Abstract:
A tracking system uses a road mounted microwave reflector as an alignment tool. The system can be used to provide primary or supplemental guidance and alignment for an self-driving vehicle, or it can be used to provide warning signals for a manually controlled vehicle. The disclosed reflector is economical and easily installed. A preferred corner reflector contains both a microwave retro reflector and an embedded tuned circuit. The system is optimized to operate reliability and accurately in conditions of inclement weather and poor visibility, particularly where GPS signals, conventional road markers and visual aids fail.

Description:
CLAIM OF PROVISIONAL APPLICATION RIGHTS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/981,728 filed on Apr. 18, 2014. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to the fields of autonomous vehicle guidance systems and vehicular steering, tracking and alignment. 
         [0004]    2. Background Art 
         [0005]    Technology that enables self-driving vehicles is not new. Many industrial vehicles such as autonomously operated forklifts receive radio frequency (“RF”) signals to guide their travel along a predetermined path. Other vehicles have used sonar and radar system for vehicle guidance and position warning. 
         [0006]    A popular low-cost, high-accuracy vehicle control system is frequently referred to as “wire-guided.” In a wire-guided system, a transmitter that operates continuously applies an RF signal to a wire embedded in the surface of a road or track. This wire emits an RF signal which is tracked by a receiver mounted on the vehicle. Wire-guided systems such as those used in warehouse forklifts can provide tracking accuracy to within 1.0 centimeter (“cm”). 
         [0007]    For example, U.S. Pat. No. 4,307,329 entitled “Wire Guidance Method and Apparatus” that issued Dec. 22, 1981, on a patent application filed by Charles L. Taylor discloses a ground vehicle guidance system for following a current-carrying guidewire. The wire-guided system disclosed in this patent provides improved steering stability over a wide range of speeds, and improved immunity to inductive field anomalies by computing vehicle heading and lateral displacement using sensor signals themselves substantially insensitive to heading variations. The sensor signals are processed to provide steering command signals having a specified relationship to vehicle steering geometry. In this way desired damping factors can be obtained for both forward and reverse travel. The disclosed system also includes improved vehicle speed limiting and stopping circuits that independently control truck operation in accordance with computed heading and with lateral displacement deviations. The patent also discloses steering systems both for vehicles using steerable wheels and vehicles steered by differential drive wheel speed control. 
         [0008]    U.S. Pat. No. 5,404,087 entitled “Automated Guided Vehicle Wire Guidance Apparatus” that issued Apr. 4, 1995, on a patent application filed by Leigh E. Sherman discloses a wire-guided apparatus for an automated guided vehicle. This wire-guided apparatus includes a first crossed coil sensor for acquiring a wire and tracking along curves in the wire as the vehicle is travelling in the forward direction. A second crossed coil sensor included in the wire-guided apparatus acquires the wire and tracking along the wire when the vehicle is travelling in the reverse direction. Third and fourth sensors included in the wire-guided apparatus track along straight runs in the wire. Circuitry included in the wire-guided apparatus switches between the first and second crossed coil sensors and the third and fourth sensors. Circuitry included in the wire-guided apparatus also generates a guidance error signal from the outputs of either: 
         [0009]    1. the first or second crossed coil sensors; or 
         [0010]    2. both the third and fourth sensors. 
         [0000]    The guidance error signal is then used to control a motor connected to a steerable wheel of the vehicle to maintain the vehicle in alignment with the wire. 
         [0011]    Wire-guided systems offer excellent performance in adverse conditions, but suffer from significant disadvantages which make them impractical for installing a long public road. Perhaps the most significant disadvantage with wire-guided systems is their requirement for powered transmitters. Transmitters are expensive and the wire-guided system cannot be used where there is no reliable electrical power, or where power is too expensive to install. Over longer distances, this disadvantage increases because multiple transmitters are required due to RF signal attenuation along the wire located far from a transmitter. Consequently, at some point the number of transmitters and the amount of electrical power becomes economically prohibitive for a wire-guided system. 
         [0012]    A second disadvantage of the wire-guided system is the necessity to bury a conductor beneath a road&#39;s surface. Burying a cable can be difficult and costly, and can impair the road&#39;s structural integrity. 
