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 [2]) ¯, 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 [2])
,
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 [2],[3]). 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 [2],[4]). 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
)