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4.1 SHARP Level 1 products 
The ESRIN AVHRR products are archived under the SHARP IB format (SHARP level 1,1989). These products 
are made of 4 minutes of raw data from all 5 or 4 geometrically located channels. Each SHARP scene contains 
the unmodified 10-bit AVHRR pixel values, and calibration information to convert pixel values for the infrared 
bands into equivalent brightness temperatures. The SHARK software calculates the calibration information for 
each scan-line as it reformats the raw data At the same time it calculates the histograms of the raw pixel values 
in each band and writes these into the SHARP product as well. 
To aid the interpretation of the imagery, SHARP products include the geographical position of the centre of each 
pixel on a subsampled lattice within the image. The lattice is included every 32nd pixel on every 16th scan-line. 
SHARP products also include the azimuth and elevation of the sun and satellite from the centre of each pixel of 
the lattice. The process of calculating the geographical position of an image is known as image navigation. 
Graphical overlays of the coastline, state boundaries and a latitude-longitude grid are included in each SHARP 
product Each overlay can be plotted over the image data independently as required. 
4.2 SHARP Level 2 products 
A level 2 product can be derived under request, (SHARP level 2,1992-a and -b). This product consist of data that 
have been further processed from level 1. Two level 2 products have been defined, known as 2A and 2B. Each 
consists of 5 bands with the same restrictions on size and the same ancillary data as level 1 products, all daytime 
passages acquired by the ECTN stations can be processed to generate SHARP level 2 products. A passage is 
considered to be day-time if the AVHRR band 1 histogram read from the SHARP level 1 trailer file has a mean 
value greater than a given unit Because of the possibility of making changes in the data calibrations procedures 
or in the geophysical data retrieval algorithms, the SHARP level 2 is not archived. 
The SHARP level 2 format (SHARP, 1992-a) has the same structure as the SHARP level 1. SHARP level 2 is a 5 
bands image in LINN format. Together with the image data, other information can be found in the prefix and 
suffix data of the imagery records. SI. iRP level 2 pixel is organized in 2 bytes (SHARP 2, 1992-a). Starting 
from the Less Significant Bit there are: 
• 10 bits used for image data. 
• 3 bits used for state boundary, coastlines and Lat/Long grids flags. 
• 3 bits used for classification flags. Presently the classification of Muirhead and Malkawi, (1989) is used. 
However, instead of the raw pixel values, which are written to level 1 products, each band in a level 2A product 
contains calibrated data from the corresponding AVHRR channel. Thus, bands 1 and 2 of a level 2A refers to 
calibrated reflectance of channel 1 and 2 using the coefficients given by Kaufman and Holben, (1993), while 
band 3 contains radiance of channel 3 and bands 4 and 5 the brightness temperatures from channel 4 and 5, 
(Lauritson et al„ 1979). 
Level 2B product is more complicated. Bands 2, 3 and 4 are identical to those in the level 2, but the type of data 
in bands 1 and 5 varies depending on the nature of the target. Each pixel in a level 2 is classified as either land, 
sea. cloud, snow/ice or sun-glint. There is also an extra, unclassified class. The class to which pixel is assi gned 
determines which type of data it holds. 
Pixels in band 1 which are classified as land contain the Normalized Difference Vegetation Index (NDVI). All 
other pixels contain die AVHRR band 1 reflectance as in level 2A product Similarly, pixels in band 5 which are 
classified as either sea or sun-glint contain the Sea Surface Temperature (SST). This SST has been calculated 
using the non linear split window algorithm from Me Gain et al„ (1990). This algorithm accounts for the satellite 
zenith angle variations within the image. The brightness temperature are corrected from the non linearity effects 
of the detectors, (Weinreb et al„ 1990). All other pixels contain the AVHRR c hanne l 5 brightness temperature.