[amsat-bb] Repeater interference on AO-92

Leffke, Zachary zleffke at vt.edu
Tue Apr 28 15:49:21 UTC 2020

Not to get too far off topic, but this is the kind of thing that an SDR in orbit could help hunt down.  Imagine one of the GOLFs, with is SDR capability.....send a few commands, and now we have a bent pipe transponder on the frequency of interest, maybe routed maybe to an S or X Band downlink (or both), plop a carrier next to the relayed passband to aid in Doppler tracking.  Record a pass with the FM interferer transmitting, using TLEs (or better orbit data if available) reverse out the Doppler between monitoring GS and the bird, then look for the inflection point in the S-curve....there's Time of closest approach.....draw the ground track and at TCA, draw lines at right angles to the ground track....transmitter lies on that line.  Do that for a few passes (or with multiple receivers, i.e. both GOLFs) and you get intersecting lines...there's your huckleberry.  Even better, if there is IQ storage capacity on the birds, time it to record over the region of interest, and downlink the digital IQ at some later time (say over X-band at high rate).  This removes channel effects of the downlink and removes the need to be in the same footprint to receive the interferer.

Similar situation and geolocation was done via FO-29 in previous years, but that was because the uplink happened to lie in the passband of FO-29.  Essentially the same algorithms as the original SARSAT system before the advent of GPS.  With something like the GOLF series, we could actively 'go on the hunt' and control things a lot better.

Also, for the record, I don't know if this is actually a capability being designed into GOLF.  Just using GOLF as the example of Amateur SDRs in orbit and yet another novel application.  My little 'send a few commands' note above completely obscures how complex something like this could wind up being (especially if storage capabilities are included)....but fun to think about!

-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 Roy Dean via AMSAT-BB
Sent: Tuesday, April 28, 2020 11:25 AM
To: amsat-bb at amsat.org
Subject: [amsat-bb] Repeater interference on AO-92

> Yes, I am also listening to the uplink freq. Next pass is in about 1 hour.
> 73 Eric KV1J

Keep in mind that somebody may be commanding it into picture mode.

Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb

More information about the AMSAT-BB mailing list