Operation of a Multi-Year, Multi-Agency Project

Jürgen Fälker, European Space Operations Centre, Darmstadt, F.R.G.

Frederick Gordon, Goddard Space Flight Center, Greenbelt, MD

Michael C. W. Sandford, Science and Engineering Research Council, London, U.K.

This article originally appeared as a chapter by the same name in Exploring the Universe with the IUE Satellite (Y. Kondo, ed., D. Reidel Publishing Company (and appears here with permission of Kluwer Academic Publishers), 1987), and has been updated for mid-1994 by Jim Caplinger.
  1. Operational Concepts
  2. The IUE System
    1. The Spacecraft
    2. The Ground System
  3. Normal Operations
    1. Proposal Selection and Observation Planning
    2. Shift Handover and Spacecraft Operations
    3. A Typical Observation
      1. Target Identification and Acquisition
      2. Telescope Focus
      3. Spectrograph Modes
      4. Camera Operations
    4. Calibration Observations
    5. Data Reduction
  4. Spacecraft Operational Constraints
    1. Sun, Earth and Moon Constraints
    2. Eclipses
    3. Spacecraft Power
    4. Radiation
    5. Temperature
    6. Momentum Wheel Speed
  5. Problem Areas
    1. Gyros
    2. On Board Computer (OBC)
    3. The FES Streak
    4. Fine Sun Sensor (FSS)
    5. Cameras
  6. Operational Performance
  7. References

1. Operational Concepts

The IUE mission is based on real time control of the satellite and instrument and real time acquisition of the scientific data. Thus both the spacecraft operations staff and the science users must support IUE operations 24 hours per day, every day. Operations are shared between two very similar ground systems, one of which is located in the USA and operated by NASA. The other, provided and operated by ESA, is located in Spain and is used by European observers. According to the Memorandum Of Understanding between the three agencies, NASA has use of IUE for two thirds of the time, and ESA and SERC share equally the remaining third. For the purpose of operations it was decided to divide each day into three shifts of eight hours duration, and it was agreed that the NASA ground system would control the satellite for two shifts, and the ESA ground system for one shift.

The receiving site for NASA's ground system was, until April 1986, located at the Goddard Space Flight Center (GSFC), Greenbelt, Maryland. Subsequently it has been removed to NASA's facility at Wallops Island, Virginia (WPS). NASA's spacecraft and science operations facilities have always been located at GSFC. Commands and received data are now transmitted between GSFC and WPS by a commercial communications satellite link.

NASA has three principal areas of responsibility in operations: first, overall responsibility for monitoring and maintaining the health of the spacecraft; second, providing a backup system for the purpose of spacecraft safety during the shift controlled by ESA's ground station; and third, operating IUE for two 8 hour shifts each day, designated US1 and US2. The spacecraft-related tasks are carried out on a 24 hour per day basis by the GSFC IUE Operations Control Center (IUEOCC). The scientific operations are carried out by the IUE Science Operations Center (IUESOC), also at GSFC, in conjunction with the IUEOCC. The IUESOC comprises a Telescope Operations Control Center (TOCC) from which astronomical observations are controlled during the US1 and US2 operational shifts. An Image Processing Center (IPC), located near GSFC, carries out the standard processing of the IUE data.

All the elements of ESA's ground segment are located at Villafranca del Castillo near Madrid (VILSPA). ESA provides spacecraft control and science operations for one shift, designated VILSPA, and in a second shift the standard data processing is carried out. Because the ESA station has limited backup facilities, e.g. only one computer capable of controlling IUE, GSFC maintains readiness to take control and keep the spacecraft safe in the event of a failure in the VILSPA station.

The operational concept of sharing, internationally, the responsibility for operations was made possible by the choice of a geosynchronous orbit for IUE. Constraints on the orbital parameters arose since continuous viewing of IUE is required from the NASA receiving/transmission site and for at least one shift per day IUE must be in view from VILSPA. To insure reliable communications, IUE must be at least 10° above the local horizon at VILSPA for more than about 10 hours per day, which is the time required for an 8 hour shift, a shift handover of about 30 minutes, and a monthly adjustment by 2 hours in the shift starting time. The latter arises since the orbit is fixed in sidereal time but for the convenience of the operations staff the shift start remains at a fixed Universal Time during each month.

An elliptical geosynchronous orbit was chosen with a period of 23 hours 56 minutes, eccentricity 0.2, inclined at 28° to the equatorial plane and with the ground track initially centered over the Atlantic at about 70° west longitude. With this positioning the VILSPA viewing time usually averages 12 hours. As the orbital plane precesses westward due to natural perturbations the viewing time falls and when the 10 hour minimum is reached an orbit adjustment must be made. A maximum of 15 hours may be achieved at its most easterly position. Partly in consequence of these orbit adjustments, and partly as a result of other natural perturbations the ground track of the orbit has gradually evolved during life of the mission as shown in Figure 1. For example, as of late June 1994, the eccentricity has dropped to 0.13, while the inclination has risen to 35°.

Fig. 1. The IUE Ground Track.

An early project objective was that the IUE system would be an international research facility, available to a wide community of astronomers, and be organized much in the way that many ground based observatories are operated. The operations management plan and the ground system were specifically oriented towards achieving this. IUE has been operational since its launch, 26 January 1978, for over 16 years at the time of writing. It continues to provide excellent facilities to the scientific community. It can be unequivocally stated that all scientific objectives of the mission have been met or exceeded. The mission's success was made possible not only by the outstanding performance of the spacecraft and its scientific instrumentation, but also by the excellent cooperation of the technical and scientific staff of the participating agencies. These achievements have been widely acknowledged in the general scientific community as being unique in space exploration.

2. The IUE System

The operation of IUE, like any complex scientific satellite, involves many systems. The space segment includes the spacecraft and the payload systems. The ground segment includes: the ground station which transmits commands and receives telemetry; the operations control center, housing the ground control computers used by the controllers to analyze the telemetry and issue appropriate commands; the science operations center; the data reduction system; and a communications system to link the parts together. Descriptions of some aspects of the system may be found in Boggess et al. 1978 and Boggess and Wilson 1987. To prepare the reader for the discussion of operations that follows, this section summarizes the main characteristics of these systems and details are given of relevant systems not adequately covered elsewhere.

