European Remote Sensing Satellite, ERS-2

This document was derived primarily from ESA Bulletin 83.

Summary:

The ERS-2 satellite is essentially the same as ERS-1 except that it includes a number of enhancements and it is carrying a new payload instrument to measure the chemical composition of the atmosphere, named the Global Ozone Monitoring Experiment (GOME).
Other major instruments common to ERS-1 and ERS-2 are the Active Microwave Instrument (AMI), the Radar Altimeter (RA), the Along-Track Scanning Radiometer (ATSR), the Microwave Radiometer (MWR) and the Precise Range and Range Rate Experiment (PRARE). The AMI operates in three different modes devoted to radar imagery, and oceanic wind and wave measurements. The RA measures precisely the altitude over ocean ice and land surfaces and also measures oceanic wind and waves. The ATSR measures sea-surface temperatures and has been enhanced for ERS-2 by including visible channels for vegetation monitoring. The MWR and PRARE both support the RA mission by providing information respectively on propagation delays of the radar signal and satellite positioning.

Table of Contents:

1. Source/Platform or Data Collection Environment Overview:

Source/Platform or Data Collection Environment Long Name, Source/Platform Acronym:

Second European Remote Sensing satellite, ERS-2

Source/Platform Introduction:

The ERS-2 satellite is essentially the same as ERS-1 except that it includes a number of enhancements and it is carrying a new payload instrument to measure the chemical composition of the atmosphere, named the Global Ozone Monitoring Experiment (GOME).

Collection Environment:

Satellite

Source/Platform Program Management:

European Space Agency (ESA)

Source/Platform Mission Objectives:

Through ERS-1 and ERS-2, the European Space Agency intended to (and now does) provide:

Source/Platform Parameters:

ERS-2 was built by a consortium led by Deutsche Aerospace and was launched on April 20, 1995 on an Ariane.
Powerful radar pulses are needed to provide sufficient illumination of the Earth's surface to produce detectable echo signals from the satellites' polar orbits, which have a mean altitude of about 780 km. The spacecraft also need large antennas to be able to pick up the returning signals. Consequently, the satellites have to be rather large: they each weigh about 2.3 tonnes. The payload alone weighs about 1000 kg and consumes about 1 kW of electrical power when in full operation. The antennas, after deployment, are up to 10 m long; the main payload support structure has a 2 m x 2 m base and is some 3 m high. To support the payload by providing electrical power, attitude and orbit control, as well as overall satellite operational management, a platform module (derived from the French national SPOT programme) is attached to the payload. That module is roughly equivalent in size to the payload itself and is equipped with a deployable 12 m x 2.4 m solar array.
The solar array consists of two 5.8 m x 2.4 m wings, manufactured from flexible reinforced Kapton, on which are mounted a total of 22,260 solar cells. The two wings are deployed by means of a pantograph mechanism, and the whole array rotates through 360 degrees with respect to the satellite during each orbit in order to maintain its Sun pointing.
During the 66-min sunlit phase of each orbit, the array provides electrical power to all of the onboard systems. It also charges the spacecraft's batteries, located in a cylindrical compartment at the solar-array end of the platform, so that they can provide the energy necessary for a similar level of payload operations during the 34-min eclipse periods. The four nickel-cadmium (NiCd) batteries are sized to allow payload operations to be independent of the satellite's orbital position. Connected directly to the spacecraft's unregulated 30 V bus, they power it during the 14 eclipses that occur each day, using their combined capacity of 96 Ah. The precise management of the charge and discharge cycles is handled by the onboard computer, with the possibility of ground intervention if required.

Coverage Information:

Through their total planned ground station network, ESA expects nearly complete land-surface coverage and extensive coverage of near-polar waters. ERS-2 and ERS-1 are currently orbiting in-tandem with a 35-day repeat cycle. ERS-2 follows ERS-1 by about a day.

