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SST record 50 km cell MDS
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ATS_TOA_1P_MDSR_conf
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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
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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
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LST record 50 km cell MDS
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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
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Breakup of the Ross Ice Shelf
Land cover in the Middle East
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Mutsu Bay, Japan
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Spatially Averaged Global SST, September 1993
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Why Choose AATSR Data?
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Special Features of AATSR
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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
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1.6 Micron Histogram
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ATS_CL1_AX: Cloud LUT data
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Payload description, position on the platform
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Auxiliary Data Sets for Level 1B processing
Summary of auxiliary data sets
Calculate Solar Angles
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ATS_TOA_1P_ADSR_sa
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
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BT/TOA Sea record 50 km cell MDS
BT/TOA Land record 50 km cell MDS
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2.12.1 Hints and Algorithms for Data Use

As noted in section 1.2. of the AATSR User Guide, there are a number of important concepts associated with the AATSR products that should be understood before proceeding with reading and interpret the data.

The following list of topics is by no means complete. Additional hints and explanations will be added in subsequent issues of this handbook, based on the problems experienced by and the feedback received from users during the mission.

2.12.1.1 Granules

Certain parameters within the products are calculated per granule, rather than for every individual pixel. A granule represents a sub-division of the product, which can be of variable size depending on the parameter in question.  In general the along-track dimension of a granule corresponds to 32 image rows. However, the Summary Quality ADS in the GBTR product is based on a 512-row granule.

2.12.1.2 Tie Points

Tie point pixels are pixels associated with specific points equally spaced across a single image or instrument scan. A series of tie points can be defined for every image row, or at specific intervals in the along-track direction, thus forming the across-track dimension to a granule.

2.12.1.3 Relationship Between Measurement Data and Annotation Data in the Gridded Products

2.12.1.3.1 ATS_TOA_1P

Further information on the interpolation of pixel geolocation in the AATSR full resolution products can be found in the following document HERE.

The AATSR MDS records are re-sampled to a 1 km grid, and every pixel in every row of that grid is written into the MDS. However, the corresponding ADSs are sub-sampled with respect to this grid. Therefore, users must employ an interpolation scheme to retrieve annotation data for each pixel.

The sub-sampling schemes used in each ADS are summarised below.

ADS 0, the Summary Quality ADS (SQADS), contains one record per set of 512 image rows.

ADS 1 and 2, containing scan and pixel numbers for the nadir and forward views, are provided at points every 32 rows along track and for every pixel across track. To derive the scan number of each pixel in the rows between the along-track tie points, take the scan number of the current tie point, and increment by 1 for every subsequent row until the next tie row is reached.

ADS 3 , latitude, longitude and topographic correction, contains these parameters every 32 rows along track and every 25 km across track. Note that in this case, this set of tie points include points at the edges of the scan, at (±275 km), which may lie outside the instrument swath. This results in 23 array elements per record. Latitudes and longitudes for these tie points are simple to calculate, since this is a matter of pure geometry given the position of the tie point.

Linear interpolation should be used to establish the latitude and longitude values for the pixels lying between the tie points. For this reason, the provision of tie points outside the swath was necessary to ensure that there are always two points to interpolate between to derive the latitude and longitude of the edge pixels. Care should be taken whilst doing this interpolation in regions that cross the 180 degree meridian.

It is not appropriate to attempt to interpolate values of topography at this sampling rate, as topography does not change at a uniform rate.

ADS 4 provides the x and y co-ordinates of the INSTRUMENT (as opposed to image) pixels, before regridding onto the image grid, therefore it is arranged in terms of instrument scans instead of image rows. One scan includes pixels from both the nadir and forward views. Therefore scan pixel x and y co-ordinates are not separated into different data sets for the nadir and forward views.

These co-ordinates are provided for every 32nd instrument scan, and for every 10th pixel around the scan. The instrument pixel number of each of these tie points is given in the SPH of the product.

Note also that because this ADS is based in instrument scans, the corresponding time tag represents the time of the instrument scan derived from the telemetry source packet and reflects the start of the instrument scan, rather than the nadir time (i.e. the time at which the middle pixel of the nadir view was beneath the sub-satellite point) used in the other ADS and MDS records.

Linear interpolation should be used to derive scan pixel x and y co-ordinates of the pixels between the tie points.

ADS 5 and 6, solar and viewing angles for the nadir and forward views, are provided every 32 rows along track and every 50km across track (i.e. for the image pixel closest to the 25 km point across-track).

Linear interpolation can be used to derive the solar and viewing angles of the pixels between the tie points, although care must be taken when the azimuth angle passes through 180 degrees.

