20/12/2000 19:01:50 Subject STRV-1c&d Anomaly Investigation update. Dear Payload Sponsor, The following is a summary of the results of the preliminary investigation into the spacecraft anomaly. I hope that it will help to answer some of the questions which you have been raising. It has been very difficult to reach a stage where our assessment of what has happened appears to be borne out by the evidence. In the process, we have examined several other possible explanations but sifting through the information, which we have, has shown each to be highly unlikely. The fact that there is no power to either receiver is obviously very serious. Although we still consider it unlikely, the failure of both receivers in both spacecraft would be catastrophic. The alternative is that either the power supply has failed (again unlikely that 4 completely independent supplies should fail at a similar time), or an over-current condition has blown the fuses to Receiver-1 and tripped the power supply switch to Receiver-2, on both spacecraft. Initially, we could not see a likely way in which this could happen unless one of half a dozen components failed in a 'short-circuit' condition. Although not impossible, the nature and selection of the components made it unlikely that it would happen independently on both spacecraft without some other significant effect, possibly from the operating environment. There is no evidence to indicate adverse changes in the environment two weeks into the mission, either from the spacecraft or independent sources, and this too is therefore considered highly unlikely. We have therefore concentrated on establishing a mechanism by which an over-current could occur. We have confirmed that the receivers' fuses and trip have a smaller than intended margin over the normal operating currents but these should still have proved adequate in normal operating conditions. Both receivers also supply power to the RF Distribution Unit (RFDU). The transmitter and antenna selection circuits in the RFDU are latching relays and under normal operating conditions one coil in each relay is energised. Because the RFDU is powered from both receivers, an excessive current draw from the coils (such as by energising more than 2 coils) could be sufficient to blow/trip the receiver fuses/trip switch. Examination of the command logs sent to the spacecraft has indicated that these coils were not inadvertently powered by an erroneous command. Examination and initial testing of the switching circuitry (which has flight heritage on other missions) has indicated no fault mechanism which would inadvertently power the coils. We are now therefore focusing on the possibility that either the onboard application code has spuriously activated a third, or third and fourth, coil or noise on the 28V supply caused the current-draw in the active coils to exceed the rating on the fuses/trip. The anomaly occurred on both spacecraft when they were set to run for 6 orbits with experiments operating for the first time. If the failure is due to noise or software problems (rather than component failures), then Receiver-2 on both spacecraft should be re-usable, when we can re-set the current trip. Unfortunately, the nature of the failure is that we currently can see no way of achieving this until the spacecraft suffer an under-voltage or the OBDH a hard reset. Clearly, the spacecraft have been designed specifically to be robust to these conditions and therefore this could take some time, unless we can think of a way of initiating the latter artificially. We would also wish to be as sure as possible of the cause of the anomaly before restarting operations to reduce the risk of a re-occurrence. The plan now is to pursue the investigation further and to understand why we did not detect the potential problem during the ground testing. (It is not yet completely clear whether the testing was insufficiently thorough, the symptoms were missed or were observed but not recognised. There is certainly no record of the current trip ever being activated on Receiver-2 during the ground tests.) We hope that this part of the investigation will help us to establish a cause for the over-current. Provided we get back into the spacecraft, and there is not a component (short circuit) failure, then we have a software correction to re-set the trip automatically if the problem re-curs. Once we have a correct diagnosis of the cause, we anticipate we may be able to introduce another software correction to protect against re-occurrence. As soon as we are satisfied that we have a full and correct understanding of the anomaly and its cause, or we can no longer usefully pursue the investigation, we will make the full results available to you. On the basis of current progress, we shall try to do this by the 28 February 2001. This allows time to plan and conduct laboratory tests including the use of flight spare equipment, if required. Obviously, if anything of significance occurs in the meantime, or we make faster progress, I shall let you all know. I hope that this provides you with a clearer picture of the situation. Despite the rather difficult situation, we all wish you a very Happy Christmas and hope for better news in the New Year. Best wishes R J Blott STRV-1c&d Project Manager.