PHASE_SCREEN

        PHASE_SCREEN

    Warning:  This  tasks  is encapsulated into the "phaseeen.map" procedure
    which enables an easy viewing of the result.

    To compute the atmospheric phase errors, this tasks generate a 2-D phase
    screen on a grid with sufficient spatial resolution to sample the anten-
    na diameter (typically 4-m pixels for ALMA, and 4-m  pixels  for  Bure).
    The  phase  screen is generated in the Fourier plane with the constraint
    that its 2nd order structure function is a combination of 3  power  laws
    in  3  different  spatial ranges. The resulting phase screen is averaged
    over the effective dish diameter. Because of the size limitation imposed
    by  the FFT, very long phase screen are built as a linear combination of
    independently generated screens. This is correct since atmospheric path-
    length variations are completely uncorrelated at large distances.

    Dynamic  (anomalous)  refraction  is  directly proportional to the phase
    gradient. This tasks thus computes the phase gradient associated to  the
    phase  screen,  in  order to obtain a coherent derivation of the dynamic
    refraction term. As the phase screen, the  phase  gradient  is  averaged
    over the effective dish diameter.