2.1 The Spacecraft

The sole purpose of the spacecraft is to support the Scientific Instrument (S/I) in achieving the scientific objectives for the mission. These objectives give rise to the basic performance requirements for the spacecraft. First the satellite should be able to point the telescope anywhere within the celestial sphere except within 45° of the Sun. Second the Attitude Control System (ACS) should on command move the telescope to a new target with a slew rate of 4.5° min¯¹ axis¯¹ and guarantee that, at the end of the maneuver, the desired target star falls into the 16 arc minute field of view of the Fine Error Sensor (FES), a unit which performs the dual functions of star field mapping and guide star tracking.

Three-axis stabilization of IUE was initially accomplished by an Inertial Reference Assembly (IRA) composed of 6 gas bearing single-degree-of-freedom gyroscopes operated with pulse rebalance electronics. The ACS was designed to hold a 1 arc second diameter star image within a 3 arc second entrance aperture to permit an integrating exposure of at least 1 hour by the spectrograph camera. Now, under the two-gyro/Fine Sun Sensor (FSS) control system, the FSS is also used as a sensor for spacecraft control. The ACS uses the outputs of the attitude control sensors (gyros, FES, and/or FSS) as inputs to the control program in the On Board Computer (OBC). The OBC then controls the spacecraft attitude and slews by changing the rotation speed of the pitch, yaw, and roll momentum wheels.

The primary power for IUE is derived from the two solar cell arrays which are shown in the general layout, Figure 2. The power output is a function of the angle between the Sun and the satellite pointing direction (+x axis). The supplement of this angle is designated Beta. In normal operations the satellite is rolled about the x-axis to keep the Sun close to the x-z plane. Thus the single angle Beta is useful to define the approximate orientation of the satellite with respect to the Sun, and will be so used here.

Fig. 2. IUE Configuration.

Central to the spacecraft control are the command system and the data multiplexer unit (DMU). The Command Decoder receives the commands, checks for errors, and then routes valid commands or OBC data block loads to the correct subsystem. The DMU samples the performance data from the spacecraft subsystems and generates two telemetry streams: one is dedicated to the OBC and its control of the spacecraft; the other is the ground telemetry stream, which includes both the science data and the spacecraft `housekeeping' data. The OBC also uses the ground telemetry stream.

The final key spacecraft element is the communications system, which warrants description in some detail. Two wavebands, S-band and VHF, are used for communication between IUE and the ground. The normal data telemetry link from IUE to the ground receiving site is by S-band (2249.8 MHz). This telemetry can be transmitted at several different bit rates: 40, 20, 10, 5, 2.5 and 1.25 kilobits per second (kbs). There is a convolved, half rate, data mode used to increase accuracy. (The 40 and 2.5 kbs telemetry rates are not used operationally because there is either a timing conflict in the generation by the DMU of the separate ground and telemetry data streams, or the OBC has a timing conflict in reading the two data streams.) There are two telemetry streams, one special for the OBC and the other a ground telemetry stream that is also read and used by the OBC. The dedicated OBC telemetry rate is normally held at 20 kbs for smooth spacecraft control. The telemetry rates are selected by the operations controller. Both spacecraft housekeeping data and science data come down in this S-band telemetry. There are four S-band power amplifier/antenna combinations distributed around the spacecraft, only one of which can be switched on at a time. Two are at the bottom on the sun- and the anti-sun-side respectively, and two are on the satellite upper body in similar locations. As a result, no matter what the attitude orientation relative to a ground station, there is at least one S-band antenna that can be used. There are two VHF transmitters (for redundancy) that operate at 136.86 MHz for the transmission of satellite data. A VHF transmitter is used during range and range-rate operations to determine IUE's location and also under conditions when S-band is not available, such as: spacecraft to ground S-band data link problems; during eclipses because it uses less power than the S-band; and during spacecraft emergency operations. The maximum data rate of the VHF system is 5 kbs. There are two VHF command receivers (148.98 MHz) and decoders for redundancy on board IUE. The command bit rate is 0.8 kbs. All operations and control are achieved through this command system.

2.2. The Ground System

The IUE ground system serves two general functions: controlling (commanding) the spacecraft operations and payload utilization; and receiving and processing telemetry, both science data and spacecraft housekeeping data. Science data processing is also carried out by the IUE computers and various ancillary systems. On account of the separation of the control center at GSFC from the command and receiving site at WPS, a complex communications network is used when operating IUE from GSFC. The ESA facility at VILSPA is very similar functionally but simpler partly because it is all situated at one site.

At GSFC the command of IUE is initiated either in the TOCC, or in the IUEOCC. VILSPA also divides control between a main control room and the observatory room. Operations of an observatory-type satellite are very complex and are achieved for IUE by calling up a set of pre-coded and extensively tested procedures, each carrying out a particular sequence of operations, such as camera preparation, exposure and reading. The procedures are run on the control centers' computers and can check the status of the satellite via the telemetry before selecting the appropriate command and issuing it at the appropriate time. A medium level and user-tolerant language known as Control Center Interactive Language (CCIL) was developed which is used by the operations staff to develop new procedures to meet the requirements of the guest observers (GOs).

The overall ground system is designed in such a way as to resemble functionally the operations of a modern ground-based telescope. A Resident Astronomer (RA) provides the necessary support to the GO. The Experiment Display System (EDS), which consists of an interactive control keyboard and display terminal, is operated by a Telescope Operator (TO), usually working in the TOCC. These personnel, the RA and the TO, possess the required knowledge of spacecraft maneuvering, target acquisition, and S/I operations needed to advise the GO how to carry out critical operations in an efficient way. They also actually carry out the operations, since the GO is not permitted to use the control functions of the EDS console. The EDS provides the observer with all the information needed to plan maneuvers, identify the target, and verify the quality of the spectral image and carry out a `quick-look' analysis of it.

3. Normal Operations

Normal operations are described below in an ordered sequence, commencing with proposal selection and the preparations made prior to the observing shift by a guest observer who has been allocated observing time, and followed by shift handover and normal spacecraft operations. Next, instrument operations are described in the form of the sequence for a typical observation. Finally, calibration observations and routine data reduction are considered.

3.1 Proposal Selection and Observation Planning

The proposals received by the deadline towards the end of each year are reviewed by panels of peers. In the USA, the panels report to NASA Headquarters which makes the final approval of shift allocations. In Europe, ESA and SERC at first allocated their shares separately, but following an agreement reached in 1981, a single joint allocation committee (of peers) selects the proposals and determines the shift allocation. Allocations for collaborative proposals which require both US and VILSPA time are negotiated between the three agencies. An annual schedule commencing in July is prepared from the selected programs, and these form IUE's observing years or observing episodes as they are known in the USA. The detailed schedule for a given month, however, is usually prepared only a month or two in advance to give observers as much time as possible to incorporate their research into their observing plans and change their plans as needed.

