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

4 Sep 2009

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Is this cross-connect sequence hard coded 
in the firmware?  No way to replace that 
small bit of code directly from earth?
We used the KISS principle and since we had no programmers on
the team, we put no cotroller or CPU on board.  PCSAT is nothing
but a pair of TNC's, and pairs of TX and RX's.  And a simple 555
timer chip that reboots them if they have not beaconed in over a
minute.
The STOCK TNC's have all the command, control, telemetry, I/O we
needed.  All of our satellites to date flew with nothing but a
TNC as the "system" with the 555 backup timer as a fail safe
reset.
Bob
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On 9/4/09 9:44 AM, "Robert Bruninga" [email protected]
wrote:
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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
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in multiple redundancy.  Two identical RX=>TNC=>TX systems.
We
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thought the most important thing was the command link.
To make sure we could still access both TNC systems even
with a
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TX or RX failure we added a second UHF RX to both systems.
In
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additionl we had a relay to CROSS-CONNECT the transmitters.
THen we made the mistake.  We assumed that to recover from
an
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anomoly, the most important thing was to regain the command
link.  Hence, from cold-boot, the spare UHF receivers would
both
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come on AND the transmitters would be cross-connected.  This
assured we could access either TNC even if we had lost one
RX or
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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
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DISCONNNECT the cross conneced transmitters once we had
command.
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Well... DUH.... If the reason the spacecraft crashed back to
defaults was because it was low on power, then the last
thing
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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
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that only happens during mid-day passes during maximum
eclipse
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periods, and sometimes right at the beginning of full sun
periods in the southern hemisphere.
Our first commmand then IN SEQUENCE is

LOGON
Send command to separate the transmitters
Send command to turn off the two spare UHF reciverss

If those are successful, AND PCSAT then has a full orbit in
full
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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
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battery is exhausted and you loose the pass.
Bob, Wb4APR
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On Sep 3, 2009, at 17:34, "Robert Bruninga"
[email protected] wrote:
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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
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with
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the APRS satellites on 145.825.  We just need several of
them in
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orbit at the same time.  We have demonstrated dual-hops
several
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times whenever two or more of the APRS satellites (and
ARISS)
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are operational at the same time.  If we could get 6 to 10
of
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the University cubesats to simply carry the 3.4" square
APRS
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transponder (Byonics TinyTrck-4), then we would have a
constellation providing nearly continuous connectivity via
these
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satellites from any handheld or mobile APRS radio.  With
6,
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you
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might have to wait 30 minutes or so to make yoru contacts.
With
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10 or so, you might have to wit no more than 5 to 10
minutes
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for
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connectivity.
See www.aprs.org/cubesat-comms.html
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It's better to put that gain and power
consumption on earth.
The advantage of the APRS satellite concept and Packet, is
that
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we can use a 5 Watt transmitter on the satellite to be
able
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to
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hit any mobile or HT using its existing omni antenna
because
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the
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packet has a low dutycycle.  So running 5 watts on a
cubesat
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is
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easy, because the transmitter dutycycle is only on less
than
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say
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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
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watt
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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
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does the same generic relay, independent of callsign, then
the
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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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