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OBSERBVATIONS FROM NORWAY.

Jan: 2006.

AO-7 MODE-CHANGES.


For those that requested more informasjon on AO-7s mode changes and the 24 
hour timer that controls this change.

On page 6 of the December 1974 Amsat newsletter the timer is described thus 
...

The 24 hour timer is derived from a 3.181457Mhz crystal followed by 39 
divide by 2 counters  (you can do the math yourself.....LA2QAA).

However, after having been in contact With Jan King W3GEY ... (now VK3GEY) 
... whom was the project manager for AO-7 and helped design and build the 
satellite ... (together with Karl Meinzer DJ4ZC, among others) ... in his 
garasje 30 odd years ago, I have been given the following informasjon.

(Those satellite enthusiasts who don't recognise those callsigns ought to be 
lined up against a wall and shot!).

The timer uses a simple CD4001AD RC oscillator comprising 1 x capacitor and 
1 x resistor (!) ... not particularly accurate but then it was never meant 
to be. Space (pun intentional) was at a premium and the priority was on the 
actual RF transponders ... because even under testing on the ground there 
was a mode-B command RX desense problem ... (which was never fixed).

What must be remembered here is that this was before the days of the NE555 
timers ... (not that they're particularly accurate either) ... buffered CMOS 
and case-hardened chips were not available in those days ... imagine the 
radiation dose AO-7 has experienced in the last 31 years.

The CD4001 RC oscillator was followed by 22 divider stages. Karl Meinzer 
felt that accuracy wasn't that importan ... which it wasn't ...the emphisis 
was on low power consumption. Naturally, such a circuit was prone to drift 
but even that wasn't important either ... and anyway, who would have 
expected it to perform in a space environment more than 30 years! later ... 
(ok, so it hibernated for 20 years).

The general idea of the clock ... which is parameter 2C in the beacon 
telemetry ... was ... no, *is* to increment until it reached 96 ...this can 
be seen by the telemetry 2C parameter changing every 15 minutes ...
( ok!, pencils out ...how many 15 minute periods are there in 24 hours?) ... 
then when it reached 96 it would trip the circuit that would change the mode 
from A to B ... (or vice-versa) ... reset, and start counting again.

Amazingly!, 34 years later, while AO-7 is out of eclips, the counter is 
still clocking up the milage.

Unfortunately, the beacon is not as healthy as it once was due to lack of 
power ... ("Gators" take note!) ...
so copying the telemetry is still quite a random affair, even though to all 
intents and purposes the transponders are functioning ... this, despite the 
extreme radiation dose that is most certainly *not* good for the solar 
array.

However, the sun angle is still good so there's no reason why AO-7 shouldn't 
prove many more years of communication possibilities ... providing! we all 
use a bit of common sense and follow the recommended operating proceedures 
outlined on the AO-7 RESOURCE PAGE.

This page, and "everything ou need to know" about operating AO-7 can be 
found at ...

<http://www.planetemily.com/ao7/redirected.php>

PS.

The current mode changes ... (between A, B and C) ... occur between 08:30 
and 09:00 UTC daily. This *may* or *may* not change back to "random" after 
the spring of 2006. If you can't hear (or copy) the telemetry just check the 
AO-7 log page for the current mode.

73 LA2QAA & GM1SXX   <la2qaa@amsat.org>
----
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