Hybrid optical correlator/digital processor for target detection and discrimination

#N/A

#Coherent long-range laser radar (LADAR) systems rely on externally supplied data to cue the RF receiver processor to examine received data at a known time and frequency. Such externally supplied data can include target angle sensor data, target range, target range rate, and target velocity. This reliance on externally supplied target data reduces the utility of this type of long range LADAR as a stand-alone autonomous system. The externally supplied data is needed in these types of systems to limit the volume of space to be searched for long-range fast-moving targets to keep the amount of data to be stored and processed to a manageable level.

#In addition, LADAR systems having an all range capability would require multiple pulses to overcome speckle using pulse tone acquisition waveforms. Speckle occurs due to the surface roughness of the target having a greater dimension than the wavelength of the laser radar used to detect the target. This leads to interference in which a speckle null may occur and a target missed or lost until the target orientation relative to the the LADAR receiver has changed sufficiently to shift the interference from the null. Speckle is typically overcome by the use of multiple pulses of laser radar energy to ensure that the probability of detection of a target is nearly 100%.

#The invention will be more fully understood by reference to the following Detailed Description of the Invention in conjunction with the Drawing of which:

#FIG. 1 is a block diagram of one embodiment of a hybrid optical spectrum analyzer/correlator for an all range coherent laser radar;

#FIG. 2 is a more detailed block diagram of the acousto-optical spectrum analyzer depicted in FIG. 1; and

#The disclosure of U.S. Provisional Patent Application No. 60/351,556 is hereby incorporated by reference.

#FIG. 1 depicts a block diagram of an embodiment of a hybrid processor 100 using an analog optical spectrum analyzer/correlator and digital processing. The hybrid processor 100 receives a laser radar (“LADAR”) optical return signal and the optical receiver 102 detects the LADAR optical return signal and provides the detected optical signal to a mixer 104. The mixer 104 also receives a first mixer signal 103 and up-converts the detected optical return signal to an up-converted signal having a first frequency. In the embodiment depicted in FIG. 1, the first frequency is 7.5 GHz. The up-converted signal is filtered using a matched filter 106 having a bandwidth that is a first order matched filter based upon the whole body Doppler shift of the target, the pulse shape and the pulse width. The pulse shape and pulse width are dependent upon the transmitter characteristics and the estimate of the whole body Doppler shift of the target. As will be explained in more detail below, the whole body Doppler shift is estimated and provided by hybrid processor described herein.

#After being estimated by the hybrid processor the matched filter 106 is adjusted appropriately to remove the whole body Doppler shift, leaving only the target rotational Doppler information. The filtered up-converted signal is mixed in mixer 108 with a second mixer signal 105 to down-convert the up-converted signal to a baseband signal having a second frequency. In the embodiment illustrated in FIG. 1 the second frequency is 500 MHz. The base band signal is split at power splitter 109 with a first output resolved into in-phase and quadrature-components (“I&Q components”) by I&Q module 110. This down-conversion and resolution into I&Q components cuts the digital sampling rate that is required in half, resolves frequency fold-over and removes the imaginary value image. The range of the target determines the sampling rate that is needed. In one embodiment, the sampling rate is 1 Gigasample/sec. As will be discussed in more detail below, the estimate of the target range is provided by the optical spectrum analyzer/correlator described herein. The I&Q components are sampled, and digitized by analog-to-digital converter 112 and stored in FIFO buffer 114. DSP 116 is coupled to FIFO buffer 114 can control the FIFO buffer 114 and retrieve data therefrom as well. As will be explained below, the DSP 116 is also coupled to the optical processor 130 and receives from the optical processor 130 a cue signal indicating the presence of a target, the estimate of the target range, and the target Doppler shift. The DSP 116 is responsive to the cue signal by retrieving and analyzing the data stored in the FIFO buffer 114 to determine preselected target characteristics, such as the target velocity, range, range rate, and rotational velocity.