Comparison between measurements of insolation from pyranometers mounted on ships and estimates from satellite visible measurements during STREX are presented. In general for both hourly and daily insolation values the comparison is quite favorable. The root mean square (rms) difference between the two measurements of daily insolation was about 13 W m-2. From a detailed analysis of the errors and measurement discrepancies it was found that a large part of the difference between the two estimates of insolation could be due to inaccuracies in the surface measurements, interpolation during periods of radio interference, and ship motion. Comparisons between time-averaged insolation values over periods of about 10 days and limited statistical analysis of the pyranometric time series suggest that, on that time scale, the accuracy of the satellite estimates is of the order of 4 W m-2. Large-scale insolation fields derived from satellite during STREX will be presented in the companion paper (C. Gautlier and S. Masse, manuscript in preparation, 1984).