The following calculator determines the calibrator cycle time (the amount of
time between successive visits to the calibrator) given observer
parameters and parameters describing the phase stability.
This assumes that
self-calibration is not going to be used.
Explanation of input parameters
The calculation of the recommended calibrator cycle time depends on a number
of parameters:
Observing frequency and maximum baseline length are set by the
observer's science.
Seeing monitor phase stability: this gives the phase stability parameter
(in µm) measured by the ATCA seeing monitor. It is typically 50-2000 µm.
50 µm is typical of good winter nights, whereas 1500 µm is more typical
of poorer summer days. In thunderstorms, it can reach 6000 µm.
Phase screen speed: This gives the pattern speed of the phase screen,
and determines how rapidly the phase changes.
Typical values for the phase screen speed are 1 to 10 m/s.
The phase screen speed is traditionally thought to
be the wind speed of the turbulent layer (the turbulent layer is
that part of the atmospheric where fluctuations dominant the phase
stability). As a generalisation, the turbulent layer will be at greater
heights, with corresponding higher wind speeds (relative to the ground) in better observing
conditions.
Kolmogorov exponent β/2: The phase structure function is generally
modelled as φ2 ~ bβ. For so-called
three dimensional turbulence (which is of interest for the ATCA in all but 6km arrays)
β/2 ~ 0.83. For the ATCA in a 6km array, two dimensional
turbulence theory is thought to be valid, and β/2 ~ 0.33.
For more information, see Thompson, Moran & Swenson
(page 530 onwards in the second edition).
Original: Bob Sault (22-Nov-2004)
Modified: Bob Sault (26-Nov-2004)
