OSCAR-11 Report
22 October 2007
During the period 27 September to 22 October 2007 the satellite has been
heard from 06 October to 08 October.
The beacon switched on as expected, on 06 October, but prematurely switched
OFF sometime between 08 and 09 October. Signals were weaker than usual.
The on-board clock has continued to loose time. During the ten day OFF
period it lost 72 minutes, and there was a further loss of 48 minutes
during the last ON period. When last heard the clock error was 69.97793
days slow. The day of the week counter is unchanged with 0 representing
Monday UTC.
The premature shutdown has probably caused the watchdog timer to reset,
and start a new 20-day cycle. If that has occurred then the beacon should
resume transmissions around 29 October 2007.
I am indebted to John N7ZL, John N3NKC, Thomas
HB9SKA, and Peter ZL3TC, for their reports.
Many thanks.
The satellite is now in continuous sunlight and this will continue until
the start of November, when there will be a short eclipse season, lasting
until around 07 January. However the maximum duration of the eclipses
will only be about 12 minutes. At this level the satellite might just
survive.
John KD2BD has noticed a discrepancy between my eclipse
predictions and those from his Predict program. This could explain why
the beacon didn't switch on until mid September. The main cause of the
problem was due to the use of old Keplerian elements. In previous years
the it had been possible to use elements which were many months out of
date, with only minor effects on the eclipse predictions. This year the
orbit appears to be more critical. Different software also produces slightly
different results. Many thanks John for your comments.
The current status of the satellite, is that all the analogue telemetry
channels, 0 to 59 are zero, ie they have failed. The status channels 60
to 67 are still working. The real time clock is showing a large accumulated
error, although over short periods timekeeping is accurate to a few seconds
per month. The day of the month has a bit stuck at 'one' so the day of
the month may show an error of +40 days for some dates. The time display
has switched into 12 hour mode. Unfortunately, there is no AM/PM indicator,
since the time display format was designed for 24 hour mode. More data
is required to determine exactly when the date changes.
The spacecraft computer and active attitude control system have switched
OFF, i.e. the satellite' attitude is controlled only by the passive gravity
boom gradient, and the satellite is free to spin at any speed. When telemetry
was last received it showed that one of the solar arrays had failed, and
there was a large unexplained current drain on the main 14 volt bus. After
23 years in orbit the battery has undergone around 100,000 partial charge/discharge
cycles, and observations suggest that it cannot power the satellite during
eclipses, or sometimes during periods of poor solar attitude.
The watchdog timer now operates on a 20-day cycle. The ON/OFF times have
tended to be very consistent. The average of many observations show this
to be 20.7 days, i.e. 10.3 days ON followed by 10.4 days OFF. However,
poor solar attitude may result may result in a low 14 volt line supply,
which may cause the beacon to switch OFF prematurely, and reset the watchdog
timer cycle. When this occurs, the beacon is OFF for 20.7 days.
The Beacon frequencies are -
VHF 145.826 MHz. AFSK FM ASCII Telemetry
UHF 435.025 MHz. OFF
S-band 2401.5 MHz. OFF
Listeners to OSCAR-11 may be interested in visiting my website. If you
need to know what OSCAR-11 should sound like, there is a short audio clip
for you to hear. There is an example of the latest telemetry received
from the satellite. The website contains an archive of news & telemetry
data. It also contains details about using a soundcard or hardware demodulators
for data capture. There is software for capturing data, and decoding ASCII
telemetry. The URL is http://www.users.zetnet.co.uk/clivew/
73 Clive G3CWV
xxxxx@amsat.org
(please replace xxxxx by g3cwv)
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