[amsat-bb] Leveraging Satnogs recordings ( was: request detailed system diagram for remoting a satellite station)

[Leffke, Zachary]

Sorry, another TL;DR...but hey its Friday night, what else have I got to do.....The one sentence version of the below is 'it would be cool if the SatNOGS network could record raw IQ, and then that could all be brought together at a central processing location to do some really cool stuff.'.....here comes the longer version........

I'm also generally 'pro-SatNOGS' (they started right about when I published my Master's thesis about GS networks!  I wish they had been there sooner or I had known about them earlier....would have been a great addition to the work).  I was furiously writing to finish my thesis while they were out winning the Hack-a-day prize.  I'll admit I was jealous at first......but I quickly got over my "hate 'em cause I ain't 'em" problem because what they are up to is just too cool (and every time I talk with someone from their team face to face, they have all been super nice enthusiastic folks!).  I'll freely admit I haven't had time to participate in the network myself or with VTGS systems, though that is mainly due to us not being 'ready' in terms of things like system automation (just not enough hours in a given day....this whole sleep thing is a real downer).  That said, the VT Amateur Radio club, K4KDJ, a separate entity from our ground station (though with overlapping people, I used to be an officer in K4KDJ when I was a student) is considering setting a node up.

Here's one thought (laid out in a list) for the SatNOGS folks (This is a total 'feature request' of which I'm sure they get a lot......this is not a poke against them, more 'hey here's a fun idea' type thing.  If I had more time I'd actually try to get more involved on this front).  What follows is a total 'thinking out loud', not fully thought through, brain dump of some ideas I've been kicking around for a couple years now (I guess since at least 2013/2014.....but the existence of the SatNOGS network makes it seem more feasible now), so feel free to skip.  I know others have had similar thoughts, so I also no way claim this as solely my own idea.....(AMSAT in general and Douglas, KA2UPW and Joe, KM1P specifically were in the acknowledgments section of the thesis specifically because of some conversations in this arena and for their super positive encouragement to a newby, Bob McGwier, N4HY was my committee chair....).  I'm sure there are others I've talked about this with (maybe also Chris Thompson), these ideas have been 'marinating' over the last few years and I can't remember every conversation I've had........

1.  One thing I think would be REALLY fun to explore is raw IQ captures shared over their network.  The recorded audio is great, and I'm 100% behind what Douglas mentioned.  However, if the recording point could be moved 'upstream' a bit to the raw IQ samples, some really interesting applications could be explored.  Cross correlating the IQ streams to determine things like time difference of arrival and frequency difference of arrival can lead to things like orbit determination...something that might be of interest for folks burning Ion thrusters (or solar sails) and don't want to wait for TLE updates.....I'm thinking for future HEO efforts on this front, but the general theory/application could be applied across the various orbital regimes from LEO out to GEO and beyond.  The familiar 'Fox Tail' on the Fox bird might make for a good 'target' for this type of thing.

2. One requirement for this to work properly is good timing references at the ground stations though (RTL SDR and FCDPP oscillators are prob not good enough), GPSDOs and other oscillators would be desirable (and are not hard to come by these days...QEX has some great articles, ebay has surplus, etc.).  If reference signals can be sent over a transponder (PN sequences to make the correlations that much simpler to identify is what I'm thinking here) with a known group delay, with precise freq/time control, then there is potential to explore applications and resolving timing ambiguities between station pairs, even if they don't have 'great' oscillators (room to experiment!).  Even with good timing references, you then have to make sure that's properly integrated with the SDR software actually executing the recordings (see #5 below for potential solution to part of that problem).  I'm sure Tom Clark, K3IO with his long history of VLBI work might have some advice on this front (along with others).  For those involved with ADSB collection over the Flightaware network (similar to SatNOGS....but different and for collecting transmissions from general and commercial aviation aircraft), they do something similar as part of their multilateration processing (using the 'known location' message bursts to resolve timing residuals between station pairs, and then using the residuals to correct the timing offsets when geolocating the messages/emitters that don't have position info in them....they call it 'MLAT').  Big difference is they don't send IQ, just 'precise' timestamps of the received messages......but I bring it up because all their Python code is open source and on github for the multilateration process....and might be a useful starting point.

3.  Raw IQ recordings come with an obvious tough requirement....storage capacity and network bandwidth.  IQ sample captures will be MUCH larger than the audio recording of the same pass.  Perhaps some type of prototype system could be set up though to 'get started' that doesn't break the bank where certain systems (maybe volunteer stations that have the oscillator/timing features mentioned) are tasked for specific experiments on this front (not normal ops that might generate way too much data).

4.  Then there is the central storage/computing resources needed to run the correlations.  With modern compute resources (including cloud services), the cost of that effort doesn't necessarily have to break the bank (especially if a willing university....with big honkin GPUs in server stacks that are currently underutilized, with a research faculty member interested in GS networks and novel applications, who is also a ham, and a lifetime member of AMSAT......... are willing to free up some system time ;-) hi hi, at least maybe for some prototyping on this front).

5. then there is the appropriate tagging of metadata necessary to take full advantage of the timing information.  For this I would recommend the Signal Metadata Format (aka SigMF) (https://github.com/gnuradio/SigMF).  While it grew out of GNU Radio (and there are OOTMs for GNU Radio support) it is not specifically tied to the GNU Radio project and can be used for anything (literally any time series of data, even for stuff that is 'not radio' if desired).  I use it extensively for signal collection work and keeping track of the myriad bits of info that needs to be tracked along with the raw IQ (the metadata is in JSON format, super easy to use).  Their 'extensions' framework is ingenious, and allows folks to 'extend' the specification for custom metadata formats not already covered in the spec.

