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I see here that the Exomars rover has only two UHF monopole antennas.

Two UHF monopole antennas are used to communicate with Mars orbiters, including the Trace Gas Orbiter.

Does it mean that it will always need an orbiter around to communicate with Earth? (I see that NASA Rover has a high-gain antenna to communicate with Earth, in addition to the UHF).

Being monopole antennas omnidirectional, does the rover need to orient these antennas to communicate with the orbiter or does it just need to have the orbiter above the horizon to communicate with it?

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  • $\begingroup$ Great question! Just a thought, you might want to ask which Mars orbiters can be used. $\endgroup$
    – uhoh
    Commented Aug 7, 2018 at 9:33
  • $\begingroup$ Thanks uhoh :) Good point, I actually have even more questions, i.e. 1) Why two antennas? Just for backup or other reasons? 2) Given that "omnidirectional" actually means it has two blind spots, are the antennas vertical (with blind spot on the zenith) or are they oriented otherwise given that they want to transmit "up"? I didn't want to clog the question too much so I left these out - hopefully the answer to the main question will solve them as well :) $\endgroup$
    – BlueCoder
    Commented Aug 8, 2018 at 8:55
  • $\begingroup$ I believe that fancy antennas that can radiate two polarizations at the same time can have almost no blind. See for example Why did Sputnik 1 have four antennas? and also What is this antenna called, and what does the polarization and radiation pattern look like?. However I don't know what the antennas for Marsy McMarsface will be like. ;-) $\endgroup$
    – uhoh
    Commented Aug 8, 2018 at 9:03
  • $\begingroup$ It seems a good idea :) In the following image they seem to have the same orientation, but maybe they can rotate? exploration.esa.int/science-e-media/img/06/… $\endgroup$
    – BlueCoder
    Commented Aug 8, 2018 at 9:26
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    $\begingroup$ Good point. I went ahead and tried to look for the "real thing". First video here is a recent protoype ("Structural Thermal Model"): bbc.co.uk/news/science-environment-43832868 So far they haven't changed it from the artist impression. $\endgroup$
    – BlueCoder
    Commented Aug 8, 2018 at 9:42

2 Answers 2

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Yes, it always needs an orbiter to communicate with Earth.

The ExoMars Trace Gas Orbiter, part of the 2016 ExoMars mission, will support communications. The Rover Operations Control Centre (ROCC) will be located in Turin, Italy. The ROCC will monitor and control the ExoMars rover operations. Commands to the Rover will be transmitted through the Orbiter and the ESA space communications network operated at ESA's European Space Operations Centre (ESOC).

The plan is to use the ExoMars Trace Gas Orbiter as the relay.

NASA's participation in the 2016 ExoMars Trace Gas Orbiter includes two "Electra" telecommunication radios. Used successfully on NASA's Mars Reconnaissance Orbiter, Electra acts as a communications relay and navigation aid for Mars spacecraft. Electra's UHF radios support navigation, command, and data-return needs. 9)

TGO's Electra radios use a design from NASA/JPL with special features for relaying data from a rover or stationary lander to an orbiter passing overhead. Relay of information from Mars-surface craft to Mars orbiters, then from Mars orbit to Earth, enables receiving much more data from the surface missions than would otherwise be possible.

As an example of Electra capabilities, during a relay session between an Electra on the surface and one on an orbiter, the radios can maximize data volume by actively adjusting the data rate to be slower when the orbiter is near the horizon from the surface robot's perspective, faster when it is overhead.

Because TGO's radios are based on a NASA design, I suspect MRO could be used as a comms relay if needed. TGO has already been used as a relay for the NASA rovers on Mars.

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  • $\begingroup$ Thanks for the answer! I haven't accepted it yet (only voted up) because I would still like to have answer to question 2 as well :) Given that Exomars will talk to TGO which has NASA "Electra" radios, maybe sources detailing how NASA rovers use their UHF antennas might be good as well (if Exomars sources are not available). $\endgroup$
    – BlueCoder
    Commented Aug 9, 2018 at 12:22
  • $\begingroup$ It seems curiosity also has two UHF radios: en.wikipedia.org/wiki/Curiosity_(rover) The wiki article also leads to this, which might have further relevant info.. descanso.jpl.nasa.gov/DPSummary/Descanso14_MSL_Telecom.pdf $\endgroup$
    – BlueCoder
    Commented Aug 9, 2018 at 12:31
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The answer by @Hobbes covers the first question well - there always needs to be an orbiter to act as a relay. (If you are only working with solar power, direct-to-earth comms is a bit of a stretch.)

All of the NASA and ESA orbiters use the Proximity-1 protocol, so there is a high degree of interoperability between Rovers and Orbiters.

You can see in this photo that they aren't pure omni-directional, but there isn't any capability to orient the antennas. As the question says, the orbiter just needs to be a few degrees above the horizon for comms to work.

There are two primarily for redundancy/back-up.

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