GEOS

The geos projection pictures how a geostationary satellite scans the earth at regular scanning angle intervals.

In order to project using the geos projection you can do the following:

proj +proj=geos +h=35785831.0

The required argument h is the viewing point (satellite position) height above the earth.

Other arguments include:

• lon_0: the subsatellite longitude point
• sweep: the sweep angle axis of the viewing instrument. Can be x or y (x is the default). (**new in 4.8.1**)

The projection coordinate relate to the scanning angle by the following simple relation:

scanning_angle (radians) = projection_coordinates / h

Note on sweep

The viewing instrument on-board geostationary satellites described by this projection have a two-axis gimbal viewing geometry. This means that the different scanning positions are obtained by rotating the gimbal along a N/S axis (or *y*) and a E/W axis (or *x*).

In the image above, the outer-gimbal axis, or fixed-angle axis, is the N/S axis while the inner-gimbal axis, or sweep-angle axis, is the E/W axis.

This example represents the scanning geometry of the Meteosat series satellite for example. However, the GOES satellite series use the opposite scanning geometry, with the E/W axis as the fixed-angle axis, and the N/S as the sweep-angle axis.

The sweep argument is used to tell proj.4 which on which axis the inner-gimbal is rotating. The possible values are x or y, x being the default.