Old technology AO-40 Doppler Frequency Corrector

Created 12 September 2001

Updated 13 September 2001 with third downconverter corrector disk and experimental L band uplink and expanded documentation

Updated 29 April with a correction to the usage text (*)

Point the MA arrow and there's the MB ballpark frequency. Plus, given the MA and a downlink frequency you can find an uplink frequency, or vice versa.

OK, so it's not that accurate, but on mode US it's pretty good to within +/- 5kHz. Works fine between about MA10 and MA245.

I took the average MA's over five orbits of the at -35kHz to +35kHz in 5kHz steps for mode U/S and -45kHz to +45kHz for mode L/S. AO-40 appears to repeat its tracks every five orbits, so I deemed this sample enough for now.

Be aware that an uplink and downlink frequency pair won't be precisely the same on successive orbits for the same MA - that's why a simple device like this can only be accurate to a few kHz.

At very low and very high MA's (ie at perigee), it's not possible to accurately predict with such a simple tool because the orbital mechanics rough approximation fails miserably in this part of the orbit. Not a big problem: the satellite's not visible to many of us anyway during this time, and the transponder's often not operating either.

Many thanks to Aldo, IZ1ANT, who has provided a third disk. If you have a downconverter, you may well find that it's not precisely on frequency. You can compensate for this by gluing in place the third disk to the top of the downlink disk.

For the three disk solution, the largest disk is the true uplink frequency, the middle disk is the true downlink frequency and the smallest disk is the downlink alignment correction disk. Once you've set the top alignment disk for your downconverter, you don't need to set it again.

I also added an experimental mode L1/S2 Doppler calculator. It's seems far less accurate (maybe +/- 15kHz), but I may well have got the conversion factor slightly wrong. I don't know what the L2 RX conversion factor is at all yet - I've not tried it when it's been switched on.

How to use the three disk model:

o    Use a piece of wire or whatever through the middle of the disks with the Uplink disk on the bottom, the S2 downlink disk next and the S2 IF downlink disk on the top. Of course, you only need to do this once.

o    Glue the top IF disk to the middle S2 disk so that the IF frequency maps directly to the received frequency, correcting for any downconverter misalignments. You only need to do this once.

o    Now point the the MA pointer of the middle disk to the closest current AO-40 MA, marked on the bottom uplink disk.

o    You'll see almost opposite the MA pointer is the MB (middle beacon) marker on the bottom disk which indicates the possible range of frequencies on the middle disk that the MB may be found. Tune your radio to check you can hear it.

o    Now pick a clear downlink frequency with your radio and read off the corresponding uplink frequency on the outer* disk. Tune your uplink radio to this. [*Thanks Mike Fontenot for pointing out a previous error here]

o    To within a few kHz, you should be there. Readjust the middle and bottom disks to reflect the true uplink and downlink frequencies.

AO-40 U/S2 Doppler Corrector

AO-40 L1/S2 Doppler Corrector (experimental)

Mail Howard, G6LVB