Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
1999-02-17
2001-10-23
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C180S169000, C342S115000, C342S189000, C356S004010, C356S005010
Reexamination Certificate
active
06307622
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to detection and ranging systems and more particularly to an optical detection and ranging system.
DESCRIPTION OF THE RELATED ART
Detection and ranging systems that utilize electromagnetic energy are referred to as (ra)dio (d)etecting (a)nd (r)anging systems, or “radar” systems. Applications of radar systems vary from detecting and ranging of intercontinental ballistic nuclear missiles for national security to detecting and ranging of trout for sports fishing. A radar system can provide an operator with the ability to “see” objects that cannot be perceived though visual means due to limitations of human vision, such as distance, visual obstruction and darkness. In addition, a radar system can function as an extra “eye” to detect objects that breach a predetermined boundary from the location of the radar system. For example, a radar system can be incorporated into a collision avoidance system in an automobile to prevent collisions by detecting any objects within a predetermined proximity of the automobile, allowing an operator to take appropriate steps to avoid the collision.
The basic principles of a radar system are elementary. A transmitter of the radar system emits an electromagnetic signal. The emitted signal is reflected if a target is present at some distance from the radar system. The reflected signal is received by a receiver of the radar system. By calculating the round trip time of the electromagnetic signal, the distance of the target can be determined. However, when noise and interference are introduced into this process, the determination of whether a received signal is the reflected signal of the emitted signal or a signal caused by noise and/or interference becomes difficult. In order to differentiate the desired signal in the presence of noise, a radar system typically emits the signal with an embedded code using discernible differences within the signal, such as phase shifts or frequency changes. The code allows the radar system to correlate the received signal with the emitted signal in order to determine whether the received signal is an echo of the emitted signal, indicating a positive detection of a target.
A common type of radar system that utilizes phase shifts to encode the signal is a pulse compression radar system with binary phase modulation. In these radar systems, the transmitted signal is a pulse that is comprised of a number of subpulses. The subpulses are of equal duration, and each has one of two predetermined phases, wherein the two phases represent digital “0” and “1”. The transmitted signals are encoded in a sequence. By correlating a received signal with multiple time-delayed transmitted signals, an autocorrelation function can be plotted as relative amplitude over time. A typical autocorrelation function includes a central peak with sidelobe peaks on both sides of the central peak. The highest relative amplitude value of the central peak indicates a point on the autocorrelation function at which the correlation between the received signal and the transmitted signal is the greatest. The corresponding time of that point is the round trip time required for an emitted pulse to propagate from a radar system to the target and back. A presence of the central peak in the autocorrelation function equates to a positive detection of the target.
A pulse compression technique is an attractive feature for a radar system, because generation of high peak power signals are avoided. Instead, the pulse compression technique utilizes a long pulse with lower power to efficiently apply the average power capability of the system. In the radar system, this long pulse is received and then multiplied by the time-delayed transmitted pulses to efficiently utilize the power of the entire pulse. After the multiplication, the received pulse is integrated and plotted on a display. The length of the pulse determines the ratio between the central peak and the sidelobe peaks. A longer pulse with a greater number of subpulses increases this ratio, which in turn increases the signal-to-noise ratio of the radar system. However, different sequences for the transmitted pulse exhibit different autocorrelation function characteristics. Optimally, a code sequence that produces sidelobes with minimal peaks is desired. However, there are only a limited number of codes that exhibit the desired sidelobe behaviors. Barker codes generate low sidelobes in the autocorrelation function. Unfortunately, the longest Barker code that exhibits the minimum sidelobes behavior is a thirteenth-order code sequence.
Typically, the electromagnetic signals that are transmitted by radar systems are radio frequency (RF) signals. However, optical signals have been utilized instead of RF signals in radar systems. U.S. Pat. No. 5,141,308 to Danckwerth et al. describes a radar system that employs laser beam pulses to detect the presence and range of objects. The radar system of Danckwerth et al. is a pulse compression type radar system that generates the laser beam pulse by a semiconductor laser diode. In operation, the laser beam pulse emitted by the laser is modulated in accordance with a selected code sequence. Portions of the emitted laser beam pulse are reflected back to the system by targets and are received by a photodiode. The photodiode converts light energy of the received pulse to electrical current. The current is demodulated and correlated with the selected code sequence that has been time-delayed. Using the information from the correlation, the range of the target is displayed on a display and counter circuit.
Although prior art systems operate well for their intended purposes, what is needed is a cost-effective detection and ranging system having a high level of effectiveness with respect to rejecting of undesired signal interference and having a reduced system complexity.
SUMMARY OF THE INVENTION
A method and a system for detecting and ranging objects utilize summed and difference signals to determine whether a target is present at a predetermined distance from the system. The summed and difference signals represent corresponding points on two discriminator functions that are derived by summing and subtracting two autocorrelation functions. The two autocorrelation functions are identical functions, except that one has been shifted by a one-bit period. By analyzing the summed and difference signals, the system is able to detect objects that cross a boundary zone located at the predetermined distance from the system. In one application, the invention can be incorporated into an automobile to detect objects, such as other vehicles, within a predefined region surrounding the automobile for back-up sensing, blind spot sensing, and pre-collision detection for vehicle safety systems.
In order to detect the presence of a target, the system includes a transmitter that transmits a unique electromagnetic signal. The system also includes a receiver that is configured to receive the transmitted signal, if the transmitted signal is reflected by the target. Preferably, the transmitter includes a number of optical pulse emitters, such a unique electromagnetic signal is an optical signal. The optical pulse emitters may be laser diodes or high frequency modulated light emitting diodes. Accordingly, the receiver preferably includes a number of photodiodes to receive the reflected optical signal. Each of the optical pulse emitters may be uniquely associated with a particular photodiode. In the preferred embodiment, the transmitter also emits a leader segment prior to transmitting the optical signal. The leader segment is a signal that is utilized by the system to establish an amplitude reference prior to processing the reflected optical signal, thereby controlling any transient effect in the system. The leader segment may be a constant half-powered optical signal. Alternatively, the leader segment may be a series of full-powered pulses separated by spaces to yield an average power equal to half power to establish the amplitude reference.
The transmitted optical sig
Buczinski Stephen C.
Infineon Technologies North America Corp.
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