Migrated Geostationary Satellite View projection from trac

1 files changed, 63 insertions, 1 deletions

diff --git a/docs/source/projections/geos.rst b/docs/source/projections/geos.rst
index 07779386..96884238 100644
--- a/docs/source/projections/geos.rst
+++ b/docs/source/projections/geos.rst
@@ -3,8 +3,70 @@
 ********************************************************************************
 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  
+   :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.svg
+   :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.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.