Since we use the chopper wheel method for calibration, it is convenient to write the radiometer equation in units of , the `antenna temperature corrected for rear spillover losses and atmospheric attenuation'. The r.m.s. noise fluctuation is then:
The system temperature (in the scale) is calculated by OBS via
with the mean physical atmospheric temperature (see ) ¯ , and
with the zenith opacity in the signal sideband ,
the airmass ,
the sideband gain ratio (see Table 2)
the sky brightness temperature ,
the mean physical atmospheric temperature (see ) ,
the ambient temperature , and
the receiver temperature (see Table 2) .
Zenith opacities are strongly correlated with the most important atmospheric absorbant, water; the relevant parameter is the amount of precipitable water vapour (pwv --see ,). For 20% of the time during winter (summer) the pwv is below 2 mm (4 mm). The `average' pwv is below 4 mm during the winter regime and below 7 mm during the summer regime (see e.g. IRAM technical reports ,). Zenith opacities for typical winter and summer conditions are given in Table 3.
Table: Zenith opacities for typical weather conditions at selected frequencies
Note that January and February are dedicated in principle to 0.8 mm and bolometer observations. Especially during this period the night-time opacities are better than the day-time opacities.
Table 4 gives examples of the rms noise after 10 minutes of total integration time (ON + OFF position). To calculate the system temperature some typical values were chosen: Elevation , gain ratio between 0.001 and 1 (depending on receiver and frequency), forward efficiency , cabin temperature K, mean atmospheric temperature = 250 K.
Table: Examples of rms noise values after 10 minutes of integration (assuming )