---------- From: "Jim Towler"
In general terms, this is largely true, and for a 2m uplink, little or no tuning is required on AO-51/Echo etc. However, a number of earlier emails spoke of the more critical need for AO-16 as it appears to have tight filters compared to a typical FM uplink. Careful control of Doppler for the uplink would allow a more full use of the entire filter bandwidth, rather than using FM-Narrow, and having weak audio.
Hmmmmm. I would like to claim that I am a victim of the news article about the test mode for AO-16 on the ARRL website, but that would be admitting that I am gullable enough to believe the FM uplink did not need Doppler shift compensation. That it works at all on a system that was intended to be digital is a blessing.
I still like the idea of attempting CTCSS....
I think it is a great idea. I can imagine a comparison of three modes using it. The first is to use CTCSS in the uplink to model what was previously tried with SSB and pilot carrier. That is of course on the assumption that the pilot carrier did not need to be zero beat with the SSB signal. If you can find out about their experiements you might find suggested frequency and power levels to use.
The second mode would be to use the Costas Loop to compensate for Doppler shift and synchronously detect the DSB signal. That would give you comparison of the effectiveness of the Doppler tracking and maybe compare the noise benefit of the synchronous detector.
The third mode would be Costas Loop without CTCSS to find out if it really helps, and how much. As I indicated previously, I am guessing that the system will acquire lock on voice power peaks, so it may not require much CTCSS power to maintain the system lock. It may well be that the power required to maintain lock depends on the rate of change in the Doppler shift. Said another way, it may require more power to hold the signal at the time of closest approach than when the Bird is near the horizon, comming or going.
Not related to AO-16, but something I intended to test on FO-29 at some point, was "two-station Doppler correction"....
It is an interesting set of control systems, one for transmit and an independent one for receive. The transmit system would seem to be open loop and based on the known position of the other station and path of the Bird. The receive system could be either open or closed loop, perhaps both with the ability for the operator to select the "better" channel. It might also be possible to feed the receive side closed loop parameters into the transmit system. I will have to think a little more, but there does not seem to be any way to close the transmit loop.
Best Wishes
James n5gui