AATSR Frequently Asked Questions

AATSR stands for ‘Advanced Along-Track Scanning Radiometer’.

AATSR is one of the Announcement of Opportunity (AO) instruments on board the European Space Agency (ESA) satellite ENVISAT. It is the most recent in a series of instruments designed primarily to measure Sea Surface Temperature (SST), following on from ATSR-1 and ATSR-2 on board ERS-1 and ERS-2. AATSR data have a resolution of 1 km at nadir, and are derived from measurements of reflected and emitted radiation taken at the following wavelengths: 0.55 µm, 0.66 µm, 0.87 µm, 1.6 µm, 3.7 µm, 11 µm and 12 µm.

Special features of the AATSR instrument include its use of a conical scan to give a dual-view of the Earth’s surface, on-board calibration targets and use of mechanical coolers to maintain the thermal environment necessary for optimal operation of the infrared detectors.

ENVISAT continues the work of the ERS satellites, and its data supports earth science research and allows monitoring of the evolution of environmental and climatic changes.

ENVISAT flies in a sun-synchronous polar orbit of about 800-km altitude. The repeat cycle of the reference orbit is 35 days.

AATSR primarily measures SST to the high levels of accuracy and precision required for monitoring and detecting climate change. Together with its predecessors, ATSR-1 and ATSR-2, AATSR will establish a unique SST data set spanning at least 15 years, supporting oceanographic and climate research.

AATSR data can also be used for a number of land surface, cryosphere and atmospheric applications.

A link to further information on the applications of (A)ATSR data will be added in the next issue of this FAQ.

AATSR Cyclic Reports are made available by ESRIN to keep the AATSR community informed of any modifications to the processor, updates of auxiliary products, instrument anomalies, the status of data acquisition and processing, and the status of the calibration, validation, and quality control activities.

ATSR-1 measured in the infra-red at 1.6 µm, 3.7 µm, 11 µm and 12.0 µm and had no visible channels. ATSR-2 and AATSR include the same four infra-red channels of ATSR-1 and three additional channels at 0.55 µm, 0.67 µm and 0.87 µm.

The ATSR-2 instrument is largely the same as ATSR-1, differing only in the inclusion of the extra channels and an on-board visible calibration system.

AATSR is functionally largely similar to ATSR-2, but components have been redesigned to match the new platform environment of ENVISAT. The details of the scan, calibration systems, spatial resolution and swath have been kept as close as possible to the earlier instruments to ensure data continuity. The major advantage of AATSR over ATSR-2 is in the downlinking of data. ATSR-2 had restrictions on the amount of data that could be downlinked, due to platform constraints, which particularly affected the visible channels, whereas AATSR can send down data from all the channels continuously at full 12-bit radiometric resolution. This also simplifies the data processing for AATSR, since the processor does not have to cope with the wide range of instrument data formats that could be used for ATSR-2.

The specific differences between the ATSR-2 and AATSR source packets are discussed in section 2.5.2 of the AATSR handbook (see Q7).

A summary of the relationship between the AATSR and ATSR products is given in Section 2.2.3 of the AATSR Handbook (see Q7).

MERIS and (A)ATSR Workshop (26 – 30 September 2005)

ENVISAT/ERS Symposium (6-10 September 2004)

ENVISAT Meris and AATSR Validation Team Workshop - MAVT-2003 (20 - 24 October 2003)

Envisat Validation Workshop (9-13 December 2002)

Envisat Calibration Review (9-13 September 2002)

ERS-Envisat Symposium "Looking down to Earth in the New Millennium" (16-20 October 2000)

ATSR Project Web Site – Documentation Section

ESA Data User Element (DUE) Projects

ESA EO PI Projects

AATSR PI Team at Leicester University – Publications List

ENVISAT Library

N.B. An AATSR Reference Documents web site is due to be created in 2006. Further details will be provided in subsequent issues of this FAQ.

The EnviView tool provided by ESA (see below) can also be used to convert ENVISAT data into hdf file format. This allows the data to be automatically read by any third-party software supporting this format.

The providers of a number of COTS image processing packages such as ERDAS, ENVI and Geomatica also planned to extend their products to directly support the AATSR product format. The individual packages/providers should be consulted for details of each tool’s capabilities.

EnviView is a free application that allows Envisat data users to open any Envisat data file and examine its contents. It provides simple visualisation capabilities, and allows data to be exported to HDF for use in other software packages.

