[amsat-bb] PSAT and Bricsat FM PSK on same frequencies

Tom Schuessler tjschuessler at verizon.net
Mon May 25 03:23:21 UTC 2015


Your explanation to my questions make much more sense now that I worked
another pass of the satellites.  Today the two were sufficiently separated
that I could see one lagging behind and thus higher up in frequency because
of Doppler shift.  Your explanation on why the same frequencies for the two
birds is quite nice.  Also the frequency offset difference for the PSK
beacons as well as the different IDs work as well.

Bricsat was quite a bit weaker but that was probably because I was trying to
track PSAT with my Arrow on a tripod.  It was also a couple of Khz higher
up.  I also saw the difference in the two beacons so now I know how to tell
which from which.

I did try transmitting PSK31 USB on 28.120 + 1Khz or so and listening
through HDSDR on 435.350.  I heard faint signals of other PSK signals but
believe they were just some other terrestrial QSOs and not directed through
the satellite.  They were slipping down the DIgipan waterfall as they would
be without correction.  I was not doing correction either on 28Mhz but I was
hoping, even on playback of the HDSDR I/Q recording, to find me but I never
did.  Going to try a different software configuration next weekend to try
and hear myself live.

The best time to test PSK through these satellite transponders will be
overnight passes when possible so as to avoid terrestrial stations from
calling other terrestrials and confusing things.  Will have to wait for
times to shift for that.

73, Tom, N5HYP


Message: 11
Date: Sun, 24 May 2015 10:56:55 -0400
From: Robert Bruninga <bruninga at usna.edu>
To: amsat-bb at amsat.org
Subject: Re: [amsat-bb] PSAT and Bricsat FM PSK on same frequencies
Message-ID: <835f099c08a7be546571f9704a1ed498 at mail.gmail.com>
Content-Type: text/plain; charset=UTF-8

> ... how will this work with two FM satellites on the same frequency?

The idea is to serve users no matter which spacecraft is in view.  And to
not have the users have to always change frequencies and track different
objects.  Imagine a camper in the field.  Just point your beam towards AOS
generally and set +5 KHz tuing and when a satellite comes into view, you can
use it.

Many things separate them:
1) Distance
2) Time
3) Doppler
4) Users antenna beamwidth

When they are 5 minutes apart, the beam headings will be over 90 degrees
apart (10 to 15 dB separation), the Doppler will be typically 10  KHz
(10-15 dB separation) for a total of 20-30  dB or more to prevent capture

Since an orbit is 95 minutes long, and the satellites will drift, then 95%
of the time, this condition of at least 5 minute separation, will be met.

PSAT PSK (W3ADO-5) was built over 3 years ago and is hardwired to power up
by default (maximum chance it will work despite spacecraft packet link or
other failures)

BRIC PSK (W3ADO-6) was built just last year and has independent control by
Brno designers (can be off while sats are adjacent)  It also powers up by
default (again, for maximum availability in case of bus failure).

So I think the choice pretty much meets the goals of maximum availability to
users and as independent of spacecraft failures as we could make them.

Hope that Helps.
Bob, WB4aPR

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