         [0013]    A third disadvantage of the wired guided system is the lack of two-dimensional position information. A wire-guided system can guide a vehicle from side to side perpendicular to the wire axis, providing a single axis for steering, but it cannot provide a second axis of information containing a vehicle&#39;s longitudinal position along the length of the wire. 
         [0014]    UK Patent Application GB 2 277 152 entitled “Localizing Systems For Robotic Vehicles” that was published Oct. 19, 1994, on a patent application filed in the name of Gareth Anthony Edwards discloses a localizing system and method suitable for use in a robotic lawn mower. The disclosed system includes plurality of spaced reference stations that are associated with an area to be worked by the lawn mower. The reference stations are located in relation to the area and the operating lawn mower communicates with two or more of the spaced reference stations to determine its distance from and bearing with respect thereto. In this way the mobile lawn mower localizes itself in relation to the working area. Thus, the mobile lawn mower is capable of carrying out a task over at least part of the working area in a controlled manner. The UK patent application discloses that communication between the lawn mower and the reference stations may be effected using ultrasonic or electromagnetic radiation. 
         [0015]    European Patent Application EP 2006708 entitled “Localizing System For a Robotic Vehicles” that was published Dec. 24, 2008, on a patent application filed in the name of Jurgen Seidel and others discloses a system having a transmission unit for transmitting an electromagnetic signal toward active reflecting landmarks, and a receiver unit for receiving electromagnetic signals reflected from respective landmarks. A control unit included in the system determines a robotic vehicle&#39;s position based on the reflected electromagnetic signals received by the receiving unit. Each reflecting landmark is distinguished from other landmarks by a unique reflection characteristics, preferably a difference in reflected signal modulation intensity profile. The patent application discloses that landmarks having a different number of reflection elements and/or differently shaped reflective elements produce differing modulation intensity profiles particularly if the landmarks move such as by each landmark&#39;s rotation. In this way the intensity of the signal reflected from various landmarks differs and identification of the landmarks becomes possible by comparing reflected signal intensities. 
         [0016]    With the ever increasing likelihood of self-driving passenger vehicles traveling on public roads, safe operation has become paramount. To avoid accidents and their corresponding liabilities, and to increase public acceptance of autonomous technology, self-driving vehicles must be built to the highest safety and reliability standards. Reliable vehicle position information is more critical than ever for safety. 
         [0017]    Today&#39;s self-driving vehicles use a combination of global positioning system (“GPS”), inertial, visual, cameras, ultrasonic and radar guidance position information. With appropriate corrections GPS can provide position accuracy to within 3.0 meters (3.0 m). To achieve greater accuracy, GPS data is typically combined with map data and inertial guidance. Radar or ultrasonic sensors are used to detect nearby objects. and other moving vehicles, and while parking. The reliability of all these systems can be degraded by adverse environmental conditions. 
         [0018]    Environmental conditions which degrade the accuracy of these conventional sensors include inclement weather, foliage cover, ice, snow and fog. For example, GPS is prone to RF interference and its performance can be impaired by foliage and trees. 
         [0019]    Visual guidance via digital cameras and/or lidar helps keep today&#39;s self-driving vehicle centered in its lane, but camera vision and lidar are impaired by weather, dust, smoke, and heat. Visual guidance systems may also be blocked by other vehicles. In addition, visual guidance systems are limited to visible road markings, which may be obscured, hard to detect, missing, in poor condition, or spaced far apart. 
       BRIEF SUMMARY 
       [0020]    An object of the present disclosure is to improve self-driving vehicle performance and safety. 
         [0021]    Another object of the present disclosure is to provide a simple and economical system for use in self-driving vehicles. 
         [0022]    Another object of the present disclosure is to provide a highly reliable system having no moving parts. 
         [0023]    Another object of the present disclosure is to provide a system for use in self-driving vehicles having improved performance in adverse environmental conditions. 
         [0024]    Another object of the present disclosure is to provide supplemental guidance for self-driving vehicles that is reliable, economical and practical to install along long roads. 
