How does GLAS geolocate data and provide off-nadir pointing?
The Nominal Orbital Spatial Extent (NOSE) system will be used so that the Warehouse Inventory Search Tool (WIST) can select which granules contain data in a specific geographic region. ICESat’s normal mission consists of a repeat orbit with a 183-day period. A special eight-day repeat orbit will be flown for calibration and validation after launch and as required during its lifespan. For nominal nadir pointing (off-nadir angle less than or equal to an undetermined degree), the eight-day and 183-day repeats will be divided into tracks (each track begins at the ascending node) such that one track is a complete revolution around the earth. Each track will be divided into 73 segments of approximately 5° latitude each. The NOSE system will use the geographic coordinates of the polygon vertices to define each segment within each track. The metadata delivered with every granule will list the tracks covered in the granule and their corresponding segments. ICESat has the capability to point off
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
