Figure 1. The ERS-1 spacecraft
The first European Remote-Sensing Satellite (ERS-1) was launched from Kourou on 17 July 1991 (Fig. 1). Today, with the celebration of its fourth anniversary in orbit, ERS-1 has exceeded twice its planned operational lifetime, having completed more than 20 000 revolutions of the Earth.
The platform providing the major services for satellite and payload operation continues to perform exceptionally well. In particular, the power and thermal-control subsystems are very stable, and there is still a large reserve of hydrazine onboard for a further extension of operations. The comfortable power- budget margin allows continuous and extensive operation of all payload elements.
The healthy condition of ERS-1 is currently allowing the unprecedented tandem mission with ERS-2, as well as providing additional confidence that its data-gathering role could be maintained for several more years.
Figure 1. The ERS-1 spacecraft
ERS-1, being the first mission of its kind, has been operated in various orbital scenarios, ranging from the 3-day reference orbit used during the commissioning phase, to the 168-day repeat cycle allowing a very high density of Radar Altimeter (RA) tracks.
The latter orbit covering the so-called 'geodetic phase' has been visited twice by ERS-1 (with an offset introduced to improve the track density) and has provided scientists with a detailed mapping of the topography of the sea-surface for the first time. This topography is dominated by the structure of the gravitational equipotential surface called the 'geoid', the shape of which is largely determined by the sea floor's topography. ERS-1 is therefore providing us with a glimpse of the vast hidden parts of our planet that lie beneath the sea (Fig. 2).
At present, ERS-1 is operating in the nominal 'multi-disciplinary phase' with a 35-day repeat cycle, and is leading the ERS-2 satellite which is in the same orbit with a one-day offset.
Aside from these planned changes in repeat cycle, orbit maintenance is required to compensate for the air drag on the spacecraft, which varies with the degree of solar activity. The compensatory in-plane control interventions generally consist of two-burn thruster manoeuvres.
During 1991 and early 1992 solar activity was high and control cycles were typically in the order of 1 to 3 weeks. In the subsequent period of low solar activity, the time between in-orbit corrections has grown to one month or more. Out-of-plane manoeuvres to correct for the reduction in the inclination of ERS-1's orbit have also been required every 9 to 10 months. The ERS-1 ground track has been kept within a deadband of better than + - 1 km since launch. For the current tandem operation, the maximum distance between the ERS-1 and ERS-2 ground tracks is fine-tuned to just a few hundred metres.
Shortage of fuel, often the commodity that determines the end-of-life for a satellite, will not be a key factor in the continued operation of ERS-1. The average rate of consumption is just 0.9 kg of hydrazine per month and 255 kg of the original 300 kg of fuel still remain. This abundance of fuel is a direct result of the perfect performance of the Ariane-4 launcher back in 1991. Additional hydrazine, carried as a contingency for fuel-hungry orbital corrections in the event of a dispersion in launcher injection parameters, was therefore not needed and part of it can be used for other purposes, including rapid de- orbiting of ERS-1 at its eventual end-of-life.
Figure 2. Map of marine gravity anomaly derived from ERS-1's Radar Altimeter instrument
The platform's thermal-control system has worked perfectly throughout the four years in orbit. The observed onboard temperatures lie well within the acceptable operational limits, the maximum average being lower than 16 degC. The maximum temperature observed so far has been 30 degC for the solar-array drive mechanism and the gyroscopes. The observed degradation in spacecraft-radiator temperature amounts to less than 1 degC for each year in orbit (Fig. 3).
The power subsystem provides an equally positive picture. The battery compartment contains four NiCd batteries, each with a specified capacity of 24 Ah. Their actual in-orbit capacity at beginning-of-life (BOL) was 32 Ah, since when the battery compartment has been maintained at about -4 deg C, providing optimum and stable conditions for a long lifetime. Nominal ERS-1 operations have resulted in a mean depth of discharge (DOD) of around 20%, which is notably lower than the 24% anticipated before launch.
The end-of-discharge voltage at the end of eclipse is a good indicator of battery degradation. The battery supplier guarantees a minimum end-of-discharge voltage of 26 V after a four-year mission, to be compared with the 28 V presently being measured. This value has not changed for a long time, leading to the conclusion that battery health is good.
