TARGET POSITION ESTIMATION FROM CROSS TRANSMISSION REFLECTIONS OF UNSYNCHRONIZED RADARS

A vehicle, radar system for a vehicle and method of estimating a cross-transmission range of an object. The radar system includes a first radar, a second radar and a processor. The first radar transmits a test signal. The second radar is separated from the first radar by a selected distance and receive a total signal that includes the test signal received directly from the first radar and a reflection of the test signal from the target. The processor performs a non-linear operation on the total signal to obtain a cross-correlation term of the directly received test signal and the reflection signal, and estimate a cross-transmission range of the object from the cross-correlation term.

INTRODUCTION

The subject disclosure relates to determining radar parameters of an object using radar signals and, in particular, to determining a position of an object using unsynchronized radars.

In vehicular radar systems, there are often multiple radars located on the vehicle. Each radar generally includes a transmitter and a receiver. The transmitter transmits a signal that is reflected from an object and is received at the receiver in order to determine a parameter of the object, such as a position of the object. By synchronizing multiple radars, these parameters can be determined using a signal transmitted from a radar at one location of the vehicle with a reflection of the signal received at another radar at another location of the vehicle. However, synchronization of radars requires a significant amount of additional processing circuitry and power needs. Accordingly, it is desirable to provide a system and method for determining a position of the object that does not require synchronized radars.

SUMMARY

In one exemplary embodiment, a method of estimating a cross-transmission range of an object is disclosed. The method includes transmitting a test signal from a transmitter, receiving, at a receiver separated from the transmitter, a total signal that includes the test signal received directly from the transmitter and a reflection of the test signal from the object, performing a non-linear operation on the total signal to obtain a cross-correlation term of the directly received test signal and the reflection signal, and estimating the cross-transmission range of the object from the cross-correlation term.

In addition to one or more of the features described herein, the transmitter and the receiver are unsynchronized. Performing the non-linear operation further includes at least one of squaring the total signal, obtaining a scalar product of the total signal, and obtaining an absolute value of the total signal. The method further includes applying a bandpass filter to a result of the non-linear operation. The method further includes integrating the cross-correlation term to estimate the round trip delay between transmitter, object, and receiver. The method further includes applying a Fourier transform to the cross-correlation term and estimating the cross-transmission range of the object from the peak in a resulting Fourier spectrum. The method further includes combining the estimated cross-transmission range of the object from the cross-correlation term with an estimate of a self-transmission range of the object from a self-transmission echo.

In another exemplary embodiment, a radar system for a vehicle is disclosed. The radar system includes a first radar, a second radar and a processor. The first radar is configured to transmit a test signal. The second radar is separated from the first radar by a selected distance and is configured to receive a total signal that includes the test signal received directly from the first radar and a reflection of the test signal from the target. The processor is configured to perform a non-linear operation on the total signal to obtain a cross-correlation term of the directly received test signal and the reflection signal, and estimate a cross-transmission range of the object from the cross-correlation term.

In addition to one or more of the features described herein, the first radar and the second radar are unsynchronized. The processor is further configured to perform the non-linear operation by performing at least one of squaring the total signal, obtaining a scalar product of the total signal, and obtaining an absolute value of the total signal. The processor is further configured to apply a filter to a result of the non-linear operation. The processor is further configured to integrate the cross-correlation term to estimate the cross-transmission range of the object. The processor is further configured to combine the estimated cross-transmission range of the object from the cross-correlation term with an estimate of a self-transmission range of the object from a self-transmission echo. The processor is further configured to navigate the vehicle with respect to the object based on the estimated cross-transmission range.

In yet another exemplary embodiment, a vehicle is disclosed. The vehicle includes a first radar, a second radar and a processor. The first radar is configured to transmit a test signal. The second radar is separated from the first radar by a selected distance and is configured to receive a total signal that includes the test signal received directly from the first radar and a reflection of the test signal from the target. The processor is configured to perform a non-linear operation on the total signal to obtain a cross-correlation term of the directly received test signal and the reflection signal, and estimate a cross-transmission range of the object from the cross-correlation term.

