Radar speedometer

Method and apparatus for measuring the speed of a land vehicle over a surface. A narrow beam of RF energy is directed toward the surface at a substantial angle. Reflected RF energy is mixed with that transmitted to produce Doppler signals having a spectrum. The spectra of the Doppler signals are normalized by the Doppler signals being applied to an AGC circuit so that the maximum amplitudes of the spectra are substantially constant and being applied to one or more tracking band pass filters which eliminate unwanted frequency components in the spectra. From the normalized spectra, output signals are produced from which can be determined the speed of the vehicle relative to the surface.

TECHNICAL FIELD 
This invention is in the field of method and apparatus for measuring the 
speed of vehicles moving over a fixed surface using Doppler radar and more 
particularly to such methods and apparatus for measuring the speed of a 
locomotive relative to the roadbed of the rails on which the locomotive 
runs. 
BACKGROUND ART 
To date locomotives have been equipped with mechanical or electromechanical 
speedometers which essentially detect wheel revolutions per unit time and 
display this information in terms of miles per hour (mph), for example. 
Such speedometers have serious problems due to the inaccuracy resulting 
from the wear of the wheels over a period of time, because of wheel slip 
particularly at low speeds which increases wear, and finally because of 
the inherent inaccuracy of such speedometers at low speeds particularly 
below one mph. 
The need for accurate speed measurements, particularly at low speeds has 
increased as the result of introducing unit trains. The reason being that 
such trains are frequently loaded with coal, for example, by pulling empty 
cars at low speeds substantially in the vicinity of one mile per hour 
under a loading hopper. Any significant variation in speed from the 
desired speed can result in the cars being under or overloaded. 
The application of Doppler radar techniques to measure the speed of 
automobiles, for example, is well known but such applications generally 
rely on movement of the vehicle whose speed is to be measured along the 
antenna beam axis, or boresight. Applying such Doppler radar techniques to 
measuring the speed of a locomotive is not practical since oncoming or 
passing trains on adjacent tracks would be within the beam of the 
transmitted microwave energy and would produce erroneous readings. To 
prevent moving objects from entering the beam of the radar antenna mounted 
on such locomotive, the antenna is mounted on the underframe of the 
locomotive pointed downwardly at the roadbed at a substantial angle and 
preferably pointing backwards with respect to the front of the locomotive. 
With the axis of the transmitted beam of RF energy substantially coinciding 
with the direction of motion of the target or vehicle, essentially a 
single Doppler frequency is generated, or at least the amplitude of the 
single frequency so predominates that it is easily processed as a single 
frequency which is linearly proportional to the speed of the target. 
However, if the antenna which transmits and receives the RF energy is 
mounted at a significant angle with respect to the reflecting surface, the 
Doppler signals produced do not have a single frequency that substantially 
predominates but rather produces a spectrum, or distribution, of signals 
of many frequencies and amplitudes. The reason is that the transmitted 
beam of electromagnetic energy has a finite beam width so that the beam 
intersects a substantial area of the roadbed. The spectrum of the Doppler 
signals so produced differs from the ideal distribution of such signals in 
part because the reflective characteristics of the roadbed within this 
area also vary substantially so that the amplitude of the signals vary 
with time. The spectrum also differs from the ideal because of the unusual 
geometry of the reflective surface due to grade crossings, guard rails, 
and to changes in the distance between the roadbed and the antenna mounted 
on the locomotive, for example. 
Attempts to use Doppler radar techniques to measure the speed of 
locomotives have not heretofore been successful to the degree of precision 
desired over the desired ranges of speeds because the prior art methods of 
processing the Doppler signals produced by various types of roadbeds under 
all types of weather and track conditions have heretofore sought to 
identify and process the single Doppler frequency, the boresight 
frequency, which would correspond to that produced by reflecting the 
transmitted RF energy from that point of the roadbed where the axis of the 
antenna, its boresight, intersects the roadbed. 
