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Claims  |
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I claim:
1. A directional radar apparatus for detecting objects in a detection cone
and rejecting carrier waves reflected by objects located outside the
detection cone, comprising:
means for radiating carrier waves from a radiating position;
means for receiving, at a receiving position, carrier waves returned by
reflection from at least one object within the broadcast range of the
radiating means;
means for repetitively moving one of the radiating or receiving positions
on a line which lies on the extended axis of the detection cone to induce
directionally dependent frequency modulation in the carrier waves due to
the Doppler effect; and
means for selecting carrier waves reflected by objects inside the detection
cone, including filter means for blocking received carrier waves having an
induced frequency modulation less than a predetermined value.
2. The apparatus of claim 1 wherein the receiving means is sufficiently
directional to suppress carrier waves returning from directions inclined
at an angle of more than 90.degree. from the detection cone axis.
3. The apparatus of claim 1 wherein the bandwidth of the filter means
determines a vertex angle .phi. of the detection cone.
4. The apparatus of claim 1 wherein the one of the radiating and receiving
positions is moved repetitively back and forth along the line of motion by
commutating plural antenna elements arranged along the line of motion to
induce a periodic rectangular wave frequency-shift modulation of the
carrier wave, with a frequency-shift magnitude dependant on the angle
between the line of motion and the direction to the object.
5. The apparatus of claim 4 wherein the selecting means further includes
local means for providing a periodic rectangular-wave frequency-shift
modulated carrier wave and means for mixing the received carrier waves
with the periodic rectangular-wave frequency-shift modulated carrier wave
provided by the local means to produce an IF signal which is applied to
the filter means.
6. The apparatus of claim 5 wherein the radiating position is moved
repetitively and wherein the local means for providing the periodic
rectangular-wave frequency-shift modulated carrier wave includes a
receiving antenna located on the extended axis of the detection cone.
7. The apparatus of claim 5 further comprising a rectangular wave generator
for controlling the commutation of the plural antenna elements and wherein
the local means for providing the periodic rectangular-wave
frequency-shift modulated carrier wave includes a voltage controlled local
oscillator whose frequency is controlled responsive to the rectangular
wave produced by the rectangular wave generator.
8. An apparatus for determining the distance of an object comprising the
steps of:
means for broadcasting a periodic rectangular-wave frequency-shift
modulated carrier wave, said carrier wave having alternating higher and
lower frequency portions;
means for receiving the frequency modulated phase-shifted carrier wave
reflected from the object;
means for mixing the higher frequency portions of the received carrier wave
with portions of the broadcast carrier wave to produce an IF signal having
a frequency within a predetermined range;
means for separately mixing the lower frequency portions of the received
carrier wave with other portions of the broadcast carrier wave to produce
an IF signal having a frequency within the predetermined range; and
means for producing a signal related in value to the distance of the object
responsive to the difference between the average phases, over a
predetermined time interval, of the IF signals.
9. A method for detecting objects in the path of a moving vehicle
comprising the steps of:
radiating carrier waves from a radiating position carried by the vehicle;
receiving at a receiving position carried by the vehicle carrier waves
returned to the vehicle by reflection from objects in proximity to the
vehicle;
repetitively moving one of the radiating and receiving positions with
respect to the vehicle at a uniform velocity along a line of motion having
a predetermined orientation with respect to the vehicle; and
detecting carrier waves reflected by an object in the path of the vehicle
by filtering the returned carrier waves to select carrier waves having the
frequency characteristics of carrier waves radiated in directions
deviating from the direction of motion of the vehicle by less than a
preselected angle determined by the band width of the filtering.
10. The method of claim 9
wherein the line of motion is a straight line parallel to the direction of
motion of the vehicle, and
wherein the one of the radiating and receiving positions is moved
repetitively back and forth along the line of motion by commutating plural
antenna elements arranged along the line of motion to induce a periodic
rectangular-wave frequency-shift modulation of the carrier wave, with a
frequency-shift magnitude dependent on the direction of return of the
carrier wave reflected by the object.
11. A method of inhibiting collision between a moving vehicle and an object
in the path of the vehicle comprising the steps of:
radiating carrier waves from a radiating position carried by the vehicle;
receiving at a receiving position, carried by the vehicle, carrier waves
returned to the vehicle by reflection from objects in proximity to the
vehicle;
repetitively moving one of the radiating and receiving positions with
respect to the vehicle at a uniform velocity along a line of motion having
a predetermined orientation with respect to the vehicle, to induce, by the
Doppler effect, higher frequency and lower frequency portions in the
received signal;
detecting carrier waves reflected by an object in the path of the vehicle
by frequency-shifting separately down the same intermediate frequency the
higher frequency and the lower frequency portions of the returned carrier
waves and filtering to select carrier waves having the frequency
characteristics of carrier waves radiated in directions deviating from the
direction of motion of the vehicle by less than a preselected angle
determined by the bandwidth of the filtering;
measuring the phase difference between the selected carrier waves;
determining the range of the object responsive to said measured phase
difference;
measuring the Doppler shift of a signal, returned by the object, due to
relative motion between the vehicle and the object;
determining the range rate of the object responsive to said measured
Doppler shift; and
controlling the motion of the vehicle responsive to the occurance of
predetermined combinations of range and range rate of the object.
