ATSR (IRR and MWR) Design

Introduction

Each ATSR instrument has been designed for exceptional sensitivity and stability of calibration which are achieved through the incorporation of several innovative features in the instrument design:

• use of low-noise infrared detectors, cooled to near-optimum temperatures (i.e., less than 95 K) by a Stirling cycle mechanical cooler;
• continuous on-board radiometric calibration of the infrared channels against two stable, high-accuracy blackbody calibration targets and, in the case of ATSR-2 and AATSR, calibration of the visible and near infrared channels with an on-board visible calibration system;
• use of the multichannel approach to SST retrieval previously demonstrated by the AVHRR instruments;
• use of the "along-track scanning" technique to provide two views of the surface and thus an improved correction for atmospheric effects.

ATSR's field of view comprises two 500 km-wide curved swaths, with 555 pixels across the nadir swath and 371 pixels across the forward swath. The nominal instantaneous field of view (IFOV) pixel size is 1 km2 at the centre of the nadir swath and 1.5µm processing.

1.1 Along Track Scanning
Application of the along track scanning technique is the ATSR instrument's most innovative development. This works by making two observations of the same point on the Earth's surface through differing amounts of atmosphere; the "along track" view passes through a longer atmospheric path and so is more affected by the atmosphere than the nadir view.

First, the ATSR views the surface along the direction of the orbit track at an incidence angle of 55× as it flies toward the scene. Then, some 150s later, ATSR records a second observation of the scene at an angle close to the nadir.

By combining the data from these two views a direct measurement of the effect of the atmosphere is obtained, which yields an atmospheric correction for the surface data set which is an improvement on that obtained from a single measurement.

1.2 ATSR-1
ATSR-1 was launched as part of the payload of ESA's ERS-1 satellite on 17th July 1991, and was the test-bed for the along track scanning concept. It carries infrared channels at 1.6µm, 3.7µm, 10.8µm and 12.0µm, and has no visible channels. Routine ATSR-1 operations stopped when ERS-1 was put into hibernation in June 1996, but the instrument is still capable of operation as, even after nearly 7 years of use, the signal to noise performance of the detectors is higher than for a typical AVHRR at launch.

1.3 ATSR-2 and AATSR
The ATSR-2 and Advanced ATSR (AATSR) instruments are developments from the original experimental ATSR-1 instrument which, in addition to the ATSR-1's infrared channels, carry extra visible channels at 0.55µm, 0.67µm and 0.87µm for vegetation remote sensing. The evolution of ATSR-2 was constrained by the requirement to maintain the ATSR-1 precision measurement of global SST.

The ATSR-2 instrument, launched in April 1995, is currently flying as part of the payload of the ESA ERS-2 satellite, and AATSR will be launched early next century on ESA's Envisat platform.The AATSR instrument represents an orderly development of the ATSR series of instruments.

TABLE 1. ATSR-1, ATSR-2 and AATSR Spectral Channels 

Feature	Wavelength	Bandwidth	ATSR-1	ATSR-2 AATSR	Detector type
Chlorophyll	0.55µm	20µm	N	Y	Si
Vegetation Index	0.67µm	20µm	N	Y	Si
Vegetation Index	0.87µm	20µm	N	Y	Si
Cloud Clearing	1.6µm	0.3µm	Y	Y	PV InSb
SST retrieval	3.7µm	0.3µm	Y	Y	PV InSb
SST retrieval	10.8µm	1.0µm	Y	Y	PC CMT
SST retrieval	12.0µm	1.0µm	Y	Y	PC CMT

The ATSR-2 instrument for ERS-2 is largely the same as ATSR-1 except for:

• the inclusion of 3 extra spectral bands in the visible, mainly for vegetation monitoring;
• an on-board visible calibration system.

The AATSR instrument is functionally the same as the ATSR-2, but the structure and some of the other components have been re-worked to match the environment of the Envisat platform, which is somewhat different to the ERS satellites.

The major purpose of AATSR is to provide continuity of the crucial sea surface temperature data sets which have been produced by ATSR-1 and ATSR-2. Therefore, the key scientific parameters which were optimised for ATSR, are retained for AATSR. Thus details of the scan, the optical system, the basic spectral bands, thermal calibration system, spatial resolution and swath have been kept as close as possible to those of the original instrument to ensure continuity.

The major advantage AATSR has over ATSR-2 is in the telemetry bandwidth available on Envisat. For ATSR-2, the limited telemetry available on ERS-2 imposed severe limitations on the collection of visible channel data; on Envisat there are no such restrictions, so AATSR can telemeter all the visible channel data it can collect. This significantly simplifies the ground processing required for AATSR data, as the processor does not need to cope with the wide range of data formats that are possible from ATSR-2.

ATSR-1 Design Details

The ATSR consists of two instruments, an Infra-Red Radiometer (IRR) and a Microwave Sounder (MWS). Both are nationally funded experiments resulting from an ESA Announcement of Opportunity for a scientific add-on package.

The IRR is a four-channel infra-red radiometer used for measuring sea-surface temperatures (SST) and cloud-top temperatures . It was designed and constructed by a consortium, consisting of Rutherford Appleton Laboratory , Oxford University , Mullard Space Science Laboratory , UK Meteorological Office and CSIRO in Australia.

The MWS is a two channel passive radiometer designed and built under the responsibility of Centre de Recherche en Physique de l'Environnement (CRPE). The MWS is physically attached to the IRR and its data is merged with that of the IRR prior to transmission to the ground.

The ATSR-2 design follows the same principles as that for ATSR-1 except for the inclusion of 3 extra spectral bands in the visible (mainly for vegetation monitoring) and an on-board visible calibration system.

Geometry and optics

ATSR fore-contents

The ATSR (see the figure) was designed to provide the following types of data and observations:

• sea surface temperature with an absolute accuracy of better than 0.5 K with a spatial resolution of 50 km and in conditions of up to 80% cloud cover
• images of surface temperature with 1 km resolution and 500 km swath, relative accuracy around 0.1 K
• measurement of the atmospheric integrated water content (vapour and liquid) in order to compute the most problematic path delay in the signal of the Radar Altimeter.

The novel feature of the IRR is the viewing of the same area through a near-vertical atmospheric path and an inclined path of different length some way along the satellite track. Assuming the atmosphere is locally horizontally stratified and stable during the two minutes it takes the sub-satellite point to reach the along-track point, this technique permits the atmospheric correction to be more accurately determined than by previous methods. Data from these two swaths is combined to retrieve accurate and recise atmospheric corrections for the radiometric measurements.

The swaths are produced by a scanning mirror with an axis of rotation inclined 23.45deg. from the vertical. Thus, the field of view by the instrument's detectors via the scan mirror is a near elliptical path on the Earth's surface. However, not all of the scan is used to collect measurement from the surface, since it is interrupted by calibration measurements. Nominally both swaths are 500 km in width, while the two views are separated by approximately 900 km in along-track distance. The pixels making up the nadir view are 1 x 1 km, while the forward view pixels are larger, due to the viewing perspective and are about 1.5 x 2 km. Each ixel contains measurements from three channels.

The footprints of the two MWS channels are not co-incident on the Earth's surface, as one channel views slightly forward of the nadir point and the other slightly behind, although both are on the sub-satellite track. The footprints are 22.4 km x 17.6 km (forward view) and 21.2 km x 21.2 km (rear view) with a separation of 60 km.

ATSR geometry

The IRR uses spectral channels which are very similar to those on recent NOAA meteorological satellites, with many improvements in accuracy. As an imaging radiometer its four channels are fully co-registered by a common fieldstop and sense at wavelengths of 1.6, 3.7, 10.8 and 12 micro-m, defined by beam splitters and multi-layer interference filters. The instrument is housed in a carbon-fibre composite structure which ensures that the optical alignment is maintained.

A schematic view of the IRR is shown in the figure and the main technical characteristics are:

• Objective: sea surface temperature, cloud observations, land and ice surface emissivity
• Spectral channels: 4 co-registered channels at 1.6, 3.7, 10.8 and 12 micro-m
• IFOV: 1 km x 1 km (nadir), 1.5 km x 2 km (forward view)
• Swath width: 500 km
• Scanning method: mechanical - rotating plane mirror, providing two views (nadir and 47deg. to nadir) about 900 km apart; and field of view conically scanned
• Detector: single element HgCdTe and InSb
• Cooler: Stirling cycle - ensures temperatures as low as 77 K
• Radiometric precision: <0.1 K
• Predicted SST accuracy: 0.5 K over a 50 km x 50 km area with 80% cloud cover
• Calibration: two on-board black bodies referenced in the scan pattern
• Instrument housing: carbon fibre composite structure featuring an independent optical bench ensuring optical alignment

Radiation from the Earth's surface is directed by an inclined rotating flat scan mirror on to an off-axis paraboloid (see the figure). The field stop is positioned at the focus of the paraboloid to define the field of view of the instrument. Beyond the field stop the beam is separated into four components of different spectral bands. Three of these component beams (corresponding to the 3.55 to 3.93 micro-m, the 10.4 to 11.3 micro-m and the 11.5 to 12.5 micro-m bands respectively) are re-imaged by three off-axis ellipsoidal mirrors on to separate detectors. The fourth beam (1.58 to 1.64 micro-m) is re-imaged by an aspheric silicon lens on to its detector. The 1.6 micro-m channel was added to the original three channel radiometer to improve sea surface temperature retrievals by detecting cloud during day-time operation of the IRR. To minimise the cost and design effort of adding this channel the same detector/pre-amplifier combination has been used as for the 3.7 micro-m channel.

Details of the four spectral band detectors are provided below:

• Channel 1.6 micro-m 3.7 micro-m 10.8 micro-m 12 micro-m
• Waveband (micro-m) 1.58-1.64 3.55-3.93 10.4-11.3 11.5-12.5
• Detector PhotoV* PhotoV* PhotoC** PhotoC** InSb InSb HgCdTe HgCdTeDetector
• Size (micro-m square) 200 200 190 190
• Condensing Apheric Si Off-axis Off-axis Off-axis
• Optics lens ellipsoid ellipsoid ellipsoid mirror mirror mirror

• Channel Required NE deltaT per pixel at 270 K Required
• signal to noise ratio 1.6 micro-m 20 (with 1/2 % albedo) 3.7 micro-m 0.08 K 22710.8 / 12 micro-m 0.05 K 800*: PhotoV = Photovoltaic**: PhotoC = Photoconductive

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