In addition to one or more of the features described herein, the first radar and the second radar are unsynchronized. The processor is further configured to perform the non-linear operation by performing at least one of squaring the total signal, obtaining a scalar product of the total signal, and obtaining an absolute value of the total signal. The processor is further configured to apply a filter to a result of the non-linear operation. The processor is further configured to integrate the cross-correlation term to estimate the cross-transmission range of the object. The processor is further configured to combine the estimated cross-transmission range of the object from the cross-correlation term with an estimate of the self-transmission range of the object from a self-transmission echo.

DETAILED DESCRIPTION

In accordance with an exemplary embodiment,FIG. 1shows a vehicle10with an associated trajectory planning system depicted at100in accordance with various embodiments. In general, the trajectory planning system100determines a trajectory plan for automated driving of the vehicle10. The vehicle10generally includes a chassis12, a body14, front wheels16, and rear wheels18. The body14is arranged on the chassis12and substantially encloses components of the vehicle10. The body14and the chassis12may jointly form a frame. The wheels16and18are each rotationally coupled to the chassis12near a respective corner of the body14.

In various embodiments, the vehicle10is an autonomous vehicle and the trajectory planning system100is incorporated into the autonomous vehicle10(hereinafter referred to as the autonomous vehicle10). The autonomous vehicle10is, for example, a vehicle that is automatically controlled to carry passengers from one location to another. The autonomous vehicle10is depicted in the illustrated embodiment as a passenger car, but it should be appreciated that any other vehicle including motorcycles, trucks, sport utility vehicles (SUVs), recreational vehicles (RVs), marine vessels, aircraft, etc., can also be used. In an exemplary embodiment, the autonomous vehicle10is a so-called Level Four or Level Five automation system. A Level Four system indicates “high automation”, referring to the driving mode-specific performance by an automated driving system of all aspects of the dynamic driving task, even if a human driver does not respond appropriately to a request to intervene. A Level Five system indicates “full automation”, referring to the full-time performance by an automated driving system of all aspects of the dynamic driving task under all roadway and environmental conditions that can be managed by a human driver.

As shown, the autonomous vehicle10generally includes a propulsion system20, a transmission system22, a steering system24, a brake system26, a sensor system28, an actuator system30, at least one data storage device32, and at least one controller34. The propulsion system20may, in various embodiments, include an internal combustion engine, an electric machine such as a traction motor, and/or a fuel cell propulsion system. The transmission system22is configured to transmit power from the propulsion system20to the vehicle wheels16and18according to selectable speed ratios. According to various embodiments, the transmission system22may include a step-ratio automatic transmission, a continuously-variable transmission, or other appropriate transmission. The brake system26is configured to provide braking torque to the vehicle wheels16and18. The brake system26may, in various embodiments, include friction brakes, brake by wire, a regenerative braking system such as an electric machine, and/or other appropriate braking systems. The steering system24influences a position of the of the vehicle wheels16and18. While depicted as including a steering wheel for illustrative purposes, in some embodiments contemplated within the scope of the present disclosure, the steering system24may not include a steering wheel.

The sensor system28includes one or more sensing devices40a-40nthat sense observable conditions of the exterior environment and/or the interior environment of the autonomous vehicle10. The sensing devices40a-40ncan include, but are not limited to, radars, lidars, global positioning systems, optical cameras, thermal cameras, ultrasonic sensors, and/or other sensors. In various embodiments, the vehicle10includes a radar system including an array of radar sensors, the radar sensors being located at various locations along the vehicle10. In operation, a radar sensor sends out an electromagnetic pulse48that is reflected back at the vehicle10by one or more objects50in the field of view of the sensor. The reflected pulse52appears as one or more detections at the radar sensor.

The actuator system30includes one or more actuator devices42a-42nthat control one or more vehicle features such as, but not limited to, the propulsion system20, the transmission system22, the steering system24, and the brake system26. In various embodiments, the vehicle features can further include interior and/or exterior vehicle features such as, but are not limited to, doors, a trunk, and cabin features such as ventilation, music, lighting, etc. (not numbered).

