So, why is it working for some here, for others not?
I donāt know why itās not working for you if you have nothing else connected to that pi.
But looking at the resulting preamble filter on your station, that pi4 seems to be not in the best condition anyhow.
Could be other CPU / memory intensive applications running.
No prob. Itās not that important, was just out of curiosity
It was a fresh install today (Ubuntu x64) with nothing else than Piaware and the FR24 feeder on it.
htop does not deliver anything suspicous beside the required processes.
The OS should not be the problem. It was also not working in a different attempt with Raspberry OS some days ago.
During the test i also switched -C to 50 to avoid high load.
From my experience, unless the airspy mini is the one and only active device connected to USB, MLAT sync @20 is not reliable (whoever you feed). No issue with WiFi (RPI4).
Anyway, I never seen real benefit using 20 over 12. Unless today and RC17!
Iāll wait for a time with higher traffic to make test on samples but it looks better to anything Iāve experienced previously. Many indicators are going in the right direction. To be confirmed, but worsts a test if you can afford it.
I donāt think most of us have power related issues. More likely bandwidth/IO/interrupts kinda of stuff. My Pi is powered by POE so I can watch the wattage usage fluctuate, and RPImonitor shows voltage can increase with more USB devices connected.
Normal mode for me now is Airspy with bias tee to antenna enabled, Orange FA pro stick with Uputronics 978 LNA powered by USB. Voltage stays at 0.8688V and wattage fluctuates from around 9.5 to 10.25 (FA orange stick is what seems to fluctuate).
A few months ago, for curiosity, I ran for a week with Airspy -b + Uputronics + USB powered bias tee + RTL-SDR LNA, along with Orange stick for UAT + Uputronics 978 powered by USB. 2 receivers and 3 LNAs. No power issues, wattage range only increased by around 0.5 watt. RPI voltage remained just under normal voltage spec which I think is 0.8700V default. And no under-voltage power/throttle flags on the Pi 4.
Just in case the new airspy_adsb at 12 MSPS could be challenging USB bandwidth for those of us also running UAT/978 dongles, I enabled -p after my last post to see if anything changes beyond a slight bump in CPU utilization. Little did. I wonāt be fooled by short term data though. 
Preamble went up to 27 after returning to 12 MHz and a reboot
Same experience here. Try stopping the dump978-fa process. Then it will run and sync 20 and 24 MSPS. Note that piaware is sneaky enough to restart dump-978 by itself if it has not heard from it after a few minutes.
I have resorted to putting UAT on a separate pi. Oh well. Merrily chugging at 24 MSPS now.
I bought one from airspy.us shortly after I posted my question. 
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Is it worth it though? In terms of stats benefit of 20/24 MSPS. I like seeing UAT with the rest on my map!
Yes. The PC has an FA orange stick for UAT, keyboard, mouse and the airspy.
So, with tar1090 running on the pi4, I can see both 1090 and UAT on 1 map. The settings for that are quite simple. Also, the graphs on the pi4 include both the 1090 and UAT. Easy enough
I felt (in the absence of hard data) that 24 MSPS gave me more positions. Granted, this is confounded with the increased airspy_adsb versioning. This is the Misc graph where the switch is visible last Thursday.
YMMV
Yes.
(20 characters)
That is what you would expect - 20 Msps uses more CPU than 12 Maps, so there is less CPU available for the preamble filter. With a fixed -e, CPU drops with the lower sample rate.
Re MLAT - is your mini directly connected to the Pi or through a USB cable. If the latter, the shorter and heavier gauge cable, the better. Long, thin wire cables also cause MLAt problems.
With changes, you might get minor advantages only, until you can connect the LNA near the antenna. Of course, huge miracles donāt happen even after that. 
You can try disconnecting the FA-stick and run the airspy with 20 MHz. If MLAT works then you know whatās causing it (one too many SDR-stick connected).
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Is it possible that timestamping of samples does not work properly with -p ?
MLAT hates if there is any deviationā¦
Me neither, but there are quite a few posts (on other forums) regarding running the Pi4 with an SSD connected to USB3 and problems related to supplying power enough via the USB ports to other peripherals connected to the other ports. Seem to be some kind of consensus that even though the Pi4 has a better USB-subsystem than the Pi3, itās still not following the standards very well.
I have the same problem on one of my Pi4:s, itās not headless and boots off an SSD and there both degraded performance on the SSD and I have to connect the other peripherals via a powered USB hub to make them work properly (keyboard, mouse, PSU control/measurement software, oscilloscope etc).
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Perhaps. Restarted with -p and got the following in the logs after 5 minutes. Iāll check the next time it checks in with FA.
[2021-09-19 22:22 CEST] 845404 msgs recv'd from readsb (312 in last 5m); 843790 msgs sent to FlightAware
[2021-09-19 22:27 CEST] mlat-client(18178): Server status: clock unstable
[2021-09-19 22:27 CEST] mlat-client(18178): Server: 0.1 kB/s from server 0.0kB/s TCP to server 0.9kB/s UDP to server
[2021-09-19 22:27 CEST] mlat-client(18178): Receiver status: connected
[2021-09-19 22:27 CEST] mlat-client(18178): Receiver: 164.9 msg/s received 59.9 msg/s processed (36%)
[2021-09-19 22:27 CEST] mlat-client(18178): Results: 18.2 positions/minute
Thanks for the effort.
So, we have hope without -p only. ā¦but -p is necessary to push the data through the usb cable.
I hadnāt even thought to check MLAT status.
I do have some instances of being unsynchronised since updating to RC17 at 14:06 today.
Sep 19 14:39:01 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 1 nearby receivers
Sep 19 14:54:01 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 15:09:02 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 15:24:02 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 15:39:03 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 1 nearby receivers
Sep 19 15:54:03 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 16:09:03 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 16:24:03 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 16:39:03 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 738 nearby receivers
Sep 19 16:54:04 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 756 nearby receivers
Sep 19 17:09:04 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 761 nearby receivers
Sep 19 17:24:04 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 1 nearby receivers
Sep 19 17:39:04 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 749 nearby receivers
Sep 19 17:54:05 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 18:09:05 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 18:24:05 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 18:39:05 pi4-tracker piaware[765]: mlat-client(5577): Server status: not synchronized with any nearby receivers
Sep 19 18:54:05 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 1 nearby receivers
Sep 19 19:09:06 pi4-tracker piaware[765]: mlat-client(5577): Server status: synchronized with 762 nearby receivers
No sign of that happening on RC16.
Pi4B, overclocked to 2 GHz.
Airspy mini
Powered using the official PoE hat.
GAIN= auto
OPTIONS= -v -f 1 -C 84 -e 2 -w 5 -t 300 -P 5
SAMPLE_RATE= 20