FM-CW Radar ranging system with automatic calibration

A frequency-modulated continuous wave (FM-CW) ranging system for determining the range of a target includes apparatus which utilizes a target simulating delay line and target illuminating antenna, alternately switched to a difference frequency determining circuit and a scaling device to compensate for undesired changes of the FM modulation waveform.

CALIBRATION 
The present invention relates to continuous wave frequency-modulated 
(FM-CW) ranging systems and, in particular, to an automatic calibration 
system to compensate for undesired charges in the peak value of the 
modulation frequency or its rate. 
FM-CW radar ranging interrogation systems are well known in the art. In 
such systems, a radio frequency (rf) interrogation signal, frequency 
modulated with a given modulation waveform, is transmitted toward a target 
and a portion thereof is reflected from the target back to the 
interrogating system. The reflected signal as received at the 
interrogating system is delayed in time, and hence shifted in frequency, 
from the instantaneous interrogation signal by an amount, .tau., 
proportional to the range, R, of the target. For example, when the 
interrogation signal is modulated by a triangular waveform having a peak 
value, .DELTA.F, and a period, 1/f.sub.m, the frequency shift or 
difference frequency, f.sub.R (also known as beat frequency), is equal to 
the time derivative of the frequency of the interrogation signal, df/dt, 
times the round trip time delay to and from the target, .tau.. Frequency, 
f.sub.R, may be expressed as: 
##EQU1## 
where C is the speed of light. The range, R, or distance between the 
target and the interrogating system is determined by measurement of the 
frequency shift, f.sub.R. Rearranging formula (1) provides the formula for 
range, R: 
##EQU2## 
where 
##EQU3## 
if .DELTA.F and f.sub.m remain constant, K remains constant. 
In practice, such FM-CW radar ranging system have been plagued with errors 
due to drifts in the maximum frequency excursion (.DELTA.F) and/or period 
(1/f.sub.m) of the frequency modulation waveform. Such drifts are often 
caused merely by changes in ambient conditions such as temperature, power 
supply variations and timing circuit drifts. U.S. Pat. Nos. 3,968,492 
issued July 6, 1976, to G. Kaplan and 3,974,501 issued Aug. 10, 1976, to 
A. Ritzie, disclose signal processors for FM-CW ranging systems which make 
provisions for compensating for drifts in the period, 1/f.sub.m, of the 
modulation waveform. However, no provision is made for compensating for 
drifts in the frequency excursion, .DELTA.F, of the modulation waveform. 
The prior art has typically used one of four methods of compensating for 
drifts in frequency excursion. 
In one such prior art method, while the system is off line, a delay line of 
known length is temporarily connected between the transmitter and receiver 
antenna feedlines. The time delayed signal, generated by the delay line 
from a portion of the interrogation signal simulates a reflected signal 
from a target at a predetermined range. The processor is then manually 
adjusted to display the predetermined range and the delay line is 
thereafter disconnected. Such a method is limited in use in that any 
subsequent changes in the modulation frequency or amplitude which may be 
undetected require recalibration of the system, again, while it is off 
line. 
A second prior art method utilizes a delay line in conjunction with a 
frequency discriminator in a complex closed loop stabilizing system. Such 
a system is described in The Radar Handbook, M. I. Skilnik, McGraw Hill, 
1970, chapter 16, pages 29-32. However, frequency discriminators are 
relatively expensive and not suitable for large volume usage. Such 
frequency discriminator systems maintain a constant frequency excursion 
.DELTA.F, but do not provide an accurate calibration for the drifts of the 
modulation waveform frequency, f.sub.m. Such a ranging system must 
additionally employ, for example, relatively expensive crystal 
oscillators, temperaturee controlled ovens, or both. 
A third prior art method, disclosed in U.S. Pat. No. 4,008,475 issued Feb. 
17, 1977, to the present inventor, utilizes a delay line in a feedback 
network to alter one of .DELTA.F or f.sub.m to keep their product 
constant. 
