OSCAR-11 REPORT

23 March 2007

OSCAR-11 celebrated it's 23rd birthday on 01 March 2007. Congratulations to Professor Sir Martin Sweeting G3YJO, his team at the University of Surrey and associated radio amateurs for their magnificient achievement.

During the period 14 February to 22 March 2007, the satellite was heard from 18 to 28 February and from 11 to 21 March . Good steady signals have beeen heard on most passes, and good copy of the telemetry obtained.

The on-board clock has comntinued to maintain accurate time. However, there are two problems with the date and time display in the ASCII telemetry. Firstly, the date counter appears to be incrementing correctly, but the day of the month is reset to 41, at the end of each month, instead of being reset to one. A likely cause is that the unused bit representing 40 in the ASCII code for the day of the month, is permanently stuck at a one. This may be related to the '0=' display error when the month was 09, previously reported.

The second problem, is that the hours are incorrectly counting and/or displayed. Since August 2006, the hour count has never exceeded 12. During one pass recently the hour count changed from 12 to 01 instead of to 13! A possible explanation is that the time/date chip has two modes of operation, which are switchable between 12 or 24 hour modes. An internal chip fault or an external circuit fault may have switched the chip into the 12 hour mode.

Investigating this problem has been difficult, because the satellite passes over the UK at roughly the same times, each day, and only a limited number of pass times are available for analysis. If the above explanation is correct, the date changes around 04:47 UTC, each day.

The 'day of the week' counter operates reliably, zero representing Thursday. At the present time the total clock error is 37.21444 days slow.

If the satellite's watchdog timer continues to operate normally, the beacon should switch ON around 31 March/ 01 April 2007. The satellite is in full sunlight at the present time, and will remain in this state until 20 April 2007, when eclipses start again. The eclipses are expected to continue until 23 August, and if the satellite performs as it did last year, it is unlikely to be heard, for any length of time, during the eclipse season.

I am indebted to David VK5DG, David G8OQW, Etienne ZS6Y, Jeff KB2M, Geoff ZL6GA, Edward BX1AD, and Sil ZL2CIA for their reports. Many thanks.

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 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 22 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. 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:U2RPT131.CWV, to prevent duplication.

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