12. The method of claim 11 wherein the motion of the vehicle is controlled
by applying the brake.
13. The method of claim 11 wherein the motion of the vehicle is controlled
by controlling fuel intake.
14. The method of claim 11 wherein the instantaneous speed of the vehicle
is determined and the motion of the vehicle is controlled responsive to
predetermined combinations of range, range rate and vehicle speed.
15. The method of claim 11 wherein amplitude variations of the carrier
waves are detected and wherein the motion of the vehicle is controlled
responsively to said detected amplitude variations.
16. The method of claim 11 wherein signal strength of the carrier waves is
determined and wherein the motion of the vehicle is controlled
responsively to said determined signal strength.
17. A method for identifying objects in a detection cone having a vertex
angle .phi., comprising the steps of:
generating a carrier wave having a radio frequency f.sub.o ;
generating a rectangular wave of repetitive frequency f.sub.m, lower than
f.sub.o ;
radiating the carrier wave from a radiating position moved repetitively
responsive to the rectangular wave at a uniform velocity back and forth
along a straight line of motion lying along the extended axis of the
detection cone, to induce a directionally dependent Doppler shift in the
radiating carrier wave such that carrier waves radiated on the detection
cone axis alternately shift in frequency between a higher frequency,
f.sub.o +.DELTA.f, and a lower frequency f.sub.o -.DELTA.f, .DELTA.f being
the Doppler induced frequency shift;
locally providing a periodic rectangular-wave frequency-shift modulated
wave responsive to said generated rectangular wave, shifted between a
higher frequency, f.sub.o -f.sub.if +.DELTA.f, and a lower frequency
f.sub.o -f.sub.if =.DELTA.f, where f.sub.if is an intermediate frequency
greater than f.sub.m ;
receiving the radiated carrier waves reflected by objects within the
broadcast range of the carrier waves;
mixing the higher frequency portions of the received carrier waves with
lower frequency portions of the locally provided wave to produce a first
IF signal; and
filtering the first IF signal by means of a filter having a bandwidth,
B.sub.f, centered at f.sub.if, the bandwidth B.sub.f being selected to
reject IF signals having a frequency lower than f.sub.if -B.sub.f /2,
corresonding to the reduction in the Doppler shift of the carrier waves at
the angle .phi. from the cone axis;
whereby the filtered IF signal provides an indication that an object is
located in the detection cone.
18. The method of claim 17 further comprising the steps of:
mixing the lower frequency portions of the received carrier waves with the
higher frequency portions of the locally provided wave to produce a second
IF signal;
filtering the second IF signal by means of a filter having a bandwidth
B.sub.f centered at f.sub.if, the bandwidth B.sub.f being selected to
reject IF signals having a frequency lower than f.sub.if -B.sub.f /2,
corresponding to the reduction in the Doppler shift of the carrier waves
at the angle .phi. from the cone axis;
producing a signal related in value to the range to the detected object
responsive to the difference between the average phases, over a
predetermined time interval, of the IF signals.
19. A method for determining the distance to an object comprising the steps
of:
broadcasting a carrier wave having abrupt frequency shifts;
receiving the carrier wave returned by reflection from the object;
frequency converting a higher frequency portion of the received carrier
wave to produce a first signal within a predetermined frequency range;
separately frequency converting a lower frequency portion of the received
carrier wave to produce a second signal within the same predetermined
frequency range; and
producing a signal related in value to the distance of the object
responsive to the phase difference between the first and second signals.
20. A directional radar apparatus for detecting objects in a detection zone
and rejecting carrier waves reflected by objects located outside the
detection zone, comprising:
means for radiating carrier waves from a radiating position;
means for receiving, at a receiving position, carrier waves returned by
reflection from at least one object within the broadcast range of the
radiating means;
means for repetitively moving one of the radiating or receiving positions
on a line which has a predetermined orientation with respect to a
reference axis of the detection zone to induce directionally dependent
frequency modulation in the carrier waves due the Doppler effect; and
means for selecting carrier waves reflected by objects inside the detection
zone, including filter means for blocking received carrier waves having an
induced frequency modulation outside of a predetermined range. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to radar systems and methods for
directionally selective detection of objects, for measurement of range and
range rate of objects, and for control of radar carrying vehicles
responsive to the detection of objects and their range and range rates
relative to the vehicle. More particularly, one embodiment of the present
invention relates to a radar apparatus for detecting objects in a
detection cone. This directionally selective radar apparatus may be
employed in providing collision avoidance control for a vehicle carrying
the radar apparatus. The system may also employ a novel signal processing
circuit for extracting range information from a received periodic
rectangular-wave frequency-shift modulated carrier wave.
Two types of directional radar are known in the prior art: phased array
radar systems and systems employing antenna polar patterns to achieve
directionality.
