UV_OBSERVE

    UV_OBSERVE  simulates the uv coverage of an observation specified by the
    source, observatory, and hour angle range. The output is a uv table that
    can be used as input for tasks such as UV_FMODEL. Caution: this uv table
    is not standard, as it includes two addition columns  (#11  with  eleva-
    tion, #12 with integration time).

    The  weight  column  (#10) is filled with as realistic as possible esti-
    mates of the actual weights (derived from Trec, Tau, elevation).

    The visibilities columns (#8 and #9) are filled with a point source (am-
    plitude  = 1, phase = 0). Amplitude calibration errors (offset + drifts)
    can be included. Random phase noise can be added, to simulate atmospher-
    ic phase noise.

    If DO_SCREEN = YES, a phase screen is used to simulate phase noise: this
    file contains a statistically correct spatial distribution of the  phase
    perturbation  induced by the atmopshere. This screen moves above the ar-
    ray at the wind velocity. The correct phase value is computed  each  an-
    tenna at each time dump.

    If  DO_CALIB  = YES, the observations of a calibrator are simulated, as-
    suming a loop calibrator-source-calibrator-source. The corresponding  uv
    table is created.

    If DO_RADIOM = YES, a phase correction based on WVR measurements is sim-
    ulated.

    UV_TRACK family of tasks (in order of increasing complexity):

        - UV_TRACK: uv coverage + random phase noise
        - UV_TRACK_PHASE: phase noise from atmospheric screen + observations
          of calibrator +  WVR correction
        - UV_OBSERVE: also includes amplitude noise
        - UV_OBSERVE_NEW: also includes anomalous refraction