Vehicle identification system, using microwaves

A vehicle identification system, using microwave, in which every vehicle to be identified has an identification panel attached to its side, containing a very low level power drain tunnel diode transponder and a digital coder plus a small battery and a resonant frequency array. Interrogating transmitter-receivers are placed at designated ground stations. For verification purposes there is a resonant reflective array printed on the identification panel to reflect back a doppler offset signal emitted by the interrogating transmitter, thereby registering that a vehicle with an identification panel has passed, even if the transponder has failed. Unlike optical scanners used for freight car identifications, this system cannot be disrupted by dirt, ice and snow. Vehicles do not have to slow down to be interrogated, and the emitted field strength from this plate is so low that F.C.C. licensing is not required. The code stored in the panel is readily programmable, if desired.

BACKGROUND OF THE INVENTION 
This invention is particularly appropriate for embodiment in freight car 
identification system. Every freight train contains cars from many 
different railroads, and the respective cars may have many different 
destinations. An identification (I.D.) code that can be fed into a central 
computer facilitates the loading, unloading, accounting for, and 
supervising the movement of all of the freight cars in a yard or on the 
trackage of a system. 
In order to keep track of railroad cars, freight car I.D. systems have been 
devised, and are in use. These existing I.D. systems use optical scanners 
(which identify color coded identification plates on passing freight 
cars.) However, these optical I.D. systems have a serious deficiency; the 
optical scanners become disrupted when the coded identification plates 
become coated with dirt or when fog, snow or rain interrupt the optical 
scanning path. 
A microwave freight car I.D. system embodying the present invention has the 
advantage that it is oblivious to dirt, fog, snow and rain. This microwave 
I.D. system has the following additional advantages over the prior optical 
scan system: 
(1) It is self-checking, in that indication of a freight car passing (with 
an identification unit) occurs independently of the transmission of a 
digital identification code. 
(2) The train does not have to slow down for accurate identification. 
(3) The distance from the train to the identification interrogation 
equipment is not critical. 
(4) The speed of the train can be checked, as a by-product. 
(5) The I.D. word generator in the panel can be programmable to provide 
routing and delivery information, data on vehicle contents, weight, etc. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a microwave signal transmitted 
from a fixed location is detected by a tunnel diode detector located in an 
identification unit attached to a vehicle. This detected signal 
momentarily turns on the battery power, thereby actuating a binary digital 
identification word generator. The binary word pulses bias the tunnel 
diode into an oscillitory mode for tansmitting an identification code to 
the receiver located at the fixed location. Simultaneously as this 
sequence is occurring, a doppler sensor associated with the fixed location 
transmitter picks up the doppler signal reflected from an efficient array 
of dipoles located on the identification panel. The presence of this 
doppler signal indicates that an identification panel is passing and an 
identification code should be received. This doppler signal provides a 
verification capability which can locate defective identification panels. 
The power drain of the panel is so low that a 0.5 ampere-hour battery 
should last ten years. 
The advantages of a vehicle identification system embodying the invention 
will be more fully understood from a consideration of the following 
detailed description in conjuction with the accompanying drawings.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 shows the vehicle 10 in the form of a railroad freight car moving in 
the direction of the arrow 11 along the tracks 13 and having a microwave 
identification panel 12 attached to its side. The interrogating 
transponder 16 is located off to one side of the tracks 13 on a post at a 
predetermined distance from the tracks which is in the range from 3 to 15 
feet. The antenna 14 of the interrogating station 17 or 17A has a 
beamwidth of approximately 10.degree. and is pointed obliquely toward the 
oncoming vehicle at an angle "A" of approximately a 40.degree. angle to 
the side of the vehicle. At this angle an insignificant amount of 
re-radiation from the vehicle is reflected back into antenna 14. 
FIG. 2 shows a schematic electrical circuit diagram of one form of the 
vehicle-mounted transponder 21 which is located inside of the panel 12. 
The antenna 18 is shown in detail in FIG. 5. Component 20 is a tunnel 
diode oscillating-detector stage which is shown in greater detail in FIG. 
7 and which includes a tunnel diode 23 and means providing an inductor 15 
and a capacitor 25 which are resonant at the microwave frequency 
transmitted from the antenna 14. The gain of the antenna 18 and the low 
level of output power from the tunnel diode oscillating detector stage 20 
produces a sufficiently low level of radiated field strength so that no 
F.C.C. license is required. The oscillating-detector stage 20 is included 
in a transponder circuit 55 containing a transistor 22 which is biased by 
a diode 24 so that it is on the verge of drawing current. The diode 24 and 
transistor 22 are self compensating so that no current drain through the 
transistor 22 will occur over a wide temperature range in the absence of a 
signal input. 
