karn@mouton.UUCP (05/25/84)
The following is extracted from this week's Oscar-9 bulletin, giving the latest Oscar-11 news. **** UoSAT Bulletin-76 25th May 1984 **** ** UoSAT-Oscar-11 Status ** Over the weekend of 11th to 13th May, dedicated radio amateurs at Stanford Research International in California and their outpost in Greenland (headed by Bob Leonard, KD6DG, at Stanford and Finn Steenstrup, OX3FS, at Sondre Stromfjord) heard very weak signals emanating from Oscar-11's command RECEIVERS which, by their nature of operation, generate small signals on a frequency near to that which they normally listen. These signals told the University satellite team that their craft was still alive, although at this time the chances of complete recovery were not known. Although this observation, the first one confirmed since launch, did not lead directly to recovering the satellite, it did confirm that the orbital predictions provided by the NASA tracking organisations were correct and that the object being tracked by the Surrey team was indeed UoSAT-2. On the morning of 14th May at 10:24 GMT, Neville Bean, G8NOB, and Roger Peel, G8NEF, continued to command the satellite using its 144MHz uplink, to no effect. At 11:01GMT (12:01 BST), Neville made further command attempts on 438MHz, and after a brief stream of initialisation commands, the main UoSAT-2 beacon (145.825MHz) was powered up at 11:05. The signals from the spacecraft were as strong as the last ones heard from it on the 1st March, when it stopped transmitting shortly after launch. Telemetry data, from the initial two orbital passes over Guildford, appeared to be very encouraging, with temperatures around -5 to 0 degrees centigrade, as expected, and a battery voltage of 14.6 volts. The spacecraft was still spinning, but this had stabilised since the previous data received immediately after launch, ready for attitude control manoeuvres. These will not start until after the causes of the 11 week silence have been investigated, a process that itself may take many weeks in order to exercise the caution necessary in this situation. The UoSAT-2 spacecraft will be transmitting telemetry data continuously for the next few days while initial checks are made on the telecommand system, but after this other data formats will be generated using the spacecraft computer to check further the spacecraft's health. The most likely cause of the last 10 weeks silence is indicated by the very poor command uplinks. Indeed, only 8 commands were loaded into the spacecraft in the first 2 days after recovery. This accompanies a decrease in command decoder and battery temperatures, which is cyclic between -5c and -12c with a period of about 10 days. Commands are received - albeit infrequently - over only about 3-4 days of this cycle, explaining why the current investigations are proceeding cautiously. Since temperature fluctuations are connected intimately with sun-angle, and hence attitude, the command problems could be purely due to the spacecraft antennas pointing away from the earth during some parts of the precession cycle. Further investigations are continuing with efforts to characterise the signals received by the spacecraft receivers, which are not performing as well as expected. In addition, experiments to improve the performance of the data decoders (which convert the audio signals received into digital data for use by the telecommand logic) are under way, concentrating on pre-distortion of the uplink waveforms. Stephen Hodgart, our attitude control expert at Guildford, has been working on an elementary magnetorquing manoeuvre which can be controlled to improve the average sun angle (and hence the temperature), the antenna pointing (always towards the ground over the Northern Hemisphere), and hence the command success rate, given a minimum of ground interaction in the process. This will be attempted on one of the next few 'warm' cycles, once all the possible side-effects have been evaluated. ** Oscar-11 Attitude - from Stephen Hodgart (UoS) ** The angular motion of UoSAT-2 has been estimated from the magnetometer data extracted from the pass over Guildford on orbit 1076. This reveals a dominant transverse motion, a spin state approximating a 'flat spin' or 'tumbling', defined by a rotation of the z-axis about a spin axis in the plane of the x and y axes of the spacecraft. This arbitrary spin axis is the direction of the overall angular momentum vector and is parallel, within 1 or 2 degrees, to the plane of the orbit. The period is 42 seconds. The motion is complicated by a slow z-spin oscillation with a period of 165 secs and an amplitude of +/- 45 degrees. This oscillation couples to the dominant transverse motion causing the flat spin to modulate into a nutational cone of a wide half-angle varying between 84 and 96 degrees, in synchronism with the z-spin oscillation. Every half-cycle this cone flattens for an instant to the pure flat spin and then inverts. Further analysis using other orbital data is required to test for possible precessional change in the angular momentum, also to test for any continuous advance in the z-axis rotation in addition to its angular oscillation ** UoSAT-Oscar-9 Schedule ** The 21MHz beacon will be in use this weekend. In addition, the bulletin will run until Tuesday morning due to (yet) another public holiday in the U.K., preventing access to the ground station. Friday Load bulletin Saturday Bulletin / 1200 bd telemetry / Digitalker Sunday Bulletin / 1200 bd telemetry / Digitalker Monday Bulletin / 1200 bd telemetry / Digitalker Tuesday Checksummed telemetry Wednesday 1200 bd telemetry and Digitalker Thursday Whole-orbit telemetry data Friday Load bulletin