Geoscience Reference
In-Depth Information
Center for Atmospheric Research (NCAR) in order to mitigate effects of bird contamination
in wind velocity estimates.
Fig. 4. Conceptual drawing of RIM. The blue-colored volume shows the resolution without
RIM (i.e., range resolution is determined by the transmitted pulse width), and the red-
colored volume shows the range resolution improvement attained by RIM.
2.2 Range resolution enhancement using multiple frequencies
2.2.1 Signal processing
Figure 4 shows a conceptual drawing of RIM. The width of range gate is determined by the
sampling interval of analog-to-digital converter, and the range resolution without RIM is
determined by the transmitted pulse width. In RIM, signals sampled from multiple
frequencies are synthesized with appropriate weights in the subranges within a range gate.
Using the synthesized signal, the first three spectral moments (brightness, Doppler velocity,
and spectral width) at the subranges are estimated in order to attain range resolution
improvement. Among various methods which can be used for RIM processing (Luce et al.,
2001; Palmer, et al., 1998; Smaϊni et al., 2002), the Capon method (Capon, 1969) is widely
used because it satisfies both high range resolution and simple calculation. Hereafter signal
processing of RIM using the Capon method is described.
s
denotes a set of signals collected
by
N
carrier frequencies at an arbitrary range gate and is expressed by
T
.
(11)
s
( )
t
(
s
( ),
t
s
( ),...,
t
s
( ))
t
1
2
N
Frequencies are switched on a pulse-to-pulse basis in order to maximize the correlation of
signals sampled by different frequencies. Using numerical simulation, Palmer et al. (1999)
showed that using 3 or more frequencies are required for RIM measurements. For field
measurements, four or five frequencies are used typically (e.g., Fukao et al., 2011; Palmer et
al., 2001; Yu & Brown, 2004). The optimal weight vector is given as a solution that minimizes
the resulting average power
B
.
B
is expressed by
H
wRw
B
,
(12)
