  In-flight measurement of the antenna main beams of the Planck instruments is
a crucial input to the data analysis pipeline. We study the main beam
reconstruction achievable through external planets using a flight simulator to
model their observation. We restrict our analysis to the 30 GHz LFI channel but
the method can be easily extended to higher frequency channels. We show that it
is possible to fit the antenna response from Jupiter and Saturn to obtain an
accurate, robust, simple and fast reconstruction of the main beam properties
under very general conditions, independently of the calibration accuracy.
  In addition, we find that a bivariate Gaussian approximation of the main beam
shapes represents a significant improvement with respect to a symmetric
representation.
  We also show that it is possible to combine the detection of the planet's
transit and Planck's very accurate in-flight calibration to measure the
planet's temperature at millimetric wavelengths with an accuracy at the %
level.
  This work is based on Planck-LFI activities.
