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AATSR Data Formats Products
SST record 50 km cell MDS
BT/TOA Sea record 17 km cell MDS
Vegetation fraction for Land Surface Temperature Retrieval GADS
Topographic Variance data for Land Surface Temperature Retrieval GADS
Land Surface Temperature retrieval coefficients GADS
General Parameters for Land Surface Temperature Retrieval GADS
Climatology Variance Data for Land Surface Temperature Retrieval GADS
Level 0 SPH
Level 0 MDSR
Auxilliary Data SPH with N = 1
1.6 micron nadir view MDS
Summary Quality ADS
Scan pixel x and y ADS
Grid pixel latitude and longtitude topographic corrections ADS
Across-track Band Mapping Look-up Table
Configuration Data GADS
Processor configuration GADS
LST record 50 km cell MDS
Distributed product MDS
Level 2 SPH
10-arcminute mds
Limits GADS
Validation Parameters GADS
BT/TOA Land record 17 km cell MDS
General Parameters GADS
Temperature to Radiance LUT GADS
Radiance to Brightness Temperature LUT GADS
Medium/High Level Test LUT GADS
Infrared Histogram Test LUT GADS
11 Micron Spatial Coherence Test LUT GADS
11/3.7 Micron Nadir/Forward Test LUT GADS
11/12 Micron Nadir/Forward Test LUT GADS
Characterisation GADS
Browse Day_Time Colour LUT GADS
Browse SPH
Grid pixel latitude and longtitude topographic correction ADS
Level 2 SPH
Auxilliary Products
ATS_VC1_AX: Visible Calibration data
ATS_SST_AX: SST Retrieval Coeficients data
ATS_PC1_AX: Level-1B Processing configuration data
ATS_INS_AX: AATSR Instrument data
ATS_GC1_AX: General Calibration data
ATS_CH1_AX: Level-1B Characterization data
ATS_BRW_AX: Browse Product LUT data
Level 0 Products
ATS_NL__0P: AATSR Level 0 product
Browse Products
ATS_AST_BP: AATSR browse image
Level 1 Products
ATS_TOA_1P: AATSR Gridded brightness temperature and reflectance
Level 2 Products
ATS_NR__2P: AATSR geophysical product (full resolution)
ATS_MET_2P: AATSR Spatially Averaged Sea Surface Temperature for Meteo Users
ATS_AR__2P: AATSR averaged geophysical product
Frequently Asked Questions
The AATSR Instrument
Instrument Characteristics and Performance
In-flight performance verification
Instrument Description
Internal Data Flow
Instrument Functionality
AATSR Products and Algorithms
Common Auxiliary data sets
Auxiliary Data Sets for Level 2 processing
Instrument Specific Topics
Level 2 Products
Level 1B Products and Algorithms
Level 1B Products
Instrument Pixel Geolocation
The Level 0 Product
Differences Between ATSR-2 and AATSR Source Packets
Definitions and Conventions
Organisation of Products
Relationship Between AATSR and ATSR Products
AATSR Product Organisation
Data Handling Cookbook
Characterisation and Calibration
Monitoring of AATSR VISCAL Parameters
Latency, Throughput and Data Volume
Data Processing Software
Data Processing Centres
The AATSR Products User Guide
Image Gallery
Breakup of the Ross Ice Shelf
Land cover in the Middle East
Typhoon Saomai
Mutsu Bay, Japan
Deforestation in Brazil
Spatially Averaged Global SST, September 1993
Further Reading
How to use AATSR data
Why Choose AATSR Data?
Why Choose AATSR Data?
Special Features of AATSR
Principles of Measurement
Scientific Background
The AATSR Handbook
SST record 17 km cell MDS
Surface Vegetation class for Land Surface Temperature Retrieval GADS
1.6 micron forward view MDS
12 micron nadir view MDS
12 micron forward view MDS
Summary Quality ADS
Surveillance Limits GADS
Master Unpacking Definition Table GADS
1.6 micron Non-Linearity Correction LUT GADS
General Parameters GADS
Thin Cirrus Test LUT GADS
Fog/low Stratus Test LUT GADS
1.6 Micron Histogram
Browse MDS
ATS_CL1_AX: Cloud LUT data
Pre-flight characteristics and expected performance
Payload description, position on the platform
Auxiliary products
Auxiliary Data Sets for Level 1B processing
Summary of auxiliary data sets
Calculate Solar Angles
Image Pixel Geolocation
Level 0 Products
Acquisition and On-Board Data Processing
Product Evolution History
Hints and Algorithms for Higher Level Processing
Data Volume
Software tools
Summary of Applications vs Products
Geophysical Coverage
Geophysical Measurements
Visible calibration coefficients GADS
Level 1B SPH
LST record 17 km cell MDS
Conversion Parameters GADS
12 Micron Gross Cloud Test LUT GADS
ATS_PC2_AX: Level-2 Processor Configuration data
Level 2 Products
Hints and Algorithms for Data Use
BT/TOA Sea record 50 km cell MDS
BT/TOA Land record 50 km cell MDS
Level 2 Algorithms
Signal Calibration
Site Map
Frequently asked questions
Terms of use
Contact us


2.3.2 Conventions

The x and y axes of the image plane are defined as follows.

