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

Robert Bruninga bruninga at usna.edu
Sun May 24 14:56:55 UTC 2015

> ... 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

-----Original Message-----
From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of Tom
Sent: Sunday, May 24, 2015 12:00 AM
To: amsat-bb at amsat.org
Subject: [amsat-bb] PSAT and Bricsat FM PSK on same frequencies

Okay so I am a bit confused about how this will work with two satellites
both using FM modulation on the same frequency.  FM has the capture ratio
effect which basically means that if one signal is about twice as strong
as the other that signal will capture the receiver and the other one may
make a little background noise but won't be properly demodulates.  If both
of them are received at the same power level pretty much you just get
noise a.k.a. SO-50 on a crowded day.  So if these two satellites are
transmitting simultaneously whichever one happens to be stronger at the
moment will capture the receiver.  Regardless of whether one of them has a
audio frequency modulated on the carrier at 315 hertz while the other ones
at 375 you're most likely going to not here both of them simultaneously
Heard or displayed on a PSK waterfall.

This may not be an issue in the future as the satellites drift apart from
each other but right now as it seems to me they are pretty darn close so
if they were both on they would be in the beam width of any directional

Aside from the frequency of the PSK carrier itself as modulated on the FM,
I would also have to assume that the call sign of the PSK data beacon is
hopefully different.  The PSK I decoded this morning had a beacon call
sign of W3ADO-5. I would have to assume there is a different beacon call
sign for both. In my reading I have not seen what the difference between
the two would be.

Looking forward to soon trying to test an uplink on 28.120 and see if I
can hear myself. This looks like a good challenge.

73. Tom. N5HYP
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