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'■■-C'AUICAL tePOK'f SECTION
i'^Jtt ^°STGfiAOUArc SCHOOl
HQMESST, CALIFOBNIA 93640
NAVAL POSTGRADUATE SCHOOL
//
Monterey, California
MULTIPLE-TARGET CW FM RADAR ♦
David B. HOISINGTON
June 1972
Technical Report^ for Period July 1971 - June 1972
Approved for public release; distribution unlimited
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D 208.1 4/2: NPS-52HS72062
NAVAL POSTGRADUATE SCHOOL
Monterey, California
Rear Admiral Isham Linder Jack R. Borsting
Superintendent Provost
The work reported herein was supported by the Naval Postgraduate School
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4. TITLE (and Subtitle)
MULTIPLE-TARGET CW FM RADAR
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Technical Report
July 1971 - June 1972
6. PERFORMING ORG. REPORT NUMBER
7. AUTHORCs;
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David B. HOISINGTON
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Naval Postgraduate School
Monterey, CA Code 52Hs
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Naval Postgraduate School
Monterey, CA Code 52Hs
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15 June 1972
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RADAR
CW radar
20. ABSTRACT (Continue on reverse aide If neceaaary and Identify by block number)
Development of a multiple-target CW FM radar is described. This
type of radar has advantages over pulse radars particularly in port-
able, battery operated applications.
DD 1 JAN 73 1473 EDITION OF 1 NOV 65 IS OBSOLETE
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MULTIPLE-TARGET CW m RADAR
David B. Holsington
Naval Postgraduate School
Monterey, California
Abstract
Development of a multiple-target CW FM radar is described. This type of radar
has advantages over pulse radars particularly in portable, battery operated
applications.
INTRODUCTION
Althougli multiple-target CW FM radars have been
suf;gested [1], little application of FM radar in
this mode of operation seems to have been made.
Recent advances in solid-state technology have
made availnble devices that would permit the con-
struction of a multiple-target FM radar that would
be complctelv solid state except for the cathode-
ray tube indicator, and perhaps even a solid-
state replacement for the cathode-ray tube will
be available within a few years.
Tlie most obvious application for a radar of this
type would be in situations where light weight,
low power consumption, and high reliability are
of paramount importance. Light weight indicates
operation at 9 Cllz or liigher to obtain narrow an-
tenna beam width wilh reasonable antenna size.
It will be shown that doppler frequency shifts
can be of acceptable magnitude at 9 GHz for sur-
face targets. Powers in the order of 1 watt can
be obtained at 9 (iHz from solid-state devices,
and tin's is adequatt- for many applications. An
experimental program has been initiated to demon-
strate the feasibiljl^ of such a radar suitable
for use on small vi-ssi'Js down to the size of
motor whale boatP.
THEORY OF OPERATION
If the transmitted frequency of a CW radar is swept
linearly with time as shown in Figure la, the re-
ceived echos from fixed targets will have a simi-
lar pattern, but will be delayed in time by 2 R/c
where R is the target range, and c is the velocity
of propagation. If now the echo frequency is
heterodyned with the transmitted signal to obtain
the beat frequency, f , , then
= 2R d f ^ 2RAf
b c dt c T
(1)
where Af is the transmitter frequency excursion
and T is the sweep period. Figure 1 -b shows that
during the initial portion of the transmitter
sweep, echos are still being received from the pre-
vious sweep, hence the beat frequency is higher
than given by (1). After a time AT corresponding
to targets at maximum range tlie difference fre-
quencies associated with all targets are as given
by (1). Tlie receiver is gated off during the
initial interval to prevent the passage of the
incorrect frequencies. It is convenient to make
the duty cycle, (I -A), equal to 1/2 so that the
receiver is gated on for n time i equal to one
half of the sweep period, T.
Figure 2 shows the boat frfi|urncy spectrum due to
a single sweep and a single l.irgct. Tlie spectrum
Is rontiiuious and h.is a (sin x)/x envelope.
Figure 1-a Radio frequencies vs. time
Figure 1-b Beat frequencies vs. time
To separate targets at different ranges one or
more filters may be used. The ideal filter for
maximum output slgnal-to-noise ratio would be
matched to the spectrum. As a practical matter
the filter may be matched as nearly as possible
to the central lobe of the spectrum. The filter
6dB (hall amplitude) bandwidth is, then, 1.2/t or
2.4/T. This filter has rise and decay time ■
approximately equal to t, hence the signal due to
one target decays during the receiver dead time
clearing the filter to accept another target
during the succeeding on time.
