Vaisala CL31 - Theory of Operation; Basic Operating Principle; Practical Measurement Signal

Chapter 4 ______________________________________________________ Functional Description
VAISALA_______________________________________________________________________ 43
CHAPTER 4
FUNCTIONAL DESCRIPTION
This chapter provides information on the measurement principles of the
product.
Theory of Operation
Basic Operating Principle
The operating principle of Ceilometer CL31 is based on the measurement
of the time needed for a short pulse of light to traverse the atmosphere
from the transmitter of the ceilometer to a backscattering cloud base and
back to the receiver of the ceilometer.
The general expression connecting time delay (t) and backscattering
height (h) is
h = ct/2
where c is the speed of light (c = 2.99 x 10
8
m/s).
A reflection from 25,000 ft can be seen by the receiver after
t = 50.9 µs
Practical Measurement Signal
Generally, particles at all heights scatter light, so the actual return signal
may look like that shown in Figure 17 on page 44.