Future multiple uplink per aperture access schemes

Present TT&C communication with distant probes is carried out with a single link dedicated to each probe. During the pass the ground antenna is exclusively assigned to this link. The current international exploration mission model is such that several spacecraft may be observable close to each other in the sky. When several spacecraft are within the antenna beamwidth, it is possible to use a ground station antenna to communicate simultaneously with them. The advantage of this concept is the availability of a larger number of ground station support hours for the missions without the need of additional infrastructure since one station is able to provide simultaneous support to more than one mission. The implementation of multiple spacecraft support is quite straightforward for the downlink but presents several problems for the uplink. This paper describes alternatives for the implementation of multiple uplink with special emphasis on the operational requirements for telecommand, telemetry and ranging/Doppler tracking of each spacecraft and on the independence of each link relative to the others.