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.

 

Mail Howard, G6LVB

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