2.4.2 Routine Measurement Orbit
A routine SCIAMACHY orbit starts above the northern hemisphere with an observation of the rising sun. In order to acquire also light from the sparsely illuminated atmosphere at the limb in the direction of the rising sun, a sequence of limb measurements precedes each sun occultation measurement. Once the sun has risen, it is tracked by the ESM for the complete pass through the SO&C window. After about 175 sec the sun leaves the limb TCFoV at the upper edge. In order to fully exploit the high spatial resolution during occultation, measurement data readout with a high rate is required in the SO&C window.
Until the passage of the sub-solar point, a series of matching limb/nadir observations are executed. At the sub-solar point the sun, generally close to descending node crossing, has reached its highest elevation relative to ENVISAT. Whether a sub-solar measurement is actually executed depends on whether a sub-solar calibration opportunity has been assigned by ENVISAT. Because of the vignetting of the sub-solar TCFoV by the Ka-band antenna in operational position, only 3 orbits per day with sub-solar opportunities are possible, of which nominally one has to be selected. Another sequence of matching limb/nadir measurements follows. Above the southern hemisphere, the moon becomes visible during the monthly moon visibility period, otherwise matching limb/nadir observations continue. The rising moon is observed similarly to the rising sun from bottom to top of the limb TCFoV. A series of limb/nadir observations concludes the illuminated part of the SCIAMACHY orbit. Because the instrument is still viewing sunlight in nadir direction while the projected ground-track in the flight direction will already have seen sunset, the final measurements in this phase are only of the nadir type. When ENVISAT enters the eclipsed part of the orbit, dedicated eclipse observations can be executed until SCIAMACHY moves towards another sunrise and the orbit sequence starts again (fig. 2-15).