PRIOR ART STATEMENT 
The following references are submitted under the provisions of 37 CFR 
1.97(b): 
______________________________________ 
U.S. Pat. No.: 
______________________________________ 
3,833,906 Augustine 
3,895,384 Fathauer et al 
4,012,736 Angwin 
4,052,722 Millard 
Re 29,401 Aker et al 
______________________________________ 
Publications: 
H. C. Johnson, Speed Sensors for Locomotives, published by RCA 
Laboratories, Princeton, N.J., and copyrighted in 1976 by the RCA 
Corporation. 
Augustine, U.S. Pat. No. 3,833,906, discloses a Doppler radar system which 
is adapted to measure the speed of land vehicles, particularly 
automobiles. Two radar antennas are mounted on the vehicle, one of which 
is forwardly and downwardly directed. The Doppler signals from each 
antenna are summed to provide a speed signal which is compensated for 
changes in the pitch of the vehicle. 
Fathauer et al, U.S. Pat. No. 3,895,384, discloses a Doppler radar system 
which is adapted to be mounted on a tractor to measure the cumulative 
distance traveled by the tractor as well as the rate or speed of travel of 
the tractor. In Fathauer et al the problems arising from the presence of 
multiple Doppler frequencies caused by the beam width of the antenna are 
recognized. Fathauer et al teaches as a solution amplifying and clipping 
the Doppler frequencies prior to their being subsequently processed. 
Angwin, U.S. Pat. No. 4,012,736, discloses a radar speedometer for use on 
trains for example. Angwin teaches using a duty cycle detector to reject 
spurious noise. 
Millard, U.S. Pat. No. 4,052,722, discloses a traffic radar system. The 
signal processing subsystem of Millard includes a phase locked loop 
circuit to which input signals are applied directly. In addition the input 
signals are also applied to the phase locked loop circuit through a 
tuneable band pass filter. 
Aker et al, U.S. Pat. No. Re 29,401, discloses a Doppler radar system for 
measuring the speeds of motor vehciles. Aker et al's signal processing 
circuits include an AGC circuit for attenuating overly strong signals and 
a tracking band pass filter which is tuneable in steps. 
The article by H. C. Johnson discloses a radar speedometer for locomotives. 
His system includes a tracking low pass filter, the output of which is 
applied to a phase locked loop, which loop produces a spectrally clean 
output signal which in turn is used to produce signals to drive the 
standard locomotive speedometers and recorders. The article by Johnson has 
a good description of the environment in which locomotive radar 
speedometers operate. 
DISCLOSURE OF INVENTION 
The present invention provides both method and apparatus for measuring the 
speed of a land vehicle moving over a fixed surface. A narrow beamwidth 
directional microwave antenna is mounted on the vehicle and is directed 
toward the surface at a substantial angle to the velocity vector of the 
vehicle. Reflected RF energy from the surface received by the antenna is 
mixed with the transmitted energy to produce Doppler signals of varying 
frequencies and amplitudes. The distribution of the amplitudes of the 
reflected signals is a function of frequency and has a pattern of 
distribution; i.e., a spectrum. The Doppler signals are amplified by a 
high gain linear amplifier, and the amplified Doppler signals are applied 
to an AGC circuit and to a tracking band pass filter. The AGC and tracking 
band pass filters substantially normalize the spectra of the Doppler 
signals by causing the maximum amplitudes of the signals of the spectra to 
have a substantially constant voltage or magnitude and by eliminating the 
components of the spectra whose frequencies lie outside the pass band of 
the tracking filter. The signals constituting the normalized spectra are 
applied to a phase locked loop which produces an output signal. The 
frequency of this output signal has a substantially constant mathematical 
relationship with both the center frequency of the passband of the filter 
and the frequency of those signals reflected from the surface 
substantially intersected by the axis of the antenna which is defined as 
the boresight frequency. The boresight frequency in turn has a 
substantially constant mathematical relationship to the speed of the 
vehicle over the surface. 
It is therefore an object of this invention to provide improved Doppler 
radar methods and apparatus for measuring the speed of a land vehcile with 
respect to the surface over which the vehicle is moving. 
