Depending on their particular hardware architecture, RWP can be classified into three main groups (Muschinski et al., 2005):
Single signal systems are the most frequently used wind profiler type (Law et al., 2002; Steinhagen et al., 1998; Engelbart et al., 1996; Carter et al., 1995; Ecklund et al., 1988; Strauch et al., 1984). They are monostatic2pulse radars using one single carrier frequency with the hardware architecture resembling that of a typical Doppler radar system, as described in Skolnik (2001). The term single signal refers to the character-istics of the instruments sampling function, which can be regarded as an integral kernel function that maps a field describing the physical properties of the atmosphere relevant for the actual scattering process to the received radar (voltage or current) signal. For clear-air scattering, this is the scalar field of the refractive index (or permittivity) irreg-ularities.
Two signal systems are extensions of the single signal architecture, where basically two different sampling functions are realized to improve the retrieval of atmospheric properties of interest. The two techniques that have been used are the frequency-domain interferometry (FDI) using a monostatic radar with two different carrier fre-quencies (Kudeki and Stitt, 1987; Chilson et al., 1997) and the spaced-antenna tech-nique using one carrier frequency and multiple receiving antennas (Lataitis et al., 1995;
Doviak et al., 1996).
Recently, multi-signal systems have been developed and tested. They either use a bistatic combination of a single transmit and a multitude of receiving antennas to
per-2The same antenna is used for transmitting and receiving.
form digital beamforming (Mead et al., 1998; Pollard et al., 2000; Helal et al., 2001) or they transmit several carrier frequencies to achieve so-called range imaging (RIM) with a single (monostatic) antenna (Palmer et al., 1999; Luce et al., 2001; Smaïni et al., 2002; Chilson et al., 2003; Chilson, 2004). Much work remains to be done to further develop, understand and use this obviously most complex and demanding radar archi-tecture.
In the following, the discussion will be restricted to single signal RWP systems, in particular to the 482 MHz instruments used in the operational network of the DWD (Lehmann et al., 2003) as shown in Fig. 1. A block diagram of the general hardware architecture is given in Fig. 5. The central unit is the radar controller, which uses a highly stable coherent oscillator as the single reference for all signals. It generates all control signals needed to operate the radar through Direct Digital Synthesis (DDS).
The electromagnetic pulse to be transmitted is created by a waveform generator which acts essentially as an amplitude and phase modulator. After a frequency up-conversion and amplification (through a linear power amplifier), the transmit signal is delivered to the antenna and the resulting electromagnetic wave is radiated into free space. As the same antenna is also used for signal reception, a duplexer is necessary to protect the sensitive receiver electronics from the strong transmit signal. It is typically comprised of a ferrite circulator and additional receiver protecting limiters.
The antenna is a phased array comprised of coaxial-collinear (CoCo) elements (Law et al., 2003; Balsley and Ecklund, 1972). A relay-switched true-time delay phase shift-ing unit generates the necessary phasshift-ing of the individual elements required to steer the beam in three fixed directions for each CoCo sub-array, as shown on the right hand side of Fig. 3. For the five-beam pointing configuration indicated in Fig. 6, two such CoCo sub-arrays are combined. Due to the finite extent of the antenna array, the beam can not be made infinitely narrow. This results in unwanted radiation through so-called sidelobes in other than the boresight direction and can be visualized through the an-tenna radiation pattern, which shows the distribution of the total radiated powerPas a function of spherical antenna coordinatesθand φ. Fig. 4 shows the ideal radiation pattern for the DWD 482 MHz network wind profiler calculated with the method of Law et al. (1997). Note that the sidelobe level will be somewhat more irregular and higher in reality because of stochastic excitation differences of array elements due to hardware imperfections (Mailloux, 1994).