6. Finally, if all of the above can be accomplished, then there is another application that could be explored beyond the satellite tracking, signal combining.  Imagine if there were packets transmitted that no single ground station could decode due to lack of sufficient SNR.....but then all the IQ streams are brought together, the correlations are performed and the streams are time aligned, and summed (perhaps with weighting)........the random nature of the noise drives its average lower, while the not so random nature of the signal drives its average higher (massive oversimplification of the idea).....leading to higher SNR....leading to potentially high enough SNR to demodulate the otherwise 'lost' packets.  There are lots of techniques out there for this sort of thing in the DSP world, right now I'm thinking 'maximal ratio signal combining' that applies weights to the summing process based on the measured SNR at each station.

7. Finally, Finally.  Imagine the ground stations involved were using omni antennas.  Also imagine multiple birds were overhead and in the recording bandwidth.  Theres nothing stopping you from going back in time, doing a bit of preprocessing (maybe coarse tuning for the target bird using TLEs, filtering for the specific signal of interest, etc.) and then re-executing the above process.

All of the above is pretty complicated to do, and I didn't cover every little technical detail or 'gotcha' type problem......but I think it could be a lot of fun to explore as a team.....if there is enough interest, perhaps some collaboration on this front and ideas for proposals could be put together to explore more.....I'm also betting the 'super-Elmers' out there (like the Bobs, Toms, and Phils) could figure out the math for this sort of thing in their sleep.

If you're reading this line....then THANKS! For sticking with the thinking out loud brain dump on this topic......as always, feedback is welcome (on or off list).

-Zach, KJ4QLP

-- 
Research Associate
Aerospace & Ocean Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

-----Original Message-----
From: AMSAT-BB <amsat-bb-bounces at amsat.org> On Behalf Of Douglas Quagliana via AMSAT-BB
Sent: Friday, July 17, 2020 8:19 PM
To: peter at 2m0sql.com; Peter Goodhall (2M0SQL) <peter at magicbug.co.uk>
Cc: AMSAT BB <amsat-bb at amsat.org>
Subject: [amsat-bb] Leveraging Satnogs recordings ( was: request detailed system diagram for remoting a satellite station)

On Fri, Jul 17, 2020 at 4:49 PM Peter Goodhall (2M0SQL) via AMSAT-BB <
amsat-bb at amsat.org> wrote:

> Sadly Satnogs isn't suitable for the job being it's for telemetry only,
> also I don't believe it forwards telemetry to AMSAT-NA or AMSAT-UK so radio
> amateurs would be better off with a different solution.
>

Friends,

Satnogs is an incredible and incredibly underutilized resource for AMSAT.
It doesn't automatically forward telemetry, but you can go to the
appropriate Satnogs site, filter on the satellite you want and download the
audio recording of the pass (or passes) that you want. Then
lather-rinse-repeat for as many passes as you want. I would never have been
able to create as many recordings myself at my home QTH as I can get by
downloading them from Satnogs.  You can get recordings from anywhere on the
planet where there was a ground station that made a recording. (If you're
fully onboard you could even schedule somebody else to make a recording for
you on a particular date/time in the future. Think about that.)

A while back I downloaded several months worth of old Falconsat-3 audio
recordings so that I could run the recordings through my own 9600 DSP
demodulator and see if I could recover any data from the recordings, and
improve the demodulator as much as possible. I wrote some scripts to
convert and filter each ogg recording and then run them all through my
demodulator.  This whole scheme actually worked too well, as I now have
more than enough recordings to keep me busy for a long long time because it
takes my old computer DAYS to process through ALL of those recordings
[footnote 1] and to decide if the latest tweak was an improvement or not.
(Think about your last visit to the eye doctor where you watch the eye
chart through the phoropter [footnote 2] while the eye doctor makes
adjustments to the lenses and asks you if this one is now "Better? Or
worse?" -- well, this is almost the same thing but it takes three days to
find out the answer for each change. But, I digress...)

So far, of the recordings that I downloaded, the most extreme pass is

https://network.satnogs.org/observations/180707/

the audio recording is eighteen megabytes and is online at

https://ia902808.us.archive.org/7/items/satnogs-observation-180707/satnogs_180707_2018-07-12T00-08-11.ogg

The satnogs webpage for this pass shows 644 valid 9600 baud AX.25 packets
this recording so you know it is a good pass with plenty of frames in it.
By throwing everything I can think of at this recording, I can now get over
1000 good frames out of it. Oh, and, hey, if anybody else tries this
recording against their own demodulator, then whatever results you get
please send them to me so I can look for any valid frames I am missing.
And the advantage of the satnogs recordings is that you can keep tweaking
your software iteratively by running the recordings against the "new"
demodulator again, and again, and again, and compare the results.  You
could never achieve this or iterate this many times with live satellite
passes.

73,
Douglas KA2UPW/5
[1] If the whole process doesn't get interrupted... Actually, this says
more about my old slow computer than it says about the speed of the
demodulator.
[2] If you get your eyes checked then you know this device. You just might
not know the name for it.  Consider "phoropter" to be today's new
word-of-the-day.  See https://en.wikipedia.org/wiki/Phoropter
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