It is another object of this invention to provide improved Doppler radar 
methods and apparatus for measuring the speed of a locomotive with respect 
to the roadbed which accurately and reliably measures the speed of a 
locomotive over its normal range of speed and which is particularly 
accurate at very low speeds. 
It is another object of this invention to provide an improved method and 
apparatus for processing a spectral distribution of Doppler signals of 
varying amplitudes and frequencies to produce a signal whose frequency has 
a mathematical relationship with the boresight frequency of the spectrum.

BEST MODE FOR CARRYING OUT THE INVENTION 
In FIG. 1, radar speedometer 10 is illustrated schematically. Speedometer 
10 is adapted to be mounted on a locomotive which is not illustrated with 
its directional horn antenna 12 mounted on the underframe so that the axis 
14 of the cone, or beam, 16 of transmitted microwave electromagnetic 
energy will strike the surface of roadbed 18 between the rails of a 
typical railroad track which are not illustrated on which the wheels of 
the locomotive run. The angle .theta. of the beam with respect to surface 
18 is, in the preferred embodiment, substantially 45.degree.. The 
direction of motion of the locomotive with respect to roadbed 18 is shown 
by the arrows 20a, 20b, and preferably antenna 12 is mounted so that it 
points in a direction opposite to the direction in which the locomotive is 
moving. While such an orientation of antenna 12 is desirable, it is not 
necessary. Connected to antenna 12 is a conventional electromagnetic wave 
generator and receiver, transceiver 22. Transceiver 22 preferably 
comprises a Gunn diode oscillator utilized in the self detecting mode in 
which the Gunn diode transmits RF energy, detects the reflected RF energy, 
and mixes them. In the preferred embodiment the Gunn diode generates a 
microwave (CW) signal tuned to 10,525 MHz. Some of the transmitted signals 
forming beam 16 will be reflected from roadbed 18 and received within 
antenna 12. The reflected signals are mixed in the Gunn diode with the 
transmitted signals to produce Doppler signals having frequencies equal to 
the difference in frequencies between the transmitted signals and the 
received signals. The frequencies of the received signals vary with the 
speed of the locomotive and thus of antenna 12 over the roadbed, as is 
well known. 
For a ground speed radar sensor having an antenna 12 that produces a narrow 
beam 16 of transmitted microwave energy whose axis 14 lies in a vertical 
plane through the locomotive velocity vector 20 and strikes the roadbed 18 
at an incident angle .theta., the Doppler shift F.sub.d =2V.sub.g 
/cos.theta..lambda. where V.sub.g is the speed of the locomotive and 
.lambda. is the wavelength of the transmitted signal. For the ideal case 
with a transmitted frequency of 10,525 MHz and if .theta.=45.degree., then 
f.sub.d is approximately 21 Hz for each mph of locomotive speed. This is 
true only if the beam 14 has no substantial beam width. The finite width 
of beam 16, where .alpha.=10.degree., causes the Doppler frequency to be 
spread over a range of frequencies that is a function of the width .alpha. 
of beam 16 so that the Doppler frequencies produced by transceiver 22 have 
a distribution of frequencies, or a spectrum. 
The Doppler signals from transceiver 22 are applied to a conventional 
linear preamplifier 24. The purpose of preamplifier 24 is to amplify the 
Doppler signals produced by transceiver 22 to a convenient level for 
subsequent processing. As a step in the process of normalizing the 
spectrum of the Doppler signals, the signals produced by preamplifier 24 
are applied to automatic gain control (AGC) circuit 26. The dynamic range 
of AGC 26 should be large, 60 dB in the preferred example so that it can 
adjust a weak reflected signal nearly equal to the base line noise 
produced by the transceiver 22 and preamplifier 24 to the strongest 
anticipated signal. A suitable AGC circuit for this application is 
described in an article entitled "Automatic Gain Control has 60 Decibel 
Range", by N. Hecht, Electronics, Mar. 31, 1977, page 107. AGC circuit 26 
detects the highest amplitudes of the received signals of the Doppler 
spectrum and delivers output signals in which the signals forming the top 
portions of the spectrum will have a substantially constant voltage or 
amplitude regardless of the actual amplitudes of the reflected signals 
which vary with the nonuniform reflective characteristics of roadbed 18. 
