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Comparison
Contrary to tracked observations, the position switched observing mode can
be more efficient than the frequency switched observing mode. Indeed, in
frequency switch, the same time is spent in the on and off spectrum. When
subtracting them, the off brings as much noise as the on. In position
switch, the same off can be shared between many ons, in which case the
optimal integration time on the off is much larger than on each independent
on spectrum. Hence, the noise brought by the off spectrum can be much
smaller than the noise brought by the on spectrum.
For frequency switched observations,

(46) 
while for position switched observations,

(47) 
We thus have

(48) 
Position switched OTF is more efficient than frequency switched OTF for

(49) 
Moreover,
for
, and
for
. Using eqs.
and , we see that the limit on the maximum number of on
per off is set by

(50) 
i.e. the ratio of the maximum system stability time by the minimum time
required to map a telescope beam.
As for tracked observations, there are other considerations to be taken
into account. For extragalactic observations, the lines are large which
excludes the use of frequency switched observations. For Galactic
observations, the intrinsic sensitivity of the receivers may make it
difficult to find a closeby position devoid of signal. We can still use the
position switched OTF observing mode. But we then have to observe the off
position in frequency switched track observing mode long enough to be able
to add back the off astronomical signal.
Next: Acknowledgement
Up: OnTheFly observations
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20140701