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Celestrak's NORAD Two-Line Element Sets Current Data TLE page has many pre-defined categories for quick access for groups of spacecraft that share similar characteristics.

I clicked on the Tracking and Data Relay Satellite System (TDRSS) option, and many spacecraft named "TDRS" are listed:

TDRS 3, TDRS 5, TDRS 6, TDRS 7, TDRS 8, TDRS 9, TDRS 10, TDRS 11, TDRS 12

Neither the names TDRS-M nor TDRS-13 appear yet. TDRS-13 has now "graduated" from TDRS school!

There are many more included as well:

AIM, AQUA, AURA, FGRST (GLAST), GPM-CORE, HST, ISS (ZARYA), JPSS-1, LANDSAT 7, MMS 1, 
MMS 2, MMS 3, MMS 4, SORCE, SWIFT, TERRA, THEMIS A, THEMIS D, THEMIS E, TIMED, WISE

These look like spacecraft that use the TDRS satellites, and I'm guessing that they make up the User Segment of the Tracking and Data Relay Satellite System (TDRS).

Question: Does the user segment behave as simply TDRS "customers", or do they contribute in some way to the function? For example, is TDRS traffic ever routed through a user segment member, or buffered or stored there for later transmission? Since one of the user segments ISS (ZARYA) is manned at all times, are there any special TDRS management functions performed there by the ISS crew?

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    $\begingroup$ I'm trying to research this question [i.e. find a reference in the open lit] but I want to make sure I understand what you are asking. Are you asking if user spacecraft ever function as part of the Space Network by channeling traffic to or from a TDRS to another user spacecraft? IE the normal path is Ground <-> TDRS <-> user, you are asking if a path of Ground <-> TDRS <-> user1 <-> user2 is ever used? $\endgroup$ – Organic Marble Mar 28 '18 at 13:30
  • $\begingroup$ @OrganicMarble I can't distinguish clearly enough between those two options. If I said "does a member of the user segment ever touch or handle or relay or store another user member's traffic?" would that be at all clearer? $\endgroup$ – uhoh Mar 28 '18 at 13:53
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    $\begingroup$ I think so, but I'm abandoning my effort because I think the question is really not answerable, at least without a huge research effort. I'm pretty sure the answer is no for NASA, but without checking every single spacecraft... And who knows what the DOD gets up to. $\endgroup$ – Organic Marble Mar 28 '18 at 13:54
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    $\begingroup$ I can tell you that THEMIS is strictly a TDRSS customer, and we are never used as a relay for other missions or customers. Onboard storage and transmitter power are highly constrained resources, and the flight software would have to be designed and tested from the get-go to implement relay capabilities. I can't speak for the other missions you listed, but I would be extremely surprised if these considerations didn't apply to them as well. $\endgroup$ – Jim Lewis Mar 28 '18 at 22:20
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    $\begingroup$ @JimLewis this is what I would expect, but it seems like a big effort to "prove a negative" for every customer. $\endgroup$ – Organic Marble Mar 28 '18 at 22:33

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