The amplitudes of the balance of the signals of the spectrum will be 
amplified proportionately. 
The output signals of AGC 26, the spectra of which are normalized as to 
amplitude, are in the preferred embodiment further amplified by 
conventional linear amplifier 28 and the output of amplifier 28 is applied 
to tracking filters 30, 32, 34 and independent phase locked loop 36. 
FIG. 2 is a schematic of tracking filter 30. Tracking filters (TF's) 32, 34 
are similar to TF 30 except that the values of selected passive elements, 
capacitors and resistors, are chosen to provide different operating 
characteristics; i.e., frequency ranges over which the tracking filters 
are designed to operate. Tracking filters 30, 32, 34 detect the 
predominant, in amplitude, signals applied to them and lock on and track 
such predominant signals over a frequency range of 10 to 1, for example. 
If it is desired to measure speeds of the locomotive in the range of from 
0.7 mph to 125 mph, the low frequency TF 30 would be designed to track 
frequencies corresponding to speeds in the range of 0.7 to 4 mph (14.7 to 
84 Hz); medium frequency range TF 32 would be designed to track 
frequencies corresponding to speeds in the range of from 2.5 mph to 21 mph 
and high frequency TF 34 would be designed to track frequencies 
corresponding to speeds in the range of from 17 to 125 mph, for example. 
Increasing the operating frequency range of a TF decreases the number of 
TF's needed for a given application. As will be explained later the Q's of 
the TF's 30, 32, 34 are chosen so that they substantially match the 
normalized spectrum of the Doppler signals applied to them. 
The spectra of the signals produced by amplifier 28 after being normalized 
as to amplitude by AGC 26 are also applied to a conventional independent 
phase locked loop (PLL) 36. PLL 36 substantially follows the signals of 
maximum amplitude which approximates the Doppler frequency produced from 
the RF reflected from that portion of the roadbed 18 intersected by the 
beam axis 14 or the boresight of antenna 12 which frequency hereafter is 
sometimes referred to as the boresight Doppler frequency. The output 
signal of independent phase locked loop 36, a DC voltage roughly 
proportional to the boresight Doppler frequency, is applied to a frequency 
range selector logic circuit 38. The output signals of the TF's 30, 32, 
and 34 are selectively applied to one of the two input terminals of 
conventional AND gates 40, 41, 42. The other input terminals are connected 
to logic circuit 38 which operates to enable only one of AND gates 40, 41, 
42 if the speed of the locomotive is in the desired operating range, i.e., 
equal to or greater than 0.7 mph and not substantially greater than a 
maximum speed of 125 mph in a preferred example. The output signals of 
gates 40, 41, 42 are applied to the input terminals of a conventional 
three input inclusive OR gate 44. The output of OR gate 44 is applied to 
master phase locked loop 46. MPLL 46 will produce a single frequency 
output signal the frequency of which is the weighted mean of the 
frequencies of the signals constituting a normalized spectrum, and will 
substantially equal the center frequency f.sub.c of the pass band of 
whichever TF is operationally connected to one input terminal of a two 
input terminal AND gate 48. Gate 48 will be enabled whenever the Doppler 
frequency signals applied to PLL 36 equal or exceed the minimum speed that 
can be measured accurately by speedometer 10, 0.7 mph in the preferred 
embodiment. In a preferred embodiment phase locked loop circuits 36, 46 
each include a monolithic integrated circuit model CD 4046AF which is a 
product of the RCA Corporation. 
