The Scan Unit (SU) contains a rotating mirror, situated optically in front of the spectrometer, and its related mechanics and electronics. To perform global Earth coverage, GOME-2's instantaneous on-ground field of view has to be scanned in the across-track direction. The field of view may be varied in size from 320 km x 40 km to 960 km x 80 km.
The SU is physically subdivided into two assemblies: the Mechanical Assembly (SUMA) and the Electronics Assembly (SUEA). The SUMA is almost identical to that of GOME-1, with a rotating mirror installed on an axis actuated by a brushless three-phase motor. A major improvement with respect to the GOME-1 design is the presence of a wireless angular-position resolver; which removes all electrical connections between fixed and rotating parts, leading to a notable reliability gain.
Another positive consequence of such a design is the possibility to perform continuous (360°) rotations of the Scan Mirror at a speed of 10 rpm. This feature, exercised every now and then, will allow redistribution of the lubricant that could accumulate in some parts of the bearing races due to wear, so recovering the original smoothness and precision of movement. Another minor improvement is better confinement of the debris generated by the bearing wear itself.
The SUEA is a separate box, which contains all of the electronics needed for closed-loop control of the scan mirror angular position. It is able to implement five scan profiles at constant angular speed (as per GOME-1) and five new scans, compensating for the Earth's curvature and providing a constant linear scan speed on the ground. A new wide-amplitude scan corresponding to 1920 km on the ground is also implemented, which will allow complete Earth coverage in 1.5 days. All scans are completely in-flight reprogrammable, allowing an almost unlimited choice of profiles. The basic scan timing is 4.5 sec for the forward scan and 1.5 sec for the flyback.
The mirror movements are synchronised with the global instrument timings (detector integration times, and so on). In the event of a failure in the synchronisation interfaces, the Scan Unit can autonomously perform a pre-programmed series of operations, in order to partially recover the mission. The Unit's overall performance remains the same as for GOME-1; mirror positioning accuracy in fixed pointing will be better than 0.03° (corresponding to about 800 m on the ground), while in scanning modes it will depend on actual speed, but will anyway be better than 0.065°.