Tracking and Data Relay Satellite System (TDRSS) option on Celestrak's NORAD Two-Line Element Sets Current Data TLE page I've compiled current TLE inclinations for "on line" TDRS spacecraft:
COSPAR NORAD Launched Spacecraft current inc(degs) 1988-091B 19548 1988-09-29 TDRS 3 14.4405 1991-054B 21639 1991-08-02 TDRS 5 14.5306 1993-003B 22314 1993-01-13 TDRS 6 14.0861 1995-035B 23613 1995-07-13 TDRS 7 15.0545 2000-034A 26388 2000-06-30 TDRS 8 7.9573 2002-055A 27389 2002-03-08 TDRS 9 5.8274 2002-055A 27566 2002-12-05 TDRS 10 5.5187 2013-004A 39070 2013-01-31 TDRS 11 5.0141 2014-004A 39504 2014-01-24 TDRS 12 5.6613
While not included (yet) on that page, according to this answer TDRS-13 has entered into service, and can still be found in Celestrak:
2017-047A 42915 2017-08-18 TDRS 13 6.7494
While these satellites are in geosynchronous orbits, they would not be considered geostationary orbits; their large inclinations result in analemma-shaped ground tracks, and the antennas of TDRS ground-segment stations would need to trace their daily, nearly North-South movement in order to remain in contact.
This is probably not a big deal, considering the attention and resources already necessary to maintain this critical segment of so many ongoing missions.
But I am curious about the inclinations themselves. There is a cluster (TDRS-3 through TDRS-7) between 14 and 15 degrees, and another (TDRS-9 through TDRS-13) between 5 and 6 degrees. The newest member TDRS-13 is and has been close to 7 degrees for a while now, and TDRS-8 is near 8 degrees.
Question: Are these inclinations specified, station-kept and optimal in some way, or do they simply reflect non-station-kept "inclination creep" that is a natural phenomenon for circular orbits at this distance, or is there another way to uderstand this distribution in TDRS inclinations?
note: I understand that without station keeping, inclination will increase. But I'm looking for more of an answer than "It might be...". Considering that TDRS-13 started at almost 7 degrees suggests that it might be more than just an intrinsic lack of North-South station-keeping ability.
above: Screen shot of TDRS-13 analemma-shaped ground track from N2Y0.
below: Inclination and mean motion (rev/day) for TDRS satellites from the first TLE found in the first week of each calendar year. Dot for TDRS13