A resistor 26 of approximately 10 meg ohms is connected from the biasing 
diode 24 to a battery 35. This resistor diode combination 26, 24 draws 
less than one micro ampere from a 9 volt battery. This low current drain 
constitutes the average total current drain of the vehicle mounted 
transponder 21. 
The tunnel diode 23 is normally at zero bias where it acts as a detector. 
When microwave energy is received by the antenna 18, a voltage is built up 
on a capacitor 25 which biases transistor 22 into conduction thereby 
developing a voltage drop across the resistor 27. This current is further 
amplified in a second transistor stage 29 which then rapidly charges up a 
capacitor 28 through a diode 30. The voltage build-up in capacitor 28 
turns a third transistor stage 32 full on, thereby applying full voltage 
from the battery 35 to the binary identification word generator 34. 
A digital pulse stream from I.D. word generator 34 is then applied through 
an output connection 35 to tunnel diode 23 causing it to be biased by 
capacitor 25 into an oscillatory mode for transmitting from the antenna 18 
a microwave radio signal which corresponds to the digital pulse coded 
indentification word that is stored in the word generator 34. This same 
bias voltage on capacitor 25 also turns off the transistor 22, but the 
charge which was already built up across capacitor 28 will maintain the 
word generator 34 in operation for the duration of its transmission. This 
vehicle-mounted transponder circuit 21 is powered by battery 35, for 
example a 9 volt battery having a rating of approximately 0.5 ampere hours 
or more, if desired. 
FIGS. 3 and 4 show two interrogating transponder circuits. In FIG. 3, the 
interrogating transponder 17 has a directional antenna 14, for example 
such as a parabolic dish antenna, with a horizontal and vertical beamwidth 
of approximately 10.degree.. The antenna 14 is mounted at the same height 
as the I.D. panels 12 on the vehicles. Thus, the antenna 14 is aimed 
horizontally at the oblique angle "A" (FIG. 1) to the path of the vehicle. 
A circulator 38 couples a mixer 42 and a transmitter 40 to the antenna 14. 
The transmitter is a C.W. oscillator delivering about 100 mw at a 
frequency in the range from 10 to 25 GHz. Mixer 42 is a single diode mixer 
which operates from transmitter leakage. The output of the mixer diode 
feeds an audio amplifier 43 which amplifies and then rectifies the doppler 
return received from a moving identification panel 12. The I.F. amplifier 
44 is designed to match the frequency difference between C.W. frequency of 
the transmitter 40 and the frequency of the tunnel diode oscillator stage 
20 (FIG. 2) which are sufficiently offset from each other so that the 
tunnel diode oscillator cannot be injection locked by the transmitter 40. 
For example, the frequency of the tunnel diode oscillator 20 may be 10,500 
MHz and of the transmitter 10,300 MHz. The I.F. amplifier 44 is centered 
at this frequency difference and is sufficiently wide band to take into 
account any frequency drift of the T.D. oscillator 20 which might occur 
with time and temperature. 
If injection locking causes a problem in certain types of installations, 
FIG. 4 illustrates an alternative interrogating transmitter-receiver 
station 17A which can be used including a separate local oscillator 45 for 
wider frequency separation between the interrogating transmitter 40 and 
the tunnel diode oscillator 20. This arrangement also allows the use of 
two separate antennas 14-1 and 14-2 which give more freedom in positioning 
and detailed design of these two antennas so as to optimize the two 
functions and hence diminish the possibility of injection locking. The 
transmitter 40, mixer 42 and circulator 38 in FIG. 4 are the same as those 
in FIG. 3, and serve to feed a doppler return signal into the audio 
amplifier 43. 
The doppler output from the audio amplifier 43 in FIG. 3 or FIG. 4 is 
adapted to be fed into a verification circuit (not shown) for indicating 
and recording the fact that a vehicle has passed but has not responded 
with its identification code. For identification, the antenna 14-2 is 
connected to a single diode mixer 42a to which is connected the local 
oscillator 45. To the mixer 42a is connected an I.F. amplifier 47 which is 
centered at the difference between the frequency of the local oscillator 
45 and the frequency of the tunnel diode oscillator stage 20. The 
difference in frequency between the transmitter 40 and the tunnel diode 
oscillator 45 can now be 1000 MHz for example and the I.F. can be centered 
at 100 MHz. 