Phased array radar systems tend to be complex and expensive. The more
common type of directional radar for achieving spatial selectivity relies
on the use of larger and larger antenna apertures to achieve
directionality in the intensity of broadcast or in the sensitivity of
reception of the radar signal. In many applications this approach has
disadvantages relating to considerations of antenna size, and avaliability
of space, system cost, weight, aesthetics, wind load factors, etc.
Accordingly, it is an object of the present invention to provide an
inexpensive and easily fabricated directionally selective radar system.
A number of radar techniques for determining range are known in the prior
art. Among these techniques are pulse radar, which measures the transit
time of a reflected pulse, and continuous wave radar, which, in typical
embodiments, derives distance information from phase shifts in one or more
continuous carrier waves of constant frequency. Both systems have the
disadvantage that they are expensive and difficult to implement since each
requires complex signal processing. Moreover, the expense of these systems
increases in inverse relation to the distances being measured.
A known radar technique for determining range employs a rectangular
frequency-shift modulated carrier wave, i.e., a carrier wave whose
frequency changes abruptly at periodic intervals between a higher and a
lower frequency. In the known system, a broadcast rectangular-wave
frequency-shift modulated carrier wave is mixed with its returned replica
reflected by an object of interest. If the object and radar system are
moving relative to one another, higher and lower frequency portions of the
signal will each exhibit a Doppler shift. These Doppler shifts are
separated, filtered and compared with respect to phase in order to extract
range information from the phase difference. The system has at least two
disadvantages: the aforementioned signal processing is generally complex
and expensive to implement and the system cannot detect the range of an
object which is moving with a small or zero velocity with respect to the
radar system. This is so because the Doppler shifts in such situations are
negligible.
Accordingly, it is another object of the present invention to provide an
inexpensive and easily fabricated short range radar system.
It is yet another object of the present invention to provide an inexpensive
and easily fabricated radar system employing frequency-shift modulated
carrier waves.
It is yet another object of the present invention to provide a radar system
employing frequency-shift modulated carrier waves for detecting objects
with a zero or small relative velocity with respect to the radar system.
The aforementioned features of the present invention viz. economy,
directional selectivity, compactness, and short range capability, render
it well suited for use in vehicles, and particularly well suited for use
in collision avoidance systems for automobiles.
There exists a pressing need for a reliable system to eliminate or minimize
the property damage and personal injury associated with vehicle
collisions.
Vehicle control systems are known in the prior art which employ light,
radio or sonic waves to detect objects. Many of these systems are not
self-contained but require that special active or passive components be
used on the detected objects--other automobiles, guard rails, etc. This is
a significant disadvantage in that it increases the expense of the system
and requires the cooperation of other motorists, local governments, etc.,
so that components external to the controlled vehicle are provided.
Accordingly, it is yet another object of the present invention to provide a
practical self-contained vehicle radar system.
A self-contained vehicle radar system is known in the prior art, which
employs the prior art rectangular-wave frequency-shift carrier wave
detection technique discussed above. However, the system, as noted above,
is expensive and incapable of detecting objects moving at nearly the same
velocity as the controlled vehicle and is not sufficiently directionally
selective unless a relatively large directional antenna is employed. These
disadvantages present particular difficulty in vehicle collision avoidance
systems for a number of reasons. First, the collision avoidance system
must be sufficiently inexpensive so that it may be widely used. Second,
the system must be capable of detecting objects at rest with respect to
the controlled vehicle, since, for example, collision avoidance may
require response to unsafe following distances. Third, the system must be
directionally selective so that it has the capacity to discriminate
collision threats from normal traffic situations which present no
immediate threat.
Accordingly, it is yet another object of the present invention to provide
an economical and easily implemented vehicle radar system which is capable
of detecting objects with small or zero velocity with respect to a radar
equiped vehicle.
It is yet another object of the present invention to provide an easily
implemented directionally selective vehicle radar system.
It is yet another object of the present invention to provide an inexpensive
and easily implemented collision avoidance system for a vehicle.
The use of radar in moving vehicles for measuring speed and distance
relative to road obstacles has thus been discouraged by the inability of
prior art methods of implementation to provide reliable discrimination
against surrounding reflectors or scatterers (such as overpasses,
overhanging road signs, objects in side lanes or on the road side, center
dividing fences, side railings, etc.) of the probing radar signal.
Attempts to effect such discrimination have thus been mainly based on
using more highly directive antenna polar (radiation and receiving)
patterns and/or special passive reflectors or active transponder mounted
on the tail ends of other vehicles.
Highly directive antenna patterns require larger antenna apertures, which
causes problems of installation, obstruction of the radiator, aerodynamic
compatibility, and aesthetics or styling. Alternatively, phased-array
techniques for effecting spatial selectivity are generally complex and
costly. Resort to operating frequencies in the 15 GHz to 40 GHz range to
reduce the physical size of high gain antennas introduces not only the
risk of excessive signal absorption by fog, rain and snow, but also pushes
the RF power source and related processing requirements into a technology
that is still in the developmental stage from the viewpoint of
mass-producible, low-cost components.