The Basic ERS & Envisat (A)ATSR and Meris Toolbox (BEAM) is a collection of executable tools and an application programming interface (API) which has been developed to facilitate the utilisation, viewing and processing of ESA MERIS, (A)ATSR and ASAR data. ¹

tape_reader.ksh is a Korn shell script which may be used to extract Envisat products supplied on exabyte tapes.

There are a number of important concepts associated with the AATSR products that should be understood before trying to read and interpret the data. These are discussed in Section 2.12.1 of the AATSR handbook (see Q7).

Background information on issues relating to AATSR Flight Operations can be found in Sections 1.1.3.2 and 3.2.2.2.1 of the AATSR handbook (see Q7).

The AATSR IPF version number is a field within the Main Product Header of the data. Volume 5 of the ENVISAT Product Specification (Product Structures) details the contents of the product headers. This document is due to be added to the archive at http://envisat.esa.int/services/esa_doc/doc_env.html shortly.

The auxiliary file of SST coefficients used by the Level 2 processor includes distinct coefficients for each of 38 across-track distances. The process by which these 38 sets of coefficients are generated involves first deriving two sets of coefficients, for the swath centre and swath edge, and then computing intermediate values by linear interpolation with respect to air mass. (Here the air mass refers to the normalised length of the atmospheric path traversed by the line of sight - essentially the secant of the angle of incidence at the pixel.)

The air mass at the swath centre is unity (that is the normalisation), but that at the edge depends on factors such as the cone angle of the AATSR scan and the satellite height.

A study of the sensitivity of the edge of swath air mass to changes in the cone angle and the satellite height has been conducted. In general, the results are very insensitive to cone angle; satellite height is a slightly more significant parameter, but as it varies around the orbit there is not much that can be done about it other than to use a suitable average value.

Small negative values (-1 to -8) are 'exception values' used to flag errors and other special conditions in the data. These are listed in Sections 2.6.2.1.1 of the AATSR Handbook (see Q7) for L1b data.

Most topographic correction tie points over sea are set to zero (as the sea is at sea level). However, exception values (notably –999999) can arise in the current algorithm if no pixel at a given across-track position regrids to a particular topographic correction tie point. This occurs because there is no cosmetic fill in the topographic correction and the array is initialised to –999999 before calculations commence.

The orbit number in the product file name is that in which the first data contributing to the product falls. This is not a problem for NRT data, as NRT products are not expected to begin exactly at ANX.

However, in the case of consolidated data (orbits defined from ANX to ANX), if the first data slightly precedes the ascending node, then the orbit number of the file is one lower than you might expect.

Taking the following file as an example:

ATS_TOA_1PPLRA20040605_001448_000062202027_00259_11836_1706.N1.gz

The start of data (00:14:48 on 2004 June 5) is very close to the ascending node at the start of absolute orbit 11837. Therefore, although the product predominantly represents orbit 11837, the start of data falls (just) in the preceding orbit.

There is no unambiguous answer to this. Both the AST and GST products are derived from the same set of L1b data, just using different algorithms, so they are, in effect, both ‘right’. The real issue is the exact definition of the 'number of pixels' field in the AST product.

The field of the AST record designated 'Number of pixels in dual view average' contains the smallest of the numbers of pixels contributing to the averaged brightness temperatures (in the 11 and 12 micron channels and in the nadir and forward views) that were used in the SST retrieval.

In an ideal world the same number of pixels would contribute to each of the averaged brightness temperatures in the cell, and this number would appear in the field, but in practice the numbers may differ for different channel/view combinations. In particular, although the numbers of 11 and 12 micron values contributing are likely to be the same in each view, the nadir and forward views may differ because of e.g. missing scans. As the number of pixels is supplied as an indicator of SST quality, the smallest is chosen. Similarly the field 'Number of pixels in nadir-only average' would contain the smaller of the numbers of pixels in the 11 and 12 micron averages where these differ. The 3.7 micron channel numbers are ignored because this channel may or may not contribute to the SST retrieval.

The units for the visible channel bandwidth in the IODD, in the ENVISAT Product Specifications and in EnviView are given as microns, but the actual numerical value is in nm.

The units for the integrated solar irradiance values are microwatts/cm².

