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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
 Calculate Solar Angles

Solar and viewing angles required for cloud clearing are also determined at this stage. The angles are calculated at a series of tie points across the scan at increments of 25 km in the across-track co-ordinate x; these values are required for internal use within the processor, although only values at 50 km intervals are output to the product.

The following angles are calculated:

  • The azimuth of the sun at the pixel;
  • The elevation of the sun as seen from the pixel;
  • The azimuth of the sub-satellite point measured at the pixel.
  • The elevation of the satellite as seen from the pixel.

These quantities are required for use by the 1.6 micron cloud clearing test, to identify situations in which sun-glint might be present, while the solar elevation is also used to distinguish day and night measurements in cloud clearing and level 2 processing. The satellite azimuth and elevation are also used in the calculation of the topographic correction.

The solar and viewing angles are calculated at the centres and edges of the across-track bands defined in Section 2.3.1. . These correspond to a series of across-track positions separated by 25 km. Thus if we define an index k, the solar angles are calculated for nominal across-track co-ordinates

x = 25(k – 10) km

for k = 0, 20. In this formulation even values of k correspond to the band edges, and odd values of k to the band-centres. Internally within the processor values are calculated for both band centres and band edges and for every instrument scan, but only the band edge values on the granule rows are output to the product ADS.

Although defined at the nominal positions in the equation above, in practice the angles are calculated for specific instrument pixels. For each view and for every tenth instrument scan, and for each value of k, the instrument pixel whose across-track co-ordinate is closest to the nominal value given by equation (3.1) is identified, and its line of sight azimuth and elevation (measured at the satellite) are derived using the same algebra as in Section . The calculation of the angles is then carried out using the standard ENVISAT mission CFI subroutine pp_target (Reference: Document PO-IS-GMV-GS-0559, PPF_POINTING Software User Manual) with the scan time and the line of sight azimuth and elevation as parameters.

Results for intermediate instrument scans are calculated by linear interpolation with respect to scan number for each across-track distance, and the results are regridded to the image rows in a similar way to the pixel data. Calculate Topographic Corrections

For those scan pixels that coincide with tie points for which topographic corrections are required, and that are over land, the topographic height is determined from a digital terrain model and topographic corrections to the latitude and longitude are calculated. Theoretical Basis

The pixel geolocation described in Section finds the intersection of the line of sight from the instrument with the reference ellipsoid. If the land surface is elevated by an amount H above the reference surface, the intersection of the line of sight with the land surface, which is the true pixel position, is displaced towards the satellite in the direction of the projected line of sight by an amount that is proportional to the elevation H.

Let the unit vector from the pixel to the satellite be k = (kx, ky, kz). If the azimuth and elevation of the line of sight at the pixel are a, e respectively, then the components of the unit vector k are

eq 2.128

eq 2.129

eq 2.130

expressed in a local co-ordinate system in which the x axis is directed to the east, the y axis is directed to the north parallel to the local meridian, and the z axis is vertical.

The pixel position is displaced by linear amounts

eq 2.131

eq 2.132

The corrections in latitude and longitude are the corresponding angular displacements:

eq 2.133

eq 2.134

where φ is the latitude of the pixel. Here R and N are the two orthogonal radii of curvature of the Earth at latitude φ; N and R are the radii of curvature in prime vertical and in the meridian respectively, given by

eq 2.135

eq 2.136

eq 2.137

where e is the eccentricity of the reference ellipsoid, and the geodetic constant

eq 2.138

The quantity N cos φ is the radius of the parallel of latitude at φ. Algorithm Description

The topographic corrections are computed for the same tie points as the image pixel latitude and longitude. This method makes use of the satellite viewing angles for the appropriate view and tie point previously computed. The topographic height is determined from a digital elevation model.

The nominal tie points are at across-track distances x = {-275, -250, -225, ... 275} km, corresponding to an across-track index k through x = 25(k – 11) km, k = (0, ... 22). However, if k = 0 or k = 22, no viewing angles will be available from the solar and viewing angles calculation, and so these cases are omitted.

The algorithm is applied to both nadir and forward view instrument scans. For each scan, the algorithm steps are as follows.

  • For each across-track distance for which a correction is required, the index p of the instrument pixel is found whose across-track co-ordinate is closest to the required across-track distance. The latitude φ and longitude λ of this pixel are found.
  • The local altitude (over land) or bathymetry (over sea), H, at latitude φ and longitude λ is extracted from the digital elevation model.
  • The pixel is regridded to the appropriate image row. Steps 4 and 5 are performed only if the pixel regrids to a tie row.
  • If H < 0 (note this includes the case that the pixel is over sea), the latitude and longitude corrections are set to zero and Step 5 is omitted.
  • If H ³ 0 the satellite azimuth and elevation corresponding to the pixel, calculated in the solar and viewing angles module (Section, are extracted and converted to radians, and the latitude and longitude corrections d φ, d λ are calculated using equations ( eq. 2.131 ) to ( eq. 2.138 ) in Section above.

Note that the corrections are the quantities to be added to the nominal latitude and longitude to give the topographically corrected values.