Attitude Characteristics:

Both ERS-2 and ERS-1 are in a Sun-synchronous polar orbit, highly inclined to the equator, giving the satellites visibility of all areas of the Earth as the planet rotates beneath their orbits. The inclination is such that the precession of the orbit, caused by the non-spherical components of the Earth's gravity field, exactly opposes the annual revolution of the Earth around the Sun. Consequently, the orbital plane will always maintain its position relative to the Sun, crossing the equator with the descending node at about 10:30 am local time. This benefits for the satellite design, in that, for example, the solar array only needs to rotate about one axis, normal to the plane of the orbit, in order to maintain its correct alignment with the Sun.
The orbital inclination required to achieve Sun-synchronism is a weak function of satellite altitude. For a mean altitude of approximately 780 km, it needs to be about 98.5 degrees, making it a so-called 'retrograde' orbit. The orbital altitude, and consequently the revolution period, may be adjusted by use of the orbit control thrusters provided on both ERS-1 and ERS-2, so that a harmonic relationship exists between the revolution period of the satellite and the rotation period of the Earth. Consequently, after a certain number of orbits, the satellite retraces its tracks over the Earth's surface. In practice, the orbital altitude of ERS-1 has been changed, by a few kilometres, several times during the four years it has been in orbit. Five orbital patterns have been flown: two had repeat periods of three days but over different ground tracks; for most of the mission, a multi-disciplinary 35-day pattern has been used; and two others had a pattern with a 168-day repeat, offset by half of the track spacing to provide a very dense spatial coverage. Each individual orbit in those patterns lasts approximately 100 min.
ERS-1 and ERS-2 are both flying in the same 35 day orbit, over the same ground tracks. It is foreseen that both ERS-1 and ERS-2 will always remain in that orbit. The phasing of the two satellites around that orbit plane has been adjusted so that they overfly the same track with a one-day separation, ERS-1 being ahead. That provides excellent opportunities to compare the results from the two satellites.

Data Collection System:

Instrument data is not processed by the on-board computer prior to transmission. See the Data Acquisition and Processing section below for related information.

Communication Links:

ERS-2 has two telemetry systems. The platform's needs are served by a classical-type Telemetry, Telecommand and Control (TTC) system operating at S-band. That low-rate (2 kbit/s) system is used to transmit the ICU (Instrument Control Unit) formats for housekeeping purposes. Because of the high bit rates involved, the science data cannot use this link and the payload therefore includes a so-called 'Instrument Data Handling and Transmission' (IDHT) system. That system allows real-time transmission of AMI Image-Mode data, providing a regional service to local ground stations and global recording and telemetry of the other sensors.


Three data streams are transmitted from the IDHT. The first contains the high-rate data from the AMI Image Mode, with auxiliary data and a copy of the S-band telemetry data, at a total rate of 105 Mbit/s. This channel has an X-band link dedicated to it. 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 is continuously recorded by the onboard tape recorder. This recorder is replayed at 13.6 times recording speed (in reverse order to save rewind time) when over the ground stations to form a second data channel, at 15 Mbit/s. It shares the second X-band link with the live transmission of the combined low-rate data, which constitutes the third data stream.

The tape recorder has been designed to store a full orbit of continuous 1.1 Mbit/s low-rate data on 3000 ft of 1/4-inch magnetic tape, leading to a total data-recording capacity of 6.5 Gbit. When performing a data dump to high-latitude ground stations, such as the primary Kiruna station, the spacecraft's solar array might cause a brief occultation of the link, due to the system geometry. On passes when that occurs, the on-board command scheduling includes a stop in playback before the occultation, a slight rewinding of the tape, and a reac-tivation of playback mode after the occultation.
The modulation scheme used for the high-rate channel is quadrature phase-shift keying, called QPSK, which allows four distinct states per clock cycle and makes it possible to transport two bits of information per cycle. That reduces the radio frequency bandwidth required for transmission by a factor of two compared with a simpler modulation scheme. The low-rate link uses unbalanced quadrature phase-shift keying, or UQPSK, to modulate the 15 Mbit/s recorder dump and the convolutionally encoded real-time data onto a single link. If there are no recorder dump data, bi-phase-shift keying (BPSK) is used for the real-time data.
Immediately before and after recorder playback, the link is automatically switched between BPSK and UQPSK operation, with minimum impact on the real-time data stream. The ERS-1/ERS-2 ground demodulators have been designed to accommodate that mode-switching automatically.
The fact that the X-band transmission was required to have a minimum power-level fluctuation during the satellite pass led to the design of a shaped-beam antenna able to compensate for losses at low satellite elevation angles, when the distance to the ground station is long, and the attenuation due to the atmosphere's water content is high. To achieve that, the antenna reflector is shaped so that its radiation pattern compensates for the inverse-square-law variation in received power with distance as the satellite passes across the sky at the ground station. The polarization of the radiated energy is rotated to compensate for Faraday rotation due to the Earth's ionosphere.
The IDHT is physically located on the Earth-facing panel of the Payload Electronics Module (PEM), with the tape recorders mounted inside, on one of the cross-walls.

