[amsat-bb] OSCAR-11 Report

[Clive Wallis]

                    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 ninutes. At this level the satelite might just survive.

John KD2BD has noticed a descrepancy 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possible to use elements which were many months out of date, with only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, ie. 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, ie. 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 www.users.zetnet.co.uk/clivew/

If you place this bulletin on a terrestrial packet network, please
use the bulletin identifier $BID:U2RPT138.CWV, to prevent duplication.

73 Clive G3CWV   xxxxx at amsat.org (please replace xxxxx by g3cwv)