Document text
United States Patent m
Brey et al.
[ii]
[45]
4,118,701
Oct. 3, 1978
[54] FM/CW RADAR SYSTEM
[76] Inventors: James C. Fletcher, Administrator of
the National Aeronautics and Space
Administration, with respect to an
invention of Hans Brey, Cookville,
Tenn.; Philip E. Geise, Jr.,
Huntsville, Ala.
[21] Appl.No.: 730,778
[22] Filed: Oct. 8, 1976
[51] IntCl. 2 G01S9/44
[52] U.S. Cl 343/6 R; 343/9
[58] Field of Search 343/6 R (U.S. only),
343/9 (U.S. only), 6 R, 9
[56] References Cited
U.S. PATENT DOCUMENTS
2,467,455 4/1949 Aurell 343/9
3*483,557 12/1969 Skenderoff et al 343/9
3,526,893 9/1970 Skenderoff et al 343/9 X
3,569,967 3/1971 Gendreu et al 343/5 CM X
3,732,565 2/1973 ‘ Symaniec et al 343/9
Primary Examiner — Malcolm F. Hubler
Attorney \ Agent, or Firm — George J. Porter; John R.
Manning; L. D. Wofford, Jr.
[57] ABSTRACT
An FM/CW radar system with improved noise discrim-
ination in which the received signal is multiplied by a
sample of the transmitted signal, and the product signal
is employed to deflect a laser beam as a function of
frequency. The position of the beam is thus indicative of
a discrete frequency, and it is detected by the frequency
encoded positions of an array of photodiodes. The out-
puts of the photodiodes are scanned, then threshold
detected, and used to obtain the range and velocity of a
target.
1 Claim, 3 Drawing Figures
OSC a FM RAMP
MOD. " S_ GENER .
U.S. Patent Oct. 3, 1978
Sheet 1 of 2
4,118,701
VELOCITY
(DOPPLER)
READOUT
FREQUENCY FREQUENCY
U.S. Patent Oct. 3, 1978
Sheet 2 of 2
4,118,701
FIG. 2 a
44
FIG. 2b
1
4,118,701
2
FM/CW RADAR SYSTEM
ORIGIN OF THE INVENTION
The invention described herein was made in the per- 5
formance of work under a NASA contract and is sub-
ject to the provisions of Section 305 of the National
Aeronautics and Space Act of 1958 , Public Law 85-568
(72 Stat. 435 ; 42 U.S.C. 2457).
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to radar systems, and particu-
larly to an improved FM/CW radar system having an
improved signal-to-noise discrimination capability.
2. General Description of the Prior Art
Ultimately, the capability of a radar system is depen-
dent upon its ability to resolve or detect useful signals in
the presence of electrical noise. There thus exists a point
of operation with a given radar system at which either
by virtue of the level of transmitter power or operating
range (distance to a target), intelligible signals cannot be
reliably detected. FM/CW radar systems, while provid-
ing a most efficient use of power in obtaining precise
range measurements, typically require a phase lock loop
to measure frequency differences, and this in turn typi-
cally requires a 3 db signal-to-noise ratio and thus
power levels generally in excess of that necessary for
signal detection and ranging alone. Practically, the de-
tection problem is often that of finding rather narrow
band signals in the allotted range, e.g., 10 KHz, where
they are often submerged in noise.
It is an object of this invention to provide an im-
proved FM/CW radar system which will operate at
lower power levels for a given range.
SUMMARY OF THE INVENTION
In accordance with this invention, a linear FM/CW
radar is constructed wherein there is transmitted a sig-
nal of constant amplitude and which linearly varies with
frequency at a selected rate for some selected range of
detection. In the detection process, the received signal,
delayed by some discrete time (as a function of range)
and shifted in frequency due to doppler (if any), is multi-
plied with the transmitted signal producing low fre-
quency signals for the up-sweep and down-sweep given
by sweep rate, distance to target, and target velocity,
relative to the position of the radar. The output of the
multiplier providing such signals modulates a frequency
responsive light deflector through which is passed a
narrow beam of light. A plurality of photosensitive
elements are positioned to incrementally pick up the
beam over a calibrated deflection range, and thus each
photosensitive element may be responsive to a rela-
tively small band of known frequencies which may be
practically regarded as a single frequency. The output
of each photosensitive element is scanned and fed
through a threshold detector which provides an output
when there appears a signal greater than uncorrelated
noise to provide discrete signals which are processed to
extract velocity and range information.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an embodiment of
this invention. 65
FIGS. 2a and 2b contain two related curves of signals
pertinent to the operation of the system shown in FIG.
