The model HELIOSAT-2 arises as a natural evolution of the model called HELIOSAT developed by Cano and Diabate during the 80s. The progress introduced in this model corresponds to modifications on the model of clear sky used by means of the introduction of some descriptive parameter of the atmospheric quality. In both models estimation of a dimensionless parameter (n), called cloud index, is proposed on a basis of using the brilliance detected by the satellite.

The cloud index is estimated for every pixel of the image according to so called apparent or

instantaneous albedo (p ), albedo of the clouds (Pn) and ground albedo (pg) as shown in the following expression:

n = (p‘ — Pg ){p’n — Pg )1 (1)

The evaluation of the different albedos necessary for the calculation of the cloud index is obtained from the radiance (L), the geographical parameters that define the pixel in question (latitude, length and altitude) and the information about the atmospheric local quality contained in the coefficient of atmospheric turbidity of Linke (TL).

To conclude, the model HELIOSAT-2 calculates the global horizontal irradiance by means of a relation between the so called clear sky index Kc (global / global of clear sky), and the cloud index (n). The relation used in this work is a modification of the original proposal in which the local median of the cloud index is used [5]. So that the global irradiance estimated in every pixel can be calculated as:

G=KcGcs (2)

where GCs is the global irradiance of clear sky calculated by the ESRA [6] algorithm. The final daily irradiation is calculated by integration of the different images available in a day.