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desired phase and actual phase might not be exactly equal, the transmitted phase is sampled
and used in processing to precisely cohere the signal from the desired trip.
Fig. 6. Transmitted pulse sequence (vertical color lines) and the corresponding received
powers (wiggly curves). The phases of the transmitted pulses are indicated and indexed
from -1 to 3. The location of overlay at one fixed range is indicated by two black vertical
lines. The phases of the received signals from the first and second trip are indicated as well
as the phase of the second trip signal after subtracting (correcting) the phase of the first trip.
In case of overlaid echoes the phase coding allows separation of the contributions by the first
and second trip signals. This is accomplished by first cohering (correcting) the phases of the
stronger echo, then filtering it out. For example if first trip is cohered the second trip signal
spectrum (complex with magnitudes and phases) is split into eight replicas over the
unambiguous interval. Then frequency domain filtering of the first (strong) trip signal with a
notch centered on its spectrum and having a width of ¾ unambiguous interval leaves two
spectral replicas of the second trip signal spectrum. From these replicas it is possible to
reconstruct the second trip spectrum and compute spectral moments. It turns out that cohering
for the first trip signal induces 4 spectral replicas in the third trip signal and again eight replicas
into the fourth trip signal; the fifth trip signal has two replicas and can not be recovered.
Determination of the ranges where overlaid echoes might be is made using powers from the
surveillance scan (long PRT) which precedes the Doppler scan (phase coded short PRT). The
overlay trip number and powers are needed to make proper cohering-recohering order and
notch filter application. In case ground clutter is present Blackman window is applied to
time series data and clutter is taken out with a special frequency domain filter (Sec 3.3). If
there is no clutter contamination but overlaid echoes are present the von Hann window is
chosen. An example of Doppler velocity fields obtained with the SZ(8/64) phase code is in
Fig. 7 (left side). The same field obtained by processing and censoring with no phase coding
is also plotted (right side); note the large pink area in the second trip region indicative of
non recoverable velocities. Small pink areas in the first trip region (SE of radar) signify that
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