Once a GO is granted observing time with IUE, usually in the form of a number of 8 hour shifts, all targets specified in his or her proposal will be checked against Sun, Earth, Moon, spacecraft power, and thermal constraints in order to investigate target availability throughout the year. This information, combined with any time-dependent requirements specified by the GO, is used by the two ground stations to construct the schedules that assign GOs to specific shifts. In due time the GOs are contacted to carry out their observations. On arrival for his or her observing shifts the GO makes final preparations with the aid of an observatory RA, who checks the overall plan for the shift, in order to confirm the feasibility of the proposed observations, prepare any special procedures required, and establish whether suitable finder charts are available for star identification, etc.

If the GO already has some observing experience with IUE and the RA approves, the GO may carry out the observations in either a `remote' or `service' observing mode, in which case he or she does not need to be present at the ground station for the observations. In both cases, the TOCC staff keep in contact with the GO either by phone or over the computer networks as needed. Remote observers receive the raw data over the network and can display it using software similar to that used by the TOs to carry out their quick-look analysis, although they do not have the observatory's software to carry out the data reduction. (They may well have their own software to carry out the reduction themselves, however.) Service observers are not interested in looking at the raw data, and so do not need to have any special software.

3.2 Shift Handover and Spacecraft Operations

In the case of a transatlantic handover, the station assuming control will already be monitoring the spacecraft's telemetry and displaying its status when, about 30 minutes before handover is due, verbal contact is made between the control centers and details are passed to enable detailed planning of operations to commence. Handover itself is marked by the relinquishing station sending a command to change a telemetry bit that then indicates the other station has control.

Routine spacecraft operations include: maintaining communications with the spacecraft, principally by switching S-band antennas and sometimes adjusting the data rate; monitoring all spacecraft housekeeping data against given safety or operational limits for each system, carrying out corrective action when necessary, e.g. slewing to a power positive attitude to prevent excessive battery discharge; planning and execution of maneuvers ensuring pointing constraints will not be violated; correction of gyro drift using information from the FES; dumping of angular momentum by means of the hydrazine jets when the speed of a momentum wheel is too high; and recording processed telemetry data and its analysis for spacecraft trends. In addition, many of the problems described in Section 5, and the testing of new operating procedures give rise to new routine operational requirements. Finally, the operations team must always be prepared, in the event of an emergency, to take immediate action to insure spacecraft safety pending a more detailed analysis by the relevant experts.

3.3 A Typical Observation

The sequence of operations for a typical observation is as follows. The coördinates of the desired target are entered into the ground computer's maneuver generator which offers the operator the choice of several different maneuver sequences to reach the target. The operator selects one, configures the spacecraft appropriately and sends the maneuver command. On completion of the slew to the new target, the field of the FES is transmitted to the ground and displayed on the EDS, the target is identified by the observer, and IUE is moved to place the target in the required spectrograph entrance aperture. Next it is confirmed that the S/I is in the required observing mode. Then, assuming that the camera has already been prepared, an exposure may be started. During the exposure a camera in the other spectrograph can have its image read out, transmitted to ground and it can then be prepared for the next observation. At the end of the exposure the observer can choose to read out the image immediately, or, alternatively, he or she can commence the next operation without seeing the image and arrange for it to be read out later during an exposure with the other spectrograph or even during the slew to the next target. (This last option is not usually recommended as contamination of the image data frequently results.) The observer, with the help of a skilled RA to advise, can get the maximum from an 8 hour shift by planning carefully the sequence of targets in order to minimize slewing time, and also by performing camera preparation and readout during other operations if possible. This is especially important when the observer requires short exposures on several targets. Although a shift is planned in advance, many decisions have to be made on the spot. For example, there is in general no attempt made to coördinate the attitude at handover from one station (or observer) to the next, so the observer's choice of the first target in a shift is often made only 30 minutes before shift handover when the expected spacecraft position becomes known. (This lack of coördination allows the greatest possible flexibility in scheduling shifts among different observers, since otherwise a change in one observer's schedule would adversely affect the schedules of several other observers.)

The commands to generate these operations are transmitted from the ground by running procedures in the ground station's computer with appropriate parameters. The options available are described in more detail below.

3.3.1. Target Identification and Acquisition

Located at the focal plane of the telescope, the FES, in its mapping mode, provides an image of a 16 arc minute field of stars down to V = 14 mag. This has a dual role. First it serves a spacecraft function; identification of a star with known celestial position which can be used to update the ACS to remove the errors in position introduced by a slew. Second it serves the astronomical function of a finder field in which, if bright enough, the target star can be identified and moved to the appropriate entrance aperture of the spectrograph. For fainter targets a blind offset can be made from a brighter nearby object using offset coördinates prepared in advance from Schmidt sky survey or astrographic plates. The FES has an additional function during exposure since it can be used in a tracking mode to follow a field star and provide a fine guidance signal. This is essential for long exposures to avoid movement of the target star from the spectrograph aperture as a result of gyro drift or thermally induced flexure of the telescope tube. The operations of the FES are controlled by using the appropriate procedure. All this is done by the observatory staff. The only related responsibility of the observer, but an essential one, is to provide a finder map at an appropriate scale, usually reproduced from Schmidt sky survey plates, and to identify the target. Both stations also have software that allows them to display fields based on the Hubble Guide Star Catalog, which, since the whole sky is instantly accessible in this manner, has substantially increased operational efficiency.

3.3.2 Telescope Focus

The temperature of the structure of the telescope tube determines the separation of the primary and secondary mirrors and thus affects the focus of the telescope. It is possible to use a mechanism to refocus but this was only used for the initial adjustment in orbit. Thereafter the mechanism has not been used because it is not redundant and any failure in an out-of-focus position would reduce the quality of the data. Instead, the operators keep the telescope in focus by thermal control, achieved by switching heaters on and off at the back of the primary mirror and on the camera deck, following procedures established during the commissioning phase.

