SST Validation
The formal specifications require that retrieved AATSR dual view SST values achieve an absolute accuracy of better than ± 0.5 K, with ± 0.3 K (one sigma) adopted by the project as the target accuracy. The derived AATSR SST values are estimated from algorithms based on radiative transfer models (see for example Závody et al., 1995, Merchant et al., 1999), which perform a linear regression of SST to simulated brightness temperatures (BTs) with nominal band centres located at 3.7 µm, 11 µm and 12 µm, utilising either the nadir view or a combination of the nadir and forward views offered by the instrument. For well-characterized sensors like AATSR, radiative-transfer based algorithms are now established as effective alternatives to algorithms based on empirical regression (Merchant and Le Borgne, 2004). During the day the 3.7 µm channel is not used due to solar contamination and so there are four possible retrieved SST values, referred to as N2 (nadir two channel), N3 (nadir three channel), D2 (dual view two channel) and D3 (dual view three channel), respectively.
There are two operational Level 2 AATSR SST products that require validation: a 1-km gridded SST product referred to as the ATS_NR__2P product and a spatially averaged SST product (at resolutions of 17 km, 50 km, 10´, and 30´), referred to as either the ATS_AR__2P product or the ATS_MET_2P product; the ATS_MET_2P product is reduced Level 2 product containing only 10´ resolution data for meteorological users. Detailed information on these products and their content can be found in the AATSR product handbook 
. For continuity with its predecessors, ATSR-1 and ATSR-2, the ATS_NR__2P gridded product is also referred to as the GSST (Gridded Sea Surface Temperature) product and the ATS_AR__2P averaged produced is also referred to as the ASST (averaged Sea Surface Temperature) product.
Merchant, C.J., A.R. Harris, M.J. Murray, A.M. Závody, 1999. Toward the Elimination of Bias in Satellite Retrieval of Sea Surface Temperature 1. Theory, Modelling and Interalgorithm Comparison, J. Geophys. Res., 104, 23,565-23,578.
Merchant, C. J. and P. Le Borgne, 2004. Retrieval of Sea Surface Temperature from Space, Based on Modeling of Infrared Radiative Transfer: Capabilities and Limitations, J. Atmos. Ocean. Tech., 21 (11), 1734-1746.
Závody, A.M., C.T. Mutlow, D.T. Llewellyn-Jones, 1995. A Radiative Transfer Model for Sea Surface Temperature Retrieval from the Along-Track Scanning Radiometer, J. Geophys. Res., 100, 937-952.