Ceilometer CT25K
User’s Guide CT25K-U059en-2.1
50
In its normal full-range operation the CT25K ceilometer digitally samples the
return signal every 100 ns from 0 to 50 µs, providing a spatial resolution of 50
feet from ground to 25,000 feet distance. This resolution is adequate for
measuring the atmosphere, since visibility in the densest clouds is in the order
of 50 feet.
Noise Cancellation
For safety and economic reasons, the laser power used is so low that the noise
of the ambient light exceeds the backscattered signal. To overcome this, a
large number of laser pulses are used, and the return signals are summed. The
desired signal will be multiplied by the number of pulses, whereas the noise,
being random, will partially cancel itself. The degree of cancellation for white
(Gaussian) noise equals the square root of the number of samples; thus, the
resulting signal-to-noise ratio improvement will be equal to the square root of
the number of samples. However, this processing gain cannot be extended ad
infinitum since the environment changes. For example, clouds move.
Return Signal Strength
The instantaneous return signal strength is in general form (the Lidar
equation):
∫
′′
−
⋅⋅⋅⋅=
z
zdz
ez
z
Ac
Eoz
0
)(2
2
r
)(
2
)(P
σ
β
where
P
r
(z) is the instantaneous power received from distance z [W = Watt].
E
O
is the effective pulse energy (taking all optics attenuation into
account) [J = Joule = Ws = Watt - second].
c is the speed of light [m/s = meters per second].
A is the receiver aperture [m
2
].
z is the distance in question [m].
ß(z) is the volume backscatter coefficient at distance z [m
-1
srad
-1
,
srad = steradian].
e
zdz
z
−′′
∫
2
0
σ
()
is the two-way atmospheric transmittance and accounts for the
attenuation of transmitted and backscattered power by
extinction at various distances (z') between transceiver and
distance in question (z). The expression equals 1 in a clear
atmosphere (i.e., no attenuation).
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