3.3.3 Spectrograph Modes

The spectrograph is configured for the observation mainly by operating mechanisms, of which the principal ones, duplicated in each spectrograph, are the high/low dispersion selection mirror, the shutter for the large entrance aperture, and the prime/redundant camera select mirror. Special procedures are used for taking wavelength calibration images, which operate the sun shutter to move a prism reflecting the calibration lamp into the apertures, and for taking flat field images from the UV flood lamps, which require special sequences to insure they strike and warm up reliably.

3.3.4 Camera Operations

Camera operations are normally made very simple through use of CCIL procedures which carry out complex sequences involving 8 electrode voltages, scanning sequences, and checking for anomalous conditions that could lead to loss of image data or, worse, damage to a camera. For example, a single procedure call can carry out a timed exposure. Another carries out a read of the image, transmitting it to the ground computer, then automatically prepares the TV tube's SEC target by erasing the residuals from the previous image and establishing the correct bias voltage on the target for optimum performance on the next astronomical exposure.

As the target is read out the data are transmitted immediately to the ground and, shortly after completion of the whole read, a copy of the resulting image is transmitted from the control center computer to the EDS in the TOCC. There it can be examined by the observer using simple image processing facilities. The most important aspect of such examination is usually an assessment of the level of the exposure. Due to the restricted dynamic range of the SEC vidicon and the wide range of intensities present in many spectra, a repeat observation with a different exposure time may sometimes be necessary. Within 15 minutes of the termination of an exposure the observer can make a decision on the subsequent program based on a quick evaluation of the image. This time is not always wasted, since an exposure in another mode may have already been initiated on the same target, or, when it is clear that a repeat is unlikely, a new target slewed to and another observation started. In camera operations, as in planning of slewing, an efficient observer can optimize the scientific return from an 8 hour shift.

3.4. Calibration Observations

The RAs perform routine observations for the calibration of IUE. Regular images of the wavelength calibration lamp are used in the data reduction process to define the wavelength scale. The camera flat fields are monitored and occasionally some shifts are devoted to acquiring a set of images of the UV flood lamp in order to construct a new Intensity Transfer Function (ITF). The observation of a set of standard stars is regularly carried out to provide data for the absolute calibration of the fluxes measured by IUE and to monitor changes in the overall system sensitivity (see Harris and Sonneborn 1987). Other calibrations, e.g. FES photometric calibration, can be carried out using the data acquired during the GO programs. These astronomical calibrations, spacecraft engineering calibrations, and testing of new procedures are carried out during specific maintenance shifts which amount to about 8% of all the available shifts.

3.5 Data Reduction

The IUE Spectral Image processing System (IUESIPS) is used to reduce the data acquired into products that are usable by the GO and are suitable for archiving for future use by other interested astronomers. The input to IUESIPS is the raw image data that results from the initial stage of image reconstitution performed on the incoming telemetry by the computer supporting operations. The prime scientific purpose of IUESIPS is to produce data that are as free as possible from instrumental effects. This is accomplished by image processing staff using a VAX workstation and associated peripherals. The processing operations include: an implicit compensation for geometric distortion of the camera system; photometric correction of the images using pixel-by-pixel ITFs; fitting of the spectral orders to a spectral format template and extraction of the spectrum; application of intensity calibrations derived from observations of standard stars; and various optional procedures. The principal data product that is delivered to the GO and transmitted to the archives, is a magnetic tape containing the raw data image and the various stages of reduction to a calibrated spectrum.

Each ground station has to keep up with the continuing daily flow of raw data images without incurring an ever-increasing backlog. The goal is to give the GO a data tape within 24 hours of the observations, prior to departure for his or her home institution. For GSFC IUESIPS runs on a VAX workstation which receives the image data via an archive tape from the commanding ground computer. At VILSPA there is only one computer for both operations and data reduction, so the latter is carried out during the US shifts following the VILSPA observing shift.

Over the many years of the mission, various aspects of the IUESIPS reduction have been changed to take advantage of new algorithms or to calibrate changing properties in the detectors. An unfortunate consequence is that a spectrum taken early in the mission cannot be directly compared with a recent spectrum of the same target, unless both have been specifically processed using the same routines. To remedy this problem, and to make the data more readily available to the astronomical community, the staffs at both GSFC and VILSPA have undertaken to develop a data reduction system, NEWSIPS (NEW Spectral Image Processing System), which uses new algorithms and re-derived calibrations to process the voluminous data taken by IUE. All images will be processed with the same routines to create a `Final Archive' of IUE data, making direct comparisons easier for observations of the same target. Among the many additional benefits of the Final Archive are a reduction in the noise level present in many spectra, an error estimate for each extracted point in the spectrum, and more accurate relative and absolute calibrations. As of mid-1994, the algorithms for high dispersion spectra are nearing completion, while those for low dispersion are complete and are being used to process the archival SWP spectra.

4. Spacecraft Operational Constraints

Constraints have to be placed on IUE operations for a number of reasons. Many of these result in restrictions, often contradictory, on pointing. Since these generally have a significant impact on the user, they are explained in detail below. Numerous others exist mostly directed towards maximizing the useful life of IUE's subsystems, either by minimizing wear, or by avoiding risky situations. Some constraints are programed into the ground computers so that they cannot be inadvertently violated by ground station personnel.

4.1. Sun, Earth and Moon Constraints

The only celestial body that IUE is absolutely forbidden to view is the Sun, which would damage the detectors and the mirror coatings, and heat the focal plane. There is a sun shutter which should automatically close if sunlight enters the telescope tube, but it is only intended as a `last ditch' safety device in the event of a failure elsewhere. In fact, IUE is not designed to operate closer than 45° to the Sun (Beta = 135°), beyond which sunlight would enter directly into the sun shield. Prior to the failure of the fourth gyro (see Section 5.1) it was permissible to go to Beta = 0°, i.e., the anti-sun direction, but with the two-gyro/FSS system, Beta > 28° must be maintained so that the FSS can play its role in controlling the satellite.

IUE can be, and has been, pointed at the Earth and the Moon, but as far as normal operations are concerned these bodies are considered to form constrained zones as they obscure a part of the celestial sphere. Furthermore they produce a high background in the FES due to scattered light within about 10° of the sunlit limb. Unlike the Sun constraint these are not hard coded into the maneuver generator and so may be overridden.

