European Space Agency


ATSR-2: The Evolution in Its Design from ERS-1 to ERS-2

N.C.M. Stricker & A. Hahne

ESA Directorate for Observation of the Earth and Its Environment, ESTEC, Noordwijk, The Netherlands

D.L. Smith, J. Delderfield, M.B. Oliver & T. Edwards

Space Science Department, Rutherford Appleton Laboratory, Didcot, UK

The Along-Track Scanning Radiometer, or ATSR, was developed for the ERS-1 mission as an Announcement of Opportunity package by the United Kingdom and France. It consists of a four-band Infrared Radiometer (IRR) to measure the Sea Surface Temperature (SST), and a Microwave Radiometer (MWR) to measure the integrated (vapour and liquid) atmospheric water content. The IRR was developed by Rutherford Appleton Laboratory (RAL, UK) and the MWR by the Centre de Recherche en Physique de l'Environment Terrestre et Planetaire (CRPE, F).

For the ATSR-2 on ERS-2, the IRR has been upgraded by adding three more bands in the visible part of the spectrum to provide data for vegetation studies (Fig 1). The MWR is identical to that used on ERS-1, but is provided by a different industrial contractor, namely Schrack Aerospace of Austria.

Rondonia
Figure 1. A 512 x 512 km section of the tropical rain forest in Rondonia (western Brazil), as seen by ATSR-2. This image combines three channels from ATSR-2, at 0.55 m (extracted as blue), 0.67 m (green) and 1.6 m (red). The regularly-shaped, pale cream patches are areas where the rain forest has already been felled

The ATSR instrument on ERS-1

The IRR, an imaging radiometer equipped with four infrared channels operating at wavelengths of 1.6, 3.7, 11 and 12 microns, scans two 500 km swaths across the satellite's ground track, one being the nadir view and the other 800 km forward (47 degrees with respect to the nadir) along the ground track (Fig. 2). Successive swaths are displaced by 1 km due to the satellite's orbital motion.

A rotating mirror scans the two tracks once every 150 ms, each scan being sub-divided into 2000 pixels (each equivalent to 75 microsec), 555 of which contain nadir-view data and 371 forward-view data. The infrared channels and associated electronics are calibrated using two black bodies, one hot and one cold, located in the path of the scanning mirror.

With the 555 pixels in the nadir view, a resolution of the order of 1 km x 1 kmcan be achieved. Averaging over 50 km x 50 km gives an absolute accuracy of better than 0.5 K in sea-surface temperature, assuming that 20% of the pixels within the area are cloud-free. For cloud-free pixels of 1 km x 1 km, the relative accuracy is about 0.2 K.

The scanning mirror directs the incoming radiation to an off-axis paraboloidal mirror (Fig. 3). A field stop positioned at the focus of the instrument determines the field of view. Beyond this field stop, the beam diverges into the Focal-Plane Assembly (FPA), where it is spectrally divided into four infrared channels Three of the component beams, corresponding to the 3.7, 11 and 12 micron bands, are re-imaged by three off-axis ellipsoidal mirrors onto separate detectors. An aspherical zinc-sulphide lens re-images the fourth beam (1.6 micron) onto its detector (Fig. 4). Photoconductive cadmium-mercury telluride detectors are used for the 11 and 12 micron channels, and indium-antimonide photodiode detectors for the 1.6 and 3.7 micron channels.

A Stirling-cycle cooler keeps the Focal-Plane Assembly at 80 K, to provide the required low-noise performance for the detectors. Eight onboard pixel maps allow the selection and compression of IRR pixels for eight different data sets. After formatting, the data are collected by the Instrument Data- Handling and Transmission Unit (IDHT) and transmitted to ground via the X-band link.

The MWR instrument uses a 60 cm Cassegrain offset-fed antenna to view the Earth in the nadir direction at frequencies of 23.8 and 36.5 GHz. The signals received are compared with that from the reference source at a known temperature in order to minimise the effects of short-term variations in the receiver- chain gain. To calibrate the MWR, additional features are used: the sky-horn antenna is pointed towards the very low cosmic background radiation of deep space at about 4 K for 'cold reference' measurements, while the 'hot reference' is obtained from measurements within the instrument itself.

ATSR
Figure 2. Measurement principle of the Along-Track Scanning Radiometer (ATSR)

ATSR Optical Components
Figure 3. The arrangement of the ATSR's optical components

Layout of IRFPA
Figure 4. The optical layout of the Infrared Focal-Plane Assembly (IRFPA) for ATSR-2

The ATSR-2 instrument on ERS-2

In the ATSR aboard the ERS-2 mission, three additional visible channels are accommodated by adding of a second Focal-Plane Assembly, with the constraint that it was not to impact adversely on the existing channels.

