[amsat-bb] Why PCSAT is hard to recover

Robert Bruninga bruninga at usna.edu
Fri Sep 4 06:44:56 PDT 2009

> Why is pcsat having so much trouble 
> carrying its 5 watts then?  Even  
> when the satellite is in full sun?  
> Even on the Z panel? ...what went wrong.

Error in design.  Since it was our first satellite, and the
first time that hamtronics TX and RX were flown in space, we put
in multiple redundancy.  Two identical RX=>TNC=>TX systems.  We
thought the most important thing was the command link.

To make sure we could still access both TNC systems even with a
TX or RX failure we added a second UHF RX to both systems.  In
additionl we had a relay to CROSS-CONNECT the transmitters.

THen we made the mistake.  We assumed that to recover from an
anomoly, the most important thing was to regain the command
link.  Hence, from cold-boot, the spare UHF receivers would both
come on AND the transmitters would be cross-connected.  This
assured we could access either TNC even if we had lost one RX or
one TX.

The mistake was assuming that in such a recovery effort, the
first thing we would then do is TURN OFF the extra receivers and
DISCONNNECT the cross conneced transmitters once we had command.

Well... DUH.... If the reason the spacecraft crashed back to
defaults was because it was low on power, then the last thing
you want to do is QUADRUPLE the power budget by having the
recovery-defaults turn on double the number of receivers and
double the number of transmitters!

So we need FOUR times the average power just to get command and
that only happens during mid-day passes during maximum eclipse
periods, and sometimes right at the beginning of full sun
periods in the southern hemisphere.
Our first commmand then IN SEQUENCE is 
2) Send command to separate the transmitters
3) Send command to turn off the two spare UHF reciverss

If those are successful, AND PCSAT then has a full orbit in full
sun, then we can recover.  But the loggon password challenge
from the satellite is the LONGEST packet in the command
sequence, and if is not successful on the FIRST try, then the
battery is exhausted and you loose the pass.

Bob, Wb4APR
> On Sep 3, 2009, at 17:34, "Robert Bruninga"
<bruninga at usna.edu> wrote:
> >> To get good coverage you need as many
> >> LEO satellites as possible so they should
> >> each be as small as possible.
> >> Intersatellite linking could be done
> >> via automated ground stations. This
> >> eliminates the need for high-power
> >> transmtters and/or high-gain antennas
> >> on the satellites for interlinkng.
> >
> > Yep, that is what we have been trying to do now for 8 years
> > the APRS satellites on 145.825.  We just need several of
them in
> > orbit at the same time.  We have demonstrated dual-hops
> > times whenever two or more of the APRS satellites (and
> > are operational at the same time.  If we could get 6 to 10
> > the University cubesats to simply carry the 3.4" square APRS
> > transponder (Byonics TinyTrck-4), then we would have a
> > constellation providing nearly continuous connectivity via
> > satellites from any handheld or mobile APRS radio.  With 6,
> > might have to wait 30 minutes or so to make yoru contacts.
> > 10 or so, you might have to wit no more than 5 to 10 minutes
> > connectivity.
> >
> > See www.aprs.org/cubesat-comms.html
> >
> >> It's better to put that gain and power
> >> consumption on earth.
> >
> > The advantage of the APRS satellite concept and Packet, is
> > we can use a 5 Watt transmitter on the satellite to be able
> > hit any mobile or HT using its existing omni antenna because
> > packet has a low dutycycle.  So running 5 watts on a cubesat
> > easy, because the transmitter dutycycle is only on less than
> > 5% of the whole-orbit time. (average power 1/4 Watt)
> >
> > Whereas ECHO which is on all the time, has to be set at 1/4
> > TX power because it is on all the time.
> >
> > Also, EVERY APRS satellite would be on the same frequency
> > 145.825 with no doppler to track, and since every one of
> > does the same generic relay, independent of callsign, then
> > user on the ground just operates... He does not have to do
> > anything to go from one satellite to another...
> >
> > Bob, WB4APR
> >
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> > author.
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