Scan angle is defined as the angle between nadir and the antenna electrical boresight direction. Vertical polarization is defined as having the polarized vector in the sun-nadir plane when the beam is pointing at nadir. Horizontal polarization is defined as having the polarization vector in the velocity-nadir plane when the beam is pointing at nadir. Each channel of the AMSU-A is considered to form a beam. All main beam axes of the AMSU-A are coincidental, i.e., they are pointing in the same direction at the same time for any given beam position.
All beams scan in a plane perpendicular to the spacecraft orbital velocity vector.
The sense of the scan is counter clockwise as one looks along the spacecraft orbital velocity direction, namely, the antenna scans from the sun direction through nadir to the cold space direction. The instrument's FOV rotates to a data collection position, stops, collects data, and then moves to the next collection position, stops, collects data, etc. The instrument starts at each earth position 1, then goes sequentially to earth position 30, then to the cold calibration view position, and then to the warm load view position. After the warm load view, the instrument goes back to earth position 1 and the cycle begins again. See Appendix J.3 for specific scan parameters and patterns of the AMSU-A instrument.