Accordingly, it is yet another object of the present invention to provide
an easily implemented collision avoidance system capable of discriminating
collision threats from safe vehicle environments.
These and other objects and features of the invention will become apparent
from the claims, and from the following description when read in
conjunction with the accompnaying drawings.
THE FIGURES
FIG. 1 is a schematic illustration of a directional radar system embodiment
of the present invention.
FIGS. 2a, b and c are graphs of frequency shifts in a carrier wave radiated
from the embodiment of FIG. 1 as observed at points P, Y and Z of FIG. 1,
respectively.
FIGS. 3a and b are schematic illustrations of directional radar system
embodiments of the present invention employing a repetitively moving
radiating position (FIG. 3a) and a repetitively moving receiving position
(FIG. 3b).
FIG. 4 is a plan view of a highway environment and a vehicle equipped with
a collision avoidance radar system of an embodiment of the present
invention.
FIG. 5 is a schematic block diagram of a circuit for determining the range
of an object from radar signals reflected thereby.
FIGS. 6a and 6b are graphs of frequency shifts in signals employed in the
circuit of FIG. 5.
FIG. 7 is a graph illustrating the phase modulation effected by an Induced
Directional Frequency Modulation Technique of the present invention.
FIG. 8 is a schematic block diagram of an illustrative embodiment of a
collision avoidance radar system which detects range and range rate to
identify objects presenting potential collision threats.
FIGS. 9a and b are graphs of frequency shifts in signals employed in the
circuit of FIG. 8.
DETAILED DESCRIPTION
The present invention relates to radar systems and methods for detecting
the presence, range, and range rate of objects. The preferred embodiment
of the present invention is directionally selective and is capable of
identifying objects within a narrow detection cone. The claimed methods
and systems may be implemented to provide small size, low cost radar
devices for small vehicles such as boats, automobiles, buses, etc., which
are capable of automatically detecting obstacles which present a collision
threat in the path of the moving vehicle. Moreover, the system and method
are effective to exclude or suppress radar returns from other objects
which do not present a real collision threat.
Embodiments of the present invention employ radio frequency waves having an
Induced Directional Frequency Modulation (IDFM), which is a directionally
dependent frequency modulation imposed on a carrier wave during the
broadcasting or the receiving of the carrier wave. The IDFM technique has
a great number of applications for determining the locations of objects
which broadcast, receive or reflect the carrier waves. The IDFM technique
is discussed in detail in applicant's copending U.S. patent application
Ser. No. 552,568 filed Feb. 24, 1975, entitled "Navigation Aid Systems"
which patent application issued as U.S. Pat. No. 4,106,023 and is
incorporated herein by reference.
In a more general form, the IDFM technique may be used to provide
locational coordinates of transmitters, receivers or reflectors of radio
frequency carrier waves having Induced Directional Frequency Modulation.
The technique may employ a carrier wave radiating, reflecting or receiving
position that is repetitively moved along a line of motion, such as a
straight line. This motion may be actual motion of a single antenna or
antenna motion simulated by commutating a number of discrete antenna
elements disposed in a rectilinear array in a prescribed succession. A
carrier wave radiated from such a system is characterized by an induced
Doppler shift, the magnitude of which depends on the location of the
observer with respect to the line of actual or simulated rectilinear
motion of the radiating position. As used hereinafter, motion or moving of
an antenna along a line of motion comprehends both actual or simulated
motion, unless the text states otherwise.
Referring first to FIG. 1, the application of the Induced Directional
Frequency Modulation technique in a directional radar system will be
described. In FIG. 1, a transmitter 10 supplies a carrier wave of the
frequency f.sub.o to a broadcasting antenna 12 having a line of motion
represented by a line A-B of length D. A Doppler shift is induced in the
transmitted carrier wave by motion of the radiating position along the
line 13. As noted above, the motion may be simulated by moving a single
radiating antenna back and forth along the line 13 at a uniform velocity
between the end points thereof. Alternatively, motion of the radiating
position may be simulated by commutating antenna elements X in sequence so
that the radiated carrier waves have an IDFM similar to that which would
be caused if the radiating position were repetitively moved with uniform
velocity from A to B and then from B to A. As a result of the moving of
the radiating position along the line 13, the radiated carrier wave may
exhibit periodic variations in frequency, the amplitude of said variations
depending on the location of the observer relative to the line 13. For
example, an observer at point P located along an extension A-C of the line
13 would observe the full effect of the Doppler shift induced in the
carrier wave. Specifically, an observer at point P would observe frequency
variations in the carrier wave such as shown in FIG. 2a where the ordinate
is frequency and the abscissa is time. In contrast, an observer located at
point Y, off the axis A-B by an angle .phi., would observe a less wide
ranging frequency shift. The frequency shift observed at point Y is shown
in the graph of FIG. 2b. The amplitudes of the frequency shifts in the
carrier wave at point Y are reduced by a factor of cos .phi.. As a final
example, an observer at a point Z, to which the carrier wave is radiated
almost perpendicular to the axis A-B, will observe little or no frequency
shift in the carrier wave as shown in the graph of FIG. 2c.
