Even with optimal planning, observing time may be lost due to a variety of hardware or software problems.
The ground control computer simultaneously handles a large variety of tasks; it is known to crash regularly. Given the mildest problems, it may be restarted within several minutes. In the most severe situations (e.g. electrical power surges during a severe thunderstorm), it may take several hours to bring the system back on-line. As an image is read down to the ground, the telemetry stream is recorded on analog tape at the receiving station at Wallops Island, Virginia. Therefore, if the ground system crashes during the read of a camera, the image can normally be recovered at a later date from a "history replay" of the tape.
The receiving and commanding antennas at Wallops Island are computer-controlled. If the receiving antenna hardware or its control computer malfunctions as an image is being read, portions of the data will be permanently lost.
On rare occasions, the OBC may halt. This crash usually causes a loss of attitude as the spacecraft drifts away from its original pointing. Recovery may take three or more hours.
The OBC normally operates with a temperature near 52 degrees celsius. Observing targets at Betas between approximately 55 and 95 degrees causes a gradual increase in the OBC temperature. Maximum heating occurs in the winter months when the Earth nears perihelion. When the temperature in the OBC exceeds safe operating levels, the telescope must be slewed to a target outside this hot zone. The observer should therefore have some targets available at cool Betas. For more information on specific Beta limits see Sections 3.1 and 4.4.
Twice each year, in late summer and winter, IUE's orbit carries it through the earth's shadow once each day for three weeks. During shadow passages, which may last up to 81 minutes, no observations or maneuvers can be performed due to heavy battery discharge. Up to 60 minutes each day may be required to prepare the spacecraft for shadow and afterwards reconfigure it for normal science operations. During most of the shadow passages, both long-wavelength cameras are turned off to conserve power. Since the shadow helps cool the spacecraft, observing targets at hot Betas for extended periods rarely presents a problem during shadow season. The shadow occurs at the end of the US2 shift in late winter and at the end of the US1 shift in late summer.
Spacecraft ranging (see Section 2.4) is performed periodically to monitor IUE's orbit to derive accurate orbital elements for ground station antenna tracking. Any given shift may include several rangings, normally lasting from 5 to 10 minutes each. In general, 24 such rangings constitute a complete set and are usually obtained over a period of several days. These are considered to be time-critical "observations" and have priority over normal science operations. Although observatory staff attempt to perform rangings during other observing activities as often as possible, rangings may result in the loss of small amounts of observing time.
Despite all potential problems, the amount of time actually lost is quite small. Major losses of time, due to hardware or software problems, are typically about one per cent of the total observing time.
The IUE Project Scientist is informed when an observer has lost a significant amount of time (usually 10 minutes or more) due to hardware or software problems or to errors in execution. Such time loss will be considered for compensation at the request of the Principal Investigator in accordance with Observatory policy.