System temperature from the Brewster VLBA station at 7 mm on 04 June 2013 as a function of airmass (given by 1/sin (elevation)) for the RCP and LCP receivers (red and blue open circles, respectively). Overplotted are solid triangles from the lowest T_sys values in each bin of 0.3 airmass size, which are used to determine the receiver temperature by fitting a straight line to the lower T_sys envelope (formed by the binning). Small T_sys variations induced by weather (e.g., the jump seen at an airmass of ∼3) do not affect the fit.