U.S. Pat. No. 4,106,020 issued Aug. 8, 1978, to the present invention is 
directed to a fourth prior art method in which a delay line continuously 
simulates a target at a predetermined range outside practical target 
ranges. A first signal consisting substantially of difference signals 
corresponding to the predetermined range and a second signal consisting 
substantially of difference signals corresponding to target ranges within 
the range domain of the system are simultaneously generated by a mixer. 
The second signal is applied to a means for determining a parameter 
thereof representative of target range, which means also has applied to it 
the first output signal for scaling the parameter. In a practical system, 
the delay line includes small mismatches along its length that produce 
signals that can correspond to ranges in the domain of the target. This 
effect reduces the capability of the system to discern true target 
signals. 
The present invention is directed to an improvement of the above-described 
fourth method and includes means for alternatively generating signals 
indicative of target range and predetermined range and means for storing 
one of the two range signals and further means responsive to the stored 
range and non-stored range for scaling the signal indicative of target 
range by the signal indicative of predetermined range.

As shown in the FIGURE, a modulating source 10, such as one which produces 
triangular waveform 12, is coupled to radio frequency source 14 for 
purposes of modulating the radio frequency signal produced by source 14 in 
accordance with waveform 12. Waveform 12 is a plot of frequency vs time. 
The maximum frequency excursion is .DELTA.F with a period 1/f.sub.m. 
Source 14 is coupled to a suitable three port circulator 16. Circulator 16 
is coupled to a means such as electronic switch 18 for passing signals 
alternatively between circulator 16 and either antenna 20 or delay line 
30. Switch 18 may be a suitable electronically controlled S.P.D.T. r.f. 
switch. When switch arm 18a is connected to terminal 18c, antenna 20 is 
connected to circulator 16. When switch arm 18a is connected to terminal 
18b, delay 30 is connected to circulator 16. 
Antenna 20 may be a printed circuit corporate feed antenna of the type 
described in U.S. Pat. No. 3,587,110 issued June 22, 1971 to O. M. 
Woodward or a standard gain horn antenna such as a Narda Model 640. FM-CW 
signals from antenna 20 are directd to and reflected from suitably 
disposed targets within the pattern of antenna 20 such as target 22, the 
range of which from antenna 20 is to be determined. 
Delay line 30, which is operated in the reflection mode, may be of the 
accoustic type or, simply, a coaxial line. The delay in delay line 30 is 
normally for practical reasons chosen to simulate a target farther than 
the range of the most distant target as determined by the equipment 
sensitivity, physical limitations and real target travel. However, delay 
line 30 may simulate a target at a range shorter than the maximum expected 
range of a real target. 
Depending on the position of arm 18 a return signals to antenna 20 or delay 
line 30 are directed to and reflected by circulator 16 to means for 
deriving difference frequency signals, f.sub.R, notably a conventional 
mixer 24 such as the Anaran model 7G0118 mixer. Mixer 24 is also receptive 
of a sample of the radio frequency source 14 output signal provided by a 
directional coupler 26 in the line between RF source 14 and circulator 16. 
The output of mixer 24 is coupled to a high pass filter 32 arranged to 
reject frequencies produced by modulating source 10. 
The arrangement of the elements just described, exclusive of switch 18 and 
delay line 30, provides a means for enabling measurement of a parameter of 
interest such as the range of target 22 in a manner well known in the art. 
The range of target 22 can be accurately determined if the parameters of 
the signal from source 14 are known. However, as mentioned previously, if 
either or both of the modulating signal parameters f.sub.m or .DELTA.F 
inadvertently vary without some offsetting compensation (or correction of 
the parameters f.sub.m or .DELTA.F) the computed range of targets such as 
target 22 will be incorrect. In accordance with the invention, a 
compensation network 28 is provided as described below. 
The output terminal of high pass filter 32 is coupled to a track and lock 
circuit 34 and to the counting (C) terminal of means (40) enabling scaling 
of the frequencies passed by filter 32 representing the distance of real 
targets 22 from antenna 20. One such scaling means is a digital 
counter/latch 40. Track and lock circuit 34 is coupled to a divide-by-N 
circuit 42. Circuit 42 produces one pulse such as 44 for each N cycles of 
frequency received from track and lock circuit 34. Circuit 42 is coupled 
to one input terminal of an AND gate 46 the output of which is coupled to 
the reset/latch (R) terminal of counter/latch circuit 40. The output 
terminal of counter/latch 40 is coupled to output means 50 which is 
suitably in the form of a display device for displaying distance as a 
function of the count stored in circuit 40. 