ERS-1's solar-array power is regularly monitored. A survey starting from launch in 1991 is presented in Figure 4. It shows that BOL performance exceeded expectations and that the degradation in available power is presently 1% per year. The available power level will therefore approach 2300 watts after five years in orbit, thereby considerably exceeding the two-year specified design goal of 2100 watts.
The lifetime predictions for both the thermal-control subsystem and the solar array are based on typical degradation factors taking into account mainly solar ultraviolet radiation and in-orbit particle radiation effects. The ERS-1 platform, derived from the French national Spot platform, has confirmed the experience from earlier Spot flight models that factors used to model degradation in polar Sun- synchronous orbits are too conservative. Studies are therefore underway to use this knowledge to optimise the design of the future generation of polar-orbiting spacecraft.
ERS-1's attitude and orbit control subsystem consists of Earth and Sun sensors, gyroscope package, reaction wheels and magnetotorquers. This subsystem is also closely monitored and regular gyroscope main-tenance campaigns are performed to determine long-term drifts. During one such campaign, the onset of noise was noted in one operational gyroscope. Although the noise level was within acceptable limits, the active-gyro configuration has been modified to keep the gyroscope in question as a redundant unit.
The payload itself provides a further means for direct verification of AOCS performance. The Radar Altimeter (RA) accurately detects the off-nadir pointing, while the Active Microwave Instrument (AMI) determines the Doppler shift with respect to the Synthetic Aperture Radar (SAR) antenna, allowing a precise estimate to be made of the satellite's combined yaw/pitch mis-pointing. Actual mis-pointing has proved in practice to be less than 50 mdeg, and thus a factor of five better than the specified AOCS performance.
The On-Board Computer (OBC) on the ERS spacecraft runs the 'centralised flight software', which is a small package (44 kwords) incorporating all mission-essential functions. A series of spurious parity errors detected in the autumn of 1992 led to a software re-initialisation and a redundancy re-configuration for the OBC. The failure was traced to a specific memory area dedicated to storage of the payload command queue. The inherent versatility of the memory design allowed the size of the command queue to be reduced and the failed memory block to be eliminated, whilst still maintaining sufficient capacity to store the payload commands. Full redundancy for the OBC has thereby been restored and operations have subsequently continued without further incident.
Figure 3. Evolution in the spacecraft platform's average temperature as a function of lifetime
Figure 4. Evolution in solar-array performance since launch
The payload-radiator temperature has proved just as stable as the platform's thermal con-trol, with less than a 1 degC change per year (Fig. 5).
Figure 5. Average payload temperature as a function of lifetime
Instrument data-handling and telemetry (IDHT) subsystem
The handling of the payload's science data,
as well as its transmission to ground by X-band link, is provided by the IDHT subsystem. The lifetime-
critical elements in this subsystem are the 6.5 Gbit tape recorders and the travelling-wave- tube
assemblies (TWTAs).
The tape recorders have been operated in sequence for three-month periods. A nominal orbit requires one complete recording and playback cycle, which means that each unit has been subjected to approximately 10 000 cycles over the four-year period. Life tests on the ground have validated such tape recorders for almost 38 000 tape passes and 120 000 start/stop cycles. The IDHT high-rate link TWTA failed in December 1993 after 2.5 years of operation. A Failure Review Board concluded that it probably failed due to fatigue resulting from thermo-mechanical stresses induced by repeated high-voltage on/off cycles. The design had been life-tested for 18 000 switching cyles, based on predicted usage over a three-year period. The favourable power conditions aboard ERS-1, however, have allowed for extensive SAR operation. The TWTA's failure after 27 000 switching cycles therefore meant that it had already greatly exceeded its expected lifetime.
