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Figure 17 Typical Measurement Signal
The instantaneous magnitude of the return signal provides information on
the backscatter properties of the atmosphere at a certain height.
Information about fog and precipitation, as well as clouds, can be derived
from the return signal. Fog and precipitation attenuate the return signal
from a cloud. However, fog and precipitation backscatter signals also
provide data that can be used in estimating this attenuation and
computing the necessary compensation, up to a limit.
In its normal full-range operation, Ceilometer CL31 digitally samples the
return signal from 0 to 50 µs. The sampling is repeated every 33 or 67 ns,
providing a spatial resolution of 5 or 10 meters respectively from ground
level up to 7 700 meters. This resolution is adequate for measuring the
atmosphere, since visibility in the densest clouds is of the same order.
Noise Cancellation
For safety and economic reasons, the transmitted laser power is so low
that the noise of the ambient light exceeds the backscattered signal. To
overcome this, a large number of laser pulses are sent, and the return
signals are summed. The desired signal can be multiplied by the number
of pulses, whereas noise, being random, partially cancels itself. The
degree of cancellation for white (Gaussian) noise equals to the square
root of the number of samples; thus, the resulting signal-to-noise ratio
improvement is equal to the square root of the number of samples.
However, this processing gain cannot be extended endlessly since the
environment changes and, for example, clouds move.
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