A controller 52 is coupled to switch 18 to control the position of switch 
arm 18a, to track and lock circuit 34 and to inhibit terminal of AND gate 
46. The design of controller 52 will depend on the frequency with which it 
is desired to calibrate the system. For example, it may be desirable to 
switch in alternately delay line 30 and antenna 20 every few seconds, in 
which case controller 52 may be a suitable multivibrator. Alternatively, 
it may be desired to calibrate the system only when target signals are 
below a predetermined threshold, such as when target 22 is at a greater 
distance from antenna 20. Accordingly, a signal level detector 57 may be 
provided which is coupled to filter 32 to receive signals therefrom 
indicative of the range of target 22. The signal level (S.L.D.) detector 
57 feedback path to controller 52 is indicated by dotted line 54. 
Controller 52 will be assumed to produce a logic 1 signal which causes 
switch arm 18a to be positioned to terminal 18b or a logic 0 signal which 
causes switch arm 18a to be positioned to terminal 18c. A logic 1 signal 
from controller 52 disables AND gate 46 and enables circuit 34 to track 
frequencies associated with delay line 30. Conversely, a logic 0 signal 
from controller 52 primes AND gate 46 and disables circuit 34. 
Track and lock circuit 34, when receiving the logic 1 signal from 
controller 52, tracks the frequency of the signal it receives from high 
pass filter 32. When circuit 34 is receiving a logic 0 signal from 
controller 52 it continues to produce the frequency it was tracking. Track 
and lock circuit 34 may be a commercially available phase lock loop which 
incorporates an RC network which is switched in or out of the loop 
depending on the polarity of the signal from controller 52. Alternatively, 
circuit 34 may contain in order: 1. a frequency-to-voltage circuit, 2. a 
sample and hold circuit, which is responsive to a logic 1 signal from 
controller 52 to sample the voltage from the frequency-to-voltage circuit 
and, when a logic 0 signal is received from controller 52 to hold the 
voltage sampled, and 3. a voltage-to-frequency circuit which produces a 
frequency corresponding to the voltage produced by the sample and hold 
circuit. All these circuits are commercially available circuits and thus 
will not be further described. 
In one exemplary system, target ranges of interest, e.g., 6.0 to 18.5 
meters from antenna 20, produce a frequency, f.sub.R, having a value 
typically between 13 and 40 kHz while the value of the frequency, f.sub.R, 
associated with delay 30 is typically 100 kHz. In such an exemplary 
system, N is divide-by-N circuit 42 may be chosen to be 100,000 such that 
nominally a momentary reset pulse is received at counter 40 each second. 
With such a system the maximum count reached in counter 40 is equal to the 
frequency f.sub.R of the desired target. Alternatively, the value N may be 
selected such that the maximum value reached in counter 40 is exactly 
equal to the range in desired units of measure (meters, feet, etc.) of 
target 22. This is a matter of design choice as determined by the 
particular type of utilization device for output means 50 chosen. 
In operation, RF source 14 produces an RF signal continuously modulated in 
accordance with waveform 12 which is alternatively sent to delay line 30 
and to antenna 20 to be radiated thereby toward targets such as 22. Under 
the control of controller 52 delayed return signals from the target 22 as 
received at antenna 20 or signals reflected from delay line 30 are passed 
via circulator 16 to mixer 24. Mixer 24 also receives a sample of the 
transmitted RF signal from coupler 26. Controller 52 is producing 
alternating logic 1 and logic 0 signals to position arm 18a to terminal 
18b or terminal 18c, thus passing signals from delay line 30 or antenna 20 
to mixer 24. 
Mixer 24 produces difference frequencies from the signals it receives 
having frequency components f.sub.R1 or f.sub.R2, depending on whether 
controller 52 is producing a logic 0 or logic 1, which frequency 
components directly correspond respectively to the range to target 22 and 
the simulated "range" of delay line 30. If .DELTA.F and f.sub.m of 
waveform 12 remain constant, the relationship of the frequency generated 
by mixer 24 to the range of real target 22 and target simulating delay 
line 30 is also a constant. 
