Category Archives: Backscatter Profiles

Ceilometer Signal to Noise.

There are a great many factors effecting the performance of ceilometers,  but the key 2 issues for any given cloud volume backscatter coefficient  are:

  1. The eye safe limit of the ceilometer .  Infra Red Ceilometers must operate as Class 1M lasers  which limits the energy density of the beam.
  2. The noise level.      The inherent noise level of the ceilometer is the ultimate determinant of the signal to noise ratio which enables the ceilometer to discriminate  cloud boundary.

The 8200-CHS has an extremely low level of inherent noise,   which is tested for each ceilometer and is recorded as per the backscatter profile below.

8200-CHS MAIN CARD OC NOISE

The external source of noise is the shot noise of the scattered and or direct solar radiation within the spectral acceptance of the sensor .   The laser operates around 910 nm and the filtering can only limit the “out of  band”  component of the solar noise spectrum.  The ceilometer will thus t\detect cloud at higher altitudes at night.

8200-CHS Ceilometer multi level clouds

The 8200-CHS has the capability of detecting up to 4 simultaneous layers but certain types of standard messages display only 3 layers .

If the lowest layer is optically dense,  the returns from beyond the first  layer are extinguished ,  and the layers above it are not detected until a gap in the lower layer is detected.   If the cloud is not optically dense,  the return from a higher level cloud is not significantly attenuated when passing back through the lower layer,  so both layers can be detected.

Here is a case where multiple layers are  at 6620 and 7290 are both detected.