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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?

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    $\begingroup$ It's doppler, but I'll have to hunt down a reference $\endgroup$
    – Organic Marble
    Jul 9, 2020 at 0:57
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    $\begingroup$ In principle, if you can determine both distance and direction to a target of known position, you only need one; as I understand it TDRSS needs two in case one is blocked by the Earth. In GNSS we derive distance from time delay, but have no measurement of the direction. They could also make use of the fact that the spacecraft is moving according to a physically possible orbit, which adds a further constraint. I don't know if either of that is what they actually do (hence comment not answer), but I think they could in principle. $\endgroup$
    – gerrit
    Jul 9, 2020 at 7:01
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    $\begingroup$ Actual name is Tracking and Data Relay Satellite System (TDRSS) $\endgroup$
    – David Ratti
    Jul 9, 2020 at 7:27
  • $\begingroup$ @DavidRatti yikes! I don't know how that happened, I'll fix it now, thanks! $\endgroup$
    – uhoh
    Jul 9, 2020 at 8:07
  • $\begingroup$ There are some major differences to normal GNSS: First, both TDRSS and the satellite are active and can send data. And, both move on predictable trajectories. A single TDRSS would suffice, given enough time. GNSS needs four satellites because the receiver is passive and has no idea about time keeping and therefore needs an instantaneous measurement. $\endgroup$
    – asdfex
    Jul 9, 2020 at 9:47

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This is probably a partial answer because I don't understand it that well myself, but the source document is so incredibly detailed that you can probably build a TDRS yourself from it, so I'm going to post an answer for those who have an interest.

From TRACKING AND DATA RELAY SATELLITE SYSTEM (TDRSS) RANGE AND DOPPLER TRACKING SYSTEM OBSERVATION MEASUREMENT AND MODELING

Closed-loop tracking consists of uplink signal generation, forward and return frequency translation via the TDRS, signal processing by the user transponder, and downlink signal processing at WSGT. Two-way tracking involves the same TDRS on both forward and return links. Hybrid tracking involves two TDRS1s, one on the forward link and one on the return link. Hybrid tracking is available to S-band users only. Closed-loop tracking provides two-way or hybrid range and/ or Doppler measurements. A TDRS closed-loop tracking configuration and a simplified diagram of the signal generation and processing are shown in figures 3-1 and 3-2, respectively.

Services ... can provide range and/or Doppler tracking data for each user spacecraft supported. The range measurement is obtained from the time delay between the transmitted and received Pseudonoise (PN) code. The Doppler measurement is obtained from the frequency shift of the reconstructed carrier...

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  • $\begingroup$ I'll pass on the DIY option, but this is great, thanks! It sounds like there are several different options available. $\endgroup$
    – uhoh
    Jul 9, 2020 at 14:23
  • $\begingroup$ @uhoh you are indeed a wise counselor to new users. $\endgroup$
    – Organic Marble
    Oct 7, 2020 at 18:13
  • $\begingroup$ the staple was getting irritating $\endgroup$
    – uhoh
    Jan 26, 2021 at 4:05

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