Page 151 - Intro to Space Sciences Spacecraft Applications

138 Introduction to Space Sciences and Spacecraft Applications

range and is measured along the direction in which the sensor is looking
(boresight). From the aspect of the point on the earth being viewed, the angle
fiom the horizon to the satellite along the boresight is called the elevation (or
grazing) angle (w), and the complement of this angle is called the incidence
ang2e (ei). The angular distance from the nadir to the point being viewed, as
measured from the center of the earth, is called the earth angle (y).
Given the altitude of the satellite (h) and any one of the viewing angles,
the slant range and all of the other angles can be calculated using:

R2 = (h + RJ2 + RZ - 2R, (h + k) COS($ (6- 1)

and the law of sines:
R
R
h+R,
-=e=
sin(y) si@,) sin(yl+ 90)
Because the sensor may only be looking at a small portion of the available
field of view at any particular time, a description of the area in view and
a method of viewing the total area available must be developed.

Instantaneous Field of View. Just as a telescope of increasing power sees
a smaller area on the face of the moon (for example), many remote sen-
sors concentrate on a small portion of the FOV at any particular time. This
smaller portion of the FOV visible by the sensor is known as the instanta-
neous field of view (IFOV). The remote sensor’s ability to resolve spatial
targets within the IFOV depends on the spatial acuity or dimensions of the
detector’s picture element (pixel) or resolution cell which is a characteris-
tic of the physical make-up of the sensing device itself.
A passive remote sensor’s IFOV is dependent upon the wavelength
scaled dimension of the collection or focusing element (antenna or lens)
and the distance from the remote sensor to the target area being observed,
as shown by (equation 6-3), sometimes referred to as the diffruction limit
of spatial performance.

rFov=(;). (6 - 3)

where:
h = primary wavelength of observation
D = dimension of the aperture (lens, antenna, mirror, etc.)
R = range from the sensor to the target area