Description

Switching times are a critical component of scheduling a phase-referencing observation with the VLBA. Beasley and Conway (1995) give two equations for the longest allowable switching time , depending on whether the time scale is less than or greater than that for fluctuations to cross a distance equal to the scale height of the troposphere:

Here, is in minutes, is the wavelength in centimeters, z is the zenith angle, is the strength of the tropospheric turbulence ( is the amplitude of the structure function), and is in units of m, v is the velocity of the turbulence in meters per second, and L is the scale height of the troposphere. See Beasley and Conway (1995) for more detailed discussion.

Below, we provide tables and plots that can be used to estimate the phase-referencing switching time (or ``cycle time'') as a function of observing wavelength, elevation, and tropospheric conditions. Here, the switching time is defined as the time between the midpoints of the two calibrator observations before and after the target observation. A ``typical'' value of m is assumed, while a ``good'' troposphere (similar to some winter nights) is assumed to have m, and a ``bad'' troposphere (similar to some summer days) has m. For all cases, the maximum switching time is assumed to saturate at a value of 10 minutes. At the longest wavelengths, ionospheric effects will become dominant, while at intermediate wavelengths, model errors (e.g., source position errors) may limit the integrations to about this value. Note that (1) the difference between 10-minute and 20-minute cycle times is only about 6% in total on-source integration time, or 3% in signal-to-noise; and (2) elevation angles greater than will be uncommon except for the shortest baselines.