List of Sensors/Instruments:

The largest of ERS-2's sensors, the Active Microwave Instrument (AMI), is capable, in its imaging mode, of producing highly detailed radar images of a 100 km strip on the Earth's surface. This mode is also known as the Synthetic Aperture Radar or SAR mode. Because that mode consumes a large amount of energy and produces a vast amount of data which cannot be stored on board, it is only used regionally, for periods of approximately 10 min per orbit. The same instrument has alternative global measurement modes, namely the Wind (or Scatterometer) Mode in which the wind speed and direction at the sea-surface can be measured over a 500 km swath, and a Wave Mode which provides small radar images at 200 km intervals. Those images can be used to generate ocean-wave spectra, showing wave energy as a function of wavelength and direction.
A second instrument, the Radar Altimeter, provides very precise measurements of the satellite's height above the ocean, ice and land surfaces. The successful exploitation of those height data - which are to be used to study, among other topics, global ocean circulation and height profiles across the ice caps - is dependent upon precise determination of the satellite's orbit, which is derived from the onboard tracking systems. Those systems are a laser retro-reflector, which is a passive device used by ground-based satellite laser-ranging systems, and the PRARE instrument, which is a two-way microwave ranging system that uses small, dedicated ground stations. The PRARE on ERS-1 failed shortly after launch. For ERS-2, the cause of that failure has been eliminated and, furthermore, a second PRARE has been embarked.
Another payload instrument is the Along-Track Scanning Radiometer (ATSR), which consists of two parts. Detailed images of the sea surface are made by an infra-red scanning radiometer, which allows extremely precise measurements of sea-surface temperature. For ERS-2, additional channels have been incorporated to provide imagery in the visible part of the spectrum as well. The other part is a passive microwave radiometer, which is used to determine the water-vapour content of the vertical column of the Earth's atmosphere passing beneath the satellite.
ERS-2 is also carrying one completely new instrument compared to ERS-1: the Global Ozone Monitoring Experiment, GOME. That instrument provides spectra of backscattered sunlight in the ultra-violet/visible/near-infrared part of the spectrum, while scanning a swath below the satellite. Processing of those spectra, in combination with direct solar spectra which are also measured by GOME for reference, allows the determination of concentrations and profiles of many trace gases, but particularly ozone, in the atmosphere.

2. Ground Segment Information:

Tracking and Control:

(Tracking is described in the Data Acquisition and Processing Section.)

Attitude Determination and Control:

ERS-2, like ERS-1, is a three-axis-stabilized, Earth-pointing satellite. Its yaw axis is pointed towards the local vertical with respect to a reference ellipsoid, taking the Earth's oblate shape into account. The direction of the pitch axis oscillates slightly during each orbit to keep it oriented normal to the composite ground velocity vector, taking account of the Earth's rotation, to assist the operation of the AMI. The residual attitude errors are no more than 0.06 degrees on each axis for ERS-1, and ERS-2 is expected to have a similar performance. The attitude control system has the capability to be offset to compensate for any static error that may be observed, but that has not proved to be necessary.
ERS-2 has a range of attitude sensors. The long-term reference in pitch and roll is obtained from one of two continuously operating, redundant infrared horizon sensors. The yaw reference is obtained once each orbit from one of two redundant narrow-field Sun sensors aligned to point at the Sun as the satellite crosses the day/night terminator. The short-term and rate reference are obtained from an inertial core, with a pack of six gyroscopes, of which any three can be in use. Finally, there are two wide-field Sun-acquisition sensors for use in the initial stages of attitude acquisition, and in safe mode, when the satellite is Sun-pointing rather than Earth-pointing.
The primary means of attitude control is provided by a set of momentum wheels (large flywheels), 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 were permanent torques on the satellite due, for instance, to radiation pressure on the solar array, to bias one or more wheels to a nominal non-zero speed. This has not been necessary with ERS-1. Angular momentum also needs to be dumped from the wheels on a regular basis and a sophisticated system has been devised for this purpose. The onboard computer contains a simple model of the Earth's magnetic field, and is also able to control the current in a pair of orthogonal magnetic coils. These coils, called 'magneto-torquers', generate torques by interacting with the Earth's geomagnetic field. Using a servo loop and the built-in field model, the spacecraft's onboard computer continuously adjusts the magneto-torquers to keep the wheel speed close to the nominal values.
ERS-2 has a number of monopropellant-type thrusters, aligned about the spacecraft's three primary axes, in which hydrazine dissociates exothermically as it is passed over a hot-platinum catalyst. They are used in different combinations to maintain and modify the satellite's orbit and to adjust its attitude during non-nominal operations. That is normally done by using pairs of thrusters to provide in-plane thrust when slightly changing the orbital height or speed, or by turning it in yaw to obtain out-of-plane thrust when slightly modifying the orbital inclination.