1 .
DETAILED DESCRIPTION OF THE
DRAWINGS
Referring to FIG. 1 , FM modulator 10 includes an
oscillator which generates an RF radio frequency signal
12 (FIG. 2d) at the center frequency of, for example, 6
GHz, and, responsive to ramp generator 14 , the oscilla-
tor is modulated in a saw tooth or ramp pattern, as
shown, over a selected frequency deviation in the range
of ± 15%. Ramp generator 14 is timed by general cir-
cuit timing and control circuit 15 . In the example
shown, the upper or higher frequency would be 6.7125
GHz, and the lower frequency would be 5.2875 GHz.
The rate of deviation would be selected as a function of
anticipated range and speeds and typically would be
427.5 MHz per second for a range of 100 km. The out-
put of FM modulator 10 is appropriately amplified in
amplifier 16 and supplied to diplexer 18 which conven-
tionally provides the amplified signal to antenna 20
which transmits it toward a selected target. A sample of
the modulated signal and 1 MHz signal from source 21
are fed to side step circuit 22 which shifts this sample by
1 MHz in order to avoid AM and FM noise and supplies
the shifted signal to one input of product modulator 24 .
A reflection 26 from the target of the transmitted signal
(FIG. 2d) is received by antenna 20 , fed to diplexer 18 ,
and diplexer 18 conventionally provides the received
signal to amplifier 28 . Amplifier 28 appropriately ampli-
fies the signal and feeds it to a second input of product
modulator 24 , which then provides, a beat frequency
output 30 (FIG. 2b) which, in the example shown,
would provide an output of 1 MHz ± 285.7 KHz for a
range of 100 Km. The output of product modulator 24
is fed through band pass filter 32 covering this signal
range to reduce out-of-band noise signals.
In order to provide an optimum signal input for elec-
tro-optical modulator 34 , it is desirable to shift the sig-
nal frequency and signal deviation up substantially in
frequency, and thus frequency multiplier 36 multiplies
the signal frequency by a factor of 70 from 1 MHz ±
285.7 KHz to 70 MHz ± 20 MHz, which signal is ap-
plied to the modulating input of electro-optical modula-
tor 34 . A monochromatic light beam (typically 400 to
1,200 nm) is generated and collimated by a laser light
source 30 and applied as a light input to optical modula-
tor 34 . Optical modulator 34 is a type which, responsive
to an electrical signal input, produces a deflection of the
input light beam as a function of applied frequency. One
example of such a deflector is Zenith’s D-70D Acousto-
Optic Light Deflector. Since this light deflector has a
relatively small total scan angle for a given change in
signal, the scan angle is magnified as shown by means of
lens assembly 42 , representative of telescope optics
which effectively magnify the scan angular change per
unit of frequency. As shown, the modulated light beam
is deflected across 400 to 1,000 or more diodes (depend-
ing upon the resolution of the optical system) of photo-
diode array 44 which are linearly arranged with, for
example, the top photodiode 46 responsive to the fre-
quency of the highest one thousandth increment of
frequency (for a 1,000 unit array), and the bottom pho-
todiode 48 being responsive to the lowest one thou-
sandth increment of frequency. In terms of the intelli-
gence signal range of product modulator 24 of 571.4
KHz (1 MHz ± 285.7 KHz), each photodiode covers a
base frequency range of only 571.4 Hz which corre-
sponds to 200 m. An output of each photodiode is con-
nected to an input terminal of multiplexer 50 which,
10
15
20
25
30
35
40
45
50
55
60
4 , 118,701
3
under the control of address counter 52 , scans each of
the photodiodes and provides an identified output
through threshold detector 54 to encoder 56 which,
when a discrete output appears, is encoded into a digital
number representative of the frequency of the band of 5
frequencies to which a diode is position responsive. The
operation is further synchronized by timing and control
circuit 15 which times the operation of address counter
52 , memory 44, and ramp generator 14.
Encoder 56 is controlled by address counter 52 to 10
thus synchronize the encoding process with the particu-
lar diode being scanned. Threshold detector 54 is set to
pass signals just above noise threshold sufficiently to
insure that an actual signal exists. Also under the con-
trol of address counter 52 , the digital outputs of encoder 15
56 , designating the upper and lower frequencies pres-
ent, are temporarily stored in discrete addresses in mem-
ory 44 and then read out by conventional means to sum
and difference circuit 60 .
