor; when the surface pushes up the leg and hits the trigger, it shuts off the lander’s retrorockets. It also sends out two signals that touchdown has been achieved: a “tone beacon” through its UHF antenna and a “beep” through its X-band antenna. This X-band “beep” is expected to turn on about seven minutes after landing, and will be a clear indicator that InSight is functional on the surface.

On Earth, two radio telescopes will be listening for the tone beacon, which is a very basic indicator of InSight’s status: They may be able to confirm that InSight is transmitting during descent and after landing.

The DSN antennas don't have UHF receivers:

Since NASA's Deep Space Network does not operate in the UHF band, large radio telescopes around the world are utilized. The Australian CSIRO Parkes Radio Telescope supported the Curiosity UHF signal reception and DSN receivers, tools, and expertise were used in the process. In preparation for the InSight mission's landing on Mars in 2016, preparations are underway to support the UHF communications.

Normally, the UHF transmitter is used to transmit to one of the relay orbiters.

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DSN is 100% capable. The reason Green Bank is being used is a test of the Cube Sats Marco A and B. They want to relay the info through the cubesats to see how they perform in a deepspace communication function.

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    $\begingroup$ Welcome to Space! Remember it's always useful to other readers to include links to primary sources or further reading, otherwise it is difficult to confirm the veracity of your answer $\endgroup$
    – Jack
    Commented Nov 26, 2018 at 18:57
  • $\begingroup$ Irrespective if the claim made is true or false, this is not an answer because it gives no information on what cubesats have to do with why Green Bank is being used rather than one of the DSN receiving sites. $\endgroup$
    – Chris Stratton
    Commented Nov 26, 2018 at 22:11
  • $\begingroup$ Chris Stratton, if one read the original question one can assume (my mistake) that I was on a sight with people who are relatively knowledgeable in what is going on in our Space Program. The use of certain verbage indicates such, however maybe it was copied. $\endgroup$
    – R. Garza
    Commented Nov 27, 2018 at 0:11
  • $\begingroup$ @R.Garza I don't understand your comment exactly, but this looks like a better answer. $\endgroup$
    – uhoh
    Commented Nov 28, 2018 at 8:43

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