The trajectory planning system100navigates the autonomous vehicle10based on a determination of objects and/their locations within the environment of the vehicle. In various embodiments the controller34operates a plurality of radars at various locations on the vehicle10to determine a location (i.e., range, elevation and azimuth) of the object50using unsynchronized radars, in particular, using cross-transmission echoes between unsynchronized radars. The determined location can be used either alone or in combination with similar parameters obtained by single radar systems in order to provide range, azimuth and/or elevation of the object50for navigation purposes. Upon determining various parameters of the object, such as range, azimuth, elevation, velocity, etc., the controller34can operate the one or more actuator devices42a-n, the propulsion system20, transmission system22, steering system24and/or brake26in order to navigate the vehicle10with respect to the object50.

FIG. 2depicts the vehicle10ofFIG. 1performing a method of determining a self-transmission range R of object50using a self-transmission echo from a single radar of vehicle10. The vehicle10shows a first radar202and a second radar204. Each of the first radar202and the second radar can include a transmitter and at least one receiver. Alternatively, each radar can include a transducer that operates as both transmitter and receiver.

Referring toFIG. 4,FIG. 4shows details of the illustrative radar system of vehicle10. The first radar202includes a transmitter402for transmitting one or more test signals and a plurality of receivers404for receiving reflections of the one or more test signals. Similarly, the second radar204includes a transmitter406for transmitting one or more test signals and plurality of receivers408for receiving reflections of the one or more test signals. It is apparent that, while the first radar202and the second radar404are unsynchonized, a test signal transmitted from transmitter402of the first radar202can be received by the plurality of receivers408of the second radar204, and a test signal transmitted from transmitter406of the second radar204can be received by the plurality of receivers404of the first radar202. In various embodiments, the test signal is a linear modulated frequency (LFM) signal, also known as a chirp signal.

Returning toFIG. 2, the second radar204is shown illustrating the operation of a self-transmission echo for determining a self-transmission range of the object104with respect to vehicle10. While the second radar204is shown for illustrative purposes, it is to be understood that the first radar202can also determine a range of object50using a self-transmission echo, either independently of the second radar204or in combination with the second radar204. In a self-transmission echo, the second radar204transmits a test signal s(t) and receives a reflection r(t) of the test signal from the object50. Since the transmitter and receiver of the second radar204(i.e., the same radar) are synchronized with each other, the self-transmission range R is obtained from a correlation of the reflected signal r(t) with the local radar transmitted signal s(t), as shown in Eq. (1):

where μ is a delay offset between the transmitted signal s(t) and received signal r(t). Eq. (1) is suitable for range determination when the transmitter and receiver are synchronized. However, Eq. (1) does not hold when using a transmitter and a receiver that are not synchronized.

FIG. 3illustrates a method of determining the cross-transmission range R′ of the object50using a cross-transmission echo. The cross-transmission echo uses both the first radar202and the second radar204. The first radar202and the second radar204are separated by a selected or known distance and are not synchronized with each other. Additionally, the first radar202and the second radar204are within a line-of-sight of each other. The cross-transmission range is a distance travelled from first radar202to the object50and then to second radar204. In a more general sense, a cross-transmission range of an object is a range determined using a signal travelling between any two distinct radars by way of the object.

As an illustration of cross-transmission echo ranging, the first radar202generates a test signal s(t) (304) that propagates from the first radar202in all directions. The second radar204receives two signals as a result of the transmission of the test signal. First, the second radar204receives directly the test signal s(t) which has travelled directly from the first radar202to the second radar204. Second, the second radar204receives a reflection r(t) (306) of the test signal304from object50. The resultant total signal y(t) received at second radar204is given by Eq. (2):

where r(t) is the reflected signal306and s(t−r) is the directly received test signal304. The variable τ is related to the distance between the first radar202and the second radar204, which is a known quantity.

In order to obtain a cross-correlation term from the total signal y(t) of Eq. (2), a non-linear operation is performed on the total signal y(t). In various embodiments, the non-linear operation can include: squaring the total signal, performing a scalar product of the total signal, obtaining an absolute value of the total signal, etc. For illustrative purposes, Eq. (3) shows the results of squaring the total signal:

