The characteristics of a polar orbiting platform with short telemetry coverage at the high latitude stations Kiruna or Svalbard demanded highly autonomous on-board operations. This comprises not only on-board anomaly detection and initiating corrective actions as part of the instrument control but also the ability to configure the instrument status and to execute measurements without direct manual intervention from ground. Thus the preplanned measurement schedule must be executed on-board in a time tagged manner.
Scientific requirements include viewing geometries for atmospheric measurements of nadir, limb, sun occultation and moon occultation (fig. 2-10). In addition, external (e.g. dark current, sun reference) and internal (calibration lamps) observations supplement the measurement schedule. One of SCIAMACHY’s main objectives is to measure the same atmospheric volume both in nadir and limb within one orbit, i.e. limb/nadir matching.
This can be achieved by first observing an atmospheric volume at the horizon by looking slightly off the flight direction towards Earth’s rotation. Later in orbit, after a time interval Dt = 430 sec, the same volume of air crosses the sub-satellite point and can be observed under nadir conditions (fig. 2-11). The interval of Dt = 430 sec is the result of the angular velocities of Earth and spacecraft platform. In limb mode, SCIAMACHY observes the horizon 3280 km ahead of the instrument close to flight direction. Because the spacecraft’s steering law is determined by the Earth’s angular velocity at the instantaneous sub-satellite point, the line of sight does not intercept the horizon at a point where the Earth’s and spacecraft’s angular velocities lead to limb/nadir matching. Therefore, an instrument yaw steering correction is implemented in SCIAMACHY’s on-board software to compensate for the phase shift between local yaw steering and instrument line of sight in limb observations. It reflects the angular difference of approx. 27° between local sub-satellite point during limb measurement and line of sight interception at the Earth’s horizon (fig. 2-12).