Page 160 - Radar Technology Encyclopedia

150 DOPPLER EFFECT doppler bin

For a case when the target is moving and the radar is sta- Figure D56 depicts the geometry for a doppler-beam-
tionary, the relation between f and f can be written sharpening approach to the problem just described. The pre-
0
r
doppler-filtering clutter cell dimensions are
1 – v ¤ c
æ
r
f = f -------------------- ö
r 0 è 1 + v ¤ ø t c
c
eff
r R = -----------sec y
2
where v is the target radial velocity (along the line connect-
r
ing the radar and target) and c is the velocity of light. If we in range, where y is the grazing angle from the radar to the
clutter cell, and
denote the difference as the doppler frequency, f = f - f , we
d r 0
obtain DAz = R q
c AZ
1 – v ¤ c in cross range, where R is the range to the clutter cell. The
r
æ
f = f -------------------- – 1 ö c
d 0 è 1 + v ¤ c ø
r doppler shift to any point n within the range cell is given by
and f < 0 for v > 0 (increasing range). Typical target veloci- 2V m
r
d
ties are much less than the velocity of light, v << c, so this f dn = ---------- cos a n
r
l
equation can be expended into a series:
where V is the radar platform velocity, and a is the angle
m
n
2v r 2v r 2v r between the point n and the radar platform velocity vector.
æ
ö
f » f 1 – -------- + ¼ – 1 » – f -------- = – --------
d 0 è c ø 0 c l
is
where l the wavelength of the transmission. This final form v m
is used in most radar applications. a
The formula is valid for active radar, where the doppler
shift results from change in the two-way path as the target
moves. For a passive radar:
v r
f » ----
d l
v cos a 1
m
because the shift applies only for the one-way path.
For pulsed radar, the doppler effect also produces a D v = v sin Da
change in pulse width and pulse repetition frequency, but m
because the doppler shift is proportional to the frequency f , it v cos an
0
m
is only significant for the RF carrier.
The doppler effect is of fundamental importance in radar
Figure D56 Doppler-beam-sharpening geometry, airborne radar.
applications, where it is used to determine target radial veloc-
ity and to discriminate between moving targets and clutter.
If the azimuth dimension of the clutter cell is further
SAL
divided into a number of smaller cells of equal size, the dop-
Ref.: IEEE (1993), p. 381; Shirman (1970), p. 51; Dulevich (1978), pp. 236–
239; Skolnik (1980), p. 68. pler frequency difference across each cell will be shown that
Doppler ambiguity (see AMBIGUITY, doppler). æ 2V m ö
Df = ---------- ( cos a – cos a )
n è l ø n1 n2
Doppler beam-sharpening is “a special form of synthetic
aperture radar processing that uses a constant frequency refer- If the doppler filter bandwidth B is set equal to the doppler
D
ence.” It is a coherent, airborne radar technique associated spread Df , the angular resolution due to beam sharpening
n
with ground mapping and the detection of fixed land targets will be
that uses doppler filtering to synthetically reduce the radar’s lB D
azimuth beamwidth. Dq= --------------------------
2V sin a m
m
In a conventional (nondoppler) radar, airborne detection
of a small, fixed target (such as a automobile, truck, or tank) PCH
in land clutter is difficult due to the large clutter-to-signal Ref.: IEEE (1990), p. 15; Schleher (1991), pp. 469–472.
ratio attendant to the radar mainbeam illumination of the sur- The doppler bin is the filter channel of a CW or pulsed dop-
face, and the clutter cell dimensions are defined by the effec- pler radar. In a tracking radar, a single doppler bin may be
tive transmitted pulsewidth, range to the target, radar antenna used that tracks the target echo. In a search or acquisition pro-
azimuth beamwidth, and relative geometry. For example, cessor, multiple bins are formed with a filter bank, imple-
,
even with a narrowbeam antenna, e.g., 1° a conventional air- mented by parallel doppler filters or a digital signal processor
borne radar with an effective pulsewidth on the order of a few using a fast Fourier transform (FFT). DKB
2
tenths of a microsecond would be unable to detect a 5 m tar-
doppler filter (see FILTER, doppler).
get at a range of 20 km in moderate land clutter.
doppler (frequency) shift (see DOPPLER EFFECT).