AMSAT-OSCAR 40 Status report
of December 13, 2000
Nearly four weeks have now passed since the launch, and there are some good things and some bad things to report.
As you all know, immediately after separation, we could not receive the 70 cm transmitter. Consequently we programmed IHU-2 to act as repeater and use the 2m middle beacon instead as our main downlink. In addition the two S-band transmitters have been operated occasionally, but the geometry within the orbit must be right to have the antennas point into the general direction of earth. This limits the use we can make of S-band.
The main activity so far for the command stations was to learn to fly the s/c and to prepare it for the first burn with the 400 N motor. This required a reorientation - the s/c must point opposite to the perigee orbital speed vector - and a spin-up to about 9 rpm. In view of this ongoing work, it was decided to postpone the analysis of the problem with the 70cm Tx to a time after the first burn.
The reorientation of the spacecraft turned out to be more time-consuming than initially assumed. We had two problems to solve:
1. As we get to our firing attitude, the geometry is very poor for producing direction changes of the spin-vector. The magnetic field of the earth is nearly perpendicular to the spin axis meaning that we can change rpm easily, but not direction. It turned out that rounding errors in the software of the IHU were in the same order of magnitude as the desired spin-change. Thus reliable control was not possible. But careful tweaking of the parameters used in the IHU finally enabled us to solve this problem.
2. The geometry is also poor in terms of attitude determination by our sensors with earth and sun viewed almost from the same direction. In fact for a certain time the earth sensor was blinded by the sun and delivered no useful data at all. But here the YACE camera in conjunction with the IHU-2 came in very handy. By taking pictures of the earth at strategic times we could use the YACE as another earth-sensor looking to the top of the s/c. It gave us badly needed additional data.
The IHU-2 played a vital role in processing and storing these pictures. Unfortunately the IHU-2 turned out not to be so reliable as the IHU-1; every one or two days it crashes. This results in the beacon transmitting only a weak carrier, but no modulation, until the IHU-2 is manually reset from the ground. So far this posed no real problem, but it is a nuisance. Also we have now some data of how the memory is corrupted by radiation.
Eventually we hope to install additional software in the IHU-2 which allows us to avoid or at least automatically recover from these crashes. But this has to wait until things get a bit quieter. Until this time we will have to live with the IHU-2 crashes, which seem to be caused by hard radiation and typically occur, when the s/c comes out of perigee and enters the radiation belts again.
Around Sunday Dec. 10 we had finally reached the proper orientation and spin for the first 400 N motor burn to take us to 50000 km apogee altitude. After reviewing the various constraints it was decided to execute this burn on Monday, Dec. 11 in the perigee of orbit 50/51. The necessary program was uploaded - this is an automatic sequence since the s/c is at this time at the equator with only 600 km altitude. Thus no command stations can be in contact with the satellite at the time of the burn.
So perigee 50/51 came and went - but the burn did not take place. The stored telemetry was analysed and it was found that the sequencer went through all the necessary steps, but no helium pressure was built up and consequently the burn did not take place. (It takes helium pressure to open the fuel-valves.)
On Monday Dec. 11 the situation was investigated and it was found that the pressurisation command was not resulting in any action. Initially the investigation concentrated on a software discrepancy, which existed between the software to test the system in Kourou and the one used in flight. But it was found that this probably was not the cause - the valve(s) simply refused to open. We had such a problem before during the test phase of the s/c, and one of the helium valves was repaired as a consequence. All the indications are that we ran again into this type of problem - which may be caused by the age of the components we are using. We then executed multiple open and close commands, which eventually got the helium flowing. But it was also noted that the helium flow was almost an order of magnitude less than required. Still we build up pressure in the tank to nominal and then programmed the s/c to perform the burn on Monday evening.
This time the burn started properly, but there were some anomalies in the telemetry indicating that the burn did not stop at the programmed time but lasted about 3 minutes longer. Thus we achieve an orbit with about 60,000 km apogee altitude. For our ultimate plans this is no problem, we would have used the electric propulsion anyway to further increase the apogee. James Miller had written a data collection program for IHU-2, thus we have a very detailed account of all the events of this burn. A detailed analysis is under way, but we have already some first indications of the nature of the problem and also possible ways to cure it or at least to cope with it, provided that the motor was not damaged by this event.
During the next days we will investigate this matter in detail. If we have a real problem with the 400 N propulsion system, we may need to change our strategy for achieving a useful final orbit. But it may also mean only that we have to observe a number of boundary conditions during the following burns and that the situation may have no impact on the mission goals.
For the next days no attitude changes are planned, thus we will be able to start to investigate the problem with the 70cm TX. During these tests we will find out if we can operationally plan on using the 70cm TX or not. So we are living exiting times.