The relationship between the pertinent frequencies 20
and outputs of photodiode array 44 is illustrated in
FIGS. 2 a and 2b, FIG. 2a showing the transmitted and
received signals with doppler present, and FIG. 2b
illustrating the difference between the transmitted and
received signals in terms of a 70 MHz center frequency 25
and graphically showing computations for target range
and doppler (velocity). Thus, sum and difference circuit
60 provides a first signal proportional to the absolute
value of the difference between the transmitted and
received frequencies f t — f n this being equal to \{f h — /J), 30
and, as shown, this is proportional to and derived from
one-half of the peak-to-peak deflection of f — f r corre-
sponding to the appropriate diodes. Actual calculation
of range is A:/ lJC/i — /}), where k t is a known constant.
Velocity is determined by sum and difference circuit 35
60 by computing a second signal equal to \(f h — fj) — f c .
The correspondence between f a the center or offset
frequency, and photodiode 61 located in the center of
the array is periodically established in a self test mode
by routing 1 MHz signal from source 21, which is used 40
in the side step circuit, directly and without any other
received signals, to multiplier 36 . Actual velocity in the
target is determined by k 2 • \{f h + f!) — f a where k 2 is a
known constant. As will be observed from FIG. 2b, / A is
equal to f t — f r + f a and /, is equal to f c - (f t -/ r ). 45
Misalignments, as they may occur, are compensated
for in sum and difference circuit 60 . Similarly, the fre-
quencies corresponding to the maximum deviation, that
is, 1.2857 MHz and 0.7143 MHz, are routed through
multiplier 36 and optical system 34 to affix the maxi- 50
mum deviation for calibration purposes.
The system is adaptive because the ramp frequency is
varied in accordance with the measured range, thus
increasing the range resolution measured in meters as
the distance to the target decreases. 55
CW operation of the system generally enables a most
efficient use of power, and frequency modulation assists
in providing a precise range measurement. By employ-
ing the optical detection system described in which a
Fourier transform in real time is performed, coupled 60
with threshold detection, there should be achievable a
4
reduction in band width of signal processing from
10,000 Hz to 200 Hz, or perhaps less, representative of
an effective improvement in the signal-to-noise ratio
over a conventional FM/CW system of 23 db, or
greater. This improvement may thus be utilized to de-
crease transmitter power, and thus enable the construc-
tion of a lighter weight radar system of the same capa-
bility or enable the construction of a system with
greater range for the same transmitter power.
Having thus disclosed our invention, what is claimed
is:
1. In an FM/CW radar system including means for
generating and transmitting a radio frequency signal
which is varied between an upper and lower frequency
at a selected rate, the improvement comprising:
reference signal means for generating a radio fre-
quency reference signal;
side step circuit means responsive to said transmitted
signal and a signal from said reference signal means
for providing a signal sample, a signal differing in
frequency from said transmitted signal by the quan-
tity of said frequency of said reference signal
means;
receiving means for receiving a reflection of the
transmitted signal from a target and multiplying the
received signal by said sample signal to provide an
intelligence signal;
frequency multiplication means responsive to the
output of said receiving means for multiplying by a
selected multiple the frequency of said intelligence
signal, whereby the difference between upper and
lower frequencies present are increased, and being
additionally responsive to said radio frequency
reference signal for multiplying it by the same mul-
tiple;
light source means providing a beam of monochro-
matic light;
light deflection means responsive to said beam of
monochromatic light and the output of said multi-
plication means for providing, as an output, a light
beam which varies in angle of departure as a func-
tion of the signal frequency of output of said multi-
plication means responsive to said intelligence sig-
nal and providing a reference angular output re-
sponsive to said reference signal;
lens means for magnifying the angle of the angularly
modulated light output of said light deflection
means; and
light angle detection means comprising a plurality of
side-by-side frequency indicative photodetectors,
each positioned to sense a discrete angular position
of the light output from said lens means, and
thereby the output of each is indicative of an input
frequency to said light deflection means, and
whereby highest and lowest frequency signals are
indicated by the maximum spaced photodetectors
receiving signals, and said reference angular output
being identified by the photodetector receiving an
output responsive to said reference signal.
*****
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