Figure 3 is a simplified block diagram of the
complete system. Received echos are converted
to an intermediate frequency for good noise per-
formance and convenience in filtering. The
carrier oscillator is linearly swept over the
frequency band Af. A portion of this signal is
mixed with the output of the IF oscillator at the
frequency (f. - f, ) . A bandpass filter passes
the sum frequency (f + f. - f, ) , to the receiver
mixer. The other input to this mixer is from the
receiving antenna. T\\e received signal at fre-
quency (f - f, ) is mixed to form a signal at the
IF frequency, f The IF filter with 6dB band-
wldtli f = 1.2/i passes this signal to the gated
amplifier, detector and indicator.
By sloulv v.iryinc, the frequency of the IF oscil-
lator from f to (f - 2R Af/rT), i/icomine
1 I max "
>;l>'.nals from cich range intervnl from zero to
^^o - t>
(f + -)
O T
Figure 2. Beat-frequency spectrum due to a
swingle sweep and a single target.
CARRIES
OSCILLATOR
/^ TRANSMITTING
<ft>
?).
<^
7-H AWTENNA
BAKDPASS
FILTER
» ^
IF
OSCILLATOR
' ^
y
./<:>^
/" RECEIVING
(f, -. f, -
vw^
(^ - V
V' ANTENNA
riLTEK
RAMMIDTH f
-^
GATED
AMPLIFIER
t
TO DFTECTOR
■*" AND INDICATOR
Figure 3. System block diagram
maximum range are successively converted to the
frequency f. and are thus separated to yield tar-
get range information. The oscilloscope PPl dis-
play is deflected radially in synchronism with the
IF oscillator frequency sweep, and deflected in
azimuth synchronously with the antenna sweep.
The range discrimination of the systems can be de-
termined from equation (1) . Targets of equal
strength can be separately detected if their beat
frequencies are separated by f,. It follows then
the range discrimination, 6R, is given by
cTff 1.2c
6R =
2Af
Af
(2)
Now for TAf>>l it can be shown that the width of
the transmitted spectrum at half amplitude is
Af[2]. It is not surprising that for equal band-
width, the range discrimination of tlie CW FM radar
Is the same as that of the pulse radar.
The radar shown in Figure 3 lias the advantage of
simplicity, but Information rate is limited since
only one range interval can be examined at a time.
Information rate could be increased by using a
multiplicity of filters. It is significant to de-
termine the time required to search all range and
azimuth bins. Assume that the maximum range of
interest is N times tlie range discrimination. If
successive range intervals examined are separated
by 6R, and 1/T range are intervals examined per
second, then the time required to sweep from 0 to
maximum range, t , is
t " NT
s
(3)
If the antenna is rotated one beam width, e„ , in
D
time t , then the search tine required to rotate
s ' ^
the antenna through 360° is
360NT
e
B
(4)
where 9 is in degrees. For example, if N is 200,
B
T is 400 usee, and 6 is 3", then t is 9.6
seconds. This time is increased if N is increased,
or If, as is desirable, some overlap is permitted
between successive range intervals examined. The
search time decreases if a sector less than 360°
is scanned.
The doppler shift due to target motion is
2v f /c where v is the radial component of ve-
r o r
locity. If v is 20 yards/sec, representing a
very fast surface target, and f is 9 GHz, the
doppler shift is 1100 hertz. Typically f j, would
be in the order of 5000 hertz for a marine navi-
gation radar, so this doppler shift represents a
range error less than one fourth of the range
discrimination, an entirely acceptable error.
COMPARISON WITH PULSE RADAR
Equation (2) shows for eqoal transmitted band-
widths, CW FM and pulse radars have virtually the
same range discrimination. In the pulse radar
the receiver must have a bandwidth comparable to
the transmitted bandwidth. The CW FM radar, on
the other hand, has an IF bandwidth equal to
f,, a small fraction of the transmitted bandwidth.
Required gain can therefore be obtained with a
relatively small nimber of stages. Moreover, the
range discrimination may be cliangfd simply by
changing &f, the transmitter fn-qui-ncy deviation.
To change range discrimination of a pulsi' radar
one generally changes pulse width, receiver band-
width, and the pulse repetition frequency.
It can be shown that for equal useful average
povers transmitted, target illumination times,
equal receiver noise figures, antenna gains, inte-
gration efficiences, integration times, and opti-
mized bandwidths, pulse and CW FM radars have the
same maximum range on a given target. Here the
OiJ FM radar has a 3dB disadvantage if the receiver
duty cycle is 50 percent. Additional loss of up
to 3dB may occur if the echo spectrum is not cen-
tered in the IF filter. The pulse radar, on the
other hand, has a 7dB loss when in a typical case
25 pulses from a given target are integrated by
means of a cathode-ray tube. Typically, then, the
range performance of a CW FM radar equals that of
a comparable pulse radar when the average trans-
mitted powers are the same.