         [0025]    Briefly, a transceiver detects the presence of road mounted microwave reflectors to accurately position a vehicle regardless inclement weather or outside interference. This system provides high accuracy at very low cost while being easy-to-install on existing roadways. While this system overcomes drawbacks of other guidance methods, it is best used to augment all of the various guidance techniques and to improve guidance system performance in adverse environmental conditions. For example, the system might be used only where increased performance and accuracy is needed, for example on a mountain pass which frequently experiences snow and ice. 
         [0026]    The disclosed transceiver is adapted for inclusion in a guidance system for a self-driving vehicle that is driveable along a road having passive corner reflectors secured thereto. The transceiver includes a transmitter having a transmitting antenna. When the transceiver operates the transceiver&#39;s transmitting antennae projects a steered, pencil-shaped transmitted beam ahead of the self-driving vehicle. The transmitted beam is swept rapidly from side-to-side across the road in front of the moving self-driving vehicle, Movement of the self-driving vehicle combined with sweeping of the pencil-shaped transmitted beam from side-to-side across the road in front of the moving self-driving vehicle causes the transmitted beam to intermittently impinge upon corner reflectors secured to the road. The transceiver also includes a receiver having a directional receiving antenna that points toward a location where the swept transmitted beam will intermittently impinge upon corner reflectors secured to the road. The receiving antenna receives that portion of the transmitted beam impinging on corner reflectors which individual corner reflectors echo back toward the receiving antenna. 
         [0027]    Advantageously, system is simple and economical since it requires only a single axis track of reflectors. If the position and spacing of reflectors is known, in combination with an accurate clock vehicle speed can be accurately determined and checked. 
         [0028]    However, the spacing between corner reflectors need not be a fixed distance. In areas where the road forks or makes sharp turns, for example, corner reflectors may be spaced closer together. In long, straight stretches of road, corner reflectors may be spaced further apart. 
         [0029]    Corner reflectors having different resonant frequencies may be used for indicating the presence of particular types of landmarks, e.g. freeway exits, fire hydrants, etc. 
         [0030]    These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a plan view of a single face of corner reflector in accordance with the present disclosure; 
           [0032]      FIG. 2  is a plan view of a corner reflector&#39;s individual faces that are depicted in  FIG. 1  arranged as three faces of a tetrahedron; 
           [0033]      FIG. 3  is a cross-sectional perspective view of a road having the corner reflector of  FIG. 2  implanted flush therein; 
           [0034]      FIG. 4  is a cross-sectional perspective view of the corner reflector of  FIG. 3  implanted in a conventional highway reflector; 
           [0035]      FIG. 5  is a block diagram depicting a preferred embodiment of a transceiver in accordance with the present disclosure; 
           [0036]      FIG. 6  is an elevational view of a self-driving vehicle having a transceiver of the type depicted in  FIG. 5  mounted thereon for transmitting a RF signal to be echoed from corner reflectors implanted in the road as depicted in  FIG. 3 ; 
           [0037]      FIGS. 7-9  are similar to  FIGS. 1-3  that depict a preferred face for inclusion in a corner reflector and corner reflectors that include the preferred face, the preferred face being configured to provide improved resonance at a transmitter&#39;s center operating frequency; 
           [0038]      FIG. 10  is a spectrum diagram illustrating performance of the corner reflectors depicted in  FIGS. 8 and 9  having the resonant structure best illustrated in  FIG. 7  compared with a similar corner reflector that lacks the resonant structure; and 
           [0039]      FIG. 11  is a block diagram illustrating inclusion of a vehicular transceiver in accordance with the present disclosure in a vehicle guidance system. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]      FIG. 6  depicts a self-driving vehicle  22  having a transceiver  24  in accordance with the present disclosure mounted thereon. The transceiver  24  projects a steered, pencil-shaped transmitted microwave beam, indicated by a dashed arrow  26  in  FIG. 6 , ahead of the self-driving vehicle  22  toward a corner reflector  28  implanted in a road  32 . As depicted by a curved, dashed arrow  34  in  FIG. 5 , the transceiver  24  sweeps the transmitted beam from side-to-side across the road  32  in front of the self-driving vehicle  22 . As depicted in  FIG. 6 , movement of the self-driving vehicle  22  along the road  32 , indicated by an arrow  36  in  FIG. 6 , combined with sweeping of the pencil-shaped transmitted beam from side-to-side across the road  32  in front of the moving self-driving vehicle  22  causes the transmitted beam to intermittently impinge upon corner corner reflectors  28  secured to the road  32 . When the transmitted beam impinges on a corner reflector  28 , the corner reflector  28  echoes a portion of the transmitted beam back toward the transceiver  24 , indicated by a dashed arrow  38  in  FIG. 6 . In addition to projecting the transmitted beam, the transceiver  24  receives that portion of the transmitted beam echoed back toward the transceiver  24  from corner reflector  28 . 
