The satellite concept is based on the re-utilisation of the Multi-mission Platform, developed within the French SPOT programme. This platform provides the major services for the satellite and payload operation, in particular attitude and orbit control, power supply, monitoring and control of payload status, telecommunications with the ground segment.
To meets its mission objectives, ERS-1 has been placed in a near-polar orbit at a mean altitude of about 780 km with an instrument payload comprising active and passive microwave sensors and a thermal infra-red radiometer. The satellite (see the figure) is large, weighing 2400 kg and measuring 12 m x 12 m x 2.5 m, making it the largest and most sophisticated free-flying satellite built so far in Europe.
ERS-1, fully deployed, inside the Interspace Test Facility. Toulouse, France.
The spacecraft platform provides the major services required for satellite and payload operation. These include attitude and orbit control, power supply, monitoring and control of payload status, telecommunication with ground stations and telemetry of payload and platform housekeeping data. In addition, the PRARE instrument is mounted on the platform.
The platform was modified with respect to the SPOT programme to
meet the unique needs of the ERS-1 mission. The major modifications
included extension of the solar array power and battery energy
storage capability; modification of the attitude control sub-system
to provide yaw steering and geodetic pointing; and the development
of new software for payload management and control.
There are two main parts to the payload module (see the figure): the Payload Electronics Module (PEM) and the Antenna Support Structure (ASS).
The PEM is an aluminium face-sheet/honeycomb structure supported by nine internal vertical titanium beams. The central beam lies at the intersection of two internal cross-walls, so that the PEM is effectively divided into four separate compartments. Each outer panel is dedicated to a particular instrument, to simplify integration logistics. The payload is separated from the platform by a non-load-bearing electromagnetic shield. An aluminium-honeycomb panel closes the opposite end of the structure, tabilising the beams and providing the interface to the ASS at the beam locations. These provide a load path from the ASS to the platform.
The ASS (see the figure) is composed of carbon-fibre-reinforced plastic (CFRP) tubes, with titanium at the high-load bearing structural elements. The lower part of the assembly consists of five tripods, three support points for the SAR antenna\view and two intermediate support points for the upper assembly. A CFRP plate at the top, which carries the Scatterometer antennae, is supported by three further tripods attached to the intermediate points and the SAR central point. The Altimeter's antenna is attached at three node points by a triangulated strut system.
The payload module also contains a dedicated Instrument Data Handling and Transmission (IDHT) system (see also Data Communications, which permits the SAR image mode data to be transmitted directly to the ground stations and the data from the LBR instruments to be transmitted to the ground in real-time or recorded on one of two on-board tape recorders. The tape recorders have been designed to store a full orbit of LBR data on 3000 ft of 1/4-inch magnetic tape, leading to a total data recording capacity of 6.5 Gbit. The IDHT is located on the Earth-facing panel of the PEM, with the tape recorders mounted inside one of the cross-walls.
ERS-1 carries instrumentation consisting of a core set of active microwave sensors supported by additional, complementary instruments (see figures):
The ATSR and PRARE are jointly referred to as the Announcement of Opportunity package, which resulted from proposals for additional instrumentation from the scientific community. In summary, the total ERS-1 instrument package provides a complementary set of sensors to enable all-weather, day / night, high accuracy observations of the Earth.
To carry out its mission ERS-1 must orbit so that its instruments can scan along predetermined paths designed to give optimum coverage for a set number of orbits. To achieve this, ERS-1 is a three-axis-stabilised, Earth-pointing satellite in yaw steering mode (YSM). The elliptical orbit is sun-synchronous, near polar, with a mean altitude of 785 km, an inclination of 98.5deg. and a local solar time at the descending node of 10.30 a.m.
