[amsat-bb] Observations on AO-91 AFC

Matthew Stevens matthew at mrstevens.net
Sun Nov 26 06:49:51 UTC 2017

Update from the 0617z pass just now. Using a D72 and arrow, I tried transmitting adjusted for doppler shift, then changing from that doppler adjusted frequency first to 15khz low, then 10khz, 5, and finally back to the correct frequency. All on a much quieter pass than earlier, only 6 or 7 stations on at this time of the morning.

At -15khz, I was still easily able to access the repeater. However my audio was extremely muffled sounding, and difficult to understand. Another station on the pass reported it sounding “extremely garbled.” I shifted to -10khz, and got the report back that my audio was clear and understandable. To my ears (full duplex), it still sounded a bit muffled but much better than before. Shifting back to -5khz, and then the approximate correct doppler adjusted uplink, I couldn’t really tell the difference between -5 and 0.

So, conclusion. The AFC seems to be working as advertised! With a 5w HT and arrow I had no problem getting in on a slow pass even 15khz off freq, albeit with terrible audio. However, while 10khz off was acceptable, +/-5khz was what I would consider “good.”

My advice is to use the AFC as an aid, not a crutch. Don’t count on it to correct a large tuning error very well, especially on a busy pass with more than one user transmitting simultaneously.

Keep tuning for doppler on your uplink like you have been for AO-85, but don’t worry too much about being exact with it on AO-91. The AFC will “fix” it if you’re off a few kHz.

As a side note, once again tonight I was hugely impressed with the audio quality, ease accessing the repeater, and horizon to horizon performance of the new sat! Amsat really scored with this one.

- Matthew nj4y

Sent from my iPhone

> On Nov 25, 2017, at 22:52, Scott <ka9p at aol.com> wrote:
> Matt’s observations beg the question of what the design basis of the AFC system might be- single signal or a number of signals of varying amplitude and frequency.
> In the end it may not matter much, but if understanding better how it works leads to more enlightened use of the bird than a few of the (obviously there’s still learning going on) passes this weekend, it could be very helpful.
> Thanks for any info or guidance you engineering guys may have, and again, congrats on a super job.
> 73 Scott ka9p
> Make something good happen!
>> On Nov 25, 2017, at 8:09 PM, Matthew Stevens <matthew at mrstevens.net> wrote:
>> 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
>> http://www.amsat.org/pipermail/amsat-bb/2017-November/065569.html),
>> 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
>> here!
>> 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
>> value.
>> Here is my recording:
>> https://drive.google.com/open?id=1c8XYCoxFCKCyR4dtfwJCyM9tjfo1mLKd
>> 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 :-)
>> 73,
>> Matthew nj4y
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