Data Acquisition and Processing:

The ERS Payload Data Ground Segment is composed of the following facilities:
  1. ESRIN ERS Central Facility (EECF)
  2. ESA Ground Stations network
  3. ESA Processing and Archiving Facilities (PAFs)
  4. National and Foreign Stations (NFS).

1. ESRIN ERS Central Facility: (EECF)

The EECF, located in Frascati, Italy, includes User Services, the Product Control Service (PCS), and the Payload Reference System. It provides:
  • The user interface (the help and order desks)
  • Definition of tasks for the whole ERS ground segment
  • Mission planning in conjunction with the Mission Management and Control Centre (MMCC) at ESOC
  • Management of a facilities network for the acquisition, archiving, processing and distribution of fast-delivery and off-line products
  • Quality checks of fast-delivery and off-line products
  • Routine monitoring of sensors
  • Coordination of the network of national and foreign stations
  • The interface to the industrial consortium charged with the promotion and distribution of data to commercial users
  • Maintenance of data-processing software for the entire ESA network.

    •  

2. ESA Ground Stations network:

The ESA ground station network has been set up to allow the maximum coverage over the European area for the Synthetic Aperture Radar (SAR) and the global LBR payload data acquisitions.
The ERS-2 payload data network is the same as the one used for ERS-1. It is managed by ESRIN and includes six ground stations, located at Salmijaervi (Kiruna, Sweden), Fucino (Italy), Maspalomas (Canary Islands, Spain), Tromso (Norway), and Gatineau and Prince Albert (Canada).
Except for Salmijaervi, which is operated by ESOC and is fully dedicated to ERS operations including telemetry, tracking and control (TT&C) activities, all of the other stations are multimission in nature. Under contract to ESRIN, they perform the ERS-1 and ERS-2 payload data acquisition, processing and dissemination, as well as hosting the ESA equipment for the requisite data exploitation. They also provide similar services for other international Earth-observation satellites, such as Landsat (USA), Spot (France), JERS-1 (Japan), and Tiros (USA).
A station's typical daily activities can be summarised as follows:
  • Satellite tracking and scheduled data acquisition
  • Recording of the data on high-density magnetic tapes
  • Processing of the fast-delivery (FD) products to be made available within three hours of data sensing, to nationally nominated centres
  • Processing of scheduled products for distribution to users
  • Reporting on the activities to the EECF
  • Transmission to the Product Control Service at ESRIN of relevant parameters and products for routine sensor performance monitoring.

    •  

3. ESA Processing and Archiving Facilities (PAFs):

The PAFs will continue to be the core of the product distribution system for ERS-2. Their role can be summarized as:
  • Long-term ERS-1 and ERS-2 payload data archiving and retrieval
  • Generation and distribution, on request, of the off-line geophysical standard products to users as instructed by the EECF via product orders
  • Support to ESA for sensor data calibration, data validation and long-term sensor performance evaluation.

