AO-40 Doppler and Middle Beacon observations
Created 22 June 2002
Here's the results of some AO-40 MB Doppler and middle beacon (MB) observations taken between MA6 and MA128 on 19/20 June 2002.
Excel spreadsheet (zipped, 2.1Mb)
Raw data in tab delimited text (zipped, 528Kb)
The equipment used includes a rubidium source accurate to about 0.001ppm. (1 part in 10^9 or 2.4Hz), although far more significant as a limiting factor to accuracy is the 10Hz frequency step in the AR-5000/AO40Rcv feedback loop.
AO40Rcv is connected to the AR-5000 using a serial port, and configured for the AR-5000 up/down commands. In addition, commands are included to send out the absolute VFO frequency back along the RS-232.
A special RS-232 cable was made up to allow writing on one COM port from AO40Rcv, and reading the frequency back on another COM port with some homebrew 'glueware'.
The glueware is a bit of Visual Basic which acts as both a DDE client to Nova and as a COM port reader. It processes the DDE strings and RS-232 strings in real time, writing the observed and actual frequencies out to a file together with the time.
From the graph below, the frequency at the satellite appears to be about 2401.328MHz at the current time, but this is for a 1000Hz centre beacon frequency on a USB receiver. Zero-beat would give a nominal frequency of 2401.329MHz. The figure published most often is 2401.323MHz.
There is some delay (or damping) in the feedback loop from AO40Rcv which makes the magnetorquing appear to produce a 'triangle' frequency change effect. In reality it is more of a 'square' as the magnetorquers are switched on and off. However, the magnitude of the frequency changes during magnetorquing is representative.