ADS 7, the visible calibration coefficients ADS, is a Global Annotation Data Set, and therefore contains only one record per product.

2.12.1.3.2 ATS_NR_2P

The ADSs from the L1B product are carried forward into the Level 2 full resolution product, with the exception of ADS 7. The ADSs in the L2 product use exactly the same sub-sampling as for the L1b ADSs above.

2.12.1.3.3 ATS_AR_2P

The AATSR spatially averaged product does not contain any annotation data.

2.12.1.4 The Switchable Product Concept

The GST product (ATS_NR_2P) is switchable, meaning that the content of the MDS depends on the settings of the land/cloud flags for each pixel. The MDS also contains 2 image fields, the Nadir Field and the Combined Field. The contents of these fields, as reflected in the current version of AATSR GST product, are defined as shown in table 2.36 .

Table 2.36 Contents of ATS_NR_2P Distributed Product MDS
Nadir View Cloud Flag Forward View Cloud Flag Surface Type Nadir Field Combined Field
Clear Clear Sea Nadir only SST Dual-view SST
Clear Cloudy Sea Nadir only SST 11 µm BT
Cloudy Cloudy or clear Land or sea CTT
(currently 11 µm BT)
CTH
(currently set to zero)
Clear Cloudy or clear Land LST NDVI

2.12.1.5 Placeholders

The following placeholders are currently used in certain fields of the Level 2 AATSR products to allow for the development of new or improved retrieval algorithms over a longer timescale.

2.12.1.5.1 GST Product (ATS_NR_2P)

Table 2.37 ATS_NR_2P Placeholders
Parameter Name Current Field Contents (i.e. placeholder)
Cloud Top Temperature 11 µm Brightness Temperature
Cloud Top Height zero

2.12.1.5.2 AST Product (ATS_AR_2P)

Table 2.38 ATS_AR_2P Placeholders
Parameter Name Current Field Contents (i.e. placeholder)
Averaged Land Surface Temperature 11 µm average Brightness Temperature
Averaged Cloud Top Temperature mean BT of coldest 25%of cloud pixels in cell

2.12.1.6 Visualising Spatially Averaged Cells

The AST product contains spatially averages surface parameters for two different cell grids and two different cell sizes: 10 arc minute and 30 arc minute, and 17 km and 50 km. The larger cells are derived by averaging over a group of 9 small cells.

The 10 and 30 arc minute cells are not aligned to the 1 km grid of the input GST product from which they are derived. Put very simply, the processing for this product involves creating a 10 arc minute cell, assigning pixels to that cell, and closing and outputting the cell when this process is complete. Due to the misalignment between the two grids, it may be necessary to create a new cell for a pixel that falls outside the boundary of the old cell, before the old cell is completely full. Similarly, it is possible for cell #2 to fill up and be output before cell #1. Therefore, these cells are not output from the processor in a natural grid pattern and should not be treated as a standard array. Instead they should be displayed on a map projection, according to the latitude and longitude co-ordinates associated with each cell.

The 17 km cells are aligned directly with the input 1 km grid and the pixels contributing to each cell are allocated on a systematic basis. Therefore each cell is filled up and output one after the other. The 17 and 50 km cells should therefore be treated just like the 1 km cells in the GST and GBTR products. They can be read into a 2-D raster grid in a systematic order from bottom left to top right.

2.12.1.6.1 Spatially Averaged Cell Latitude and Longitude

The latitude and longitude given for each spatially averaged cell represents that of the lower left-hand corner of the cell.

In 50/17 km cells, this coincides with the first nadir pixel contributing to the cell. In the case of the 50/17 km cells, the cell boundaries are aligned with the image rows and columns, and so the pixels can be systematically assigned to cells from left to right. In this case the first pixel to be assigned to the cell is the lower left corner pixel. The cell time tag is taken as the scan time of the same pixel. In this case 'lower left' means in relation to the conventional image orientation, with the y axis in the direction of satellite motion. This is the south-west corner on ascending arcs, but the north-east corner on descending arcs.

In the case of the 30/10 arc minute cells, the cell latitude and longitude refer to the geometrical co-ordinates of the south-west corner of the cell, and are always multiples of the cell size. This point does not necessarily coincide with an image pixel, and the first pixel to fall in the cell does not usually fall in this corner of the cell. The image rows are oblique with respect to the cell boundaries, and because the orbit is retrograde the cells will fill from east to west. The first pixel to be assigned to the cell, again the pixel which defines the time tag of the cell, is likely to fall in the south-east corner on ascending arcs and the north-east corner on descending arcs.

