1 PHASE_SCREEN Generate a 2-D phase screen to simulate atmospheric contribution
        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.

2 LS_FILENAME
        TASK\FILE "Name of the long output   phase screen"  LS_FILENAME$

    This  phase screen is made of the short output phase screens. It has pe-
    riodic boundary conditions at least in its largest  direction.  The  1st
    plane  of  this data cube stores the phase screen, the 2nd and 3rd plane
    store the gradient components.

2 LA_FILENAME
        TASK\FILE "Name of the long averaged phase screen"  LA_FILENAME$

    Same as the long phase screen but convolved with the phase antenna beam.

2 SS_FILENAME
        TASK\FILE "Name of the short output   phase screens" SS_FILENAME$

    There are as many planes as needed to make the long phase screen by lin-
    ear combination of the short phase screens.

2 SA_FILENAME
        TASK\FILE "Name of the short averaged phase screens" SA_FILENAME$

    Same  as  the  short  phase  screen but convolved with the phase antenna
    beam.

2 PIX_SIZE$
        TASK\REAL "Pixel  size [m]" PIX_SIZE$[2]

    Size of the phase screen pixel. It should be a fraction (typically  1/3)
    of the antenna diameter.

2 L_SCREEN_SIZE$[2]
        TASK\REAL "Long  screen size [m]" L_SCREEN_SIZE$[2]

    Size of the phase screen which will really be used in the simulation.

2 S_SCREEN_SIZE$[2]
        TASK\REAL "Short screen size [m]" S_SCREEN_SIZE$[2]

    Size  of  an intermediate phase screen. This size should be small enough
    so that the intermediate phase screen FFT transform is easily computable
    on  your computer. However the size should be large enough to be able to
    consider that atmospheric pathlength variations are completely  uncorre-
    lated from one side to the other of this intermediate phase screen.

2 SF_VALUE_300M$
        TASK\REAL  "Structure  function  value  at  300m (in square degree)"
    SF_VALUE_300M$

    The atmospheric phase rms on the 300-m baseline.

2 EXPONENTS$[3]
        TASK\REAL "Structure function exponents" EXPONENTS$[3]

    Exponents of the SQRT(2nd order structure function of the phase).  There
    should be 3 values that may be identical.

2 BASE_RANGES$[2]
        TASK\REAL  "Ranges  of  baselines  for  the  previous exponents [m]"
    BASE_RANGES$[2]

    EXPONENTS$[1] will be used between below BASE_RANGES$[1]  meters,  EXPO-
    NENTS$[2]  will  be used between BASE_RANGES$[1] and BASE_RANGES$[2] me-
    ters and EXPONENTS$[3] will be used above BASE_RANGES$[3] meters

2 FOURIER_SPACE$
        TASK\LOGI "Compute the power spectrum directly  in  Fourier  space?"
    FOURIER_SPACE$ NO

    Switch to test an alternative way to compute the phase screen. This must
    be set to NO as this is *not* working. This option  and  the  associated
    code should be removed.

2 DIAM$
        TASK\REAL "Antenna Diameter [m]" DIAM$

    Diameter  of interferometer antenna used in the convolution of the phase
    screen with the phase antenna beam.

2 SF_VERIF$
        TASK\LOGI "Verify the structure  function  of  the  output  screen?"
    SF_VERIF$

    Toggle computation of the structure function of the output screen, main-
    ly for test purposes.

1 ENDOFHELP






















