The Geometry Module is in charge of simulating the spacecraft orbit and attitude, as well as the generation of the observation geometry of each instrument.
There are four different types of scenes:
- Earth pointing scenes: Following the SEPS (SMOS End-to-end Performance Simulator) example, Earth’s surface targets is classified as: land (bare soil, vegetated soil, snow covered, inland water bodies…), and ocean (roughed sea, iced sea) , and all them is affected by atmospheric and ionospheric effects. Note: in general atmospheric effects increase with frequency, while ionospheric effects decrease with increasing frequency, and are only noticeable at low microwave frequencies. Depending on the frequency and the required accuracy they could be neglected or not.
- Sky pointing scenes: These scenes are used for calibration purposes, either by turning the satellite upside down and making the instrument point to the zenith direction, of by switching the Earth-looking antenna and the “sky-horn”, as it is done in the microwave radiometers accompanying the radar altimeters. Sky scenes should take into account the cosmic background (2.725 K), the galactic radiation, and the Sun and the Moon (whenever applicable).
- Limb pointing scenes: These scenes are generated by defining the atmospheric characteristics and can also include a reference background spectrum (e.g., cosmic background, galactic radiation, Sun, or Moon when applicable). For limb sounders, special care must be taken when modelling the atmospheric emission (see for example).
- Other scenes: A way to calibrate small radiometer drifts and/or cross-correlate different instruments consists of comparing the brightness temperature histograms and forcing the minimum value to be the same for all (vicarious calibration). This is a particular case of the Earth pointing scenes over regions that are known to provide the coolest places on Earth in term of TB.
The Geometry Module is in charge of simulating the spacecraft orbit and attitude, the instrument line-of-sight, the observation area and the auxiliary geometrical data to feed the Scene Generator, like the footprint size, orientation, and local incidence angle.
The Geometry Module, and the interaction between its Blocks, is shown in the following diagram: