Sapphire telemetry receiver during flight
Channel: Alexandru Csete
Date: 2013-07-31

Screen recording of the Sapphire telemetry monitor starting at T-10s and for approximately 2 minutes.

he monitor window shows a 4 MHz wide spectrum as received during the ascent and descent of the Sapphire rocket. The two peaks are the signals from the two transmitters, each transmitting on their own frequency. Only the highlighted signal is decoded; however, we can toggle between which transmitter is being monitored.

There is some very strong fading during the flight. In the beginning the fading is caused by the signal becoming weaker where after the operator increases the RF gain manually. Later during the flight we can see a lot of fading caused by rotation of the rocket (roll) and also vibrations probably caused by free fall.

Since the telemetry system uses frequency shift keying (FSK) we can tolerate fading as long as the signal level stays above the decoder threshold and the fading is significantly slower than the keying frequency (250 kHz). Therefore the receiver keeps decoding data despite the string fading during the flight.

TX0 or TX1: Telemetry from transmitter (fills out empty slots)
GNC: Telemetry decoded from guidance navigation & control system
AAU: Telemetry decoded from Autonomous Abort Unit

Source code and documentation for the Sapphire Telemetry System is being published here: https://github.com/csete/stlm