4.2. Eclipses

One unavoidable constraint arising from the choice of orbit is that IUE experiences twice yearly seasons of solar eclipses caused by the Earth. This season of intermittent Earth shadow lasts from some 23 to 26 days, and during each season a daily eclipse occurs that varies from a few minutes partial eclipse up to 80 minutes total eclipse. During total eclipses the solar cells deliver no power and so IUE must operate from its batteries alone. To prevent excessive discharge of the batteries, operational constraints are imposed which restrict the collection of scientific data. There are adjustments to the shift times to share the burden of lost time between GSFC and VILSPA and to minimize the overall inconvenience. The annual shadow seasons are designated Winter and Summer, which occur during January/February and July/August respectively. The Summer eclipses are longer and deeper. While the first objective during eclipse is to maintain command and housekeeping telemetry contact, it has to be done in the least power-consuming manner possible. There are a set of flight operations directives (FODs) for use during eclipses which define various satellite configurations that progressively consume less power. These are employed as required to insure that the maximum planned depth of discharge of the batteries is not exceeded, but always keeping in view the goal of minimizing attitude disorientation, so that recovery time for science operations at the end of the eclipse is minimized. One power conservation measure that is adopted is to switch the telemetry downlink to VHF during deep shadow since VHF uses less power than S-band. Plans for shadow operations are made using predictions provided to the IUEOCC by the Flight Dynamics Group at GSFC. In addition to solar eclipses caused by the Earth, the Moon also eclipses the Sun. However these tend to be brief, partial eclipses and early in the mission did not require spacecraft reconfiguration. The two-gyro/FSS uses the Sun as one input to the control system, so all eclipses now require special spacecraft control configurations which use only the FES and gyros.

4.3. Spacecraft Power

Figure 3 shows how the solar array power output has changed with time due to the degradation of the solar cells in their radiation environment; such degradation was, of course, anticipated and its effects planned for. To insure long battery life it is essential that IUE is at a power positive attitude for nearly all the time, except for the unavoidable eclipses described above. At launch, the IUE power requirements were about 180 W, but at the time of writing with the two-gyro/FSS mode of operation in use (see Section 5.1), the requirement is between 131 W and 154 W, depending on the nature of the operations taking place. In February 1994, the power positive region was in the range 40° < Beta < 101° for the higher power requirement, while the range was 28° < Beta < 122° for the lower power level. These regions are narrower in August when the Earth is at aphelion.

Fig. 3. Solar Array Output.

Although the batteries can be used to supplement the solar cell power and support operations over a wider range of Beta, this is minimized to prolong battery life and conserve their capacity for the more critical eclipse seasons. An FOD limits the number of times IUE may be used in a power-negative or power-neutral situation in any one year and the duration of any one such session. There are also constraints on the magnitude of current drain and other related operational parameters.

4.4. Radiation

The principal source of radiation affecting IUE is the Earth's out trapped electron belt. Compared to a circular geosynchronous orbit which would lie on the upper side of this belt, IUE's elliptical orbit dips into a region of higher electron flux at perigee. Towards apogee, however, there is a large portion of the orbit where the radiation is substantially less. Based on calculations of the expected radiation, the sensitive elements of the IUE systems were provided with shielding to the maximum extent possible within weight and design constraints (see Boggess and Wilson 1987). At a late stage, because of concern about radiation levels, a particle detector was added to IUE. In April 1991, this detector began to fail, and the staff at GSFC developed an alternate method in which the electron flux levels measured by the weather satellite GOES 8 are analyzed to determine the approximate radiation level around IUE. This voltage is monitored on the ground and at the predefined safety limit of 3.6 V the operating cameras are switched to the standby mode. This limit is only occasionally reached, and at this radiation intensity the background induced in the camera precludes useful exposures, even short ones. The orbit is such that the radiation principally affects the US2 shift which is consequently mainly scheduled for programs with short exposures and for maintenance. In addition to camera background, the radiation was expected to adversely effect some of the electronics and possibly the magnesium fluoride camera faceplates by the end of the three year design life. Recently, accumulated radiation damage to the DMU has restricted operations to temperatures below 26.2°C. This limit is usually avoided by insuring that the OBC remains below its own temperature limits (see below.)

4.5. Temperature

It is essential to keep the temperature of the various IUE components and subsystems controlled within certain limits. To monitor these temperatures, there are thermistors located in critical areas; their output is delivered to the telemetry via a subcommutator. In the IUEOCC, the thermistor outputs are converted to temperatures for display on the monitors. Preset limits are built into the computer system, and these cause the temperature displays to flash or blink when the limit is reached. The limits are set so that the operating analysts are warned that the parameter is outside its normal range. Usually no immediate action is required at a blink limit; however, if the `redline' limit, an absolute value set by the design engineers, is exceeded then corrective action must be taken. The OBC and DMU are cases in point. Since their temperatures are affected by the sun angle, Beta, and also by the Earth-Sun distance, there is a thermal constraint on Beta that is most restricting at perihelion. Presently there is no limit on science operations if the OBC is below 54.6°C or if the DMU is below 26.2°C; but if the temperature stabilizes at one of these values then IUE must be maneuvered outside the range 40° < Beta < 105° within one hour in order to cool off. There are also special circumstances, e.g. prior to performing an orbit adjustment, when it is expeditious to cool the OBC before taking a particular action, even though it may be below the blink limit. There are some 90 telemetered temperature points that are subject to blink or redline constraints covering such components as batteries, gyros, electronics, propulsion systems, solar arrays, etc.

4.6. Momentum Wheel Speed

An FOD covers the control of the momentum wheels to provide smooth spacecraft control and to prevent excessive wear of the bearings. At the start of the mission the wheel speeds were normally kept between 250 and 1000 rpm in either direction. The wheel speeds are permitted to run through zero or to saturate during maneuvers. When the combination of gyros 1, 3, and 5 was no longer available for emergency attitude recovery, the momentum stored in the wheels had to be reduced so that they could always bring IUE to a safe hold attitude using the analog Sunbath mode. At that time the wheel speed for normal operations was changed to 200 to 500 rpm for roll and yaw and 200 to 1000 rpm for pitch. To date there have been no mechanical problems with the momentum wheels, and even if one of the orthogonally oriented ones should fail there is a spare that could contribute to any of the three axes.

