[amsat-bb] HEO opertunity (APRS?)

Robert Bruninga bruninga at usna.edu
Mon Oct 24 23:25:04 UTC 2016

> The University of Louisiana has a slot ... to GTO for a 1u.
> WE are asking the community what payload should we fly...

You could always fly an APRS digipeater.  PSAT works well at LEO, but is
only 300 mW.  Our next 1U APRS satellite will have a 2W transmitter and
will be quite strong horizon to horizon to an HT on the ground.

But if you raise the cubesat TX power to 4W, and use Oscar class stations
running 50w instead of 5W HT's on the ground, you can pick up 23 dB more
performance which is exactly what you need to operate to GTO at 35,000km.

The problem is the TX duty cycle.  All of the APRS satellites operate with
a quite low TX duty cycle because Hams only cover about 10% of the Earth's
surface so the average transmitter power is low.  On GTO, half the users
in the world can see it half the time and if they all start banging it
with chatter, then a 1U wont have the average power.

Does your GTO remain attached to a large spacecraft?  Does it have more
than the average 1 Watt Solar power that a cubesat gets?  If so, then the
duty cycle might not be a problem.

ON THE OTHER HAND, every APRS digipeater has a DUPE-WAIT setting that is
normally 30 seconds for mobiles to prohibit duplicating packets.  You
could extend this to X miuntes to make sure no one tries to get through
more than one packet per X minutes.  Or you can think of otherways to
control the user load.

Again, I am assuming 2m on 145.825 MHz.  Although that is full of existing
users trying to hit the PCSAT, PSAT and ISS, no users without an OSCAR
class station and 50W will even be heard out there at GEO.  SO that is a
good way to share the channel.

Unless I overlooked something, I think it could work.

It would be a great emergency response and disaster preparedness link.
For low duty cycle comms into and out of an area.

You could build the whole system includeing a 10W PA using the parts from
a Byonics Microtrak TT4.  That is what we use (without the 10W PA) for our
PSAT-2 and follow-on designs.

See http://aprs.org/psat-2.html

Bob, Wb4APR

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