The output from the I.F. amplifier 44 or 47 feeds into suitable computer 
means (not shown) which serves to read the binary coded word for 
identifying the successive vehicles passing the interrogation station 17 
or 17A and which includes suitable memory and read-out capability for 
displaying a record of the various cars and of their locations. Other 
similar interrogation stations at other locations in the railroad system 
in which the tracks 13 are included are also connected to the computer 
means. In this way the identity, location and movement of all of the 
various vehicles 10 can be monitored for improved railroad operations. 
As shown in FIG. 5, the microwave identification panel 12 on the vehicle 
includes a plurality of spaced, parallel conductive reflector dipoles 50. 
The reflective dipoles, 50, are .lambda./2 long and are separated 
laterally by a spacing of .lambda./2 where .lambda. is the wavelength in 
the dielectric medium 65 of the interrogating signal being transmitted 
from the fixed station 17 or 17A. The dipoles are positioned approximately 
n.lambda./2 away from a conductive ground plane 52. The precise dimensions 
produce a maximum reflection at roughly 40.degree. off a plane 
perpendicular to the dipoles. Thus this array of dipoles 50 acts as a 
tuned reflective means for producing efficient reflection of microwave 
energy of a predetermined frequency range to reflect such energy at a 
predetermined angle "A". This reflected energy is slightly shifted in 
frequency due to the motion of vehicle 11, thereby producing an audio beat 
frequency signal which is amplified by the audio amplifier 43. 
The antenna 18 is attached to the tunnel diode module 20 and is 
approximately 8 inches long. The antenna's distance above the ground 
plane, 52, is optimized to best suit the specific system configuration 
used. The battery 35 and the transponder circuit 55 are also imbedded in 
the plastic material. The ground plane, 52, is a conductive layer covering 
the entire side of the panel that is attached to the side of the vehicle. 
The dipoles 50 and 18 are conductive strips of metal imbedded in a plastic 
dielectric material (FIG. 5) which overlies the ground plane 52. 
Specifically antenna 18 consists of a tapered strip of metal foil enclosed 
in plastic. The taper is optimized for greatest radiation efficiency. This 
is an end fire antenna which has its maximum radiated field along the axis 
of the antenna at an angle of 36.degree. relative to the ground plane 
which is provided by conductive base 52. The half wave resonant dipoles 
50, are laterally spaced approximately .lambda./2 and are located above 
the ground plane by an amount that will maximize the re-radiation at an 
angle of approximately 36.degree. above the ground plane and along the 
axis of dipoles 50. This distance is approximately n.lambda./2, where n is 
selected to produce an overall height of dielectric block 63 compatible 
with economic considerations. The longitudinal separations of the dipoles 
is approximately .lambda., from center to center. The precise separation 
will effect the angle of most efficient re-radition. 
The dimensions shown in FIG. 5 and 6 are for an operating frequency of 10.3 
GHz. 
The ground plane 52 is a 1/32 inch steel plate onto which is bolted by 
machine screws 64 and sealed with gasket 62, a molded polystyrene block 63 
with a compartment 59 formed for batteries 35 and circuitry 35. The 
antenna elements 18 and 50, consisting of thin strips of metal foil, are 
bonded with adhesive to the top of block 63 and a cover sheet of 
polystyrene 65 is bonded with adhesive to the top of block 63 to seal the 
unit and protect the antenna elements. 
The tunnel diode detector/oscillator shown as item 20 in FIG. 2 is shown in 
detail in FIGS. 7, 7A, 7B and 8. Cylindrical casing 132 contains an 
output/input probe coupler 136 which extends through the central hole 147 
in a ceramic window 134. This probe coupler connects to antenna 18 shown 
in FIG. 5. This connection is made by a slit 141 in probe coupler 136 
which then slips onto dipole 18. This ceramic window pane 134 has a 
metallized coating around its central hole 147 to which the probe coupler 
136 is soldered. Also, the periphery of the ceramic window is metallized 
and is soldered to a port 149 in the end wall 151 of the cylindrical 
casing 132. Into this casing 132 goes a tuning strap inductor member 130 
(see also FIG. 7A), a conductive spacer ring 128, and a tunnel diode 126. 
