The IUE satellite was launched on 26 January 1978 into an elliptical geosynchronous orbit, in which it is always visible from Goddard Space Flight Center (GSFC). Science operations are conducted in real-time from GSFC for 16 hours each day. During the remaining 8 hours, operations are conducted by the European Space Agency at the Villafranca Satellite Tracking Station (VILSPA) near Madrid, Spain. Due to the ellipticity of the orbit and the subsequent daily variations in the satellite's elevation above the horizon, VILSPA can view the satellite for only a portion of each day. Since IUE's orbit is fixed with respect to sidereal time, the handover times between the GSFC and VILSPA observing shifts are normally changed by 2 hours each month. The monthly UT and local starting times of US1 and US2 shifts are given in Table 2.2.
Spacecraft power is provided by two large solar panels. At most orientations the power generated by the arrays is more than sufficient to run the spacecraft. When the spacecraft is tipped at a large angle from the sun, however, it may be necessary to draw additional power from the spacecraft batteries. The primary function of the batteries is to provide power to IUE during Earth shadow periods. Shadow seasons occur when a portion of the IUE's orbit passes through the Earth's shadow once each day for 3 weeks in the late summer and winter. Additional observing constraints are invoked during these periods (see Section 3.3) to ensure that the batteries are fully recharged before the next day's shadow.
| US1 | US2 | ||||
| Month | UT | local time | UT | local time | |
| Jan | 15:00 | 10:00 EST | 23:00 | 18:00 EST | |
| Feb | 13:00 | 08:00 EST¹ | 21:00 | 16:00 EST¹ | |
| Mar | 11:00 | 06:00 EST¹ | 19:00 | 14:00 EST¹ | |
| Apr | 09:00 | 04:00 EST² | 17:00 | 12:00 EST² | |
| May | 07:00 | 03:00 EDT | 15:00 | 11:00 EDT | |
| Jun | 05:00 | 01:00 EDT | 13:00 | 09:00 EDT | |
| Jul | 03:00 | 23:00 EDT | 11:00 | 07:00 EDT | |
| Aug | 01:00 | 21:00 EDT¹ | 09:00 | 05:00 EDT¹ | |
| Sep | 23:00 | 19:00 EDT¹ | 07:00 | 03:00 EDT¹ | |
| Oct | 21:00 | 17:00 EDT² | 05:00 | 01:00 EDT² | |
| Nov | 19:00 | 14:00 EST | 03:00 | 22:00 EST | |
| Dec | 17:00 | 12:00 EST | 01:00 | 20:00 EST | |
| ¹ Shift times may be adjusted during the | |||||
| 3-week shadow seasons. | |||||
| ² Note time change between EST and EDT on the | |||||
| first Sunday in April and the last Sunday in October. | |||||
Figure 2.2: IUE spacecraft
maneuver axes. A positive
rotation around a given axis is in the direction specified by the
"right hand
rule". Pitch is the angle between the sun and the telescope pointing.
At zero
degrees pitch, the telescope would be pointed at the sun while a pitch angle of
180 degrees corresponds to the anti-solar position. The Beta angle is then
defined as 180 degrees minus the pitch angle. A "sunline slew"
is a
combination of yaw and roll motion with the Beta angle remaining
constant.
The spacecraft is three-axis stabilized with a normal 1-arc-second pointing accuracy. Control of the telescope pointing and execution of all spacecraft motion is the primary function of IUE's on-board computer (OBC). The OBC also performs other important functions, such as controlling camera exposures. Prior to August 1985, the OBC used data from three or more of IUE's six gyros to control attitude. Following the fourth gyro failure on August 17, 1985, a backup attitude-control system has been used. The new system (Femiano 1986) uses the two functional gyros and the spacecraft Fine Sun Sensor (FSS). The FSS measures the orientation of IUE with respect to the sun. In the two- gyro/FSS system, the FSS is used primarily to control the roll motion about the telescope optical axis. The two-gyro/FSS system has operational capabilities and accuracies essentially identical to the original three-gyro control system, with the exception that the telescope can no longer be pointed within 15 degrees of the anti-solar position. The two-gyro/FSS has been described and compared with the original three-gyro system by Sonneborn (1985) and Femiano (1986).
