System for Processing Reflected Energy Signals

1. Field of Invention

The present invention relates to an improved system for sensing, receiving, and processing reflected energy signals such as laser light pulses. More specifically, the invention concerns a detecting and ranging system for transmitting pulses of energy, detecting energy signals and efficiently identifying a reflected "return" signal within the detected signals with reduced processing time and reduced memory usage.

2. Description of Related Art

Many different systems have been used in the past to intelligently guide projectiles such as missiles. After the laser was developed, the defense industry put laser technology to use in the form of laser detecting and ranging (LADAR) guidance systems.

In LADAR guidance systems, brief laser pulses are generated and transmitted via a scanning mechanism. Some of the transmitted pulses striking a target of interest are reflected back to a receiver associated with the transmitter. Such LADAR systems are commonly installed in projectiles such as missiles to determine the type-and location of a target. The target and surrounding area are scanned to produce an image that comprises multiple pixels. The image can then be analyzed to extract three-dimensional targeting information. The time between the transmission of a laser pulse and the receipt of the reflected laser pulse ("return pulse") is used to calculate each pixel's range.

In known LADAR systems, electronic circuitry begins a ramp function concurrently with the transmission of the outgoing pulse. The ramp function is halted when a return pulse is received. Thus, the height of the resulting ramp is directly proportional to the range to the target. Although the contour and magnitude of the return pulse contain useful information, variations may result in uncertainties as to when the ramp function should be stopped.

Ideally, a return pulse has a finite duration, and an analog ranging system terminates the ramp function mid-way through the pulse. But since the magnitude of a pulse cannot be determined in advance, it may be difficult to distinguish a return pulse from other light. Therefore, some systems set a threshold to differentiate between return pulses and other light. This may result in an inaccurate determination of range, especially where the distance between the target and the receiving optics is great, and the return pulse accordingly has a small magnitude.

Other known systems start a counter when a laser pulse is transmitted and terminate the counter when the return pulse is detected. The value of the counter is thus proportional to the distance to the target. This method suffers from some of the same threshold uncertainties as the ramp methods. In addition, the return pulse may be corrupted by noise. Moreover, the width of the return pulse will be changed as a function of the slope of the target. Thus, there remains a need for a system that reduces the uncertainties in determining a range to a target.

Another need exists in this area of technology because engineers that design missile guidance systems are almost always interested in reducing the weight, size, and expense of the associated electronics. In particular, since random access memory ("RAM") modules are often bulky and expensive, it would be desirable to have a missile guidance system that utilizes RAM more efficiently, and therefore requires less RAM.

In designing missile guidance systems, engineers are also concerned with increasing the resolution afforded by such systems. This may be accomplished by increasing the sampling rate of the system, or, in other words, increasing the frequency of the LADAR pulses emitted by the system. In systems that utilize the time between LADAR pulses to analyze stored return signals, increasing the frequency of the pulses requires that the analysis of the stored pulses must be conducted more quickly. Therefore, it would be advantageous to have a missile guidance system that requires less computation time to analyze its return signals.

The present invention is directed at the problems set forth above. The invention produces guidance correction signals by processing reflected energy signals generated by a detecting and ranging transceiver such as a LADAR receiver. In an illustrative implementation, the invention includes certain pulse capture electronics that initiate firing of a laser transmitter and determine the time-of-flight (i.e. range) and intensity of the returning laser pulses. Also, the pulse capture electronics direct this data to a processor for execution of specific targeting algorithms. The pulse capture electronics comprises a number of signal processing stages, including an analog-to-digital comparator, a RAM buffer clocked by a counter, an intelligent convolution circuit, and a peak detector.

In accordance with one aspect of the invention, an external computing device is used to decrease the time required to process signals received from the RAM by defining a "window" in the RAM within which past return signals have been stored, and "locking" on to this window to reduce time spent searching for future return signals in RAM. After identifying a return pulse, the computing device stores the memory address of the beginning of the return pulse; based on the average beginning memory address of several previous return pulses, the computing device predicts the beginning memory address of the next return pulse. This memory address is used to define a "window," within which it is likely that the next return pulse will be stored. As new return pulses are received, the window is periodically updated, enabling the computing device to effectively "lock on" to the average return pulse. In this way, the signal analysis for each return pulse is limited to a particular region of interest, since data located outside the window in the RAM is ignored.