When controller 52 is producing a logic 0 and thus frequency f.sub.R1 is 
present, it is passed by high pass filter 32 and advances counter/latch 40 
by, for example, one count for each cycle or each X cycles of frequency, 
where X is a fixed integer. Frequency f.sub.R1, when present, is also 
applied to track and lock circuit 34. However, when frequency f.sub.R1 is 
present, the logic 0 signal from controller 52 blocks circuit 34 from 
tracking frequency f.sub.R1. 
Counter/latch 40 receives periodic reset pulses from divide-by-N counter 42 
via AND gate 46 so long as controller 52 is producing a logic 0. The 
leading edge 44a of a pulse 44 from divide-by-N circuit 42 is passed by 
AND gate 46 to prime counter/latch 40 to hold the count in counter latch 
40. The trailing edge 44b of pulse 44 produced by divide-by-N 42 resets 
counter/latch 40. 
When controller 52 is producing a logic 1, and therefore frequency f.sub.R2 
is being produced by mixer 24, AND gate 46 is disabled and thus 
counter/latch 40 maintains the last count received. Further, frequency 
f.sub.R2 is passed by high pass filter 32 to track and lock circuit 34. So 
long as the logic 1 signal from controller 52 is present at the C terminal 
of track and lock circuit 34, it tracks frequency f.sub.R2. During the 
period that circuit 34 is tracking f.sub.R2 counter/latch 40 is counting 
f.sub.R2 cycles. However, since controller 52 has a disable pulse on AND 
gate 46, the counts in counter/latch 40 are not passed to the latch 
portion of circuit 40. 
When controller 52 again produces a logic 0 signal circuit 34 continues to 
produce the frequency f.sub.R2 which it had been tracking. Frequency 
f.sub.R2 stored in circuit 34 is divided by circuit 42 to reset 
periodically counter/latch 40. Since the frequency f.sub.R1 at the C 
terminal of counter/latch 40 nominally represents the distance of target 
22, the number of cycles of f.sub.R1 counted by counter/latch 40 just 
before being reset are a measure of that frequency and also correspond to 
the range of target 22. The frequency stored in the latch portion of 
counter/latch 40 may be displayed or otherwise utilized by a suitable 
output means 50. 
If the range of target 22 remains fixed but, for example, maximum frequency 
excursion, .DELTA.F, of waveform 12 increases by, for example, 10 percent, 
then, in accordance with formula 1, the frequency f.sub.R1 increases by 
the same 10 percent. However, in such a situation, since the same waveform 
which interrogates target 22 is also applied to delay line 30, the 
frequency f.sub.R2 will also increase by the same 10 percent and therefore 
the counter/latch 40 will be reset 10 percent sooner in time (than if 
.DELTA.F had not increased) and thus contain the same count when reset as 
was the case before .DELTA.F increased in value. A similar situation 
occurs with any change in frequency, f.sub.m, of waveform 12. Thus, the 
difference frequency associated with any target, such as 22, is scaled by 
the difference frequency associated with delay line 30 in a manner which 
compensates for any changes in .DELTA.F and/or f.sub.m of waveform 12. 
It will be understood that delay line 30 may be replaced by the combination 
of a second antenna similar to antenna 20 and an additional real target at 
a known distance. The additional real target (not shown) may be positioned 
out of the RF beam illumination from antenna 20 and in the RF beam 
illumination from a similar antenna (not shown) at a known fixed distance. 
The new target may be inside or outside the range of target 22. It will, 
thus, be understood that the phrase "means simulating a target at a known 
range" as used in various ones of the claims is intended to encompass a 
real target at a known range. Furthermore, the output of track and lock 
circuit 34 may be connected to the (C) terminal of counter/latch 40 while 
the high pass filter 32 will then be connected directly to divide-by-N 42. 
In this arrangement an inverting means, such as an inverter, may be added 
in path 60 between controller 52 and switch 18. That is, circuit 34 will 
be arranged to track and lock on signals associated with target 22 rather 
than delay line 30.