IASI Observation Strategy
The IASI instrument was launched on 19 October 2006 aboard the first European Meteorological Operational satellite (MetOp-A) on a sun-synchronous polar orbit (equatorial crossing time on ascending node at 21:30) with an inclination of 98.7° to the equator and at an altitude from 800 to 850 km. The orbital period of the MetOp satellite is 101 minutes, corresponding to approximately 14.3 revolutions per day. The repeat cycle is 29 days and 412 orbits.
The IASI viewing geometry is illustrated in Figure a. In the normal mode, the instrument looks in the ground direction with the optical axis scanning the atmosphere perpendicularly to the satellite orbit track. Each across-track scan is made of 30 FOR's (Field of Regard) acquired using a step and stare scanning strategy that covers a ±48.33° range, symmetric with respect to the nadir direction, in steps of ~3.33° (58.178 mrad). The total swath at ground is approximately 2200 km ensuing 99% global coverage twice a day. Each FOR consists of a matrix of 2x2 IFOVs (Instantaneous Field of View) with diameter of ~0.84° (11 mrad ≤ d ≤ 14.65 mrad) and centres located at 15.3 mrad (~0.88°) from the instrument optical axis (see Figure b). On the ground, each cell of the 2x2 matrix corresponds to a circular pixel of 12 km diameter at sub-satellite point; at the edge of the scan the across-track and along-track sizes of the individual pixel are 39 km and 20 km respectively. The FOR size at nadir is 50 km x 50 km and the distance between two Earth FOR at sub-satellite point is about 50 km.
Figure a
IASI Viewing Geometry (Simeoni et al., 2004)
The scan starts on the left side with respect to the flight direction and has a total duration of 8 sec. Each scan includes 30 atmospheric views at equally spaced time intervals of 8/37 sec (~216 msec), with interferogram acquisition time of 151 msec, and observations of an internal calibration target and of deep space each lasting approximately 216 msec (overall calibration time equal to 1514 msec). The step-by-step scanning of Earth views is corrected by satellite velocity compensation, so that the position of the IFOV at ground does not move during the interferogram acquisition time.
The spatial density of the IASI measurements is estimated to be of 1000 samples for a 10° x 10° area, as shown for instance in Figure c by the area within the blue line..
Figure b
IASI Elementary Field of View
Figure c
IASI measurement pattern