2.12.1.6.2 Confidence Words and Exception Values

The confidence words and exception values used in the AATSR products are listed in section 2.6.2.1.1. , section 2.7.2.1. and section 2.7.2.2. .

Confidence words provide, on a pixel-by-pixel basis, a bit setting for a particular condition that has occurred in one or more channels of the pixel in question. For example, if the nadir confidence word bit 6 is set, this means that saturation occurred in the pixel and the exception value -5 would be expected in one or more of the channel pixel values.

This is the case for NATURAL pixels. The exception values are set, corresponding to the various error conditions, as these conditions are encountered during pixel processing. In some cases (e.g. Scan Absent; Not Decompressed), the error condition will occur in all 7 channels if it occurs in any. In other cases, (e.g. Saturation in Channel; Calibration Parameters Unavailable), only one channel may be affected. For each view, however, there is only one confidence word per pixel, so the confidence word does not necessarily indicate how many of the channels are affected. It is set if any one or more of the channels show the exception condition. To find which ones, the individual channel images must be inspected.

In the case of COSMETIC pixels, the confidence word bits are not set to match any exception values that may occur. The confidence flags are set during pixel regridding. As each pixel is regridded, the flags that will comprise the relevant confidence word are set. No confidence flags are created at this stage for pixels that are at that time unfilled. This includes pixels that may afterwards be cosmetically filled.

During the cosmetic fill process, adjacent pixel values are copied into the unfilled pixels wherever possible. However, cosmetic fill is based entirely on geometric considerations; i.e., each channel of the cosmetically filled pixel is set to the corresponding value of the neighbouring pixel regardless of whether or not the latter is an exception value. The cosmetic fill flag is then set, but no other flags are. If the pixel remains unfilled, the unfilled pixel flag is set. It therefore follows that for the case of a cosmetically filled pixel, only the cosmetic fill flag in the confidence word is set, even if the actual pixel values are set to exception values.

2.12.1.6.3 Topographic Correction

There will be a narrow strip at the edge of the swath for which no topographic correction is calculated. The topographic corrections are calculated at the same tie points as the image pixel latitude and longitude, so that they can occupy the same ADS. However, as already stated above, this scheme includes tie points outside the instrument swath. The topographic corrections depend on the direction of view, and it is not meaningful to define a view direction for a pixel that is not observed. The topographic corrections are therefore calculated in a similar manner to the solar and viewing angles, for the measured instrument pixel closest to the required tie point. If there is no such pixel, then this tie point will contain an exception value.

A topographic correction is not calculated for land below sea level. It is, however, calculated for lakes above sea level. This is because the current algorithm sets the latitude and longitude corrections to zero if the height value read in from the DEM is negative (i.e. the condition height < 0 is used as a test for sea, and the possibility that a land surface may be below sea level is ignored). The topographic correction is set to zero for these areas.

2.12.1.7 Implications of true IFOV on LST measurements (based on Notes from E. Noyes, University of Leicester, August 2005)

Although data from the AATSR is re-gridded onto a regular 1 km grid, the instrument field of view (FOV) actually extends beyond the edges of the 1 km image pixels on the ground. In addition, the FOV is not uniform. As the viewing angles deviate from true nadir (0º zenith), the FOV on the ground becomes distorted. This is particularly apparent in the forward view, where the instrument pixel size approaches 2.5 km2 on the ground due to the increase in zenith angle. Figure2.15 illustrates the approximate FOV for the 11 μm channel image pixels at the centre and edges of the swath in both the nadir and forward views. Different FOV maps are produced for each of the seven AATSR channels.

full size
Figure 2.15

If the Earth's surface is completely uniform over the entire instantaneous FOV of the AATSR, the nature of the true FOV is not important. However, for much of the earth, the surface is heterogeneous on the scale of an AATSR pixel. For example, variations of Land Surface Temperature (LST) of the order of 10 K have been observed over the scale of a few metres (Prata, 1994).

It has been demonstrated that biases of the order of several tenths of a degree can result in LSTs obtained over a heterogeneous site, if the true FOV of the AATSR is not considered.

From a users point of view, the surface measurements provided by the AATSR may not represent what users believe they represent. There will be a tendency for users to consider the AATSR data as an average value over the 1x1 km image pixel, whereas in reality, the quantity measured may be affected strongly by the FOV. Further information on this effect can be obtained from the University of Leicester or the AATSR team at the Rutherford Appleton Laboratory (initial contact to be made through EOHelp@esa.int).