5. Problem Areas

The problems that have occurred with IUE can generally be divided into two categories, those arising in spacecraft systems and those in ground facilities. With regard to the latter, most are related to an aging hardware system and operational software shortcomings which continue to be found throughout the lifetime of the program. However, the ground system problems, hardware or software, can be and are dealt with, although often at the expense of considerable effort. We will not dwell here on these ground system problems, for they are for the most part those common to any complex computer system. Although retrieval and repair of failed spacecraft has been demonstrated, this is very costly and can at present only be done for a low Earth orbit case. Until the development of robot repair missions, any anomaly occurring aboard the IUE spacecraft must be dealt with by changes to the commands transmitted to the satellite or by adopting workaround procedures or by switching to redundant subsystems.

In planning the operations of IUE a very high priority has been given to the safe preservation of the satellite and conservation of resources within the constraints of providing a scientifically effective and productive mission. Even with this approach anomalies can and have occasionally occurred in several systems, but those in three systems, the gyros, the OBC, and recently the FES, have caused the most concern and are described below. Other systems have degraded in an expected (non-anomalous) fashion, e.g. the solar cells which result in Beta constraints as described in Section 4.3. Problems have also occurred in the S/I. Those occurring in the camera system have been potentially the most serious and these are also described below.

5.1. Gyros

The most troublesome operational system (from the point of near catastrophe) has been the gyros, which are essential to the attitude control of IUE. Thanks both to careful and original design concepts and also to subsequent foresight and ingenuity by a number of people associated with the program, it has been possible to maintain the performance of the ACS. As a result, satisfactory scientific data collection has continued for over 16 years from launch to the time of writing. The original planning foresight was to build a six gyro package (see Figure 4) that was designed to maintain three-axis control as long as any three gyros were operational. This mode was used for 7.5 years during which three of the gyros failed.

Fig. 4. Inertial Reference Assembly.

The first three gyro failures occurred as follows. During the third eclipse season, in the Spring of 1979, three of the then six operating gyros (Gyros 2, 4 and 6) were turned off to conserve power. At the end of the eclipse season Gyro 6 failed to restart, although a number of attempts were made. (Hope has not been permanently abandoned for Gyro 6, as will be seen below.) In the middle of 1981, maneuver accuracy decreased and telemetry analysis indicated that Gyro 1 was suspect. It was taken off-line and a further analysis finally led to the conclusion that the feedback loop was open and the gyro was not recoverable. Gyro 1 was designated a permanent loss in March 1982. In July of the same year IUE began to slowly drift. Gyro 2 telemetry showed a rapid increase in current indicating that the gyro had stalled. At this point Gyro 2 was written off and the control software changed so that the spacecraft would be operating with the remaining Gyros 3, 4, and 5, at which point IUE had no spare gyros left. Seemingly, one more failure would mean the end of scientific observations. The IUE mission managers had anticipated this possibility even before the loss of the third gyro and had asked that the attitude control experts should prepare contingency plans.

They concluded that a method of attitude control could be worked out that used two active gyros and the Fine Sun Sensor (FSS). A program was planned and teams formed to develop, build and test such a system. The testing was done on simulators, as tests on the spacecraft were considered an unacceptable risk. By Spring 1983 the two-gyro/FSS control system had been carried as far as it could go, short of spacecraft usage. As it turned out, for three years (from July 1982 until August 1985) the satellite operated satisfactorily on the three remaining gyros. Then on 17 August 1985 the critical situation was eventually reached: Gyro 3 failed. The spacecraft's gyro body angles began to drift in a situation when they should have remained fixed and the Gyro 3 current fell from a normal operating level of 65 mA to 2 mA. The duty IUEOCC Operations Director immediately recognized the situation and took the action necessary to place the satellite in the safe, Sunbath, mode. The Sunbath mode uses analog information from the sun sensors to orient the satellite to Beta = 67° which gives the maximum output from the solar array. However the satellite slowly rotates about the yaw axis (z-axis) thus eventually requiring an attitude recovery exercise. After due consideration, it was decided to try to restart Gyro 3, but to no avail. A conference was called of all experts who could contribute to the situation, and it was decided to try to restart Gyro 6, but not so vigorously as to cause further damage in this already critical situation. If this failed, then the trusted but untried two-gyro/FSS system would be brought into play. Gyro 6 did not restart and the new system was now the only hope. It worked! Not smoothly at first, but with careful nurturing, it evolved in a few more weeks into a system that enabled IUE to gather scientific data essentially as well as in the old days of the three-gyro mode. At the time of writing it continues to do so, always with new improvements being evolved. This was a very great and ingenious achievement by the staff involved.

Even this two-gyro system is not seen to be the terminal control system, for considerable progress has been made in the development of a promising one-gyro/FSS system, and it is possible that, in the event of another gyro failure a workable system could be operational within a few months. The one-gyro/FSS system, unfortunately, may not be useable with the recent presence of the FES streak (see Section 5.3). Finally, even a no gyro operation may be possible using the FES and FSS and making only small slews from star to star but science operations would be greatly restricted.

5.2. On Board Computer (OBC)

The OBC is the other spacecraft component identified above as prone to operational anomalies. Indeed, prelaunch, the OBC was the cause of major concern due to malfunctions when running `hot'. If the ACS can be considered the nerve and muscle of IUE, then the OBC is certainly its brain, for its function is to tell the ACS and other systems what to do and when to do it. It does this through a number of installed software routines called `workers' which are in turn controlled by the computer executive system program. These workers are algorithms designed to perform specific spacecraft tasks. When there is a problem involving the OBC, it usually seems to involve the malfunctioning of the executive system hardware, with its subsequent effect on the performance of at least one of the workers. Over the lifetime of the satellite there have been numerous OBC malfunctions. While not always understood (mainly due to limited monitoring telemetry), most have not had any major impact on operations; i.e. the effect of the malfunction was easily and quickly corrected, usually by the simple retransmission of a command. However, there have been cases when attitude control was lost and recovery required a great deal of effort on the part of the RAs and the IUEOCC personnel. During the first three years of operation, 1978-80, the OBC was prone to crashes that caused loss of attitude. The OBC software analysts, OBC hardware engineers and operations personnel identified the conditions under which these crashes occurred. The software analysts then inserted code to detect the failures when they occurred, and restart the OBC control. As various failure modes occurred with time, the OBC code was modified until OBC crashes were no longer a significant operational problem. This worked so well that continuing failures, `hits', would have been undetected so a counter was inserted to record them. Many times there were several hundred `hits' per day from which recovery was automatically made without impact. The `hits' have decreased to only a few per day in recent years.