         [0041]    Referring now to  FIG. 5 , as is well known to those skilled in the art of microwave transceivers, the preferred transceiver  24  includes:
       1. a plurality of transmitting antennae  42 ; and   2. at least one receiving antenna  44  that connect to a receiver that is included in the transceiver  24 .
 
The transceiver also includes a horizontal row of phase shifters  52 , one phase shifter  52  respectively connected to each of the transmitting antennae  42 , and a network of power dividers  54  connected between the phase shifters  52  and a source of microwave power  56 . The power dividers  54  supply equal amounts of RF power received from the source of microwave power  56  to each of the phase shifters  52 .
       
 
         [0044]    As is those skilled in the art know, each phase shifter  52  may be implemented using an ensemble of detour lines having lengths chosen so that PIN diodes, connected between pairs of detour lines, may be appropriately switched for delaying the microwave signal received by the phase shifter  52  from the power dividers  54  by a specified amount. The PIN diodes produce the desired delay by routing the received microwave signal through appropriately selected detour lines. 
         [0045]    Configured in this way, the transmitting portion of the preferred transceiver  24  constitutes a passive electronically scanned array (“PESA”), also known as passive phased array. A PESA has a central radio frequency source (such as a magnetron, a klystron a travelling wave tube, or a frequency synthesizer), sending RF energy into the phase shifters  52  via the power dividers  54  for retransmission into the transmitting antennae  42 . Signals supplied to the phase shifters  52  form microwave energy received from the source of microwave power  56  into the steered, pencil-shaped transmitted microwave beam that, as indicated by the curved, dashed arrow  34 , sweeps from side-to-side across the road  32 , preferably in a sawtooth pattern. Microwave energy formed into the transmitted beam preferably includes microwave frequencies that are no less than two gigahertz (2.0 GHZ). The frequency should as high as possible to minimize the physical size of the corner reflector  28 , but not so high that serious attenuation will occur when the corner reflector  28  is covered with ice, water, and/or road debris. 
         [0046]    It should be apparent to one of ordinary skill that the source of microwave power  56  can supply any frequency in the SHF band. 
         [0047]    Higher frequencies yield the advantage of a smaller corner reflector  28 , and lower frequencies have the advantage of a lower cost transmitter and less path-loss attenuation due to water. ISM bands allow unlicensed operation in most countries. The 24 GHz ISM band is a possible frequency range which works quite well with a corner reflector  28  as small as one-half inch (0.5″) in diameter. 
         [0048]    The phase shifters  52  for sweeping the transmitted beam from side-to-side may be as simple as a tuned circuit incorporating a varactor diode, where the diode is tuned by an oscillating voltage corresponding to the sweep frequency. A more elegant type of phase shifters  52  uses a different fixed reactance in each phase shifter  52 . For this type of phase shifter  52 , sweeping the transmitted frequency over a relatively small range causes the transmitted beam to swing in an arc. Because synthesized microwave transmitters are very common and inexpensive, this second configuration for the phase shifters  52  provides a practical transmitter having a relatively low parts count. Since the bandwidth required to steer the beam is small compared to the bandwidth of the corner reflector  28 , there is no significant reduction in that portion of the transmitted beam echoed back from corner reflectors  28  toward the transceiver  24  due to frequency sweeping. Moreover, sweeping the frequency of microwave RF projected from the transceiver  24  may be exploited advantageously for distinguishing the corner reflector  28  from other microwave reflecting objects such as metallic candy wrappers. 
         [0049]    To reduce costs, the entire transmitting circuit, including phase shifters  52 , power dividers  54 , and transmitting antennae  42  may be constructed on a single glass-epoxy printed circuit board, with the majority of the power divider and antennae structures, and parts of the phase shifters  52  constructed of copper traces. The power dividers  54  may be either resistive or electromagnetic. 