The pulse radar transmits high peak power in short
pulses to obtain a given average power. High volt-
ages are therefore required in the relatively
complicated modulator. In the CW FM transmitter
the power is constant, and only relatively low
voltages are required in the transmitter. Solid
state oscillators now available can furnish the
power required for a moderate (10 mile) range
marine navigational radar while operating from the
storage battery voltages available on most small
craft. A high degree of modulation linearity is
required in the CW FM transmitter, but the re-
quired linearity is not necessarily hard to
obtain [3].
Pulse radars do not suffer from any range ambi-
guity provided echos are not received from targets
at a time greater than the pulse repetition period.
CW FM radars may have a range ambiguity problem
when weak target returns are received at ranges
somewhat smaller or larger than the return from a
very strong target. Figure 2 shows the IF spec-
trum of a given target. If the IF filter is at a
frequency 2/t higher than the center of thla
spectrum in an effort to locate a target at a
range difforing by 2r/Af, the filter will respond
to the side bands of the strong-target spectrum.
The filter output in this case will be In the
order of 26 dB less than if the strong target re-
turn were centered In the filter, but this
spurious response could make it difficult to de-
tect the return from a navigational buoy with a
relatively large object such as a ship nearby.
This range ambiguity can be reduced, for example,
by shaping the IF spectrum of targets to reduce
the amplitude of the side bands. This could be
accomplished by modulating the receiver gain
before the IF filter with a smooth weighting func-
tion rather than by employing square-wave gating
as has been assumed. Further work needs to be
done in this area.
EXPERIMENTAL PROGRAM
As the first step in demonstrating the feasibility
of a solid-state CW FM radar, a hybrid unit was
constructed from laboratory components. The
basic systiiii block diagram was as indicated in
Figure 3. Operation was at a frequency of about
9 GHz with a power output of 2 watts. The trans-
mitter frequency deviation could be set at any
value up to 12 Mllz. The receiver intermediate
frequency was 30 MHz, and the IF filter was a
linear phase (Gaussian) filter with a 6 dB band-
width of 5.1 kHz. Receiver noise figure was 13.8
dB. Circular parabolic reflectors with a 3 de-
gree beam width were used for both transmitting
and receiving antennas. These antennas could not
conveniently be scanned, so a type A (amplitude
vs range) presentation was used.
Figure U shows a typical display obtained with
the antenna oriented as shown in Figure 5.
Figure 6 shows a high resolution display obtained
with the antenna oriented as shown in Figure 7.
Note that objects separated by AO yards are com-
pletely resolved. This display was obtained with
a Af of 12 MHz for a theoretical range resolution
from equation (2) of 33 yards.
Figure 4. Long-range display
3° BEAMWIDTH
STACK
KMBY TOWER
POINT CABRILLO
4000 YDS
POINT ALONES
PACIFIC
OCEAN
MONTEREY
COAST GUARD PIER
MONTEREY WHARF
SPANAGEL HALL
Figure 5. Kadar field of view (long-range)
Figure 6. Short-range display of campus
BEAM WIDTH
INGERSOLL
HALUGAN
BULLARD
ROOT
j 343 YDS
305YDS
4-
RADAR
rj- i
i , 173YDS
133YDS
Li_L
SPANAGEL HALL
Figure 7. Radar field of view (short-range)
CONCLUSIONS
The feasibility of a CW FM radar with a range
resolution of about 30 yards has been demonstra-
ted. It remains to be shown that such a radar
can be built with all solid-state components in a
small, lightweight package. Development of the
necessary solid-state circuitry is proceeding as
time and limited funding pemit.
ACKN()WLtD(TMENT
Appreciation is expressed to Lieutenant Ronald P.
Lewis, tISN , who performed much of the experimental
work reported in this paper as part of the require-
ments for a Master of Science tliesis.
REFERENCES
1. Luck, D. G. C. , FM Radar, McGraw-Hill Book
Company, Inc., New York, 19A9
2. Berkowitz, R. S., Modern Radar, P. 225, John
Wiley & Sons, Inc., New York, 1965.
3. Lewis, R. P., A Developmental CW FM Multi-
Target Radar, M.S. Thesis, Naval Postgraduate
School, Monterey, California, 1971.
INITIAL DISTRIBUTION LIST
No. Copies
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2. Library, Code 0212 2
Naval Postgraduate School
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3. Dean of Research 1
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Naval Postgraduate School
Monterey, CA 93940
4. Professor David B. Hoisington 10
Electrical Engineering Dept.
Code 52
Naval Postgraduate School
Monterey, CA 93940
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