How is accurate time of round-travel determined from downlinked ranging signal?
AFAIK Ranging signal, used to determine distance of a spacecraft is usually a reference wave modulated and uplinked, passed through PLL (Phase Locked Loop) thus demodulated and then again modulated to a downlink signal which is demodulated by PLL at receiving ground station.
What are the steps that follow in calculating the distance of the spacecraft? How exactly is time of round trip calculated accurately ? How is time tagged to the wave downlinked?
A brief reference to Radiometric Tracking Techniques for Deep Space Navigation - C Thornton, J Border (Wiley, 2003) helped me to get this:
A Phase Locked Loop at receiving station produces a reference signal coherent with received signal. This reference signal is used by the ranging assembly to demodulate the downlink signal. The received range code is compared against a model of the transmitted range code to determine the round trip transit time. range measurements are quantized in steps referred to as range units(RU). The size of an RU depends on the frequency of the highest component of code, and is currently about 28cm. Doppler data are obtained by differencing the received reference signal with the station frequency reference.
Maybe I am connecting the dots wrong and completely unable to understand the process.