# 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 4.8.0**)

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

scanning_angle (radians) = projection_coordinate / 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 sweep-angle axis, is the N/S axis (`y`) while the inner-gimbal axis, or fixed-angle axis, is the E/W axis (`x`).

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

The `sweep` argument is used to tell `proj.4` which on which axis the outer-gimbal is rotating. The possible values are `x` or `y`, `x` being the default. Thus, the scanning geometry of the Meteosat series satellite should take `sweep` as `x`, and GOES should take `sweep` as `y`.