Geostationary Satellite View

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

Classification

Azimuthal

Available forms

Forward and inverse, spherical and ellipsoidal

Defined area

Global

Alias

geos

Domain

2D

Input type

Geodetic coordinates

Output type

Projected coordinates

Geostationary Satellite View

proj-string: +proj=geos +h=35785831.0 +lon_0=-60 +sweep=y

Usage

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.

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

scanning_angle (radians) = projection_coordinate / h

Note on sweep angle

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).

Gimbal geometry

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 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 y, and GOES should take sweep as x.

Parameters

Required

+h=<value>

Height of the view point above the Earth and must be in the same units as the radius of the sphere or semimajor axis of the ellipsoid.

Optional

+sweep=<axis>

Sweep angle axis of the viewing instrument. Valid options are “x” and “y”.

Defaults to “y”.

+lon_0=<value>

Longitude of projection center.

Defaults to 0.0.

Note

The default convention is to interpret this value as decimal degrees. To specify radians instead, follow the value with the “r” character.

Example: +lon_0=1.5708r

See Projection Units for more information.

+R=<value>

Radius of the sphere, given in meters. If used in conjunction with +ellps, +R takes precedence.

See Ellipsoid size parameters for more information.

+ellps=<value>

The name of a built-in ellipsoid definition.

See Ellipsoids for more information, or execute proj -le for a list of built-in ellipsoid names.

Defaults to “GRS80”.

+x_0=<value>

False easting.

Defaults to 0.0.

+y_0=<value>

False northing.

Defaults to 0.0.