The receiver is of the classical superheterodyne type (Tsui, 1989). A rather broad-band low-noise amplifier with an excellent noise-figure is necessary to raise the signal level of the weak atmospheric return for further processing. After frequency down-conversion to an intermediate frequency (IF), the signal is bandpass-filtered, demod-ulated and A/D converted for further digital processing in the radar processor. To maximize the per-pulse signal-to-noise ratio (SNR) for optimal signal detection, the bandwidth of the bandpass filter is matched to the transmitted pulse (Zrni´c and Doviak, 1978). Actual technical implementations differ, for example the received signal can be
Balun
Ground plane Feed point
Radome
COCO dipole elements, ~ /2
/4, typical spacing
Ideal current distribution magnitude
x
y z
N
2 P(2,N)
.643-(40 cm)
H
Figure 3: Single coaxial-collinear antennas with ideal current amplitude distribution (left) and array arrangement (right). A planar CoCo array generates a linear polarized electromagnetic wave, with the electric and magnetic field vector oscillating in the so-called E-plane or H-plane, respectively. The antenna beam is steerable in the H-plane through a different phasing of the CoCo lines. (Graphics courtesy of D.C. Law.)
Figure 4: Surface plot of the ideal normalized antenna radiation pattern|P(θ,φ))|2/P0 in logarithmic scaling (dB) for an oblique beam of the 482 MHz RWP at Bayreuth.
digitized either at IF (so called digital IF receivers) or at base-band, after further analog down-conversion by a quadrature detector (analog receiver).
Single signal RWP use the simple method of Doppler beam swinging (DBS) to de-termine the wind vector. At least three linear independent beam directions and some
R adar C omputer
P C I B us
R adar C ontroller Digital IF
DS P I,Q Demodulator
and F IR F ilter 14-bit A-D C onverter DDS 3 Local
Oscillator DDS 2 S ample
C lock DDS 1 Timing
C lock Timing and C ontrol E ngine P ulse, Amplitude,
and P hase Modulator
B lanker, IF Amp and B andpass
F ilters 422 MHz
Oscillator T/R
S witch LNA F inal
Antenna Array
Figure 5: Simplified block diagram of the DWD 482 MHz radar wind profiler. (Graph-ics courtesy of J.W. Neuschaefer.)
assumptions concerning the wind field are required to transform the measured ’line-of-sight’ radial velocities into the wind vector. This principle will be briefly shown for a five beam system as depicted in Fig. 6.
In the vicinity of the radar, the wind fieldv with components (u,v,w) in a cartesian coordinate system can be approximated through a Taylor series expansion in the hori-zontal coordinates up to the linear term:
v(x,y,z)≈v(x0,y0,z) +∇hv(x,y,z)|x0,y0·∆r. (11) For simplicity it is further assumed (without loss of generality) that the antenna beam directions are aligned in parallel to the cartesian coordinate axes (i.e. x East, y North).
If the radial velocity measured in the ’line-of-sight’ of a radar beam described by unit directional vectornis written as
vr=v·n (12)
E N W V
S
15° 15°
15° 15°
3°
Figure 6: Beam pointing configuration of a five-beam DBS radar wind profiler and with δx=δy=ztan(α0)cos(α0), the differences of the radial winds of the four oblique beams at heightzare obtained as
[vrE−vrW](z) =2u0(z)sin(α0) +2∂w
∂x(z)δx(z) (13) [vrN−vrS](z) =2v0(z)sin(α0) +2∂w
∂y(z)δy(z). (14)
Here, α0 is the zenith distance of the oblique beams, z is the height above ground and the subscripts denote East, West, North and South, respectively. It is immediately obvious that one assumption is required to determine the (horizontal) wind components (u0,v0)above the radar, namely
∂w
∂x = ∂w
∂y =0. (15)
That is, the horizontal shear of the vertical wind must vanish to retrieve the horizontal wind without errors. A discussion of the problems associated with the DBS method can be found in Koscielny et al. (1984); Weber et al. (1992); Goodrich et al. (2002).
The condition (15) is of course not always fulfilled, in particular not in a convec-tive boundary layer. The problem and the resulting measurement errors have recently been investigated by Scipion et al. (2008, 2009b), they are even noticeable in NWP data assimilation (Cardinali, 2009). However, the assumption is usually correct for mean winds averaged over a longer time interval. Cheong et al. (2008) have used data obtained with a volume-imaging multi-signal wind profiler in a convective boundary layer to show that for this particular case the assumptions inherent in the DBS method were valid for a wind field averaged over 10 minutes. More work is certainly required to obtain reliable estimates of this error under a variety of conditions.