Radio Tracking of the ISS - 922 MHzWhile en-route to the ISS, the Soyuz piloted spacecraft and the Progress cargo transports transmit routinely at 922 MHz. The signal manifests itself as a tone that can be heard through a receiver set to the "CW" reception mode. The Doppler effect is very noticeable at this frequency and the signal drops by something like 40 kHz in the space of five minutes or so. With a conventional receiver, rapid re-tuning is needed to keep it in earshot.
Here is the Doppler curve for Progress M-62 on rev 31, 2007 December 25 as viewed from Lincoln in the UK. It exhibits a classic low-orbit "S" shape and the time of closest approach to the receiving station comes in the middle of the near-straight section, just after 11:26 UTC. Data was collected as an output stream from a Software Derived Receiver. Measurements from the Doppler CurveUsing the data that produced the curve it is possible to determine the precise time of closest approach, the true transmission frequency, and the range at closest approach.
This plot shows the rate of change in frequency as time passed. The missing segments are the result of variations in signal strength. There is some fluctuation visible near the lowest point that is presumably due to instability in the transmitter. The time when the curve changes from a negative to a positive slope is Closest Approach, which occurred at 11:26:05 UTC. At that point, the rate of change in frequency due to Doppler is -350 +/-3 Hz/s. FrequencyRelating the closest Approach time to the original Doppler curve gives the precise transmission frequency at 922.765 MHz. In the past, Soviet authorities gave the frequency as 922.763 MHz and it would seem that it varies slightly from craft to craft. In the plot below that was gathered real-time, the mid-point is truly at 922.763 MHz. In this case, the data was collected as an image using the output from a Sofware Derived Receiver which is capable of monitoring a wide frequency range at any given moment.
DistanceAnother parameter that can be calculated is the range at the time of Closest Approach. The orbit announced by Russia that equates to the time of the observation is 243 x 266 km. The average along-track orbital velocity is 7.75 km/s but it varies between 7.76 and 7.74 km/s - at perigee and apogee respectively. The range at Closest Approach computes as 528 km +/-6 km. The error margin allows for not knowing exactly where Progress M-62 was situated in repect to perigee, and the uncertainty in the rate of change in the frequency. | |||||||||
