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   Instantaneous field of view of an optical sensor.
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   Form:
   iFOV = IFOV( altitude, f, a, d, lambda )
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   Inputs
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   altitude       (1,n) Altitude
   f              (1,1) Focal length
   a              (1,1) Detector size
   d              (1,1) Aperture
   lambda         (1,m) Wavelength, numbers or 'visible', 'near infrared'
                        'far infrared'

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   Outputs
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   iFOV           (1,1)  Output data structure

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