Again about Doppler Shift
by Miles Mann

The following is a follow up to John's good article on Mir and the problems with Doppler shift. Many of us have transceivers that are "channelized" (tuned in discrete steps of several kilohertz each). This means you cannot make any fine tuning adjustments to your receiver or transmitter frequency.
Most Mobile/HT transceivers are limited to the smallest frequency change of 5 kHz or 2.5 kHz channel steps. Doppler shift will cause the Mir transmit frequency (145.985 MHz) to appear as if it is 3.5 kHz higher in frequency.
If you tune to 145.555 MHz, you may improve you reception (for a 5 kHz rig).
Consult your transceiver's operating manual for information on establishing "odd-splits" and program in the following consecutive frequencies into your transceiver's memories:

For transceivers with 5 kHz steps:

For transceivers with 2.5 kHz steps:

When Mir is approaching your QTH, use channel #1. Then when Mir is over head, use channel #2. When Mir passes your QTH use channel #3. For best results, use a satellite tracking program that displays Doppler shift in real-time. InstantTrack, available from AMSAT-NA, is one such program.
This information will assist you in determining when it is best to change memory channels.

As you may have noticed, I do not recommend adjusting you uplink frequency for the 5 kHz transceivers. This is because you may have better results if you leave your receiver tuned to 145.985 MHz. The Doppler shift is only at the +3.5 kHz setting for a short period of time at AOS prior to an overhead pass. Shortly after AOS, the downlink signal will drift lower, and in five minutes or less when the spacecraft is overhead, the Doppler shift will be zero for a brief period of time.
The downlink signal will then drift lower, down to -3.5 kHz over the next five minutes.

The receiver on Mir can work ground stations when the ground station is off frequency by as much as 3 kHz. This is because the receiver on Mir has a relatively wide passband. In some situations, a wide receiver is good, and we are lucky to have such a receiver on Mir. If the receiver on Mir had better selectivity, it would actually make it harder for ground stations with channelized transceivers to access Mir. The following are specifications for the transceiver on Mir along with a few others for comparison:

What makes the Mir transceiver work so well is the wide -6dB @ 15kHz receiver passband. This helps the receiver copy signals that are off frequency by as much as 3 kHz. The bad thing, however, is that Mir requires a wide, clear channel. The normal channel spacing required for 2-meter FM is 20 kHz, but because of the Doppler shift, the Mir station requires 25 kHz channel spacing.

Remember, only 1 station can connect to the R0MIR-1 PMS at a time. ALL others must wait.

Miles Mann, WF1F
Dr. Dave Larsen, N6CO