         [0050]    While similar to a conventional radar the transmitting antennae  42  might be used for receiving that portion of the transmitted beam echoed from the corner reflector  28  back toward the transceiver  24 , to simplify the transceiver  24  it preferably has the separate receiving antenna  44  depicted in  FIG. 5  thereby allowing continuously projecting the transmitted beam. The design of the receiving antenna  44  is not critical and can be a directional horn, shielded dipole, or any similar directional antenna with a forward lobe sufficient to receive reflected signals over the entire range of arc, i.e. plus or minus sixty degrees (±60°). Presently, the preferred receiving antenna  44  is a “horn” to waveguide adapter. This configuration for the receiving antenna  44  receives the reflected a portion of the transmitted beam throughout an aperture that is approximately: 
         [0051]    1. plus or minus sixty degrees (±60°) horizontally; and 
         [0052]    2. ten degrees (10°) vertically. 
         [0000]    Stated alternatively, the receiving area is sensitive only in a rectangular area pointed in the same direction as the transmitting antennae  42 . In this way the receiving antenna  44  picks up only RF directly reflected from the corner reflector  28 , and ignores interfering signals from outside the rectangular area. The receiving antenna  44  should be shielded to prevent receiving interference from directions other than the intended sensing area. 
         [0053]    It should be apparent that a set of conventional microwave corner reflectors may be used for position sensing. In using a conventional corner reflectors, however, two difficulties arise. First, the corner reflector must have high reflectivity which usually means a large surface area thereby making the corner reflector unsuitable for implantation in a road. Second, the corner reflector must be uniquely distinguishable from other objects that are highly reflective to microwave signals such as a foil candy wrapper. 
         [0054]    Disclosed herein are corner reflectors  28  that are small, sealed and can be embedded flush with the surface of the road  32 . Flush surface mounting allows the corner reflectors  28  to survive installation on roads which may be exposed to snowplows. The disclosed corner reflectors  28  utilize a corner reflector having a tuned circuit. The tuned circuit provides an identifying resonant reflective response which allows unique identification by the transceiver  24 . 
         [0055]    Disclosed herein are corner reflector  28  that can be installed in any rotational orientation about its vertical axis without affecting its performance. To map locations of corner reflectors  28 , a surveyor may use a combination of maps, road markings, and fixed differential GPS signals. As illustrated in  FIG. 3 , installing corner reflectors  28  requires only a small cavity in the road  32  approximately one-half inch (0.5″) deep. This small cavity may be quickly drilled in cement or simply pressed into asphalt. Epoxy may be used in affixing each corner reflector  28  to the road  32 . The disclosed corner reflector  28  may be covered with epoxy, and the road  32  may be lightly repaved without impairing operation of the corner reflector  28 . Since the corner reflector  28  is beneath the surface of the road  32 , road cleaning equipment such as snow plows will not dislodge it. 
         [0056]    The high reflectivity and sharp resonance characteristics of the preferred corner reflector permit using a relatively low power transmitter operating at a frequency that matches the peak resonance frequency of the preferred corner reflector. 
         [0057]      FIGS. 1-3  depict various aspects of a corner reflector  28  that might be implanted in the road  32 . Each corner reflector  28 , depicted in  FIGS. 2 and 3 , is assembled by juxtaposing three identical triangularly-shaped (3) corner reflector faces  72 , one of which is depicted in  FIG. 1 , to form a tetrahedron that is open at its base. As illustrated in  FIG. 3 , when the corner reflector  28  is implanted in the road  32  the open face of the tetrahedron faces upward. Each corner reflector face  72  may be conveniently fabricated by etching a pattern, such as that depicted in  FIG. 1 , into one of the conductive layers of a double sided sheet of printed circuit board material. For inclusion in the corner reflector  28 , the printed circuit board material&#39;s insulating layer must be made from a material, such as the preferred material teflon, having characteristics suitable for use at microwave frequencies. 