The non-linear operation introduces terms that are the square of the reflection and the square of the test signal as well as a cross-correlation term, 2r(t)s(t−τ). Thus, performing the non-linear operation generates the cross-correlation term than can be used to determine a cross-transmission range from unsynchronized radars, using Eq. (1) or a similar equation. For a LFM test signal, the cross correlation term gives a sinusoidal signal such as sin(2πft+φ) or a complex exponential signal. exp(j2πft+φ). In either case, the frequency f is proportional to the round trip delay between the transmit antenna, the object and the receive antenna. Thus, the round trip delay is determined by estimating the frequency f of the cross-correlation term by applying a Fourier transform on the cross-correlation term. The peak value of the Fourier spectrum is the related to the round trip delay for a single object. For multiple objects at different positions, the Fourier transform produces multiple peaks, one for each of the multiple round trip delays related to the multiple objects. The Fourier transform is implemented by either a Discrete Fourier transform or a Fast Fourier transform, in various embodiments. One can apply a bandpass filter to the output of the Fourier transform. Cross-transmission range determination using cross-transmission echoes is discussed with respect toFIG. 5.

FIG. 5shows a flow chart500illustrating a method for determining a range R to an object204using unsynchronized radars. In box502, a test signal s(t) is generated at a first radar. In various embodiments, the test signal s(t) is a linear frequency modulated signal or “chirp” signal. In box504, a total signal y(t) is received at a second radar unsynchronized with the first radar, the total signal being the summation of the test signal directly received from the first radar and a reflection of the test signal from the object.

In box506, a non-linear operation is performed on the total signal. The non-linear operation produces the square of the reflection signal, the square of the directly-received signal and the cross-correlation term that includes a product of the reflection signal and the directly-received signal shifted in time by a time delay. The square of the reflection signal and the square of the directly-received signal are high frequency terms and DC (low frequency) terms. Therefore in box508, a filter is applied to the signal in order to remove these terms. In various embodiments, the filter is a band-pass filter.

In box510, the filtered signal (i.e., the cross-product term) is transformed into a frequency space using, for example, a Fourier transform. In box512, the peaks of the cross-product term are located in frequency space. The peaks in the Fourier domain are proportional to and/or related to the cross-transmission ranges of the object. In box514, an offset (cτ) is deduced, the offset being introduced by the direct path delay from the transmitter to the receiver. The peak in the Fourier spectrum is a measurement of the difference between the round trip delay (along the path from the transmitter to the object and to the receiver) and the delay τ along the direct path (between the transmitter and the receiver). The round trip delay measurement is adjusted by τ or by the corresponding distance measurement CT in order to eliminate the offset from the cross-transmission range calculations.

In box516, the cross-transmission range measurements and the self-transmission range measurements are combined into a uniform beamforming signal. For a radar system including the first radar202and the second radar204, both first radar202and second radar204can be used to determine a self-transmission range using a self-transmission echo. Furthermore, cross-transmission ranges can be determined using two cross-transmission echo ranges. Since, each radar has multiple transmitters and receivers, matrices A1(θ), A2(θ), A3(θ), A4(θ) correspond to the multiple transmitter and receiver paths for self-transmission echoes and cross transmission echoes. The combined ranging signal z(θ) is shown in Eq. (4):

where y1is a self-transmission range determined from a first radar, y2is a self-transmission range determined from a second radar, y3is a cross-transmission range determined by transmitting a test signal from a first radar and receiving the reflection at the second radar and y4is a cross-transmission range determined by transmitting a test signal from a second radar and receiving the reflection at the first radar. The combined ranging signal z(θ) can be provided to the processor44ofFIG. 1to indicate a position of the object50which is used to control navigating of the vehicle10with respect to the object50.

FIG. 6depicts a diagram illustrating the results of combining the results of self-transmission ranging and cross-transmission ranging. Curve601shows peaks for an angular location of the object50using only a self-transmission ranging from a single radar. Curve603shows peaks for angular location of the object50using a combination of self-transmission and cross-transmission ranging methods disclosed herein. The beam width of each curve indicates an estimated location and resolution of the object obtained via the respective ranging method. The beam width of the curve603(from about −5° to +5°), which uses cross-transmission ranging, is decreased by at least a factor of 2 over the beam width of the curve605(from about −15° to +15°), which uses only the self-transmission ranging. Thus the angular resolution of the object50is increased by at least a factor of 2.

It is to be understood that use of additional cross-transmission echoes, as is possible using the multiple receivers of the radar system ofFIG. 4can increase a number of range estimations during cross-transmission ranging and can therefore be used to further reduce beam width or, equivalently, to increase the angular resolution of the object.