GPS satellite ephemeris measurement

How are ephemeris values for GPS satellites obtained? I have failed to find any information about how this essential data is obtained - ie whether by optical or radar measurements or somehow by using only the satellite signals received by the monitoring stations. In the latter case I'd be puzzled because although the stations are at accurately known positions they don't have access to GPS master time and it seems to me you need both to determine accurately the satellite positions and clock corrections. I'd be very thankful to know of any easily accessible source of information about this aspect of the GPS.