I know that Exo Mars TGO is intended to be a relay for Exo Mars Schiaparelli and Exo Mars Rover.
However the orbiter could also serve as a relay for other missions.
Is there currently any plan for doing so ?
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Yes, Electra radio package is provided by NASA/JPL, and NASA fully expects to utilize it's capabilities for it's own future missions.
Plans call for the TGO to use its Electra radios for communication with ESA's 2018 ExoMars Rover and Russia's Lander as well as with NASA's 2016 Mars lander and 2020 Mars rover.
2016 Mars lander is the Phoenix-derived InSight ( now delayed to 2018 ), and of course 2020 MSL/Curiosity heritage rover.
Maybe it's important to mention that not all Mars orbiters have equal contribution opportunities as relays, due to their chosen science orbits. So TGO will add valuable capacity thanks to it's favorable orbit.
TGO's Electra radios use a design from 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.
NASA's Curiosity Mars rover and Mars Reconnaissance Orbiter (MRO) already use Electra technology for relay of data. NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, in orbit since 2014, also carries an Electra radio.
Due to improvements in the newest Electra radios and reduced interference levels compared with MRO, TGO's relay radios are expected to have a relay signal about twice as strong as MRO's. Compared to MAVEN's highly elongated orbit, TGO has a planned orbit similar to MRO's relay-favorable orbit at about 250 miles (400 kilometers) in altitude and nearly circular in shape.