The first constraint is linked to the orbit.
An orbit with a permanent viewing of the Sun is mandatory for the helioseismologic measurements (internal structure) and is an optimum for the instrument thermal stability (diameter measurements).
A Sun synchronous orbit (SSO) with an ascending node at 6 h and an altitude of about 725 km meets these requirements; the satellite pointing accuracy is of 0.01 arc degree around the Sun's axis. Thanks to the SODISM guided primary mirror, the solar image centring is achieved within a precision of 0.1 arc second.
A second contrainst is linked to the launch date.
To be in the best conditions for the study of the relation between the solar luminosity and the diameter variations, it is preferable to operate during the rising phase of the solar cycle. All other PICARD objectives being also linked to solar activity, observations are scheduled to start at the beginning of the next solar cycle (cycle 24).
The launch took place on June 15, 2010. The mission minimum lifetime is 2 years. The objective which consists in studying the relationship between the diameter and the solar luminosity suggests a longer mission.
Insertion of PICARD mission in the future solar missions context
The schedule of the next solar missions is very favourable to the PICARD mission. On board SORCE, the solar spectrum and total solar irradiance measurements should be still in operation as well as for the SoHO mission. PICARD will ensure the continuity of the measurements up to the turn over by NPOESS mission in 2011. Moreover, the International Space Station operates solar instruments since the beginning of 2008, gathering measurements of the total solar irradiance and of the solar spectrum (Thuillier, et al., 2005; Schmidtke et al., 2005). Other missions will enable the observation of the external layers of the solar atmosphere such as Solar B and STEREO which launches are scheduled in 2006. Their lifetime should ensure concomitant measurements with PICARD. Solar Dynamics Observatory is scheduled for launch at about the same time as PICARD. A strong synergy exists with PICARD, because SDO does not have radiometric measurements, but will measure the distribution of the solar magnetic field and will achieve a deep sounding of the solar interior. These two missions will enable to relate the internal dynamic with the energy emerging from the photosphere.