Another aspect of the invention involves a real time return system to decrease the time required to process signals received from the RAM. In this embodiment, the pulse capture electronics include a peak detector to detect the maximum amplitude of the return signal and to store the value supplied by a counter at that point. The value of the counter, then, provides the approximate location of the return signal in the RAM. Having this knowledge, the convolution circuit is able to more efficiently read data from the RAM, since the data of the RAM can be effectively narrowed to the area of interest.

In another aspect of the invention, a timing delay system is provided to decrease the required size of RAM by delaying storage of data until a return pulse is actually received. With the timing delay circuit, analog to digital conversion of the return pulse is delayed, permitting a peak detector to identify the receipt of a return pulse on a non-delayed timeframe, and, in advance, to activate the RAM for storage of the delayed converted digital pulse.

The nature of the invention, as well as its objects and advantages, will become more apparent to those skilled in the art after considering the following detailed description in connection with the accompanying drawings, in which like reference numerals designate like parts throughout, wherein:

FIG. 1 is a block diagram of pulse capture electronics 66 of the present invention;

FIG. 2 is a flowchart of the process of the windowing system of the invention;

FIG. 3 is a block diagram of a real time return circuitry 300 of the invention; and

FIG. 4 is a block diagram of a timing delay system or circuit 400 of the invention.

When the present invention is applied in the context of light signals, the invention may utilize optical equipment such as that described in the above-identified parent '748 application. In the following description, specific bus sizes, data words lengths, data sample times, clock speeds, and other such features are given by way of example only, and should not be construed to limit the scope of the invention.

The circuitry of the present invention produces guidance correction signals by processing reflected energy signals generated by a detecting and ranging transceiver. The exemplary implementation discussed below operates in conjunction with a LADAR system to process reflected light signals. As shown in FIG. 1, the invention includes certain pulse capture electronics 66.

The pulse capture electronics 66 initiate firing of a laser transmitter 86. In an illustrative implementation of the invention, the laser transmitter 86 may transmit one or more pulses of coherent, monochromatic light. In addition to initiating firing of the transmitter 86, the pulse capture electronics 66 determines the time-of-flight (i.e. range) and intensity of the returning laser pulses. The pulse capture electronics 66 includes a number of signal processing stages, such as an analog-to-digital comparator 68; a RAM buffer 72 clocked by a counter 90; an intelligent convolution circuit 74; and a peak detector or peak detect circuit 76. The pulse capture electronics 66 may also include an optional data buffer 70, which in an illustrative embodiment comprises an emitter-coupled logic (ECL) buffer. In the preferred embodiment, the above-mentioned stages of the pulse capture electronics 66 comprise a multichannel, monolithic, digital integrated circuit with an imbedded analog comparator front end.

The comparator 68 includes an avalanche photo diode 80, an amplifier 82, and a number of nonlinear comparators 84. The avalanche photo diode 80 converts light pulses received by optical equipment (not shown) into an electrical input signal; this signal is amplified by the amplifier 82 and directed to the comparators 84. If the laser transmitter 86 is configured to transmit modulated light signals, then a circuit such as a capacitive envelope detector (not shown) is interposed between the amplifier 82 and the comparators 84 to generate a signal representative of the envelope of the received modulated signal.

Multiple comparators 84 are provided, and each comparator 84 is provided with reference voltages spaced in logarithmic intervals over the range of expected voltage signals. In an illustrative embodiment, comparators 84 comprise seven flash converters, where the converters detect signals above 10 mV, 20 mV, 40 mV, 80 mV, 160 mV, 320 mV, and 640 mV, respectively. In the illustrative embodiment, seven comparators 84 are utilized to provide seven outputs, each output with a frequency of 1 GHz. Every 1 nS, each comparator 84 whose voltage threshold is exceeded turns on, such that the comparators 84 together produce a type of signal, known as a "thermometer code."

The output of the comparators 84 is received by the buffer 70. The buffer 70 converts the "thermometer code" into a 3-bit binary word, time de-multiplexes the signal to 125 MHz, and provides eight 3-bit words in parallel to the RAM 72. Each word supplied to the RAM 72 represents a 1 nS sample of data.