Finally we would like to express our compliments and our appreciation to the command stations for the superb job they are doing. Each day they are facing new difficulties requiring a very large amount of time and dedication. They are doing a SUPER job
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AMSAT-OSCAR 40 Status report of December 13, 2000 Nearly four weeks have now passed since the launch, and there are some good things and some bad things to report. As you all know, immediately after separation, we could not receive the 70 cm transmitter. Consequently we programmed IHU-2 to act as repeater and use the 2m middle beacon instead as our main downlink. In addition the two S-band transmitters have been operated occasionally, but the geometry within the orbit must be right to have the antennas point into the general direction of earth. This limits the use we can make of S-band. The main activity so far for the command stations was to learn to fly the s/c and to prepare it for the first burn with the 400 N motor. This required a reorientation - the s/c must point opposite to the perigee orbital speed vector - and a spin-up to about 9 rpm. In view of this ongoing work, it was decided to postpone the analysis of the problem with the 70cm Tx to a time after the first burn. The reorientation of the spacecraft turned out to be more time-consuming than initially assumed. We had two problems to solve: 1. As we get to our firing attitude, the geometry is very poor for producing direction changes of the spin-vector. The magnetic field of the earth is nearly perpendicular to the spin axis meaning that we can change rpm easily, but not direction. It turned out that rounding errors in the software of the IHU were in the same order of magnitude as the desired spin-change. Thus reliable control was not possible. But careful tweaking of the parameters used in the IHU finally enabled us to solve this problem. 2. The geometry is also poor in terms of attitude determination by our sensors with earth and sun viewed almost from the same direction. In fact for a certain time the earth sensor was blinded by the sun and delivered no useful data at all. But here the YACE camera in conjunction with the IHU-2 came in very handy. By taking pictures of the earth at strategic times we could use the YACE as another earth-sensor looking to the top of the s/c. It gave us badly needed additional data. The IHU-2 played a vital role in processing and storing these pictures. Unfortunately the IHU-2 turned out not to be so reliable as the IHU-1; every one or two days it crashes. This results in the beacon transmitting only a weak carrier, but no modulation, until the IHU-2 is manually reset from the ground. So far this posed no real problem, but it is a nuisance. Also we have now some data of how the memory is corrupted by radiation. Eventually we hope to install additional software in the IHU-2 which allows us to avoid or at least automatically recover from these crashes. But this has to wait until things get a bit quieter. Until this time we will have to live with the IHU-2 crashes, which seem to be caused by hard radiation and typically occur, when the s/c comes out of perigee and enters the radiation belts again. Around Sunday Dec. 10 we had finally reached the proper orientation and spin for the first 400 N motor burn to take us to 50000 km apogee altitude. After reviewing the various constraints it was decided to execute this burn on Monday, Dec. 11 in the perigee of orbit 50/51. The necessary program was uploaded - this is an automatic sequence since the s/c is at this time at the equator with only 600 km altitude. Thus no command stations can be in contact with the satellite at the time of the burn. So perigee 50/51 came and went - but the burn did not take place. The stored telemetry was analysed and it was found that the sequencer went through all the necessary steps, but no helium pressure was built up and consequently the burn did not take place. (It takes helium pressure to open the fuel-valves.) On Monday Dec. 11 the situation was investigated and it was found that the pressurisation command was not resulting in any action. Initially the investigation concentrated on a software discrepancy, which existed between the software to test the system in Kourou and the one used in flight. But it was found that this probably was not the cause - the valve(s) simply refused to open. We had such a problem before during the test phase of the s/c, and one of the helium valves was repaired as a consequence. All the indications are that we ran again into this type of problem - which may be caused by the age of the components we are using. We then executed multiple open and close commands, which eventually got the helium flowing. But it was also noted that the helium flow was almost an order of magnitude less than required. Still we build up pressure in the tank to nominal and then programmed the s/c to perform the burn on Monday evening. This time the burn started properly, but there were some anomalies in the telemetry indicating that the burn did not stop at the programmed time but lasted about 3 minutes longer. Thus we achieve an orbit with about 60,000 km apogee altitude. For our ultimate plans this is no problem, we would have used the electric propulsion anyway to further increase the apogee. James Miller had written a data collection program for IHU-2, thus we have a very detailed account of all the events of this burn. A detailed analysis is under way, but we have already some first indications of the nature of the problem and also possible ways to cure it or at least to cope with it, provided that the motor was not damaged by this event. During the next days we will investigate this matter in detail. If we have a real problem with the 400 N propulsion system, we may need to change our strategy for achieving a useful final orbit. But it may also mean only that we have to observe a number of boundary conditions during the following burns and that the situation may have no impact on the mission goals. For the next days no attitude changes are planned, thus we will be able to start to investigate the problem with the 70cm TX. During these tests we will find out if we can operationally plan on using the 70cm TX or not. So we are living exiting times. Finally we would like to express our compliments and our appreciation to the command stations for the superb job they are doing. Each day they are facing new difficulties requiring a very large amount of time and dedication. They are doing a SUPER job