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4.1 Observation strategy 
The observation strategy plays a basic role in the 
achievement of 10 mas accuracy on the astrometric 
parameters. Such a strategy is, in turn, strongly 
connected with the chosen scanning law, as it is 
explained in the following. 
The attitude of the spacecraft follows a complex 
"revolving scanning" motion, called the nominal 
scanning law, which ensures a complete sky coverage 
over the lifetime of the mission and allows to reduce the 
thermal perturbations on the payload. 
The spin direction is maintained constant at 55° angle 
with respect to the Sun direction, and rotates with 
angular velocity of 120 as/sec (120° for hour), and a 
period of 3 hours. The axis has a precession motion 
around the solar directions with constant precession 
rate = 0.17 arcsec/s (4.14 revolution/year). A pictorial 
representation of the satellite scanning law is given in 
figure 9. 
A complete great circle in the sky is scanned in 3 hours 
by the instrument line of sight, and the closure 
conditions on the great circle allow accurate 
determination of the most critical geometrical 
instrument parameters (in particular the basic angle 
between the two viewing direction) with a time 
resolution corresponding to the spin period. 
The path scanned on the sky in one year by one of the 
lines of sight of the Astrometric Instrument following the 
nominal scan law is shown in Figure 11. 
As a star enters the preceding (or following) FOV, its 
interference fringe pattern is recorded on the main CCD 
field. Then, the first reduction step consists in using this 
fringe pattern to accurately determine the photocenter 
position of each stellar image with respect to the 
optical axis. 
Figure 11: Path described in the sky by one line of sight 
in one year 
4.2 Strategy of reduction 
In order to formulate the observation equation we note 
that the GAIA measurement has a very high resolution 
in only one direction, i.e., that of the satellite 
instantaneous scanning velocity. Therefore, the 
measure of a star position as seen in the instantaneous 
FOV, neglecting deviations from flat-field geometry 
caused by instrumental effects, which nevertheless 
must be accurately calibrated, can be expressed by the 
following equation: 
x ik = n k * £* ( 3 ) 
where x is the projection along the scanning direction of 
the star distance from the FOV center, r is the unit 
vector representing the geometric direction to the stellar