# 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 (y 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, y being the default. Thus, the scanning geometry of the Meteosat series satellite should take sweep as x, and GOES should take sweep as y.