LDI Intellectual Property.
Not for secondary distribution or replication, in part or entirety.
DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
SECTION 1 - GENERAL SYSTEM DESCRIPTION 1-37
the centre of that beam. Therefore, detecting the direction by selecting the beam with the largest amplitude can
never be an incorrect thing to do. One has to avoid thinking of the beam as excluding echoes from other direc-
tions and realize that all that is needed is that a beam favours echoes more as their angle of arrival becomes
closer to the centre of that beam. In fact with a four element array the summed amplitude in a wrong direction
may be nearly as strong as it is in the correct beam, however, given that the same four complex amplitudes are
used as input it cannot be stronger.
1:79. The phase shifts required to sum echoes into each of the seven beams depend on four variables:
a. the signal wavelength,
b. the antenna geometry (separation distance and orientation),
c. the azimuth angle of arrival, and
d. the zenith angle of arrival.
The antenna weighting coefficients are unity amplitude with a phase which is the negative of the extra phase
delay caused by the propagation delay, thereby removing the extra phase delay. The phase delays for antenna is
resulting from arrival angle spherical coordinates (
j
,
j
) which corresponds to the direction of beam j, are de-
scribed (using Equation 1-24 by the following:
ij
= (2 sin
j
/
) d
ij
'
where
ij
is the phase difference between antenna i’s signal and antenna 1’s signal,
j
is the zenith angle (0 for
overhead), and d
ij
' is the projection of the antenna separation distance (from antenna i to antenna 1) upon the
wave propagation direction. The parameter d' is dependent on the antenna positions which can be placed on a
Cartesian coordinate system with the central antenna, antenna 1, at the origin and the X axis toward the North
and the Y axis toward the West. With this definition the azimuth angle is 0° for signals arriving from the
North and:
d
ij
' = (x
i
cos
j
+ y
i
sin
j
)
Since antenna 1 is defined as the origin, x1 and y1 are always zero, so
i
has to be zero. This makes antenna 1
the phase reference point which defines the phase of signals on the other antennas. The correction coefficients
i
are unit amplitude phase conjugates of the propagation induced phase delays:
ij
= 1.0 i(f,x
i
,y
i
,
j
,
j
) = 1–
ij
Because they are frequency dependent, these correction factors must be computed at the beginning of each CIT
when the beamforming mode of operation has been selected. A full description as well as some modeling and
testing results were reported by [Murali, 1993].
1:80. Although the received signal is resolved in range/height before beamforming, the beamforming tech-
nique is not dependent on isolating a signal source before performing the angle of arrival calculations. If two
sources exist in a single Doppler line then these components (the amplitude of the Doppler line can be thought
of as a linear superposition of the two signal components) then some of each of them will contribute to an en-
hanced amplitude in their corresponding beam direction. Conversely, the Drift technique assumes that the inci-
dent radio wave is a plane wave (thus requiring isolation of any multiple sources).
To Next Page
Loading...