[amsat-bb] Observations on AO-91 AFC

Matthew Stevens matthew at mrstevens.net
Sun Nov 26 02:09:52 UTC 2017

I’ve done a bit of study on the Fox series AFC. I never really knew
much about it up until this point, but with all the discussion on the
amsat-bb about AO-91’s AFC function, and the alleged lack of uplink
Doppler tuning needed – I figured I should educate myself a bit more.

I also wanted to test it out some. My observations (which I’ve
outlined in another BB post
making hundreds of QSOs on AO-85 using various antennas, radios, and
power settings have been that there is definite need to tune the
uplink frequency for Doppler shift. My initial experiences working
passes on AO-91 from a few minutes after commissioning, using an HT
and whip, then an HT and Elk, then and HT and arrow, did nothing to
change my mind about tuning for the AO-91 uplink.

However, even though I already made a bunch of QSOs on AO91, I didn’t
really have enough data to make any kind of real conclusion about the
AFC on AO-91. So, I figured I’d try some experiments and post them


Today was the first pass I had worked on AO-91 with anything other
than a 0.05-5w Kenwood D72 HT for a radio. I broke out my Icom 821H
for a 90 degree lunchtime pass, turned up the power to the full ~35w,
and used my handheld arrow II antenna. Having already heard the
absolute zoo that has been daytime AO-91 passes, I was curious first
to hear how it went using more power (as far as being able to get in
over the dead carriers and multiple hetrodyning uplinks), and second,
how the AFC functioned with a lot of high-power users.

What I found: The AFC *does* seem to work! However, there are some
usage caveats that I have to note. When there is a single signal
coming into the bird, it seems to compensate fine for being off
frequency (within reason). I didn’t try too wide of a tuning range,
since compared to some stations I heard today I didn’t have enough
power to be way off (more on that in a minute). But I did try +/-
about 5khz, and it seemed to compensate just fine if I was the only
signal coming in.

That’s the rub however. There was virtually no time today where I was
the only signal coming into the bird! And when there was a bunch of
different signals, basically, the strongest uplink was the one that
determined the AFC tuning. You can hear that happening in the
recording which I’ll link to below. There was more than one instance
where I began to transmit over a carrier or other signal that I was
hearing (being careful not to transmit over any actual audible callers
or QSOs in progress). You can hear my audio go from poor to crystal
clear as the AFC (apparently) adjusted to my carrier – and I did not
adjust my transmit frequency at the time. I assume that my signal was
stronger than the others, and the AFC “picked” mine.

You can also hear a signal from XE3ARV, who had by far the strongest
signal I heard during the pass. Because his uplink signal was so much
stronger than anyone or anything else, it sounded like the AFC
adjusted to whatever his uplink frequency was. If I, or anyone else
was transmitting at the time, you can hear it IMMEDIATELY go from the
other clear (or scratchy) signal (depending on what state the AFC
tuning was in at the time), to a crystal clear downlink from XE3ARV.

Another observation of note was the downlink signal from 5K0T. At the
beginning of the pass you can hear a very clear signal from their
obviously strong uplink. As the pass progressed, the audio quality on
their downlink deteriorated a lot. The only reason I can see for this
is that they may have not been adjusting for Doppler during the pass,
and were possibly 10khz or more from the correct uplink frequency
(perhaps transmitting at 435.240 the entire pass?). I can’t confirm
this as I wasn’t there to see what was happening on their end, but
that is one possible explanation to why their signal sounded like it
did at the end of the pass. This hypothesis also is supported by
having heard another station on a much quieter, early morning pass the
other day who I know was transmitting 30khz off frequency. His audio
sounded very similar to 5K0T at the time.

Yet another observation is that when there are a lot of users on the
pass, it seems to be beneficial to adjust for Doppler. There were
several times during today’s pass where I did tune my radio to the
correct, Doppler adjusted uplink frequency. If there was a high level
of QRM, tuning for Doppler seemed to help my signal be received by the
satellite, as opposed to tuning +/- 5khz from the Doppler adjusted

Here is my recording:

TL;DR observations:

1. AFC does work within some limitations.

2. With multiple strong signals into the bird, the strongest “wins” as
far as determining the AFC adjustment. This makes other signals tuned
to other frequencies sound “staticky”

3. Transmitting very far off frequency makes your downlink audio sound bad.

4. Adjusting for Doppler seems to help your signal capture the sats
receiver better than other users who are operating at a similar power
level to you.

Conclusion so far: In practice, AFC is good for correcting minor
Doppler tuning errors. However, unless there is only one user
transmitting into the sat at a point in time, it does not eliminate
the need to tune your uplink for Doppler.

I would be curious to hear other observations and interpretations from
AO-91 passes. I think it’s helpful to support observations with
recordings, so others can listen and draw their own conclusions about
your interpretation of what has occurred.

Also, these are just my initial impressions from operating passes…and
these are subject to change over time :-)


Matthew nj4y

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