How does GLAS geolocate data and provide off-nadir pointing?
Analysis of altimetric data acquired by the GLAS instrument requires accurate determination of the laser spot location on the Earth's surface (ice, land, water, clouds) or geolocation of the laser spot. The spot location with respect to the Earth's center of mass (geocenter) is determined by both the orbital location of GLAS in an appropriate reference frame and the direction of the laser beam described in the same reference frame. With these two position vectors, the location of the laser spot can be inferred in typical geodetic coordinates (geodetic latitude, longitude, and height above a reference ellipsoid) using a Terrestrial Reference Frame whose origin is coincident with the center of mass of the Earth.
The nominal laser pointing direction is the geodetic nadir (perpendicular to a surface defined by an ellipsoidal model of the Earth), but off-nadir pointing up to 5° is a requirement. ICESat's normal mission consists of a repeat orbit with a 91-day period. A special 8-day repeat orbit was flown for calibration and validation immediately after launch and will be flown as required during its lifespan. For nominal nadir pointing (off-nadir angle less than or equal to an undetermined degree), the 8-day and 91-day repeats are divided into tracks (each track begins at the ascending node) such that one track is a complete revolution around the earth. The orbit altitude is about 600 km, the perigee is fixed in an average sense near the north pole, and the inclination is near 94°. Note: The laser never points directly at nadir because of saturation effects that occurr at nadir, so it points just off nadir where each degree off-nadir corresponds to 10 km on the Earth.