OSCAR-11 REPORT
17 March 2008
OSCAR-11 celebrated its 24th birthday on 01 March 2008. It was designed, built and launched, within a time scale of six months, using commercially available components. Congratulations to Professor Sir Martin Sweeting G3YJO, his team at the University of Surrey and associated groups of radio amateurs, for their magnificient achievement.
This report covers the period from 17 February to 17 March 2008. The satellite was heard from 03 to 13 March. Previously the transmissions which started on 11 February were terminated prematurely after only four days. The watchdog timer cycle appears to have continued for its full 21 days, thus restarting transmissions on 03 March.
Signals have been very variable in strength sometimes very strong, sometimes undetectable, even at high elevations. Low signal strength was particularly noticeable at AOS and LOS, and also towards the end of the transmission period, which on this occasion, lasted the full ten days.
The next transmissions should start on 24 March. However, permanent eclipses start a few days beforehand, which will probably cause the transmission period to be terminated prematurely, and may even prevent it being heard at all. After March it is unlikely that the satellite will support any sustained period of operation, and will only transmit for a short time, possibly less than a single orbit, every 21 days.
The real time clock now serves as a crude indicator of the state of the satellite. Between 04 and 13 March showed an average loss of over two hours per day. time every orbit. When last heard the clock was 83.0958 days slow.
I am indebted to Peter ZL3TC, Mark KU7Z, John KB2HSH, Antonio EA1CXG, Edward BX1AD and Andrew VK5LAC for their reports. Many thanks.
The satellite is now in continuous sunlight, and this is expected to continue until mid March, when eclipses will start again. Unfortunately eclipses will then become a permanent feature of the orbit, which is likely to prevent any periods of sustained operation after March.
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.
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:U2RPT143.CWV, to prevent duplication.
73 Clive G3CWV xxxxx@amsat.org (please replace xxxxx by g3cwv)