Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
1991-09-18
2003-06-03
Hellner, Mark (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S028500, C342S132000
Reexamination Certificate
active
06573982
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an improvement in laser radar systems and more particularly, but not by way of limitation, to a method and arrangement for compensating for frequency jitter in a laser radar system by using a double-sideband chirped modulator/demodulator system.
2. Description of the Prior Art
Because of its resistance to jamming and interference from outside sources, and its superior range and angular accuracy, light has replaced electromagnetic energy in many applications including communications and measuring systems. Laser rangefinder techniques have been successfully shown to determine the ranges of targets at distances up to five miles with accuracy of 2 to 10 meters. As with conventional radar systems, laser systems may be classified into two basic categories: (1) direct, or incoherent detection, and (2) heterodyne or coherent detection.
The theory of the former dictates that the best signal-to-noise ratio is provided when the transmitted energy is concentrated into the shortest possible pulse. This yields a good range measuring and resolution capability. On the other hand, coherent detection requires highest possible average transmitted power for the best signal-to-noise ratio, irrespective of pulsed width. Also, accurate target radial velocity measurements can be obtained in the latter case.
It has long been known that radar resolution and accuracy are functions of the signal bandwidth, being AM or FM in nature, regardless of the transmitter waveform. Thus, a continuous power, or long pulse mode of operation heterodyne system, may also yield good range measuring and resolution capability when using a wideband signal. One technique for modulating a wideband signal is to frequency modulate the long pulse, and this has been termed a “chirped” signal and the associated receiver includes matched delay vs. frequency components to compress the return pulses. A more complex receiver is required to extract the wide band information available from this type of reflected signal, as opposed to the direct detection system. These receivers, as used in conventional radar systems, are designated as matched-filter signal processing receivers. The advantages of such signal processing techniques in radars are:
1. More efficient use of the average power available to the transmitter.
2. Increased system accuracy, both in the ranging and velocity measurements.
3. Reduction of jamming vulnerability.
Also in the past, a laser range finder using heterodyne detection and chirp pulse compression has been described. U.S. Pat. Nos. 4,662,741 and 4,666,295 describe such a system as a laser range finder using a wideband signal consisting of a linear FM chirp pulse of relative long duration. The matched filter at the receiver end is a Surface Acoustic Wave (SAW) device, which compresses the relative long FM chirp pulse into a narrow one (of the same bandwidth), from which the range and velocity information may be extracted. The duration of the compressed pulse is approximately the inverse of the bandwidth of the original signal. Thus, as the amount of frequency that is chirped increases, so does the resolution of the range and velocity measurements.
However, current radar systems which utilize single-sideband linear FM chirp modulation to provide target range measurements are susceptible to frequency jitter effects which are small random shifts in the return signal frequency produced by target/platform motion, laser frequency instabilities, or rapid beam scanning in the presence of diffuse target-induced speckle. Speckle relates to the problem of signal fluctuations of an imaging laser beam when reflected from an extended rough target surface. The dynamic speckle generated by the scan mechanism introduces additional frequency broadening which directly effects range precision of the system. Physically the signal fluctuations are caused by random constructive and destructive interference of wavelets from various scatterers on the target surface as the transmitted beam moves across the target surface during the finite period of receiver signal integration of each data pixel. In a radar system that uses a linear frequency modulation (FM) chirp followed by pulse compression at the receiver, the transmitted pulse width is normally a few microseconds during which the laser beam moves a substantial distance on the target surface. Substantial speckle phase variations are introduced in the received signal due to the relatively long dwell time of a single pixel. This effect becomes the dominant determinate of range precision for a high speed scanning system. Thus, it is apparent that a need exists for frequency jitter compensation in laser radar systems which utilize linear FM chirp modulation.
SUMMARY OF THE INVENTION
In accordance with the present invention, the range precision for long transmitter pulses such as for a linear FM chirp waveform is materially improved by transmitting an up and down linear chirp pair simultaneously and using a proper demodulation process. Thus, the linear portion of the speckle phase variations is automatically compensated and the range precision of the radar system is significantly improved.
An arrangement for accomplishing the above involves applying a double-sideband linear chirped waveform to a laser transmitter beam for frequency modulation accordingly. One sideband is a frequency up-chirp and the other is a frequency down-chirp. The modulation of transmitter may be by a suitable electro-optic device, such as a Bragg cell. The two chirped sidebands are preferably generated by employing up- and down-chirp surface acoustic wave (SAW) dispersive filters or by ramping two voltage controlled oscillators (VCO's). The reflected return beam is detected ,after mixing with a local oscillator laser beam, and separated into two channels corresponding to each sideband and are then demodulated. Demodulation is preferably accomplished by using SAW dispersive delay filters to generate the compressed pulses.
Each of the demodulated pulses will be affected by frequency jitter, as described above, except that a frequency shift which advances the pulse in one channel will delay the pulse in the other channel by exactly the same amount. The compressed pulses are then input into suitable standard peak detection circuits for range-to-pixel measurements. Frequency jitter compensation is obtained by averaging the peak-detection readings from the two channels for each pixel. In addition, simultaneous, single pixel doppler information is extracted by taking the difference in the readings for the two channels.
Other features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description constructed in accordance with the accompanying drawings and wherein:
REFERENCES:
patent: 3549256 (1970-12-01), Brienza et al.
patent: 4096478 (1978-06-01), Chavez
patent: 4666295 (1987-05-01), Duvall, III et al.
patent: 4743110 (1988-05-01), Arnaud et al.
patent: 4960324 (1990-10-01), Schofield
Alkov Leonard A.
Hellner Mark
Lenzen, Jr. Glenn M.
Raufer Colin M.
Raytheon Company
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