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 |
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).
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.
- +R=<value>¶
Radius of the sphere given in meters. If used in conjunction with
+ellps,+Rtakes precedence.
- +x_0=<value>¶
False easting.
Defaults to 0.0.
- +y_0=<value>¶
False northing.
Defaults to 0.0.