In a preferred embodiment of the present invention, a directional radar
apparatus is provided for detecting objects in a detection cone such as
the cone 14 shown in FIG. 1. The detection cone is defined by a
predetermined vertex angle .phi. whose magnitude determines the
directional selectivity of the radar system. The orientation of the cone
is determined by the orientation of the line of motion 13; specifically,
the cone axis lies along the extension B-C of the line of motion 13.
Detection of objects within the detection cone 14 may be accomplished in
two different ways as shown in FIGS. 3a and 3b. In FIG. 3a, Induced
Directional Frequency Modulated carrier waves are radiated from a
radiating position moved along a line of motion 13 as was discussed in
connection with FIG. 1. These carrier waves may be reflected by objects
anywhere within the broadcast range of the system. Taking for example an
object 16 located within the detection cone 14 in FIG. 3a, carrier waves
radiated from the line of motion are returned by reflection from the
object 16 to an omnidirectional receiving antenna 18. Carrier waves
received by the omnidirectional receiving antenna 18 may be discriminated
on the basis of the Induced Directional Frequency Modulation exhibited by
said received carrier waves. This may be done, for example, by filtering
the received carrier waves to block received carrier waves having an
Induced Directional Frequency Modulation less than a predetermined value.
An alternative mechanism for accomplishing substantially the same result is
shown in FIG. 3b. In FIG. 3b, carrier waves of a frequency f.sub.o are
omnidirectionally broadcast by an omnidirectional transmitting antenna 20
located in close proximity to a receiver 22 having a receiving position
moved along the line of motion 13. Carrier waves from the omnidirectional
transmitting antenna 20 may be returned to the apparatus by reflection
from the object 16. A Doppler frequency shift is induced in the received
carrier waves by the motion of the receiving position of receiving antenna
22. As with the system shown in FIG. 3a, carrier waves reflected by
objects within the detection cone may be selected responsive to their
Induced Directional Frequency Modulation.
The above described techniques may be applied to provide a collision
avoidance method and apparatus suitable for use with vehicles. As shown in
FIG. 4, transmitting or receiving antenna elements may be positioned on
the hood of a vehicle 22. Alternatively the antenna elements could be
located on the rooftop of the vehicle. A forward facing transmitter or
receiver 24 is also carried by the vehicle.
In a preferred embodiment of the present invention a radio frequency
carrier wave having a frequency f.sub.o is commutated along the antenna
elements of the line of motion 13' in such a way so as to simulate a
single transmitting antenna in motion at a uniform speed in repetitive
alternating directions; i.e., forward with respect to the vehicle followed
by rearward with respect to the vehicle. As was noted above, such
simulated motion of a transmitting antenna causes the radiated carrier
wave to execute a periodic rectangular-wave frequency-shift modulation
where the magnitude of the frequency shift is dependent on the location of
the observer with respect to the line of motion 13'. If some of the
radiated carrier wave is reflected by an object, such as the truck 26 on
the axis A-C, a return signal may be received by the antenna 24, which
signal will exhibit a rectangular-wave frequency-shift modulation similar
to that of the transmitted carrier wave. Objects in the detection cone
14', such as the truck 26, may be discriminated from objects outside the
detection cone responsive to the magnitude of the Induced Directional
Frequency Modulation of the returned carrier wave.
Radar systems are typically employed to determine the range of distance of
an object from the radar installation, and such information is, of course,
necessary in providing collision avoidance. A receiving system for
extracting range information is described in connection with FIG. 5. This
system employs a broadcast rectangular-wave frequency-shift modulated
carrier wave such as shown in FIG. 6a, including but not limited to
periodic square-wave frequency-shift modulated carrier waves produced by
the IDFM technique. In the system of FIG. 5, a carrier wave of frequency
f.sub.o is square-wave frequency modulated to shift its frequency
.+-..DELTA..OMEGA. rad/sec about the average frequency of .omega..sub.o
rad/sec and can be expressed as
e.sub.T+ (t)=cos [(.omega..sub.o +.DELTA..OMEGA.)t+.phi..sub.o ] during the
+ shift (A-1)
and
e.sub.T- (t)=cos [(.omega..sub.o -.DELTA..OMEGA.)t+.phi..sub.o ] during the
- shift (A-2)
where .phi..sub.o is a very nearly constant phase shift of no practical
consequence. The corresponding target Doppler-shifted and returned
replicas are
e.sub.R+ (t)=cos [(.omega..sub.o +.DELTA..OMEGA.)(t+2R/c)+.phi..sub.r
](A-3)
and
e.sub.R- (t)=cos [(.omega..sub.o -.DELTA..OMEGA.)(t+2R/c)+.phi..sub.r
](A-4)
where variation with time of the distance R to the target accounts for
range-rate Doppler shift, and .phi..sub.r is an inconsequential, very
nearly constant phase term.
The returned signal, e.sub.R (t), reflected by the target or object is
received by the receiver 26 of FIG. 5. In the receiving technique of FIG.