         [0058]    The particular configuration for the corner reflector face  72  depicted in  FIG. 1  includes a meandering conductor  74  that originates at one vertex of the triangular-shape and extends almost entirely across the corner reflector face  72  to the side of the triangular-shape opposite the starting vertex. The meandering conductor  74 , which provides the corner reflector  28  with its antenna, is formed by connecting a sequence of sinusoidal curves end-to-end. In practice, the meandering conductor  74  includes more sinusoidal segments than depicted in the illustrations of  FIGS. 1-3 . For example, the meandering conductor  74  included in a corner reflector  28  having a 12 GHz center frequency will have eight (8) sinusoidal segments. The number of sinusoidal segments decreases as the operating frequency of microwave RF increases, and increases as the size of the corner reflector  28  decreases. 
         [0059]    The particular configuration for the corner reflector face  72  depicted in  FIG. 1  surrounds the meandering conductor  74  with an open area open area  76 . For the particular corner reflector face  72  depicted in  FIG. 1 , the open area  76  is itself is surrounded by an un-etched portion  78  of the double-sided printed circuit board material&#39;s conductive layer. Though not separately depicted in  FIG. 1-3 , the conductive layer on the opposite side of the double sided printed circuit board material from the meandering conductor  74  is not patterned so it forms a ground plane for the patterned side of the corner reflector face  72 . 
         [0060]    When three (3) of the corner reflector faces  72  are assembled into the corner reflector  28 , the vertices of the three (3) corner reflector faces  72  at which each meandering conductor  74  originates are connected together electrically with a dot  82  of solder. 
         [0061]    To provide a corner reflector  28  suitable for implantation into a road  32  as depicted in  FIG. 3 , or incorporation into a conventional highway reflector  86  as depicted in  FIG. 4 , the assembled corner reflector faces  72  of the corner reflector  28  may be conveniently molded into a hemispherically-shaped package  88  made from material that does not significantly attenuate microwave signals. 
         [0062]      FIGS. 7-9  depict a preferred embodiment for the corner reflector  28  and the corner reflector faces  72  assembled thereinto. Those elements depicted in  FIG. 7-9  that are common to the corner reflector  28  and the corner reflector faces  72  illustrated in  FIGS. 1-3  carry the same reference numeral distinguished by a prime (“′”) designation. 
         [0063]    As depicted in  FIG. 7 , the preferred corner reflector faces  72 ′ that are assembled into the corner reflector  28 ′ differ from the corner reflector faces  72  depicted in  FIGS. 1-3  by:
       1. omitting the portion  78  that surrounds the meandering conductor  74 ; and   2. including a rectangularly shaped bar  92  of the conductive layer that connects to the meandering conductor  74 ′ and is located at the end of the meandering conductor  74 ′ furthest from the starting vertex thereof.
 
Connected to the meandering conductor  74 ′ as illustrated in  FIG. 7 , the bar  92  inherently forms part of the antenna of the corner reflector  28 ′. Furthermore, juxtaposed across the insulating material from the ground plane included in the corner reflector face  72 ′ described previously for  FIGS. 1-3 , the bar  92  establishes an approximately fifty picofarad (50 pf) capacitor with the ground plane. Due to the location of the bar  92 , this capacitor is located at the end of the meandering conductor  74 ′ furthest from the starting vertex thereof. Configured in this way, the combined inductance in the meandering conductor  74 ′ and the bar  92  can be understood as forming a series resonant circuit preferably at the frequency of the microwave RF projected from the transceiver  24 .
       
 
         [0066]    A curve  96  in a  FIG. 10  spectrum diagram shows the resonant characteristics exhibited by the preferred corner reflector  28 ′ having a resonant frequency of approximately 8.0 GHz. A curve  98  in the  FIG. 10  spectrum diagram shows the response of a similar corner reflector whose faces lack the resonant producing structure best illustrated in  FIG. 7 . As depicted in the  FIG. 10  spectrum diagram, the corner reflector  28 ′ exhibits a resonant spike exceeding 30 dBm at its 8.0 GHZ resonant frequency. The receiver portion of the transceiver  24  can use this spike in reflected RF to easily and uniquely distinguish the corner reflector  28 ′ from interference sources such a metallic foil candy wrapper. 
