$LastChangedDate: 27-Juil-2011 $

# CL_gm_pixelSize

Footprint dimensions for a conic sensor at an angle to the vertical

## Calling Sequence

       [f1,f2]=CL_gm_pixelSize(sat_alt,tilt,target_alt,[er])

## Description

- Computes the size factors of the footprint of a conic sensor at an angle to the descending vertical.

  If the footprint is small enough, it can be considered as the intersection between a cone of revolution and the plane tangent to the planet surface (and containing the target location).

  Then:

  - The footprint outline is a circle with radius D when the sensor is looking to the descending vertical.

  - It is an ellipse with semi-major axis f1*D and semi-minor axis f2*D when the sensor is tilted at an angle (tilt) to the vertical.

  Two effects are responsible for the change of size: increase of distance (reflected in f1 and f2) and increase of incidence angle (reflected in f1 only).

- Note:

  - A spherical planet is assumed.

## Parameters

sat_alt:

  Satellite altitude [m] (1xN)

tilt:

  Angle to the descending vertical [rad] (1xN)

target_alt:

  (optional) Target altitude [m] (default is 0)(1xN)

er :

  (optional) Planet radius (default is %CL_eqRad) [m]

f1:

  Size factor in tilt plane(1xN)

f2:

  Size factor in direction perpendicular to tilt plane(1xN)

## Authors

- CNES - DCT/SB

## See also

- CL_gm_visiParams

## Examples

    // Example 1
    sat_alt = 700.e3;
    tilt = CL_deg2rad(20);
    [K1,K2]=CL_gm_pixelSize(sat_alt,tilt)
    // same computation, with CL_gm_visiParams :
    sat_radius = %CL_eqRad + sat_alt;
    target_radius = %CL_eqRad;
    // distance for the given tilt
    [dist] = CL_gm_visiParams(sat_radius,target_radius,'sat',tilt,'dist')
    // elevation for the given tilt
    [elev] = CL_gm_visiParams(sat_radius,target_radius,'sat',tilt,'elev')
    f1 = dist/(sat_alt * sin(elev))
    f2 = dist/sat_alt