ERS-1 Satellite Concept (1992)

Introduction

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.

Platform

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.
The ERS-1 platform (see the figure) consists of:

• Service Module carrying the house-keeping sub-systems, and interfaces with the propulsion module, payload, solar generator and the battery compartment.
• Propulsion Module carrying the propulsion units of the
• Solar Array Sub-assembly consists of two 5.8 m x 2.4 m wings, on which are mounted a total of 22260 silicon solar cells supplying power greater than 2000 W.
• Following launch the two wings were deployed by a pantograph mechanism (see the figure) and the whole array rotates through 360deg., with respect to the satellite, during each orbit to maintain sun pointing. During the sunlit phase (66 minutes) of each orbit, the solar array provides electrical power to all of the on-board systems and charges the spacecraft's batteries.
• Payload consisting of an Active Microwave Instrument comprising a SAR (image and wave modes) and a Wind Scatterometer; a Radar Altimeter; an Along Track Scanning Radiometer; a Precise Range and Range-rate Equipment; and Laser Retro-reflectors.

Payload Module

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):

• Active Microwave Instrument (AMI) combining the functions of a Synthetic Aperture Radar (SAR) and a Wind Scatterometer. The SAR operates in image mode for the acquisition of wide-swath, all weather images over the oceans, polar regions, coastal zones and land. In wave mode the SAR produces imagettes (about 5 km x 5 km) at regular intervals for the derivation of the length and direction of ocean waves. The Wind Scatterometer uses three antennae for the generation of sea surface wind speed and direction.
• Radar Altimeter (RA) provides accurate measurements of sea surface elevation, significant wave heights, various ice parameters and an estimate of sea surface wind speed.
• Along Track Scanning Radiometer (ATSR) combining an infra-red radiometer and a microwave sounder for the measurement of sea surface temperature, cloud top temperature, cloud cover and atmospheric water vapour content.
• Precise Range and Range-rate Equipment (PRARE) is included for the accurate determination of the satellite's position and orbit characteristics, and for precise position determination (geodetic fixing).
• Laser Retro-reflectors (LRR) allow measurement of the satellite's position and orbit via the use of ground-based laser ranging stations.

See also ERS-1 Orbit determination with other technologies.

Technnical Parameters

Orbit	Type: Near-circular, polar, Sun-synchronous Altitude: 782 to 785 km Inclination: 98.52 deg. Period: About 100 minutes Orbits per day: 14.3 Repeat cycle: 3-day, 35-day and 176-day
Instruments	Active Microwave Instrument comprising a SAR (image and wave modes) and a Wind Scatterometer;  a Radar Altimeter;  an Along Track Scanning Radiometer; Precise Range and Range-rate Equipment; and Laser Retro-relectors
Mass	Total mass: 2157.4 Kg Total payload: 888.2 Kg Total platform: 1257.2 Kg Active Microwave Instrument: 325.8 Kg Radar Altimeter: 96.0 Kg IDHT: 74.0 Kg ATSR: 55.3 Kg PRARE: 12.0 Kg Laser Retro-reflectors: 2.5 Kg
Electrical Power	Payload peak power: <=2600 W Payload permanent power: <=550 W Power supply voltage: 23-37 V On-board energy: 2650 WH max
Attitude and Orbital Control	Type: 3 Axes stabilised earth pointed Absolute rate errors: <=0.0015deg. sec (3 sigma) Maximum errors: bias 0.11deg. (pitch/roll) 0.21deg. (yaw); harmonic and Random 0.03deg. (pitch/roll)  0.07deg. (yaw) Prediction accuracy: 30 m (radial), 15 m (cross), 1000 m (along) Orbit restitution: 5 m (radial), 15 m (cross) 60 m (along)
Data Handling	On-board computer: word length: 16 bits Payload memory capacity: 20 K words max Payload data exchange: OBDH type bus Number of payload users: redundant
Communications	Transponder: coherent S-band (2 kbit/s) Transmission power: 50 to 200 mW max Telemetry rate: 2048 bit/s Telecommand rate: 200 bit/s Data down link: - X-band (105 Mbit/s high rate link for AMI image mode)  - X-band (15 Mbit/s low rate link for real-time and playback of LBR data)  - on-board recorders provide 6.5 Gbits storage - S-band telemetry links for housekeeping data - PRARE uses own ranging links for its telemetry data

Orbit Information

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:

                     3-day           35-day         168-day
semi major axis      7153.138        7159.496       7147.191
inclination          98.516 deg.     98.543 deg.    98.491 deg.
mean altitude        785 km          782 km         770 Km  
orbits per cycle     43              501            2411

The transition from one orbital configuration to another, to adjust the repeat cycle, requires up to two weeks to stabilise the orbits to within 1 km of the nominal ground track. With the transition made at certain times in the orbital cycle stabilisation of orbit to within 5 km can occur after 24 hours. LBR and SAR operations are re-started 48 hours after the start of the orbit change manoeuvre. However, because of the need to update various parameters both on-board the satellite and on the ground the quality of fast delivery LBR products are not guaranteed for two weeks after any manoeuvre.

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.

Data Communication

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.

Satellite Operation

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:

• managing the platform and its payload, including overall power regulation, power distribution and thermal control of the platform subsystems, the PEM, the antennae and the AOCS
• monitoring the spacecraft, in order to detect and neutralise any critical failure and thereby preserve the mission
• scheduling the mission programming commands transmitted from the ground
• reporting to the ground either on the real-time status of the platform and payload, or from dedicated memory where any significant event history has been recorded.

Next article: ERS-2 takes over where ERS-1 left off (1995)

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry