Full text: Mesures physiques et signatures en télédétection

Biophysical indices from ATSR-2 
Geneviève Rondeaux, Mike Steven, Jerry Clark* 
Geography Department, University of Nottingham, Nottingham NG7 2RD, U.K. 
*Department of Physiology and Environmental Science, University of Nottingham, 
School of Agriculture, Sutton Bonington, Loughborough LE12 5RD, U.K. 
The Along Track Scanning Radiometer (ATSR)-version 2, to be carried on the ERS-2 satellite, is 
designed to provide continuity of ATSR-1 (ERS-1) thermal data, but will also include new visible and near- 
infrared spectral bands which are of interest for vegetation monitoring. In many respects this instrument will 
be similar to the current NOAA-AVHRR system, with a pixel size at nadir of about 1 km, but with narrower 
spectral bands. The main difference is that ATSR-2 will produce a pair of images at two viewing angles: 0 and 
55 degrees. 
Pre-operational studies have recently started on the development of data products from ATSR-2. For 
vegetation purposes, the first and essential step is the control of soil background effects. This paper focuses 
on testing different formulations of well-established data products such as the NDVI and Soil Adjusted 
Vegetation Indices (SAVIs) for a large range of soil reflectances. A formulation of an index has been chosen 
because of its low sensitivity to the soil: this is the Simplified TSAVI, defined as (NIR-R)/(NIR + R+0.16), 
which gives satisfactory cover estimation when only the general type of the soil is known (either peat soil or 
mineral soil). This index is then used as the basis of further investigation of the dual-angle approach in 
vegetation monitoring. 
Key Words: ATSR-2; Bidirectional reflectance; Vegetation Indices; Soil effects 
1. Introduction: ATSR-2 for ERS-2: Climate research & Vegetation monitoring 
The Along Track Scanning Radiometer ATSR (-version 1) is a four-channel infrared radiometer (1.6 
nm, 3.7, 11 and 12 /im), designed to measure sea-surface temperatures at unprecedented levels of accuracy. 
Its performance is due, essentially, to two characteristics: the continuously self calibrating on-board system for 
each channel, and the two-look angle capability of the sensor. Several minutes after acquiring a forward view 
of a scene, the satellite is directly above the same point and acquires a nadir view. These two measurements 
are made with differing atmospheric path lengths, and provide information about the effect of the atmosphere. 
The ATSR sensor, on-board the ERS-1 satellite, represents an important contribution to climatic research. 
ATSR-version 2 will provide continuity of ATSR-1, but will also carry several enhancements. In particular, 
it will include three extra channels in the visible and near-infrared wavelengths, chosen for their usefulness in 
vegetation monitoring. ATSR-2 is due to be launched by ESA on the ERS-2 satellite in late 1994. 
Previous and current studies using AVHRR and Landsat TM data have established the importance and 
utility of vegetation remote sensing. AVHRR is used principally at a global scale, monitoring environmental 
problems, for example, deforestation, desertification, pollution, climatic changes. Landsat, with a more limited 
coverage, is used preferentially for local and regional land use studies, often agricultural. ATSR-2 should 
combine the global coverage of an AVHRR-type instrument with the improved spectral coverage of a Landsat 
TM-type instrument. Improvements may be summarised as: better spatial resolution than AVHRR, narrower 
spectral bands than AVHRR and TM, on-board radiometric calibration, and possibility of improvement in 
atmospheric correction. ATSR-2 will measure four bands of reflected solar radiation (table 1), with a pixel size 
at nadir of 1 km. As the main difference with others, it will produce a pair of images at two viewing angles: 
0 and 55 degrees. 

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