A solar diameter metrology measurement: the Picard microsatellite program

The PICARD microsatellite mission will provide 3 to 4 years simultaneous measurements of the solar diameter, differential rotationand solar constantto investigate the nature of their relations and variabilities. The major instrument, SODISM, is a whole Sun imaging telescope of O110 mm which will deliver an absolute measure (better than 4 mas) of the solar diameter and solar shape. Now in Phase B, PICARD is expected to be launched by 2005. We recall the scientific goals linked to the diameter measurement with interest for Earth Climate, Space Weatherand Helioseismology, present the instrument optical concept and design, and give a brief overview of the program aspects.