Another class of anomaly in the OBC was concerned with camera control. A VILSPA study of this phenomenon in June 1980 suggested, but not conclusively, that these problems might be associated with passage of IUE through transitional regions of the radiation belts. These OBC-related camera anomalies still occur on occasion, but are usually caught quickly enough to minimize the effect on the science.

One type of anomaly that has occurred twice, each time causing major concern, is the failure of the OBC to carry through a successful orbital adjustment (DELTA-V) to keep IUE on station. The first time that this occurred was on 12 January 1984. Prior to this DELTA-V there had been eight without incident. The magnitude of the DELTA-V is controlled by the duration of the firing of IUE's high thrust jets. The durations are calculated and the values are inserted into the Worker 19 command sequence which controls the operation. In January 1984 it was planned to fire the appropriate jets for 8.2 seconds but the program aborted after 1.64 seconds, leaving IUE in a slow spin. It took over four hours to restabilize the spacecraft and several hours more to determine its attitude. After analyzing the orbital data a new DELTA-V was planned and successfully carried out on 14 February 1984 without doing anything different except cooling the jets by orienting them away from the Sun prior to firing. The next DELTA-V in November 1984 also proceeded without incident but July 1985 brought a repeat of the fault of January 1984. After collective consultation with the experts it was concluded that differences between successful attempts and failure lay in the yaw phase data. The operational plan was modified so that the next try would emulate the successful DELTA-Vs, which had a negative yaw angle start. On 9 August the exercise was repeated and was successful. It is not known for certain whether the problem had been correctly identified since only eight days later the failure of the fourth gyro occurred and Worker 19 had to be extensively modified for use in the two-gyro/FSS mode. The two DELTA-Vs carried out since have both been successful.

5.3 The FES Streak

As stated earlier (see Section 3.3.1), the FES can, in theory, identify stars as faint as V = 14 mag. There have been some problems in recent years which have come to significantly affect the FES's abilities. On 5 February 1991, VILSPA reported the presence of a uniform scattered light background in the field of view, comparable to what one might see from V = 12 mag stars filling the field. This occurred during the Winter shadow season which, among other things, subjected IUE to greater temperature changes than it experiences at other times of the year, and may have caused this anomaly to appear. The speculation was that some of the thermal insulation had peeled off of the telescope tube and was now reflecting sunlight into the tube.

The scattered light intensity correlated very well with Beta, in the sense that higher Beta yielded a higher background, and was virtually non-existent at lower Beta. There were also fluctuations in intensity as time went on for any given Beta which were not predictable. Although initially alarming, the scattered light's presence did not have a substantial effect on IUE's observing capabilities. Faint targets were acquired as blind offsets, long wavelength high dispersion and short wavelength high and low dispersion spectra were not contaminated, and long wavelength low dispersion spectra were contaminated only for exposures longer than several hours.

Then on 14 September 1992 the scattered light anomaly became even more anomalous. At the end of a maneuver to a target near the Orion Nebula, the staff noticed that there was a bright streak of light in the field of view, comparable to a V = 5 mag star. They were not initially alarmed because they believed this feature was simply the Earth's limb, which they knew was nearby. However, this feature did not go away as they expected it to, and the staff became very concerned that a potentially mission-threatening event had occurred. The feature disappeared without any trace a few hours later while it was being investigated, leaving the staff to wonder if it would be seen again. A month or so later, it did return for a few days, then it was absent, but over time it became a regular feature in the field of view. As it started to make its presence felt, the streak (as it is known and loathed) became more of a problem, filling up to 80% of the field of view and at times saturating the FES detector in parts of that field. The staff concluded that this new problem, like the previous scattered light trouble, is being caused by some of the thermal insulation scattering sunlight, and sometimes Earth light, into the telescope tube. This insulation, while causing the streak, is not physically blocking the tube, since stars can still be seen throughout the field of view; the streak simply adds to the star's intrinsic brightness.

Remarkably enough, science efficiency and data quality have not been substantially affected since the streak's appearance. The staffs at both VILSPA and GSFC quickly developed workarounds for acquiring targets. The same variation with Beta angle that was seen with the scattered light is present with the streak, although there is no Beta at which it disappears altogether. Also, the streak has changed appreciably in the past year, and may be getting less intense as time goes on. As with the previous background, only long wavelength low dispersion spectra show any contamination, but that contamination can now appear in less than one hour. The staffs at both ground stations, as well as several independent observers, are working to find ways of reducing or eliminating this contamination from target spectra. The fact that spectra of the streak are similar to spectra of solar-type stars supports the conclusion that the illumination sources are the Sun and Earth.

The greatest impact has yet to be realized. The one-gyro/FSS system that was previously developed relies criticaly on the FES's ability to identify stars brighter than about V = 9 mag to maintain IUE's stability while on target. The streak intensity at Betas > 75° can wash out all but the brightest stars, which frequently leaves nothing visible in the field of view. This situation is not a problem with the two-gyro/FSS system since IUE can function quite well without the FES in the attitude control loop for the better part of an hour. After that, a short maneuver (on the order of a few arc minutes) is needed to find a star to verify IUE's pointing. But if there is another gyro failure, it is quite likely that, at best, IUE will be confined to observing targets at low Betas, and then only when bright enough stars are in the field of view.

5.4. Fine Sun Sensor (FSS)

Since the FSS is now a key part of the ACS, used to provide both roll and pitch information, its anomalies have assumed greater importance than before. It will play an even greater role in the attitude control if the one-gyro/FSS has to be installed. The FSS contains dual systems, each of which consists of an electronics package and dual detector heads. The two systems each cover an angular range of ±32° in pitch and roll centered on Beta = 45° and Beta = 105° respectively. There is an anomaly in the low-Beta FSS that prohibits operations below Beta = 28° since the two-gyro/FSS control system was put into operation.

5.5. Cameras

Four problems have occurred with the cameras. The first two, microphonic noise and scan control logic failures, have been largely overcome by changed operating procedures. The third has been the almost certain loss of use of the back-up or `redundant' camera in the short wavelength spectrograph (SWR). Fourthly, a high voltage discharge developed in the redundant long wavelength camera (LWR) and it can now only be operated at reduced gain.

