For heterodyne observations, estimate the sensitivity with the radiometer equation:
where Tsys is the system temperature, t is the integration time, ∆f is the channel width, and K ≈ 2, depending on the switching scheme and observing strategy. Invert this equation to estimate the necessary observing time. Do not neglect to add overhead for setup, pointing, calibration, etc.
Estimate system temperature on the TA* scale with:
If you're observing small sources, further adjust for the main beam efficiency.
If the atmospheric optical depth in the two sidebands is different, please review the discussion of sideband correction.