Measures have since been introduced into the mission-planning system at ESOC, in Darmstadt (D), to limit the number of such switching cycles by optimising the image-acquisition sequences. The redundant high-rate TWTA has experienced just over 11 500 cycles in 1.5 years, while the low-rate TWTA in operation since 1991 has been sub-jected to 20 000 cycles. Assuming a life-time capability of 30 000 switchings, the remaining lifetimes for both TWTAs are estimated to be slightly more than two years. Given that the defective TWTA is still capable of handling low-rate trans-mission as a backup unit, the IDHT should still be capable of providing excellent data continuity for future operations.
The instrument availability figures have been extraordinarily high, typically ranging between and 95% and 100% (Fig. 6).
Figure 6. Availabilities of the ERS-1 instruments since launch
The Active Microwave Instrument (AMI)
High-voltage arcing is a common problem with High-Power
Amplifiers (HPA) of the type needed on ERS. There have been an average of five such events per month
since ERS-1's launch, which is well within the specified allowable four arcs per 100 hours. A period of
excessive arcing in 1994, however, rendered the AMI inoperable, but after a few days of rest the
system behaved correctly once more and the good performance has continued ever since.
The long life of the prime HPA is unprecedented, exceeding the total duration of the various HPA on- ground life tests. Procedures for a reconfiguration to the redundant HPA and instrument recalibration are in place to provide a smooth transition should any such problem arise.
Monthly reports are produced concerning the radiometric stability of the AMI wind scatterometer (SCATT) over the Amazon rain forest, and the same parameter measured for the SAR over a calibration site in the Netherlands (Figs. 7a, 7b). Both parameters have shown excellent stability for the AMI in both the SCATT and SAR modes over the four years since launch.
The AMI has acquired more than half a million radar images in those four years, which constitutes a true demonstration of the wealth of data being provided to users by the ERS-1 payload.
Figures 7a and 7b. Radiometric stability of the AMI wind scatterometer, calibrated weekly over a specially selected area of the
Amazonian Rain Forest (extending from 2.5 to 5 degS in latitude, and from 60.5 to 75 degW in longitude)
The Radar Altimeter (RA)
The Radar Altimeter boasts a near-perfect in-orbit availability figure close to
99%. Over the past four years, software updates have allowed the Altimeter's tracking capability over
non-ocean surfaces like coastal zones, land and ice, to be further improved. The ERS-1 RA is now also
capable of keeping track over medium-rough terrain, as well as terrain with rapidly changing backscatter
or major changes in echo shapes. Consequently, the instrument has been available for longer periods of
unperturbed operations (Fig. 8).
Figure 8. The tracking performance of the ERS-1 Radar Altimeter, for the period 30 July to 3 September 1993 (cycle 97)
The Along-Track Scanning Radiometer (ATSR)
The ATSR and Microwave Sounder has experienced only
one setback during the four years of operation, namely loss of the Infrared Radiometer's 3.7 micron
channel in mid-1992. High-quality images have continued to be acquired, however, by the 11, 12 and
1.6 micron channels. The specified accuracy of 0.5 degC for the global day/night sea-surface
temperature product has been maintained, and so the impact of one channel's loss on the ATSR's
scientific return is regarded as minimal by the Principal Investigators concerned.
The Microwave Sounder has an excellent track record and its product, the atmospheric integrated water content, is used to enhance the quality of the Radar Altimeter's output.
Selected ESA Publications are now available via an ESA Publications Division 'home page', on a server run by the Agency's Information Retrieval Services Division - Exploitation Department, at ESRIN in Frascati, Italy.
The address of the host is:
http://esapub.esrin.esa.it/esapub.html
Further information can be obtained from the IRS Help Desk at ESRIN:
Telephone: +39.6.94180777
E-mail: irshelp@mail.esrin.esa.it (Internet)
or
irshelp@esrin.bitnet
Despite being a completely new type of satellite, ERS-1 has provided excellent performance, exceeding all expectations and more than proving, throughout its four years of operation, the novel and sophisticated nature of its payload and data products. A key aspect for users is data continuity and there again ERS-1 has paved the way for future missions, by demonstrating the extremely high operational availability of such a polar-orbiting platform and payload.
ERS-1's resources are still far from being depleted and, given the so far largely unexploited redundancy, the satellite has the potential to back-up ERS-2 operations until the end of 1996 and beyond.
ESA Bulletin Nr. 83.