The Scanning Reference System (SRS)  [ x   y   z ]  is defined by the viewing directions fP and fF according to Eq. (2). The instrument angles (ϕ,ζ) are the spherical coordinates in the SRS of the observed (proper) direction u towards the object. See Fig. 4 for further definition of the viewing directions. The field sizes are greatly exaggerated in this drawing; in reality the astrometrically useful field is 0.72° × 0.69° (along    ×    across scan) for each viewing direction. The basic angle is Γ_c = arccos(fP′fF), nominally equal to 106.5°.