The timing of the RAM 72 is controlled by the counter 90, which may comprise a 125 MHz counter. The RAM 72 is initiated when the laser transmitter 86 is triggered by a firing line 88. After the RAM 72 is initiated, the RAM 72 stores data received from the buffer 70 while receiving a clocking signal from the counter 90. The clocking signal is continued for a period of time corresponding to the maximum range of the system, whereupon the RAM 72 stops storing data. The time of flight of the laser pulse to a target and back to the electronics 66 is relatively short compared to the rate at which pulses are transmitted. Therefore, the pulse capture electronics 66 of the present invention advantageously utilizes this fact by sampling at 1 GHz and then processing the stored samples more slowly, for example, at 20 MHz.

Between receipt and storage of the next transmitted pulse, the stored samples are read and processed by the intelligent convolution circuit 74 and the peak detect circuit 76. Specifically, the convolution circuit 74 reads an entire sample of data, wherein a "sample" includes all data corresponding to light detected during the clocking of the RAM 72. The convolution circuit 74 includes a linearize circuit 74a, to convert each of the seven binary levels back to the appropriate linear threshold level (i.e. 10 Mv, 20 mV, 40mV etc.). The linearize circuit 74a may comprise a programmable read only memory (PROM) equipped with an appropriate lookup table. The convolution circuit 74 also includes a matched filter 74b to extract range and intensity information from the data it receives, despite the noise and pulse-width variations of such data. The filter 74b may comprise a Zoran model ZR33891 finite impulse response circuit. To operate the filter 74b, it is loaded with a set of coefficients representative of the expected shape (or "template") of the return pulse. The filter 74b produces evaluation numbers indicative of the degree of correlation between the template and the return signal. The operation of convolution circuits is well-known in the relevant art, and an ordinarily-skilled artisan, having the benefit of this disclosure, would be capable of implementing the convolution circuit 74 in the present invention. The peak detect circuit 76 processes the evaluation numbers from the convolution circuit 74 to determine the point in time where a "best match" occurs between the return pulse and the template. By identifying the portion of the sampled return signal that best matches the stored template, the peak detect circuit 76 effectively determines the time at which the return pulse was received. The peak detect circuit 76 may be constructed with various standard transistor-transistor-logic (TTL) circuits.

In one embodiment of the invention, an external computing device (not shown) is utilized to decrease the time required to process signals received from the RAM 72. This is accomplished by the computing device executing a number of programming lines, the source code of which is shown in order of execution in an Appendix that is attached hereto and incorporated by reference. Specifically, the computing device effectively defines a "window" in time within which the return signal is found, and "locks" on to this window to reduce time spent searching for meaningful data. Such a computing device may comprise a Motorola model DSP56000 digital signal processor. In an exemplary implementation, the window may include 100 memory locations. FIG. 2 illustrates the operation of this windowing system in greater detail by depicting a routine comprising a group of tasks 200 performed by the convolution circuit 74. After the routine begins in task 202, task 203 establishes the location of the addresses in the RAM 72 where data concerning the return pulse will be stored. Then, task 204 reads the data corresponding to a return pulse from the RAM 72. Task 204 continues to read data samples from the RAM 72 until query 206 determines that a return pulse was found. After query 204 identifies a return pulse, task 208 stores the memory address of the beginning of the return pulse; then, in task 210, the convolution circuit 74 and peak detector read the entire return pulse and analyze it.

Then, task 212 averages the marked beginning of several previous return pulses; based on this average, task 212 predicts the memory address at which the next return pulse will begin. Task 212 takes this predicted memory address, and subtracts a selected number of memory locations (e.g. 50 memory locations) to define the beginning of a "window" within which it is likely that the next return pulse will be stored. After another laser signal has been transmitted and received, task 214 begins reading data from the RAM 72 beginning with the marked window value. If query 216 determines that no return pulse was detected in the defined window, the program returns to task 203 to re-initialize the RAM 72, and start over. However, if the return pulse is identified in query 216, task 218 stores the memory address of the beginning of the return pulse, and the program continues in task 210. The process of identifying a return pulse, marking a window to predict the memory address of the next pulse, and reading and analyzing the next return pulse, if one can be identified, is repeated throughout the operation of the pulse capture electronics 66.

