[amsat-bb] Re: Why PCSAT is hard to recover

Armour, Randy (ITS) Randy.Armour at nashville.gov
Fri Sep 4 08:06:05 PDT 2009


I would like to reiterate the sentiment Roger states.  This list is
occasionally overtaken by emotionally charged "shoulda-woulda-coulda"
threads. Bob's clear and concise posts, that are understandable by those
of us that are not satellite experts, continue to make the Amsat-bb list
worthy of a daily read.  

Randy
KI4LMR

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Roger Kolakowski
Sent: Friday, September 04, 2009 9:22 AM
To: bruninga at usna.edu; 'Patrick Green'
Cc: amsat-bb at amsat.org
Subject: [amsat-bb] Re: Why PCSAT is hard to recover

Thank you Bob...

You always have a refreshing, clear, informative response for any
inquiry
into your systems.

The fact that you recognize a design error publicly reassures us
neophytes
that new designs are built considering past events.

It's always a pleasure hearing one of your explanations.

Roger
WA1KAT

----- Original Message -----
From: "Robert Bruninga" <bruninga at usna.edu>
To: "'Patrick Green'" <pagreen at gmail.com>
Cc: <amsat-bb at amsat.org>
Sent: Friday, September 04, 2009 9:44 AM
Subject: [amsat-bb] Why PCSAT is hard to recover


> > 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
> 1) LOGON
> 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
> with
> > > the APRS satellites on 145.825.  We just need several of
> them in
> > > orbit at the same time.  We have demonstrated dual-hops
> several
> > > times whenever two or more of the APRS satellites (and
> ARISS)
> > > are operational at the same time.  If we could get 6 to 10
> of
> > > 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
> these
> > > satellites from any handheld or mobile APRS radio.  With 6,
> you
> > > might have to wait 30 minutes or so to make yoru contacts.
> With
> > > 10 or so, you might have to wit no more than 5 to 10 minutes
> for
> > > 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
> that
> > > we can use a 5 Watt transmitter on the satellite to be able
> to
> > > hit any mobile or HT using its existing omni antenna because
> the
> > > packet has a low dutycycle.  So running 5 watts on a cubesat
> is
> > > easy, because the transmitter dutycycle is only on less than
> say
> > > 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
> watt
> > > 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
> them
> > > does the same generic relay, independent of callsign, then
> the
> > > 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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> the
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>
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