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Re: how abt ao-40 beacon on mode-S?



Achim Vollhardt (DH2VA) wrote:

> I have some thoughts to think about (to guide the discussion to
> something more sat-like..): why not cutting some power off the mode-V
> beacon and power up the 2.4 gig tx, lets say to 50% nominal power for
> letting all the mode-s freaks test their equipment. just the beacon,
> no transponder. Signals will be weak because of poor sat-attitude but
> I guess receiving this beacon will be the next challenge for the fan
> community before actually working over the sat (first listen, then
> talk..).

Probably because it wouldn't be a useful test at present, assuming I
understand the attitude of the spacecraft correctly.  (May be a BIG
assumption on my part, but at least you now know the rationale for my
point of view.)

The spacecraft is presumably oriented with the bottom facing the
Earth.  There is no S-band antenna on the bottom of the spacecraft.
Only VHF, UHF, and L-Band omnidirectional antennas.  With the
UHF downlink not working, the team has configured the system for
receiving on UHF and L-Band, and transmitting on VHF.  (As an aside,
that makes me wonder if the command team is sending their commands
on UHF or L-band.  If UHF uplink is also broken, it might point to
a problem with the UHF omni antenna or a switching relay, as has
been speculated by someone else previously.  If the UHF uplink works
but the UHF downlink doesn't, then that points to a problem with
the UHF transmitter or power amplifier.  But that's another issue.)

To transmit on S-band, it would be necessary to use the high-gain
S-band antenna which is on the top face of the spacecraft, which
(I believe) is now pointed away from the Earth.

If you could hear the beacon in that manner, then you DO have some
assurance that your system has "great ears" and will have no trouble
hearing the S-band transponder when it is enabled for routine use.

If you CANNOT hear the beacon in that manner, you STILL have NO IDEA
if your station "measures up" to the performance level that will allow
you to use the S-band transponder.

So I guess the value of the test is so limited that the operations
team has wisely devoted their energies to other things.  (They *DID*
turn on the S-band transmitters briefly to confirm that they work,
but that's all.)

The original plan was to use the 400 N motor to do the initial orbit
modifications, and that is on the bottom of the craft, meaning that
the omni antennas would stay pointed more-or-less towards Earth at
the time of the burn.  Now the plan has changed, to start the orbit
modifications with the ArcJet motor.  That motor is on the top
surface of the satellite.  So I presume that there is a plan to flip
the satellite to face in the opposite direction (top pointed towards
Earth) before the ATOS is fired.  My guess is that this is why there
was hope given that we may be allowed some very limited transponder
access during the first 9 months or so before the target orbit is
reached and the solar panels are fully extended.

When the gain antennas are pointed our way, THEN it may be worthwhile
for the control operators to try sending telemetry over the other
beacons to give them a better test, and if so, hopefully they would
consider telling us in advance so people with the higher bands could
listen for them.

I hope that helps (and I hope it's right!)

John (KBØZEV)
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