In order to prepare for the failure of one of the remaining gyroscopes, the Observatory staff has developed an operating system, consisting of software for the On Board and ground computers, requiring only one gyro, the Fine Sun Sensor, and the FES (Arquilla 1991). This has undergone a successful spacecraft test.
Presently the OBC processes data from the two functional gyros and the FSS and commands the reaction wheels so that either the telescope pointing stays fixed or the desired maneuver is performed. The reaction wheels act as flywheels to store angular momentum. By changing speed, the three reaction wheels cause the spacecraft to rotate about the desired axis.
The spacecraft moves from target to target by executing a series of OBC-controlled slews at a rate of 3 to 6 degrees per minute. These spacecraft-oriented axes are pitch, yaw, and roll (see Figure 2.2). Only maneuvers preserving optimum illumination of the solar panels are allowed. Under the two-gyro/FSS system, maneuvers are executed as a sequence of slews in pitch (changing Beta, or sun angle) and at a constant sun angle ("sunline" slew). The wheel speeds must stay within certain limits, so after most maneuvers, angular momentum must be added to or removed from the wheels. This is done by firing small hydrazine jets and is known as a "wheel unload". Since the wheel speeds also slowly change with time while maintaining telescope pointing, wheel unloads may also be needed prior to maneuvering after a shift-long exposure.
At the Telescope Operations Center (TOC), the Telescope Operator (TO) under the supervision of the Resident Astronomer (RA) controls all aspects of the scientific instrument operation (target acquisition, telescope focus, exposures, and camera reads) and many spacecraft functions (maneuvering, gyro trims, offset guiding, etc.). The TO runs control programs, known as procedures, on the ground command computer to check ground system configuration and spacecraft telemetry for proper status of the relevant systems before transmitting the appropriate commands to the spacecraft. The procedures also record scientific and engineering data which are archived with each spectrograph or FES image. The commands are transmitted at VHF frequencies to the spacecraft. IUE transmits a continuous telemetry stream to the ground at S-band frequencies, normally at a data rate of 20 kilobits/sec. The signal is received by an 18-meter antenna at the IUE tracking station located at the NASA Wallops Flight Facility, Wallops Island, Virginia. The helical VHF command antennas are also located at Wallops. The commands and incoming telemetry are relayed between GSFC and Wallops through a communications satellite. Once the telemetry reaches GSFC, it is processed by the ground computer and displayed on consoles in the Operations Control Center (OCC) for the various spacecraft subsystems as well as in the TOC.
During GSFC observing shifts the RA and TO in the TOC are in continuous voice communication with the OCC located in Building 14 at GSFC. The Operations Director (OD), two Systems Analysts, the Data Operations Controller (DOC), and computer operators monitor the various spacecraft sub-systems and the ground computer to ensure proper operational status at all times. The OCC has ultimate responsibility for the health and safety of the spacecraft and for all spacecraft engineering functions 24 hours per day. The OCC staff is in continuous voice communication with the Wallops tracking station technicians who operate the command and receiving antennas.
IUE's orbit is frequently monitored to provide accurate pointing predictions for the ground commanding and receiving antennas. A VHF signal is transmitted from the ground and retransmitted by the spacecraft, giving an accurate distance and velocity of the spacecraft (this is referred to as "ranging"). Gravitational perturbations cause the gradual westward precession of the orbit's semi-major axis. A station-keeping maneuver is performed at intervals of 6 to 9 months to prevent the satellite from passing outside the range of ground antennas at GSFC and VILSPA. At this time, the large hydrazine jets are fired to slightly modify the semi-major axis so that the orbit drifts eastward again.
