Could anyone throw some light on what is the minimum number of satellites that I require to find the location of a RF emitter on earth from a LEO orbit satellite constellation having a single receiver antenna on board each of them? What are the techniques available other than TDoA and FDoA? Are triangulation or trilateration the only way to go about it? Kindly elucidate the basic known and unknown variables in these scenarios as well. Thanks in advance.
Only one satellite is required. Phased-array antennas are quite accurate, and the satellite can easily reference the location of your RF emitter in $\theta - \phi$ coordinates against the known location of any reference signal emitter. Since you know the satellite's altitude, that plus the relative az-az location suffices to find the RF emitter's location.
A single pass of a single satellite is sufficient.
If the transmitter is stationary and broadcasts for a few minutes (continuous or intermittent) the satellite can plot Doppler shift over time which is unique for every position on earth.
The Argos system works that way and is widely used for tracking applications and emergency locator beacons. For a more detailed description, see the Transit navigation system, which worked on the same principle but with the transmitter on the satellite and the receiver on the ground.