ERS-1 has a range of altitude sensors. The long-term reference in pitch and roll is obtained from an infra-red Earth sensor. The yaw reference is obtained once each orbit from a narrow field Sun sensor, which is aligned to view the Sun when the satellite is at a particular point in its orbit. The short term attitude and rate reference is obtained from three of six gyroscopes. Finally, there are two wide-field Sun-acquisition sensors for use in safe mode, when the satellite is Sun, rather than Earth, pointing. The primary means of attitude control is provided by a set of reaction wheels, which are nominally at rest. They can be spun in either direction, exchanging angular momentum with the satellite in the process. It is also possible, if there are permanent torques on the satellite due, for instance, to radiation pressure on the solar array, to bias one or more wheels to be nominally not at rest, but rotating at a defined speed. Monopropellant-type thrusters aligned about the spacecraft's three primary axes are used in different combinations to maintain and modify the satellite's orbit and to adjust its attitude during non-nominal operations.
The ERS-1 mission will also include a number of adjustment manoeuvres to synchronise the orbital period with various requirements for ground coverage. The orbital parameters for the three planned repeat cycles of 3, 35 and 168 days are as follows:
For the roll tilt mode (RTM) mode campaign in early April 1992 the satellite body is rotated by 9.5deg. allowing operation of the AMI SAR imaging mode at 35deg. incidence angle. The attitude control system performances of the roll tilt mode are not significantly different to the nominal YSM with the same angular rates and harmonic and random errors and only slightly different static errors, i.e. about 0.05deg. maximum difference. During the roll tilt sequences the satellite is in a Fine Pointing Mode1 (FPM), which features no yaw steering and Earth centroid pointing, rather than geocentric pointing.
ERS-1 has two telemetry systems. An S-band (2 kbit/s) Telemetry, Telecommand and Control (TTC) system to transmit the ICU formats for housekeeping purposes and an X-band Instrument Data Handling and Transmission (IDHT) system for the science data.
Three data streams are transmitted from the IDHT (see the figure). The first, a dedicated X-band link, contains the high-rate data from the SAR image mode, with auxiliary data and a copy of the S-band telemetry data, at a total rate of 105 Mbit/s. The other sensors have their data combined, again with a copy of the S-band data and satellite ephemeris information, into a (comparatively) low-rate data channel, operating at 1.1 Mbit/s, which will be continuously recorded by an on-board tape recorder. This recorder will be replayed at 13.6 times recording speed (in reverse order to save rewind time) over he ground stations, to form a second data channel, at 15 Mbit/s. It will share the second X-band link with the live transmission of the combined low-rate data, which constitutes the third data stream.
The satellite is controlled by the Mission Management and Control Centre via the Kiruna ground station. The satellite operation schedule is up-linked and stored on the satellite's on-board computer (OBC) for the next 24 hours, through a set of time-tagged macro-commands. The timely execution of these macro-commands is controlled by the OBC.
The platform has an automatic reconfiguration capability in case of failure. If the reconfiguration fails, the spacecraft goes into safe mode, with the payload switched off, the solar array parked in the canonical position, and the satellite placed in a Sun-pointing attitude, awaiting further intervention from the ground. The payload element has no automatic recofiguration. In case of failure the identified instrument is switched off waiting for reconfiguration by ground control. The OBC, the Instrument Control Units (ICUs) and the RA tracker are in-flight programmable. The AMI, RA and IDHT all have redundant sub-systems, with the exception of their antennae.
ERS-1 carries a significant number of software packages run by different processors spread throughout the platform and the payload. In the platform, the OBC runs the 'Centralised Flight Software', which incorporates all the basic functions needed to conduct the mission in an optimal fashion. In addition, each payload element (AMI, RA, ATSR and IDHT) contains its own decentralised ICU. These five computers are linked by the On-board Data Handling (OBDH) bus, and communicate with each other via a high level packetised protocol. This set of interdependent computers fulfils a critical requirement.
ERS-1 is an extremely complex satellite, with a great many modes, parameters and logical conditions to be set and respected throughout each orbit. It is required to have autonomy for a full 24 hours, and this is only achieved by providing intelligent payload elements controlled by a capable central computer. The main functions of the OBC flight software are:
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