# [amsat-bb] FM signal on FO-29?

Zach Leffke zleffke at vt.edu
Mon Nov 9 17:59:31 UTC 2015

```No worries, I thought on it a bit more and I think a cubic polynomial is
the right fit.  I also found some python tools for regression
calculations that I think will be useful for this.  Also, I think this
is pretty similar to how the COSPAS/SARSAT system used to locate lost
ships (EPIRBs) and downed Aircraft (ELTs) before the proliferation of
GPS and its inclusion in the locator beacons.

Concerning the lines of bearing.  I'm still churning this one in my
head, but I think basically point of closest approach will give us a
line that is perpendicular to the ground track at that point.  That
should be the moment when range rate and thus doppler hits zero. Like
you said take enough measurements over enough orbits and see where the
lines begin to cross.  I know there are some more algorithms for this,
like Brown's Least Squares Triangulation Algorithm.

The work flow that's forming in my head is as follows:
1.  Measure the signal doppler shift.
2.  Work backwards through the transponder to get the uplink doppler as
it enters the receiver (to remove doppler between measuring G/S and FO-29).
3.  use regression to get the cubic polynomial S-curve of doppler.
4.  Find the inflection point in the S-Curve (the zero crossing) that
gives the instant in time for PCA.
5.  Go back to SGPs+TLEs to determine subsatellite point and ground
track at that instant.  Which gives us the line of bearing for that orbit.
6.  Repeat 1 - 5 over multiple orbits to get multiple lines of bearing.
7.  Use triangulation algorithms to determine the likely lat/long of the
emitter as well as the confidence interval (Error elliptical probable).

Things that kill this plan are multiple emitters on different
frequencies.  If its a taxi then the emitter is moving around which will
dilute precision in the position estimate.  Frequency drifts in the
emitter and spacecraft can introduce errors.  Stale TLEs will cause more
errors.

A stretch goal might be to turn this into a Master's Level research
topic for a graduate student, especially when trying to characterize all
the sources of error and how that impacts the final estimate. If we
could develop a working tool that AMSAT could use to locate illegal
emitters, maybe that could provide a body of evidence that could be
taken to say the IARU, to then maybe put pressure on the host countries
to crack down on illegal use of the Amateur bands (likely a pipe dream
to see actual political movement though, or see any real reduction in
the QRM, but one can hope!).

Fun Stuff!

-Zach, KJ4QLP

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

On 11/9/2015 9:54 AM, Robert Bruninga wrote:
>
> No, I’m all talk and no do… compared to whiz kids and Matlab… Im old
> fashioned enough to just plot it on graph paper and LOOK for the
> middle hi hi…
>
> *From:*Zach Leffke [mailto:zleffke at vt.edu <mailto:zleffke at vt.edu>]
> *Sent:* Sunday, November 08, 2015 6:55 PM
> *To:* Robert Bruninga
> *Cc:* amsat bb
> *Subject:* Re: [amsat-bb] FM signal on FO-29?
>
> Good point Bob.  I haven't fully worked through the problem yet, but
> thats pretty much right in line with what I'm thinking.  Thats what I
> meant by saying watching the 'rate of change of doppler.'  When the
> slope of the doppler s-curve is at a maximum, that should be the point
> of closest approach.
>
> Actually, that brings up a question.  Do you know what type of
> equation would fit the doppler S-Curve profile?  something that could
> be used to generate a regression equation from a few doppler observations?
>
> -Zach, KJ4QLP
>
> On 11/08/2015 06:12 PM, Robert Bruninga wrote:
>
>     Zach
>
>     You dont need to know the senders exact freq, just a plot of his
>     freq during the pass will form an "S" curve and once you have
>     enough of the "S", you can know his center freq, and hence his
>     closest point of approach.  That gives a line of bearing.  Anothe
>     pass gives another one, and so on...
>
>     bob
>
>     On Sun, Nov 8, 2015 at 5:54 PM, Zach Leffke <zleffke at vt.edu
>     <mailto:zleffke at vt.edu>> wrote:
>
>     well.. when I say students, I meant graduate students at VT that
>     happened to be in the lab when I was doing the experiment.  They
>     work with me and Bob, so a few crude words here and there aren't
>     uncommon to them.
>
>     But good point, probably not the best thing for a public demo if
>     your audience is a bit younger.
>
>     Actually, I've seen so much FM activity (basically every time I've
>     monitored FO-29 since we first came online in late september) that
>     I've been toying with the idea of trying to locate where the
>     source emitters actually are located based on doppler shift data.
>     We know the doppler between the receiving ground station and
>     FO-29, so we can back that out. We know the transponder mapping,
>     so we can work through that to determine what the center frequency
>     is as the signal enters the transponder uplink receiver.  What we
>     don't know is the uplink doppler, because we don't know where the
>     emitter is and we don't know what exact center frequency they are
>     on (but I bet you its in 5kHz steps, maybe 2.5kHz).  So we have
>     two unknowns.  I'm betting there's a way to work through it
>     though, and with enough observations and by watching the rate of
>     change of the doppler, I bet there's a way to make an educated
>     guess on what their center freq and location are.
>
>     Or if someone listening knows Spanish and/or Portugese, maybe we
>     could get lucky and hear what cross streets the taxi is going to
>     (if it is in fact a taxi).
>
>     -Zach, KJ4QLP
>
>     On 11/08/2015 05:30 PM, Clayton W5PFG wrote:
>
>     I wouldn't recommend playing the FM audio heard via FO-29 to a
>     group of children. Normally it's not English. It's most likely NOT
>     a religious broadcast based on their choice of crude words.
>
>     73
>     Clayton
>     W5PFG
>
>     On 11/8/2015 16:13, Zach Leffke wrote:
>
>     So yes, in the last couple weeks I've seen a LOT of FM activity on
>     FO-29.  And based on my experience with Friday's pass, which was
>     ascending, I'm leading towards the Central/South America QRM theory.
>     Lots of strong FM activity as the pass started and the satellite was
>     over the lower latitudes, but as FO-29 ascended over higher latitudes
>     towards the north pole, the FM activity died down.
>
>     -Zach, KJ4QLP
>
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>
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