    •  
Each PAF receives the relevant ERS-2 payload telemetry data on a regular basis from the ground stations and ensures the long-term archiving, the routine production and the distribution of the data. Their activities are managed and monitored from ESRIN. There are four PAFs, managed under contract to ESA:
  • F-PAF in Brest, France
  • UK-PAF in Farnborough, UK
  • D-PAF in Oberpfaffenhofen, Germany
  • I-PAF in Matera, Italy

4. National and Foreign Stations (NFS):

In addition to the ESA ground station network, a number of national ground stations, i.e. belonging to countries participating in the ERS Programme, and foreign ground stations, i.e. belonging to non-participating countries, have been set up around the world, or are planned, in order to acquire ERS-1 and ERS-2 SAR payload data. The current situation is as follows:
Ground Station                                  Status
---------------------------------------------------------------

Tromso          Norway                          Ready
West Freugh     UK                              Ready
Gatineau        Canada                          Ready
Prince Albert   Canada                          Ready
O'Higgins       Antarctica (Germany)            Campaign only
Libreville      Gabon (Germany)                 Campaign only
Neustrelitz     Germany                         1995
Aussaguel       France                          Campaign Only
Cotopaxi        Ecuador                         Ready
Hiderabad       India                           Ready
Alice Springs   Australia                       Ready
Hobart          Australia                       Ready, only acquisitions
Hatoyama        Japan                           Ready
Kumamoto        Japan                           Ready
Syowa           Antarctica (Japan)              Ready
Fairbanks       USA - Alaska                    Ready
Cuiaba          Brazil                          Ready
Pretoria        South Africa                    Ready
Taipeh          Taiwan                          Ready
Pare Pare       Indonesia                       Ready
Norman          USA - Oklahoma                  1995
Beijing         China                           Ready
Tel Aviv        Israel                          Ready, no MOU
Riyadh          Saudi Arabia                    Ready, no MOU
Nairobi         Kenya (Teleos)                  1995, no MOU
Singapore       Singapore                       1995, no MOU
McMurdo         Antarctica (USA)                Ready, no MOU
Bangkok         Thailand                        Ready, no MOU
Most of the stations have been used for ERS-1 and will be used again for ERS-2 under the terms and conditions of a standard Memorandum of Understanding (MOU) with ESA.

The ground stations receive, from the EECF in Frascati, the input data needed to acquire, process and distribute the SAR data and they report back to the EECF on their station activities and status. The stations generate and distribute products developed nationally to ESA principal investigators, pilot projects and commercial users.

Latitude Crossing Times:

When descending, ERS-2 crosses the equator at about 10:30 a.m. local time.

3. References:

ESA Bulletin #84

4. Glossary of Terms:

See ASF's Glossary for terms related to ASF. See the EOSDIS Glossary for a more general listing of terms related to the Earth Observing System project.

5. List of Acronyms:

This list defines acronyms found within this document. See ASF's Acronym List or the EOSDIS Acronyms List for more.
AMI:
Active Microwave Instrument
ASF:
Alaska SAR Facility
ATSR:
Along-Track Scanning Radiometer
BPSK:
Bi-Phase-Shift Keying
CCRS:
Canadian Centre for Remote Sensing
EECF:
ESRIN ERS Central Facility
ERS:
European Remote Sensing satellite
ESA:
European Space Agency
ESOC:
European Space Operations Centre
ESRIN:
European Space Research Institute
FD:
Fast Delivery
GOME:
Global Ozone Monitoring Experiment
ICU:
Instrument Control Unit
IDHT:
Instrument Data Handling and Transmission
JERS-1:
First Japanese Earth Resources Satellite
MMCC:
Mission Management and Control Centre
MOU:
Memorandum of Understanding
MWR:
Microwave Radiometer
PAF:
Processing and Archiving Facilities
PCS:
Product Control Service
PEM:
Payload Electronics Module
PRARE:
Precise Range and Range Rate Experiment
QPSK:
Quadrature Phase-Shift Keying
RA:
Radar Altimeter
RADAR:
RAdio Detection And Ranging
SAR:
Synthetic Aperture Radar
SPOT:
Satellite Pour l'Observation de la Terre (France)
TTC:
Telemetry, Telecommand and Control
TT&C:
Telemetry, Tracking and Control
UQPSK:
Unbalanced Quadrature Phase-Shift Keying

 


(From the Alaska SAR Facility's Homepage)
Your comments and suggestions are greatly appreciated! Please e-mail:
[email protected] - December 15, 1995
 

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