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| -rw-r--r-- | docs/source/projections/geos.rst | 64 |
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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. |