5, e.sub.R+ (t) and e.sub.R- (t) are first both shifted down to the same
IF frequency, .omega. if rad/sec, by means of a synchronously
frequency-shift modulated, voltage-controlled local oscillator 34. The
results are
cos [.omega..sub.if t+(.omega..sub.o +.DELTA..OMEGA.)2R/c+.phi..sub.if
](A-5)
and
cos [.omega..sub.if t+(.omega..sub.o -.DELTA..OMEGA.)2R/c+.phi..sub.if ]
Since these occur during time epochs 1/2f.sub.m sec apart, (i.e. one half
of the period of the rectangular wave), delaying one of the two so that it
occurs during the time interval of the other, yields
cos [.omega..sub.if t+(.omega..sub.o -.DELTA..OMEGA.)2R/c+.phi..sub.if
+.omega..sub.if /2f.sub.m ] (A-6)
The phase difference between (A-5) and (A-6) is
4.DELTA..OMEGA.R/c-.omega..sub.if /2f.sub.m. If this phase difference is
expressed as .phi..sub.+ .DELTA..OMEGA.-.phi..sub.- .DELTA..OMEGA., then,
with the .omega..sub.if /2f.sub.m term ignored as superfluous, we have for
the range, R,
R/c=[.phi..sub.+.DELTA..OMEGA. -.phi..sub.-.DELTA..OMEGA.
].sup./4.DELTA..OMEGA. (A- 7)
In the system of FIG. 5, the carrier wave e.sub.R (t) is received by the
receiver 26 and is applied to a first mixer 28 and a second mixer 30. A
rectangular wave having a period equal to 1/f.sub.m is applied to the
circuit at terminal 32. The mixers 28 and 30, a voltage-control led local
oscillator (VCLO) 34 and a controlled gating switch 36 are employed to
shift the higher frequency and the lower frequency portions of the
received carrier wave reflected by an object down to the same IF
frequency, f.sub.if. The downward shift to the IF frequency is
accomplished as follows. The voltage-controlled local oscillator 34 is
controlled by the rectangular wave applied at the terminal 32 and produces
a periodic rectangular-wave frequency-shift modulation such as is shown in
FIG. 6b. The signal changing this frequency-shift modulation is
selectively applied to the mixers 28 and 30 by the controlled gating
switch 36 responsive to the level shifts in the square wave applied at the
terminal 32. The mixers 28 and 30 are effective to frequency shift the
higher frequency portions of the received carrier wave and the lower
frequency portions of the received carrier waves, respectively, down to
the same IF frequency. The downward frequency-shifted higher frequency
portions are applied to the bandpass filters 38; and the downward
frequency-shifted lower frequency portions are applied to the bandpass
filter 40. Advantageously, the bandpass filters may be crystal filters
having a bandwidth B.sub.f Hz centered about f.sub.if. The filtered,
downward frequency-shifted lower frequency portions of the received
carrier wave are delayed by an electronic delay circuit 42, the magnitude
of the delay being one half the period of the square wave applied at
terminal 32. The frequency-shifted and filtered higher frequency portions
of the received carrier wave are phase-shifted 90 degrees by a .pi./2
phase shifter 39 and then applied to one input terminal 41 of a multiplier
44, and the frequency-shifted, filtered and delayed lower frequency
portions of the received carrier wave are applied to a second input
terminal 43 of the multiplier 44. An output signal of the multiplier 44 is
applied to a lowpass filter 46, the output signal of the lowpass filter 46
being related in value to the phase difference between the two signals
applied to the multiplier 44. From this phase difference, the range of the
reflecting object can be calculated.
The function of the various waveforms and components of the circuit of FIG.
5 may be better understood by further reference to FIGS. 6a and b. In
particular, as illustrated in FIG. 6b, the fixed amplitude of the
rectangular-wave modulation of the frequency of the VCLO is adjusted so
that the resulting instantaneous frequency of the VCLO differs during each
half period of the modulation by (.omega..sub.if /2.pi.).+-.no more than
B.sub.f /2 Hz from the instantaneous frequency of the return signal from a
reflecting obstacle only when the reflecting obstacle is located within
the cone 14 defined by angle .phi. shown in FIG. 1. The return signal from
the object will exhibit the maximum IDFM frequency shift of
(.DELTA.f).sub.max =.+-.2f.sub.m (D/.lambda.)Hz only when the centroid of
the object's backscattering cross-section falls exactly on the direct line
extension of the radiating line of motion, such as would be the case for
an object at point P of FIG. 1. An object at point Y lies at an angle
.phi. with respect to the direct straight-line extension of the line of
motion and reflected carrier waves therefrom would exhibit a frequency
shift given by .+-.2f.sub.m (D/.lambda.) cos .phi..
With continued reference to FIG. 6, the instantaneous frequency of the VCLO
is
(f.sub.o -f.sub.if)+2f.sub.m (D/.lambda.) during t.sub.0 to t.sub.1,
t.sub.2 to t.sub.3, etc.
and
(f.sub.o -f.sub.if)-2f.sub.m (D/.lambda.) during t.sub.1 to t.sub.2,
t.sub.3 to t.sub.4, etc.
