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.. _geos:
********************************************************************************
Geostationary Satellite View
********************************************************************************
+---------------------+----------------------------------------------------------+
| **Classification** | Azimuthal |
+---------------------+----------------------------------------------------------+
| **Available forms** | Forward and inverse, spherical and elliptical projection |
+---------------------+----------------------------------------------------------+
| **Defined area** | Global |
+---------------------+----------------------------------------------------------+
| **Implemented by** | Gerald I. Evenden and Martin Raspaud |
+---------------------+----------------------------------------------------------+
| **Options** |
+---------------------+----------------------------------------------------------+
| `+h` | Satellite height above earth. Required. |
+---------------------+----------------------------------------------------------+
| `+sweep` | Sweep angle axis of the viewing instrument. |
| | Valid options are ``x`` and ``y``. Defaults to ``y``. |
+---------------------+----------------------------------------------------------+
| `+lon_0` | Subsatellite longitude point. |
+---------------------+----------------------------------------------------------+
.. image:: ./images/geos.png
:scale: 50%
:alt: Geostationary Satellite View
The geos projection pictures how a geostationary satellite scans the earth at regular
scanning angle intervals.
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``).
.. image:: ../../../../images/geos_sweep.png
:scale: 50%
:alt: 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 x, and
GOES should take sweep as y.
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