The information required for the interpolation of pixel geolocation can be found in the AATSR Technical note entitled "Interpolation of Pixel Geolocation in AATSR Full Resolution Products" referenced in section 2.12.1.3.1 of the AATSR handbook (see Q7). The contents of this note are also reproduced in Appendix A of this FAQ.

The pixel co-ordinate resulting from the interpolation (as described in the above-referenced technical note) corresponds to the lower left corner of the pixel.

In the across-track direction the co-ordinate corresponds to the left-hand edge, while in the along track direction, the sampling of the tie points is such that the tie points correspond to the lower edge of the pixel (i.e. the edge corresponding to the lower Y co-ordinate).

The short field names, such as pix_nad, used in EnviView and the Formats sections of the AATSR handbook (see Q7) have not been assigned uniquely; the same names have been used, in different products and MDS, for different things.

The data definition document for the AATSR processing software defines a unique parameter ID for each AATSR product field. The unique parameter names of the values referred to are as follows:

The Level 2 processor generates the averaged brightness temperatures by averaging all of the valid image pixels from the Level 1b product that fall in each 10 arc minute cell. Image pixels in the Level 1b product are classifiable into three mutually exclusive types: NATURAL, COSMETIC, and UNFILLED; every pixel is one of these types, and this classification is purely geometrical, and independent of whether or not the pixel contains valid data in any channel.

Essentially, NATURAL pixels are those to which a measured instrument pixel has been mapped during regridding. (The majority of image pixels should be of this type.) A pixel to which no instrument pixel maps during regridding is either COSMETIC or UNFILLED. A COSMETIC pixel has been filled by the cosmetic fill algorithm; it will have at least one natural pixel as neighbour. Otherwise the pixel is UNFILLED.

In the BT/TOA MDS, the quantity [AST-MDS15-7] (pix_nad) is the total number of filled (i.e. NATURAL or COSMETIC) pixels in the 10 arc minute sub-cell, regardless of whether or not the brightness temperatures are valid. Similarly [AST-MDS15-8] (pix_ss_nad) is the number of filled pixels that are over sea, and [AST-MDS15-9] (clpix_ss_nad) is the fraction of these that have been flagged as cloudy. (Since the cloud clearing algorithms depend upon the pixel data, the last of these quantities has some dependence on whether or not the pixel data is valid.) [AST-MDS15-7] (pix_nad) should equal [AST-MDS15-8] (pix_ss_nad) if the sub-cell is entirely sea, but not if it is intersected by coastline.

The quantities [ECM-MDS1-8] and [AST-MDS3-8], both designated pix_nad in EnviView, are identical; that in the Meteo product, [ECM-MDS1-8], is a copy of [AST-MDS3-8]. They represent the number of valid pixels that contribute to the nadir SST in the cell. This number cannot exceed, but may be less than, pix_ss_nad, since some of the filled sea pixels may contain invalid data. Its detailed definition is the smaller of the number of valid pixels that contribute to the averaged brightness temperatures at 11 and 12 microns; this definition reflects the theoretical possibility that a given pixel might have a valid BT in one channel but not in the other, although this is unlikely over clear sea.

To be addressed in the next issue of this FAQ.

At night, noise in the visible channels cannot be distinguished from pixel exception values, and can therefore be masked into the confidence flags. This is not strictly an error in the AATSR processing scheme, but rather an unexpected result of adopting the ATSR-2 processing scheme for AATSR (ATSR-2 visible channel data were not available at night).

The AATSR pixel exception values are small negative numbers (ranging from -1 to -8). At night, the noise in the visible channels can mimic these exception values, so that the interpretation of negative visible channel reflectances is ambiguous. Also, because the confidence flags are derived from the union of the exception values across all channels, this can cause the confidence flags to be wrongly set. Note that the latter effect affects any user who is using the confidence flags to filter the data, regardless of whether he is interested in the visible channels or not.

Users are advised to take note of this effect and to take into account these effects if using the confidence flags or visible channel reflectances in night-time data.

This is also due to the presence of noise in the visible channel data at night.

NDVI values are calculated whenever visible channel data are present, and as visible channels are present in the AATSR telemetry at all times, the NDVI is calculated for both day and night data. There is no trap for night-time data.

However, at night, the visible channels are dominated by noise, making these NDVI values essentially meaningless. Users are therefore advised to disregard the values in the AATSR NDVI field at night.

Poor cloud clearing