The Infrared Focal-Plane Assembly (IRFPA) on ERS-2 differs somewhat from that on its predecessor ERS- 1. The mirror used to reflect radiation into the 1.6 micron detector has been replaced by a dichroic beam- splitter. This allows the visible beam to pass out of the IRFPA (Fig. 4), via a sapphire window and radiation-resistant doublet relay lens, and enter the Visible Focal-Plane Assembly (VFPA, Fig. 5). There the beam is split into three, using dichroic beam splitters, before being focussed by zinc-sulphide triplet lenses onto the visible-channel detectors. The centre wavelengths of these three channels are 0.555, 0.659 and 0.865 microns, respectively.

The visible channels are calibrated with a Visible Calibration Unit, as shown in Figure 7. The opal MS20 diffuser, located behind the solar input baffle and radiation-resistant glass window, is illuminated by the Sun during some parts of ERS-2's orbit. Mirror M1 reflects the diffuse beam onto the plane mirror M2, located between the nadir view and one of the black-body units in the path of the scanning mirror. The size of the M2 mirror determines the aperture stop in this calibration system, adding 16 visible-calibration pixels to the ATSR-2 data stream. Calibration takes place close to the time of local satellite sunset, when the Sun is 13 below the tangent to the Earth's surface at the satellite's nadir point. The nadir- and forward-viewing baffles are designed to exclude stray radiation from entering the calibration system, which would degrade its accuracy.

Three new amplifiers have been added to the pre-amplifier unit to cope with the three visible channels on ATSR-2, and three corresponding Single Channel Processors have been incorporated into the electronic system.

The increased data flow on ATSR-2 called for a new set of data-compression algorithms. In addition, uncompressed infrared and visible data can be transmitted in a high-data-rate mode, which provides double the normal throughput. This mode is limited, however, to the periods when other payload instruments are not making full use of the X-band data capacity.

The possibility with the original ATSR of choosing between eight fixed pixel-selection maps is replaced for ATSR-2 by a facility for uploading different pixel formatting maps from the ground, thereby providing greater operational flexibility. Two pixel maps can be loaded at any given time, which allows two different maps to be used during an orbit, for example one over the sea and a different one over land. It also allows swath-width modulation and a reduction in the number of detector channels to be traded-off against better resolution in the remaining channels in low-data-rate mode.

Major mechanical modifications were made to the ATSR-2 Infrared Radiometer. The carbon-fibre structure has been substantially re-designed, the vestigial ATSR-1 optical bench has been removed, and all optical elements are now mounted directly onto the structure.

With the addition of the Global Ozone Monitoring Experiment (GOME) for the ERS-2 mission, and the need to interface this experiment to the satellite via the ATSR-2's Digital Electronics Unit (DEU), it became important to add more redundancy to the latter as it now interfaces with the IRR, the MWR and GOME. A second identical DEU was therefore added to the payload module, together with a DEU Switching Unit (DSU in Fig. 9).

Optical Layout of VFPA
Figure 5. The optical layout of the Visible Focal-Plane Assembly (VFPA) for ATSR-2

Optical Components of Calibration System
Figure 7. Optical components of the visible calibration system

Configurations
Figure 9. Schematic of the configurations of the IRR, MWRand DEU aboard ERS-1 and ERS-2

The ATSR products

The main application objectives for the original ATSR instrument aboard ERS-1 are:

With the new features that have been incorporated into the ATSR-2 instrument carried by ERS-2, the following additional objectives are being addressed:

ATSR-2 Image
Figure 6. This ATSR-2 image, recorded on 8 May 1995 over Central Italy and Sicily, is a false-colour composite, compiled from the uncalibrated data in the 0.67 m (as a blue extract), 0.87 m (green) and 1.6 m (red) spectral channels

ATSR-2 View of Gulf Stream
Figure 8. ATSR-2 view of the Gulf Stream, which gives Europe its temperate climate, acquired on 16 May 1995. It shows the eastern seaboard of North America, stretching from NewYork (at the top) to Charleston, South Carolina (at the bottom). Off the coast is the warm Gulf Stream (in red), which comes up from the south and meets the cold Labrador current off Cape Hatteras. The sometimes quite wide transition zones stand out very clearly, as do the swirling eddies and broken-up currents that occur further on. The varying colours of the clouds near the top and bottom edges of the picture are also due to temperature differences


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Right Left Up Home ESA Bulletin Nr. 83.
Published August 1995.
Developed by ESA-ESRIN ID/D.