Therefore, the instantaneous frequency of the signal return from an object
at point P will differ from the instantaneous frequency of the VCLO by
f.sub.if during t.sub.0 to t.sub.1, t.sub.1 to t.sub.2, t.sub.2 to t.sub.3,
etc.
The instantaneous frequency of the signal returned from an object at point
Y will differ from the instantaneous frequency of the VCLO by
f.sub.if -(1-cos .phi.).times.2f.sub.m (D/.lambda.) during t.sub.0 to
t.sub.1, t.sub.2 to t.sub.3, etc.
and
f.sub.if +(1-cos .phi.).times.2f.sub.m (D/.lambda.) during t.sub.1 to
t.sub.2, t.sub.3 to t.sub.4, etc.
For numerical illustration, note that if .vertline..phi..vertline.=5.73
degrees, then (1-cos .phi.).perspectiveto.5.0.times.10.sup.-3. If,
further, (.DELTA.f).sub.max =2f.sub.m D/.lambda.=10.sup.6 Hz, then (1-cos
.phi.).times.2f.sub.m (D/.lambda.).perspectiveto.5.0 kHz. therefore, if
B.sub.f /2 is less than 5 kHz and the bandpass filter response cuts off
sharply, the return from an obstacle at an angle .phi.=0.1 rad will be
suppressed. For B.sub.f /2 of less than 50 Hz, returns from obstacles at
values of .vertline..phi..vertline. down to 0.6 degree would be
suppressed.
The controlled gating switch 36 of FIG. 5 may be controlled by the same
clock signal that controls the VCLO frequency shifts and the line of
motion commutator which simulates motion of the radiating position. These
controls by the clock are performed in such a way that the VCLO signal is
applied to the first mixer 28 during the intervals t.sub.0 to t.sub.1,
t.sub.2 to t.sub.3, etc. in FIG. 6, when the input return signal frequency
is on its positive frequency shifts and the VCLO frequency has been
shifted to differ by .omega..sub.if /2.pi.Hz from the maximum
positive-shifted frequency of the input return signal. Similarly, the
controlled gating switch 36 connects the VCLO signal to the second mixer
30 in FIG. 5 during the intervals t.sub.1 to t.sub.2, t.sub.3 to t.sub.4,
etc., in FIG. 6, when the input return signal frequency is on its negative
frequency shifts and the VCLO frequency has been shifted to differ by
.omega..sub.if /2.pi.Hz from the maximum negative-shifted frequency of the
input return signal. In this way, each of the bandpass filters 38 and 40
in FIG. 5 is presented with a periodic train of bursts of a sinewave at
.omega..sub.if /2.pi.Hz, the bursts for the filter 38 corresponding to the
positive frequency shifts of the input signal and the bursts for the
filter 40 corresponding to the negative frequency shifts of the input
signal. Each periodic burst of sinewave at the input terminals of the
filters 38 and 40 has a spectrum consisting of discrete lines at
(.omega..sub.if /2.pi.).+-.10.sup.5 n Hz, n=0, 1, 2, 3, . . . for return
signals from obstacles positioned exactly along a straight line extension
of the transmitting line of motion 13 of FIG. 1 and at
(.omega..sub.if /2.pi.).+-.(.DELTA.f).sub.max .multidot.(1-cos
.phi.)+10.sup.5 n Hz, n=0,.+-.1,.+-.2, (A-8)
for carrier waves reflected by objects positioned at a radial angle of
.phi. radians relative to the line of motion. Accordingly, for values of
.phi. such that
.+-.(.DELTA.f).sub.max .multidot.(1-cos .phi.).+-.10.sup.5 n=0
or
cos .phi.=1-(10.sup.5 n)/(.DELTA.f).sub.max, n=0, 1, 2, 3, (A-9)
a reflector positioned on a radial making an angle .phi. with the line of
motion will cause a return signal that contributes a spectral line within
the passbands of the filters 38 and 40 of FIG. 5. However, note that even
for n=1 and (.DELTA.f).sub.max =10.sup.6 Hz, such a reflector would be
approximately 26 degrees off the orientation of the line of motion 13 and
would surely be effectively rejected by a receiving antenna having
directional characteristics such as the antenna 24 of FIG. 4. Receiving
antenna directivity is desirable not only for rejecting side components of
returns from persistent reflectors located at the above defined off-course
angles, but also for providing gain to enhance the received power level
from on-course reflectors. This directionality of the receiving antenna
need not be nearly as refined as the directional detection which may be
obtained by employing the circuit of FIG. 5 in an IDFM system.
It will be readily apparent that the receiving technique illustrated in
FIG. 5 applies in general to the determination of range based on radiated
periodic rectangular-wave frequency-shift modulated waves such as shown in
FIG. 6(b). The source of the sensing signal may well be the VCLO itself
radiated out through one or more antenna elements connected continuously
to the signal source, in which case the square wave frequency shift is a
direct modulation rather than an IDFM produced by reciprocating motion of
a radiating position along a line of motion as described in connection
with FIG. 1. Evidently, only in the case of IDFM will the frequency shifts
of the received carrier waves be direction dependent as discussed above.