The IUE TV camera tubes are very sensitive to mechanical vibrations particularly at frequencies in the range 500 Hz-10 kHz. This sensitivity arises because the support structure of the SEC target is a 2 cm diameter film of aluminum oxide 50 nm thick, which has high-Q resonances in this frequency range. The target, connected to a very sensitive amplifier, acts as a condenser microphone. During the commissioning of the S/I, severe microphonics frequently occurred. This was tracked down to the panoramic attitude sensor (PAS) which was used to sense the Earth limb in order to determine IUE's attitude during the initial attitude acquisition process. After use, the PAS had been left switched on with its scanning prism rotating but, on discovery, was switched off.

Occasionally important data were lost due to a microphonic `ping' lasting 10 seconds that affected the LWR images by producing a band of interference across the image. It was eventually discovered that this `ping' was induced by the warm up of the heater in the readout gun of the LWR tube. By increasing the delay between turning up the heater voltage and the commencement of the readout scan, it was possible to insure the `ping' occurred before the scan started.

The second problem occurred in the setting up of the digital logic circuits that perform the readout scan of the LWP camera. When the circuits have remained unused for some hours, e.g. during a long exposure, the serially loaded set-up coördinates fail to pass a specific bit in the line scan register. When the scan is initiated the register then counts correctly but from the wrong starting point so that only part of the SEC target is read out. The solution to this, which took quite some time to develop into a reliable form, was to include in the operating procedure the initiation of a dummy scan to insure the logic was unlatched prior to commencing the readout scan.

The third problem has been the intermittent operation of the SWR camera tube arising from a loss of the G1 on voltage, which controls the electron readout beam current. The SWR camera was selected at the outset to be set up and calibrated in orbit for routine use in the short wavelength spectrograph. In the middle of this commissioning period the intermittent operation first occurred. At that time the decision was made to use the SWP camera routinely and this has operated flawlessly to date. At intervals, attempts were made to operate the SWR camera and it was soon observed that successful operation was much more likely when the camera electronics module was cold. As time has gone by successful reads have become less frequent even at low temperatures, and it seems unlikely that the SWR camera would be of any use in the event of the failure of the SWP. One possible cause of the failure is the loss of the clock pulse input to the G1 modulator. Such problems had occurred on the ground due to a poorly mating connector.

The fourth major problem has been the high voltage discharge or `flare' that has developed in the LWR camera. A proximity focussed intensifier, whose output is coupled by fiber-optic windows to the TV tube, acts as a UV-to-visible wavelength converter. The high electric field (3.8 kV mm¯¹) is near the maximum that the materials used for the photocathode and the anode can stand without a discharge developing. Any enhancement of the electric field caused, for instance, by a sharp point on the photocathode or contamination by a low workfunction electron emitter can result in an electron (or ion) discharge that gives rise to a point of light in the output image of the converter. The flare intensity increases rapidly as the voltage across the converter is raised, the image is enlarged due to scattering and halation, and further flares with a higher threshold voltage may appear. The phenomenon caused very considerable problems during the development of the IUE cameras. It was only after considerable labor by the project, and the eventually-successful converter manufacturer, ITT, that acceptable converters were made. The development was very successful. The high photocathode efficiency and very low background count rate (in the absence of radiation), which were better than specification, directly contributed to the performance of IUE. A difficult compromise had to be made between tube gain, safety margin from flares and reasonable production yields. The converters were formally rated at a maximum of 6 kV and some testing for flares and background was carried out at that voltage, but for all other testing and operations a voltage of 5 kV was selected.

In 1983 a weak flare was discovered in long exposures on the LWR camera. The center of the discharge was just outside the field of view of the TV camera but well within the working area of the converter, so only a crescent of light was picked up by the TV tube. Subsequently the flare steadily increased in intensity and therefore affected shorter exposures. The size of the light patch has also greatly increased. Fortunately the LWP had been calibrated and was made available to observers since it offered improved sensitivity in some parts of the spectrum. In October 1983 the decision was made to switch to the LWP for all normal observations. At first, use of the LWR was still permitted at 5 kV to complete series of observations of variable objects, but from April 1985 operation has been restricted to 4.5 kV which is at present below the flare threshold. This has reduced the gain by 27%.

On one occasion in 1976, during laboratory acceptance tests on the LWR converter before coupling to its TV tube, an extremely weak pinpoint of light was detected when making a 2 hour exposure at 6 kV but this did not recur on subsequent tests. The position of that flare is exactly coincident with the deduced center of the flare now seen through the TV camera. It is assumed that some aging process has lowered the flare threshold voltage. The extreme difficulty in producing completely flare-free converters forced the project to take the risk of including this one in the flight equipment. With hindsight this gamble has been well justified by five years of trouble-free operation by the LWR. Obviously such aging effects as have been seen in the LWR now raise the question of whether flares may in time appear in other cameras. These were not tested above 6 KV so there is no information as to whether they may have had incipient flare points which could appear at higher voltages.

6. Operational Performance

Despite the problems described above, IUE is still operating at high efficiency nearly 16 years into a mission which was originally designed for three years with consumables sized for five years. The ability to reconfigure IUE and achieve the mission objectives in quite different ways has been one of the important factors contributing to its extended life.

The scientific efficiency of IUE was not high in the first month or two after launch. The operational procedures and the ground computer software needed some debugging and extensive tuning to reduce unproductive overhead time when IUE was essentially waiting for the next command to be uplinked. As the operations staff gained confidence in the use of the improving software IUE soon achieved an average efficiency (defined as the fraction of time spent collecting astronomical photons) of about 60%. It was expected that the increased constraints on pointing in recent years would tend to reduce efficiency but this has been compensated by increased attention to efficient scheduling. At some point in the future it may be necessary to employ integrated operations so as to maintain efficiency by forming an optimum observing sequence from all the target lists of several GOs in one time interval.

At the time of writing, IUE is still in very good shape, with its scientific performance essentially unimpaired since launch over 16 years ago in January 1978. The targets it had observed by March 1986 included:

These observations have resulted in the production of almost 95,000 spectral images, collected on behalf of more than 1200 astronomers, and have resulted in over 2100 papers being published in refereed journals.

The IUE archive has assumed increasing importance (see Giaretta et al. 1987); any image can be obtained after expiration of the 6 months exclusive data rights held by the original observer. Currently over 70,000 spectra per year are requested and the rate of requests now exceeds the rate of acquiring new spectra.

7. Abbreviations

It is convenient and customary among the IUE community to use abbreviations when referring to the various complex systems. We have restricted their use here to the more common ones. A list follows of all those used in this paper both those specific to IUE and those in more general use.
Last updated: 29 July 1997
Jim Caplinger