The radio frequency (R.F.) sub-assembly 119 for the tunnel diode 126 
includes an R.F. bypass capacitor 124 formed by a cylinder of conductive 
metal 125 coated with a thin dielectric layer 124 with an outer annular 
conductive cylindrical sleeve 122 pressed over this dielectric layer 124. 
For example, the metal cylinder element 125 may be aluminum with an 
anodized coating 124 on its cylindrical periphery. An alternative way in 
which to form the thin dielectric layer 124 is to use a very thin sleeve 
of low-loss insulating plastic, for example, such as the fluorocarbon 
resin plastic obtainable commercially under the trademark "Teflon", which 
is then sandwiched between the conductive cylinder element 125 and the 
sleeve 122. 
The outside diameter (O.D.) of the sleeve 122 is slightly smaller than the 
inside diameter (I.D.) of the casing 132 so that this sleeve 122 can slide 
into the casing 132. Also, the lower rim 129 of the sleeve 122 extends 
down slightly beyond the flat lower end 127 of the conductive element 125 
so that the conductive spacer ring 128 does not inadvertently shortcircuit 
the capacitor 124. The upper rim 149 of sleeve 122 extends up beyond the 
element 125 so that the housing 120 can not touch the cylindrical element 
125. 
The tunnel diode 126 is inserted and held in position between the flat 
lower end 127 of the cylinder element 125 which acts as a ground plane and 
the central portion 135 (FIG. 7A) of the resonant tuning inductor member 
130, which is spaced from this ground plane by the spacer ring 128. This 
ring 128 presses the circular rim 137 of the inductor member 130 down upon 
the end wall 151 of the doppler module casing 132. Thus, the outer ends of 
the inductor strap 131 are grounded to the casing 132 and are grounded to 
the ring 128. 
There is a cylindrical resistor housing 120 having a d.c. stabilizing 
resistor 118 mounted therein. The resistor has one of its leads secured to 
a terminal screw 121, and the other lead is soldered to the inner surface 
of the cylindrical housing 120. Thus, this resistor 118 is in circuit in 
parallel with the capacitor 124, being shunted between one terminal 168 of 
the diode 126 and ground. This resistor may, for example, have a 
resistance value in the range from 10 to 50 ohms. The terminal screw 121 
may be held by an insulating washer 123 if desired, or this washer may be 
omitted, the screw being threaded into an axial socket 117 in the 
conductive cylindrical element 125. Also attached by this terminal screw 
is one of the leads 114 to transponder circuit 55. The other lead 114 is 
secured to the interior of the cylindrical housing by soldering. 
In the doppler module sub-assembly 119, the cylindrical housing 120 abuts 
against the cylindrical sleeve 122 which in turn abuts against the spacer 
ring 128 resting on the circular rim 137 seated on the end wall 151. A 
ring nut 116 screws down into the threaded region 133 in the upper end of 
the module casing 132 for pressing and locking the conductive components, 
120, 122, 128, 137 and 151 firmly together. 
As shown enlarged in cross section in FIG. 7B the probe coupler 136 has an 
enlarged head 160 which has a cup shaped top surface 162. The enlarged 
central portion 135 of the inductor strap 131 rests upon the rim 164 and 
spans over the cup 162. The tunnel diode 126 generally has an inverted top 
hat configuration with conductive terminal surfaces 166 and 168 on its 
bottom and top. The conductive cylindrical element 125 presses down on the 
terminal surface 168, while the resilient central portion 135 of the 
inductor strap acts like a dished spring element pressing firmly and 
resiliently up against the other terminal surface 166, thereby maintaining 
good electrical contract with both ends of the tunnel diode 126 in spite 
of any expansion or contraction due to ambient temperature changes. 
In FIG. 8 is a schematic electrical circuit diagram showing the 
oscillator/mixer circuit 150 with the tunnel diode 126 in connection with 
the mid-point of the inductor 131 formed by the diametrically extending 
conductive strap 131 (FIG. 7A) of the tuning strap member 130. The 
terminal 166 of diode 126 seats against the enlarged central region 135 of 
this strap. The terminal screw 121, the positive lead 114 and the resistor 
118 are electrically connected through the cylindrical element 125 to the 
terminal 168 of the diode 126, while the lower end of the diode is 
connected to the other (ground) lead 114 through the inductor strap 131. 