In operation antenna 12 will be mounted so that is is substantially 18 
inches above the conventional ballast of track bed 18 and is pointing 
rearwardly with respect to the front of the locomotive and at an angle 
such that the angle of incidence of the microwave radiation beam 16 
transmitted from antenna 12, is in the preferred embodiment substantially 
45.degree.. It is well to note here that the transceiver 22 will generate 
the same spectra when the direction of the locomotive travel is coincident 
with the velocity vector 20 or exactly opposite. As mentioned above the 
frequency of the CW signals radiated or transmitted from antenna 12 is 
substantially 10,525 MHz. 
Applications of the Doppler radar to measure the speed of a vehicle moving 
in either direction along the axis of the beam of radiation produces 
essentially a single Doppler frequency, or at least the amplitude of such 
a frequency so predominates, i.e., has the greatest amplitude so that the 
boresight Doppler frequency is easily identified and processed. FIG. 3 is 
a plot of amplitude versus frequency of such a system as might be produced 
by the Speed Measuring Apparatus of U.S. Pat. No. 3,118,139 which issued 
on Jan. 14, 1964, for example. 
However, if the antenna which transmits and receives the electromagnetic 
waves is mounted at a significant angle, as is antenna 12 of speedometer 
10, a complex spectrum of Doppler frequencies is produced. FIGS. 4A-F are 
plots of the Doppler signals as produced at the output of transceiver 22 
at the speeds indicated. In FIGS. 4A-F the solid vertical lines were 
generated by the test equipment and represent the calculated boresight 
Doppler frequency for the speed of the locomotive. 
A perusal of FIGS. 4A-F demonstrates that the return signals at any angle 
of incidence in the range of .theta..+-.1/2.alpha. including the boresight 
angle .theta. will not predominate in amplitude. The information 
illustrated in FIGS. 4 amply confirm this. Therefore with the beam 
significantly angled as in the present invention, the frequency spectrum 
generated must be considered as an entity and normalized before it can be 
processed to determine the actual speed. Other then the boresight 
frequency the balance of the spectrum will have frequencies which are the 
result of the transmitted signals being reflected from targets not lying 
on the boresight so that the cosine corrections for such signals differ. 
Thus angling the antenna with respect to the reflecting surface causes a 
spectrum of signals to be produced rather than a single frequency. Because 
of the variations in frequency and amplitude of the reflected or Doppler 
signals, to accurately determine the speed of a locomotive, for example, 
it is necessary to normalize the spectrum of the Doppler signals if the 
output of the speedometer is to be an accurate and reliable representation 
of vehicle speed. The plots of FIGS. 4 display the obvious characteristics 
of amplitude and frequency variations associated with the spectra of 
Doppler signals produced by the transceiver 22. By selecting a reasonable 
directive horn antenna 12 one can assure that the microwave energy 
directed toward and reflected from reflective objects near the boresight 
14 will statistically develop Doppler signals of higher amplitude than 
those produced by targets further away. However, the spectra of the 
Doppler signals are distorted from the ideal by the relatively greater 
amplitudes of the low frequency components or signals which are developed 
by reflections at angles closer to the vertical than the boresight angle 
.theta., by the varying reflectivity of the road bed, random noise, 
varying geometry of reflecting objects, etc. In FIG. 4D the base line 
noise at the low frequencies approximates in amplitude the base line noise 
of the higher frequencies seen to the right of the boresight frequency. In 
FIG. 4C the amplitudes of the lower frequency signals greatly exceed the 
base line noise and are about one-half the maximum amplitude of the 
spectrum. These variations are caused in part by changes in the reflective 
characteristics of the roadbed. Therefore, to facilitate accurately 
determining the speed of the vehicle from the spectrum of Doppler signals 
produced by transceiver 22 it is necessary to eliminate as many of the 
causes of such distortions as is possible to produce a normalized 
spectrum, or an idealized distribution of the Doppler signals. 
