System for simultaneously calibrating two or more solar radiation measuring pyranometers , comprising an electronic data processor configured for generating mean calibration constants of the pyranometers by carrying out the steps of: installing a reference pyrheliometer with a known calibration constant RDIR placed proximal to the pyranometers to be calibrated, and acquiring measurements of direct normal irradiance; recording the average values VDIR × CZA , VDIF ( i ) with i = 1, ..., NDIF and VGBL ( j ) with j = 1, ..., NGBL at a same and synchronized averaging rate for a plurality of sampling instants, where NDIF is the total number of i-th pyranometers measuring diffuse horizontal irradiance , NGBL is the total number of j-th pyranometers measuring global horizontal irradiance , and VDIR is the output of the reference pyrheliometer , CZA is the cosine of the solar zenith angle, VDIF(i) is the output of the i-th pyranometer , VGBL(j) is the output of the j-th pyranometer , for each of the sampling instants; fitting a linear function in the form Y = M × X + B to the data points defined by X i,j,k = VDIF ( i ) k / VGBL ( j ) k and Y i,j,k = VDIR × CZA k / VGBL ( j ) k with k = 1, ... , NREC, by calculating values of slope M and intercept B, where NREC is the total number of valid average value records; calculating calibration constants of the i-th pyranometer by RDIF i,j = - RDIR / M, and of the j-th pyranometer by RGBL j,i = RDIR / B; calculating a mean calibration constant for the i-th pyranometer measuring diffuse irradiance by RDIF ̅ i = 1 NGBL ∑ j = 1 NGBL RDIF i , j , and a mean calibration constant for the j-th pyranometer measuring global irradiance by RGBL ̅ j = 1 NDIF ∑ i = 1 NDIF RGBL j , i . installing a reference pyrheliometer with a known calibration constant RDIR placed proximal to the pyranometers to be calibrated, and acquiring measurements of direct normal irradiance; recording the average values VDIR × CZA , VDIF ( i ) with i = 1, ..., NDIF and VGBL ( j ) with j = 1, ..., NGBL at a same and synchronized averaging rate for a plurality of sampling instants, where NDIF is the total number of i-th pyranometers measuring diffuse horizontal irradiance , NGBL is the total number of j-th pyranometers measuring global horizontal irradiance , and VDIR is the output of the reference pyrheliometer , CZA is the cosine of the solar zenith angle, VDIF(i) is the output of the i-th pyranometer , VGBL(j) is the output of the j-th pyranometer , for each of the sampling instants; fitting a linear function in the form Y = M × X + B to the data points defined by X i,j,k = VDIF ( i ) k / VGBL ( j ) k and Y i,j,k = VDIR × CZA k / VGBL ( j ) k with k = 1, ... , NREC, by calculating values of slope M and intercept B, where NREC is the total number of valid average value records; calculating calibration constants of the i-th pyranometer by RDIF i,j = - RDIR / M, and of the j-th pyranometer by RGBL j,i = RDIR / B; of the i-th pyranometer by RDIF i,j = - RDIR / M, and of the j-th pyranometer by RGBL j,i = RDIR / B; calculating a mean calibration constant for the i-th pyranometer measuring diffuse irradiance by RDIF ̅ i = 1 NGBL ∑ j = 1 NGBL RDIF i , j , and a mean calibration constant for the j-th pyranometer measuring global irradiance by RGBL ̅ j = 1 NDIF ∑ i = 1 NDIF RGBL j , i . <EOS>