The inductor strap 131 and capacitor 124 act as a resonant circuit which 
determines the frequency of oscillation of the tunnel diode oscillator 
circuit 150. This frequency can be tuned by shaping the tuning strap 
member 130 (FIG. 7A). Namely, the longer and thinner that the inductor 
strap 131 is made and the larger that the semi-circular openings 143 are 
made, then lower the frequency of the microwave energy to be generated, 
and vice versa. 
The following components are made of suitable conductive and readily 
machinable metal, for example of brass: doppler module casing 132, sleeve 
122, spacer ring 128, housing 120, screw 121, and ring nuts 115 and 116. 
The tuning strap member 130 may advantageously be made of very resilient 
metal having a highly conductive surface for example such as gold-plated 
spring steel. The interior of the casing 132, end wall 151 and spacer ring 
128 may be gold-plated to provide high conductivity. The following 
components are made of low loss dielectric material, for example of 
ceramic: window 134 and insulating washer 123. 
Although the tunnel diode 126 is shown with its negative terminal 166 in 
connection with the mid-point 135 of the inductor strap 131, it is to be 
understood that this diode can be installed in an inverted position, 
namely, with its positive terminal in connection with said mid-point. In 
this latter case the polarity of the leads 114 is reversed. Furthermore, 
tunnel diodes of other mechanical configuration can be inserted into this 
assembly. 
FIG. 2 shows the tunnel doide as 23, the resonant inductor as 15 and the 
bypass capacitor as 25. The components correspond to FIGS. 7 and 8 in 
which 126 is the tunnel diode, 131 is the inductor and 124 is the bypass 
capacitor, and 118 is the stabilizing resistor. The connections 114 in 
FIGS. 7 and 8 provide connection with the transponder circuit 55 of FIG. 
2, one of these connections being a common ground. 
The predetermined oblique (acute) angle A (FIG. 1) at which the 
interrogating transponder antenna 14 of 14-1 and 14-2 is aimed preferably 
corresponds with the predetermined oblique (acute) angle A (FIG. 5A) to 
the ground plane 52 at which occurs the maximum intensity of the reflected 
energy from the array of dipoles 50, for example, this acute angle may be 
in the range from approximately 30.degree. to approximately 45.degree. and 
in this illustrative embodiment is approximately 36.degree. to 40.degree.. 
As mentioned above, the Doppler signal output in FIGS. 3 and 4 is fed into 
a verification circuit for indicating and recording the fact that a 
vehicle has passed. This Doppler signal output may also be fed into a 
circuit for calculating and recording the speed at which the vehicle was 
passing the fixed station 17 or 17A. 
It is to be understood that as used herein the word "vehicle" is intended 
to be interpreted broadly. For example the vehicle 10 may be a rapid 
transit car or it may be a truck or a trailer truck on a roadway 13 or it 
may be a container moving along on a conveyor track 13 or a container 
being carried along a roadway track 13, for example such as the containers 
adapted to be placed on shipboard or other containers. 
The identification word generator 34 shown in FIG. 2 may be programmable, 
for example by use of plug-in modules or by use of magnetic strips or 
punch cards which can be slid into a slot in the word generator. The 
programmable information may include identification data for the vehicle 
and for its contents, net weight and gross weight of the vehicle plus 
contents, routing information and instructions for shipping and delivery, 
data about time of departure, intended time of arrival, and so forth. If 
desired the identification code for the vehicle may be unchangeable, while 
the remaining data are programmable. The coded information may include 
error-correcting codes for automatically detecting that an error has been 
made. 
It is to be understood that a second panel 12' may be mounted on the 
opposite side of the vehicle 10. Alternatively, two of the panels 12 may 
be mounted back-to-back beneath or above the vehicle 10, as shown at 112 
and 112'. Such a dual-sides back-to-back panel is mounted with its plane 
extending vertical and being positioned parallel with the longitudinal 
centerline of the vehicle 10 above or below the vehicle so that the dual 
unit can be "seen" and interrogated by a fixed station from either side of 
the tracks. 
Also, it is to be understood that a panel can be mounted as shown at 12" in 
a horizontal plane and positioned with dipoles 50 and antenna 18 being 
perpendicular to the longitudinal center-line of the vehicle 10. Panel 12" 
may be mounted above the vehicle, preferably near the middle of the 
vehicle so that the panel 12" can be "seen" and interrogated by a fixed 
station located at either side of the tracks looking downward at an 
oblique (acute) angle toward panel 12".