The IDFM modulation induced by the commutation process can be viewed as a
sawtooth phase modulation, as illustrated in FIG. 7. Viewed in this
manner, the IDFM/T signal arriving at the target can be expressed in the
form
e.sub.rad (t)=cos [.omega..sub.o (t+R.sub.OT /c+s(t)/c)+.phi..sub.1 ](A-10)
where
R.sub.OT =radial distance between the center point of the IDFM/T LOM and
the reflecting object
##EQU1##
and
e.sub.sq (t)=square wave of .+-.1 with period 1/f.sub.m sec.
The variation of s(t) with t is illustrated in FIG. 7.
The IDFM/T signal returned can be received back at the transmitting vehicle
by a fixed antenna. The fixed antenna (18 in FIGS. 1 and 3a) located on
the IDFM/T source vehicle would receive a signal returned by a potential
obstacle expressible as
e.sub.ret (t)=cos {.omega..sub.o [t+2R.sub.OT /c+(t+R.sub.OT /c)e.sub.sq
(t+R.sub.OT /c)(v.sub.s /c) cos .phi.]+.phi..sub.2 } (A-11)
This is a sawtooth-wave phase-modulated signal. The average phase during
the positively sloping part of the sawtooth is
.phi..sub.+ave =2.omega..sub.o R.sub.OT /c+.omega..sub.o (1/4f.sub.m
+R.sub.OT /c)(v.sub.s /c) cos .phi.+.phi..sub.2 (A- 12)
The average phase during the negatively sloping part of the sawtooth is
.phi..sub.-ave =2.omega..sub.o R.sub.OT /c-.omega..sub.o (1/4f.sub.m
+R.sub.OT /c)(v.sub.s /c) cos .phi.+.phi..sub.2 (A- 13)
Therefore,
.phi..sub.+ave -.phi..sub.-ave =2.omega..sub.o (1/4f.sub.m +R.sub.OT
/c)(v.sub.s /c) cos .phi. (A-14)
whence
##EQU2##
The fixed receiving antenna 18 must preferably be separate and well
isolated from the commutated transmitting antennas.
An embodiment of the receiving technique of FIG. 5 which employs the IDFM
to provide collision avoidance for moving vehicles is illustrated in FIG.
8. In FIG. 8, a VCLO signal is derived directly by means of a receiving
antenna element 100 mounted at a point on a direct extension of the line
of motion 13 at a distance of a few wavelengths from either end of the
line of motion (preferably, the rear end). A gate timing control waveform
is derived from a control clock 102 for a commutator 104, which simulates
motion of a radiating position along the line of motion. The
antenna-derived VCLO signal in FIG. 7 is equivalent to what would be
received directly ahead, with a delay of a fixed amount equal to one-half
of the period of the frequency-shift modulation. The effect of this fixed
delay is to cause the VCLO frequency to be on opposite sides of the center
(unmodulated) carrier frequency (see FIG. 8a) with respect to the
frequency of carrier waves reflected by objects in the vehicle path. Under
these conditions, .omega..sub.if /2.pi.=2(.DELTA.f).sub.max, which is now
the value of the center frequency of each of bandpass filters 106 and 108
in FIG. 7. The choice of bandwidth, B.sub.f, for the bandpass filters
determines the vertex angle .phi. of the detection cone as explained
above.
With continued reference to FIG. 8, an unmodulated carrier wave at an
operating frequency of f.sub.o Hz is generated by a signal generator 110.
The carrier wave from the signal generator 110 is commutated among the
radiating antenna elements 112 at a uniform speed in alternating
directions. Antenna 100 picks up the radiated IDFM signal as seen along a
direct straight line extension of the line of motion 13. If antenna 100 is
positioned along an extension of the line segment A-B at a distance of
several wavelengths from the rearward end of the line segment A-B, then
the signal picked up by antenna 100 will be IDFM frequency-shifted in the
opposite sense relative to the IDFM signal reflected by objects located
ahead of the vehicle. This effect is illustrated in FIG. 9a. An electronic
controlled gating switch 114 is controlled by a gate timing control
waveform derived from the control clock 102 and applied to a terminal 115
of the switch. The control clock 102 also drives the antenna commutator
104. The controlled gating switch 114 is operative to alternatively apply
the IDFM signal picked up by antenna 100 as a local oscillator signal to
mixers 116 and 118 during alternate half-periods of 1/2f.sub.m sec of the
radiated IDFM signal frequency modulation. In FIG. 9a and in the text
hereafter the signal received by the antenna 100 is labeled VCLO. The
controlled gating switch 114 is timed so that oppositely frequency-shifted
segments of the VCLO received signal and the signal reflected by the
object are always mixed to yield the maximum frequency difference of
f.sub.if =2(.DELTA.f).sub.max, as illustrated in FIG. 9b. Signals from the
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