OSCAR-11 REPORT

26 January 2008

This report covers the period from 19 December 2007 to 26 January 2008. After two months of silence from 20 November, when eclipses started, the satellite resumed transmissions on 22 January 2008. This was three complete 20.7 day watchdog cycles after the satellite switched off. At the time of writing signals are consistantly strong, although the polarisation is very variable, with a cycle time of around 20 seconds, causing deep fading if the antenna polarisation is fixed. This suggests that the satellite may be tumbling.

If the current watchdog cycle continues uninterrupted, the 145.826 Mhz. beacon should switch off around 01 February and on again around 11 February. However, if low battery voltage causes the watchdog timer to reset, then the beacon should switch on 21 days later.

The real time clock in now very stable although it has lost a further 7.09 days since 20 November. On 26 January it is 81.36403 days slow.

I am indebted to Peter ZL3TC, Mark KU7Z, Gene WA4UKX, SWL Mark in CM87, Ron G4PGY, Julian WB9YIG, John HB2HSH and Edward BX1AD 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. 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:U2RPT141.CWV, to prevent duplication.

73 Clive G3CWV [email protected] (please replace xxxxx by g3cwv)