The routine of FIG. 2, then, effectively "locks on" to the average return pulse, and quickly locates each return pulse based on this average window. By maintaining and constantly updating the window, the routine accounts for fluctuations in the return pulses, i.e. the earlier or later receipt of return pulses. This routine serves to limit return signal analysis to a particular region of interest, since data located outside of the window in the RAM 72 is ignored. This process takes advantage of the fact that changes in the distance between the hardware components of the invention and the target will be minimal from one pulse to the next, due to the high frequency at which the pulses are transmitted.

Another embodiment of the invention, shown in FIG. 3, utilizes real time return circuitry 300 to establish a unique window for each pulse and to decrease the time required to process signals received from the RAM 72. Specifically, in this embodiment, the pulse capture electronics 66 include a secondary path in the form of a real time return system 300 comprising an encoder 302 electrically connected to the comparators 84, an optional filter 304 electrically connected to the encoder 302, and a peak detector 306 electrically connected to the filter 304. The function of the real time return circuitry 300 is to identify the presence of a return pulse and its approximate location; the RAM 72 will contain the precise location of the pulse. The return pulse will be many samples wide, and in an exemplary implementation the width of the return pulse may be on the order of 10 or 20 memory locations. The secondary path of the circuitry 300 operates at a lower rate than the components 80, 82, 84, and 70.

In an illustrative implementation, the encoder 302 comprises a model F100165 ECL priority encoder, or similar circuit to transform the 7-bit "thermometer" input from the comparators 84 into a 3-bit binary output representative of the magnitude of the return signal. The filter 304 comprises a filter such as a F100181 ECL adder, or another similar filter capable of averaging two or more samples together. The peak detector 304 comprises a model F100166 ECL, and operates to detect the maximum amplitude of the return signal and to store the corresponding value of the counter 90 when the maximum return signal is received.

The counter 90 is initialized when the RAM 72 is activated. When the peak detector 306 identifies a signal that indicates the receipt of a return signal, the peak detector 306 stores the value of the counter 90. This value of the counter 90 defines a window around the location of the return pulse in the RAM 72. With this knowledge, the convolution circuit 74 is able to more efficiently analyze data from the RAM 72, since the convolution circuit 74 need not read extraneous data stored in the RAM 72 prior to receipt of the return pulse.

Another embodiment of the invention, shown in FIG. 4, includes a timing delay system or circuit 400 to decrease the required size of the RAM 72. Specifically, the circuit 400 delays storage of data until a return pulse is actually received, thereby conserving memory and placing each captured return pulse in the same region of memory.

The circuit 400 includes a delay circuit 402, which may comprise a coil of wire, fiber optic link, an arrangement of transistors, or another circuit for introducing a 20-30 nS delay into the signal produced by the amplifier 82. The circuit 400 also includes a comparator 404, which may comprise a circuit similar to the comparator 84. The circuit 400 additionally includes a peak detector 406, which operates to provide an output indicative of the largest magnitude signal detected received from the comparator 404. Unless it is reset, the output of the peak detector 406 is updated only if the peak detector 406 receives a signal having a larger magnitude than the previously detected peak. Whenever the peak detector 406 is updated with such a signal of larger magnitude, a one shot 408 is triggered, thereby providing an output pulse to the RAM 72. This pulse causes the RAM 72 to begin storing data received by the buffer 70; such storage continues until the pulse produced by the one shot 408 ends.

The circuit 400 therefore effectively "triggers" the RAM 72 to store data for a preselected time period, by selectively enabling the RAM 72 when a signal of sufficient magnitude is detected. The storage performed by the RAM 72 is clocked by the counter 90, and the preselected period is determined by the width of the pulse provided by the one shot 408. By establishing the width of the one shot pulse to be greater than the expected return pulse, the RAM 72 can be controlled to effectively store the entire return pulse. After storage of a return pulse is completed, the peak detector 406 is reset.

The present invention offers a number of advantages to its uses. For example, the windowing and real time return systems facilitate reduced processing times by enabling the convolution circuit 74 to more efficiently locate the return pulsed thereby eliminating time spent processing unimportant data. The real time return system prevents wasted data storage that would otherwise occur when receipt of a return signal is thwarted due to an event such as missile pitching, rolling, or passing by an object such as a building or cliff; this is possible, since the search of the RAM 72 is coordinated with detection of an actual return signal. One particular advantage of the timing delay system of the invention is that it requires less RAM than prior arrangements.

While there have been shown what are presently considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.