The x axis is directed in the across-track direction, at right angles to the satellite ground track, and is directed towards the right of the track viewed in the direction of satellite motion. The ground track defines the origin of the x co-ordinate.

The y axis is tangent to the satellite ground track, in the direction of satellite motion.

The relationship of some of these concepts to the surface projection of the scanning geometry is illustrated in figure.2.1

Surface Projection of the Scan (15K)
Figure 2.1 Image Concepts and their Relationship to the Surface Projection of the Scan

Bit numbering. Bits within a 2-byte product field are numbered from bit 15 (most significant bit) to bit 0 (least significant bit) (reference ENVISAT Product Specifications Ref. [1.8 ] , PO-RS-MDA-GS-2009).

Relationship between nadir time (record time tag) and measurement time.

The ENVISAT Product Format Guidelines Ref. [1.7 ] (Document Ref. PO-TN-ESA-GS-0242) require that all data records in a product be provided with a time tag. However, because of the curved scanning geometry of AATSR, a given MDS record will contain pixels from a range of instrument source packets (corresponding to instrument scans). For example, each record of a full resolution product, corresponding to an image row, will contain pixels from some 100 source packets covering 15 seconds of measurement time. Similarly each cell and sub-cell of an averaged product will contain pixels from a range of source packets. The following conventions are therefore adopted:

  • Full resolution products: The time associated with each record is the time at which the satellite ground track intersects the image row; this is the time at which the satellite was vertically overhead at the centre of the row. This is described as the 'nadir UTC time' in product descriptions, and corresponds to the scan time of the nadir pixel.

    Note that the earliest pixels in time to contribute to a row of the nadir view image are those at the edge of the measurement swath. The time stored with each record of the forward view image is the nadir time of the corresponding nadir view image, although the forward view pixels that contribute to a given row are measured up to 135 seconds earlier than the corresponding nadir view pixels. Essentially, therefore, the nadir time associated with an image row is the time of the latest instrument scan to contribute to the row.

  • Averaged products: The time associated with each record is the instrument scan time of the first nadir view pixel to fall within the cell or sub-cell, as appropriate. In this definition the first pixel means the first pixel encountered in the product processing, not the first pixel in time. Usually it will be one of the corner pixels, although which one will depend on the local geometrical relationship between the cell or sub-cell and the image axes.

Image co-ordinates and mapping:

AATSR images are samples on a rectangular array of pixels uniformly spaced in the x and y image co-ordinates. The x co-ordinate of an image point is the distance between the pixel and the ground track, measured along the normal section through the pixel that intersects the ground track at right angles. If the pixel is at point P and the intersection is at point Q, this is the distance PQ. (Strictly the normal section referred to here is the section that is normal to the ellipsoid at the point of intersection Q, not at P. See the fuller discussion in Section )

The y co-ordinate is the distance measured along the satellite ground track to the point of intersection Q from an origin that depends on the product. In the case of consolidated products the origin is the ascending node of the orbit on the equator, but in the case of NRT products the origin may be an arbitrary point at the start of the product.

The across-track sampling interval Δx is 1 km. The along track sampling interval Δy = v x dt where v is the speed of the sub-satellite point along the ground track and dt = 0.150s is the AATSR scan period. The interval Δy varies around the orbit because v does. This equal time interval sampling means that the along track interval between image rows is that same as that between instrument scans.

Instrument pixels are located on the image grid by taking the integer part of their x and y co-ordinates (the latter in terms of the local sampling interval relative to a nearby tie point) and assigning the pixel to the corresponding image point. It follows that the co-ordinates assigned to an image pixel refer to the lower left corner of the pixel, thought of as a rectangular area.

Note that this is true of the latitude and longitude co-ordinates as well as the xy co-ordinates. In particular the ground track may be imagined to lie along the boundary between the 256th and 257th pixels in each image scan, and the latitude and longitude assigned to the 257th image pixel refers to a point on the ground track.

Instrument scan tie pixels:

The scan and pixel number ADS in the gridded AATSR products (ATS_TOA_1P and ATS_NR__2P) contain the x and y co-ordinates of original instrument pixels. Unlike the contents of other gridded product ADS, these values are not sampled with respect to the grid of image pixels, but refer to specific tie pixels on particular instrument scans.

Values are provided for every 32nd instrument scan, and each record of the ADS corresponds to one such scan. The x and y co-ordinates of intermediate pixels can be found from these ADS values using bilinear interpolation with respect to scan number and pixel index. Indices of the tie pixels within each scan are provided in the Specific Product Header, in the fields designated ‘xy tie points’. A value is provided for every tenth pixel, with the proviso that where the final pixel of a view is not a multiple of 10, a value for this last pixel is provided.

Nadir and forward pixels are indexed separately, the nadir view pixels from 0 to 574, and the forward pixels from 0 to 390, giving 59 and 40 tie pixels respectively, and it is this relative index that is shown in the product header. This is also the index that is given in the scan and pixel number ADS.

The relationship between the relative pixel index and the absolute scan pixel number (which is proportional to the rotation angle of the scan) can be found using the auxiliary parameters first_nadir_pixel_number and first_forward_pixel_number found in the auxiliary file of Level 1b Characterisation Data ATS_CH1_AX. For nadir view pixels,

            absolute pixel number = relative pixel index + first_nadir_pixel_number,

and for the forward view

            absolute pixel number = relative pixel index + first_forward_pixel_number .