Glossary - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 163

Scintillation Refractive index fluctuations influencing the propagation of optical
or electromagnetic waves along the path from the emitter to the receiver lead to fluc-
tuating intensities recorded by the receiver. This phenomenon, which is responsible
for the twinkling of stars at night as well, is called scintillation.

Side lobes If the wavelength of an emitted beam or pulse is comparable or larger
than the aperture of the instrument, then refraction takes place at the fringe of the
opening and side lobes are produced. Side lobes unavoidably occur with acous-
tic and electromagnetic remote-sensing techniques, but not with optical techniques.
Backscattered signals from side lobes can reach the receiver again and produce fixed
echoes (SODAR) or ground clutter (RADAR).

Signal-to-noise ratio The signal-to-noise ratio (frequently abbreviated as SNR)
is the ratio between the intensity of the received signal containing wanted physical
information and background noise.

Sounding Sounding is an active remote-sensing technique that delivers a range-
resolved information. Usually, pulses are emitted and the travel time of the pulses
is used to determine the range. The range resolution is determined by the signal
propagation speed and the pulse length. Pulses must have some length in order to
guarantee meaningful signal-to-noise ratios. The temporal resolution of this remote-
sensing method is limited by the travel time from the emitter via the scattering object
to the receiver. The opposite measurement technique is path-averaging.

Stokes scattering see “Raman scattering”.

Time delay See “travel time”.

Time-of-flight See “travel time”.

Tomography Tomography is a (usually active) remote-sensing and analysis tech-
nique which is able to infer a spatially resolved information from several intersecting
path-integrating measurements. It is often called computed tomography (CT) as
well. The spatial resolution increases with the number of the beams and the den-
sity of their intersection points. Information can only be obtained from the plane
which is spanned by the beams.
Travel time A method to determine the distance between an active optical remote-
sensing instrument and the backscattering air volume. The time delay or time-of-
flight between the emission of a pulse and the receipt of the backscattered pulsed
is measured. With the known propagation speed of the pulse, the travel time can
be converted into distance information. Alternative range determination methods
for continuous wave remote-sensing techniques are beam focusing and frequency
modulation.

Virtual temperature The temperature, T v , which, when entered into the definition
for the density of dry air, gives the true density of humid air, is called virtual temper-
ature. This temperature can be computed from the dry-bulb temperature T and the
specific humidity q by T v =

T + 0.609 q . This temperature has been introduced by