         [0067]    The size of the corner reflector  28  or  28 ′ varies with frequency of RF projected from the transceiver  24 . In general, the larger the size of the corner reflector  28  or  28 ′ the better, because the surface area of the corner reflector  28  or  28 ′ determines the amount of power echoed back from the corner reflector  28  or  28 ′ to the receiving antenna  44 . In practice, the minimum workable size for the meandering conductor  74  or  74 ′ is around one-half lambda (½λ), i.e. one-half the free space wavelength of microwave RF projected from the transceiver  24 . The free space wavelength at a frequency of 10.25 GHz is 1.5 cm, and at 24 GHz is 0.6 cm. 
         [0068]    It has been noticed that the quality factor (“Q”) of the corner reflector  28 ′ is very important for its detection by the transceiver  24 . The increased Q of the corner reflector  28 ′ is essential for achieving the response curve that appears in  FIG. 10  which assists in distinguishing the corner reflector  28 ′ from other possible sources that echo microwave RF such as metallic foil candy wrappers. 
         [0069]    Electrical performance of the corner reflector  28 ′ is further enhanced by plating a layer of silver at least one micron (1.0μ) thick onto the meandering conductor  74 ′ and the bar  92 . Oxidation of the silver plating does not appear to adversely affect performance of the corner reflector  28 ′. 
         [0070]      FIG. 11  illustrates a vehicle guidance system for today&#39;s self-driving vehicles referred to by the general reference character  100 . As described above, the vehicle guidance system  100  may include, in addition to other sensors, some combination of: 
         [0071]    1. a global positioning system (“GPS”)  102 ; 
         [0072]    2. inertial guidance  104 ; and 
         [0073]    3. an optical sensor  106  such as a camera or lidar. 
         [0000]    The various sensors included in the vehicle guidance system  100  supply output signals to a guidance controlling processor  108 . Responsive to signals received from the various sensors, the guidance controlling processor  108  produces output signals for controlling various aspects of a self-driving vehicle such as its steering system  112  illustrated in  FIG. 11 . 
         [0074]    A vehicle guidance system  100  in accordance with the present disclosure adds to its usual ensemble of sensors such as those depicted in  FIG. 11  the transceiver  24 . Equipped with the transceiver  24 , that portion of the transmitted beam impinging on the corner reflectors  28  which individual corner reflectors  28  echo back to the receiving antenna  44  is processed to thereby provide the vehicle guidance system  100  with a deviation signal that the vehicle guidance system  100  uses in controlling operation of a self-driving vehicle. 
         [0075]    Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is purely illustrative and is not to be interpreted as limiting. Consequently, without departing from the spirit and scope of the disclosure, various alterations, modifications, and/or alternative applications will, no doubt, be suggested to those skilled in the art after having read the preceding disclosure. For example, a retro reflector containing a semiconductor transceiver which returns a coded signal to the receiving antenna  44  would assist in discriminating the corner reflector from potentially interfering materials such as a metallic foil candy wrappers. Such an active corner reflector could be spaced further apart than the corner reflector  28  or  28 ′. For example, passive resonant corner reflectors  28 ′ might be embedded 30 yards apart while an active corner reflector might be embedded perhaps at each freeway exit. 
         [0076]    Another enhancement to the system is the use of varying retro-corner reflector resonance frequencies. Since the resonant frequency need not be identical for all corner reflector  28  or  28 ′, different frequencies may be used to indicate different waypoints along a road. 
         [0077]    Accordingly, it is intended that the following claims be interpreted as encompassing all alterations, modifications, or alternative applications as fall within the true spirit and scope of the disclosure including equivalents thereof. In effecting the preceding intent, the following claims shall:
       1. not invoke paragraph 6 of 35 U.S.C. §112 as it exists on the date of filing hereof unless the phrase “means for” appears expressly in the claim&#39;s text;   2. omit all elements, steps, or functions not expressly appearing therein unless the element, step or function is expressly described as “essential” or “critical;”   3. not be limited by any other aspect of the present disclosure which does not appear explicitly in the claim&#39;s text unless the element, step or function is expressly described as “essential” or “critical;” and   4. when including the transition word “comprises” or “comprising” or any variation thereof, encompass a non-exclusive inclusion, such that a claim which encompasses a process, method, article, or apparatus that comprises a list of steps or elements includes not only those steps or elements but may include other steps or elements not expressly or inherently included in the claim&#39;s text.