This excellent answer to How does satellites know it is in apogee or perigee states:
The NASA Tracking and Data Relay Satellite System (TDRSS) adds to this a constellation of satellites in such a high orbit that with just two satellites one can provide tracking data coverage for 85% to 100% of low earth orbits (Larson and Wertz), It is also much more accurate with accuracy of about 50m (3 sigma), and commonly used.
Larson and Wertz, Space Mission Analysis and Design, Third Edition (2006), §11.7.2 (page 501–507).
Until I read that I'd assumed that the job of the Tracking and Data Relay Satellite System was to relay a spacecraft's tracking data and other data.
How exactly does TDRSS generate tracking data with only two satellites? (GNSS usually requires four) Is it a one-way delay measurement a bit like GNSS, or two way using coherent transponders like the way deep space spacecraft are tracked using delay-doppler techniques? If one-way, which side transmits? Does it use pseudorandom Gold codes?