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@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Roger Kolakowski Sent: Friday, September 04, 2009 9:22 AM To: bruninga@usna.edu; 'Patrick Green' Cc: amsat-bb@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@usna.edu To: "'Patrick Green'" pagreen@gmail.com Cc: amsat-bb@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
- 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 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@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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