In this application the word "normalized" is used. Its meaning is its 
mathematical meaning, or more precisely, its statistical meaning which is 
"to reduce to a standard". Typically raw data from various samples will be 
processed to eliminate the distortions or abnormalities caused by the 
nature of the samples or the sample taking methods so that the underlying 
values of the samples are expressed free of those distortions or 
abnormalities. The distortions or abnormalities present in the spectrum of 
Doppler signals produced by transceiver 22 fall into three general 
categories, amplitude, low frequency components, and spurious random 
noise. Amplitude distortions caused by variations in the reflective 
characteristics of the road bed are substantially eliminated, or 
minimized, by AGC 26. The spectrum of signals produced by AGC 26 can be 
considered as normalized in amplitude. The distortions caused by the low 
frequency components of the spectrum of Doppler signals are substantially 
eliminated, or minimized, by tracking filters 30, 32, 34 since the low 
frequency components will not be in the pass band of the filter. For the 
same reason, most of the components of the signals classified as random 
noise outside the band pass of TF's 30, 32, 34 will also be rejected. The 
spectrum of the signals produced by TF's 30, 32, 34 can be considered as 
normalized in frequency. Thus normalizing as used in describing this 
invention is the processing of a spectrum of signals as received to 
substantially eliminate, or minimize, the abnormalities of the received 
spectra to produce a normalized spectra which substantially approximates 
the ideal. 
In FIG. 5 the spectrum corresponding to FIG. 4B is plotted together with 
the band pass characteristics 49 of the tracking filter at 20 mph. By 
matching the Q of the pass band of the tracking filter so that is 
substantially matches the normalized spectra, or idealized distribution of 
the Doppler signals, it can be seen that the two are substantially matched 
and that the center frequency f.sub.c of the filter will substantially 
equal the boresight frequency as calculated. In this application Q is 
substantially equal to the width of the band pass of the filter between 
its 3dB points divided by its center frequency. 
Referring to FIG. 2, which is a schematic of the low frequency tracking 
band pass filter 30, the normalized in amplitude spectrum of Doppler 
signals is applied to input terminal 50 and is then applied to the filter 
portion of tracking filter 30 which includes op amp 52. The output signals 
of op amp 52 are applied to output terminal 54 of TF 30 and are 
capacitively coupled to a feedback network which includes op amp 56, a 
pair of comparators 58, 59, where the normalized input signals applied to 
input terminal 50 are combined with the feedback signal coming from a 
phase shifter circuit which includes op amp 56. The combined signals are 
then applied to comparator 60. The output of comparator 60 is applied to 
the gate of the FET 62 which modifies the center frequency f.sub.c of 
tracking filter 52 so that f.sub.c of TF will substantially correspond to 
the statistical center, the boresight, frequencies of the spectrums of 
signals which are applied at terminal 50. 
In FIG. 2 in a preferred embodiment the comparators 58, 59 and 60 are each 
one-fourth of an MLM 139AL and the op amps 52, 56 are each one-half of an 
MLM 1558AL both of which are commercially available from Motorola Inc. As 
inferred above, the normalized spectrum of the Doppler signals has the 
characteristics of Q, since it can be described in the same terms of 
frequency and band width as is used to define the Q of a pass band filter. 
For example, in FIG. 5 the band width of the tracking filter is 50 Hz at a 
center frequency of 200 Hz which produces a Q of 4. The Q of the received 
spectrum, which depends primarily on the beam width of the directional 
antenna and the geometry of the antenna installation, can also be 
considered to have substantially a similar Q. To operate properly, the 
circuit values of the selftuning band pass filters 30, 32, 34 are selected 
to have Q's approximating or matching the Q's of the normalized spectrums 
of Doppler signals applied to them. In the preferred embodiment the proper 
Q's are in the range of from 4 to 10. In FIG. 5 only those signals 
constituting the normalized spectrum, or the idealized distribution of the 
Doppler signals, represented by the dashed line will appear at the filter 
output terminal. The signals of the received spectrum whose frequencies 
are not within the pass band of the filter will be rejected or not passed. 
The rejected signals are noise and low frequency components primarily. The 
normalized spectrum is reduced to a single frequency signal by the master 
phase locked loop 46 to which the input of the tracking filters are 
applied. 
From the foregoing it should be evident that various modifications can be 
made to the described embodiments without departing from the scope of the 
present invention.