Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2002-10-23
2004-03-02
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S442000, C342S444000, C342S445000
Reexamination Certificate
active
06700536
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to a system for determining a direction of incident electromagnetic signals. More particularly, this invention is directed to a system for determining the angle of arrival of a radio frequency signal, in one or two dimensions, utilizing a pair of simple wideband broadbeam spaced apart antennas in a receiver which includes an analog spectral separator. Further, this invention directs itself to analog spectral separation schemes utilizing narrow band filtering of radio frequency or intermediate frequency signals that are digitized utilizing analog to digital converters having a relatively low sampling rate. Still further, this invention is directed to a method for determining a direction of incident electromagnetic signals where received signals from a pair of antennas are processed by analog circuits to provide a plurality of signal samples that provide a frequency domain representation of the received signal from which phase difference information can be extracted and utilized to compute an angle of arrival for the received signal.
2. Prior Art
Many applications require the ability to determine the location of an emitter of electromagnetic signals, especially those associated with the military. Military aircraft, for example, have a need to sense the radio frequency environment for real-time defensive and offensive purposes. The direction from which an attack may be coming is critical information required by the pilot, which may be ascertained by identifying the direction from which a particular radar beam is being emitted. A number of methods for determining the angle of arrival for such radio frequency signals have been developed. One such method is the triangulation of coordinated receiver data from multiple widely displaced aircraft. For individual aircraft, several techniques have been developed, including an amplitude-difference approach, an interferometer approach, and a time difference of arrival approach. When simple broadbeam wideband antennas are installed, these approaches generally determine the angle in just one dimension. Most commonly, the antennas are separated horizontally, and often the measured angle of arrival is treated as being equal to the azimuth angle based on the assumption that the signal originated near the horizon, making the elevation angle approximately zero.
The amplitude-difference angle of arrival measurement approach is a commonly used approach for military fighter aircraft because it is relatively simple and inexpensive to implement. The amplitude-difference approach is based on the utilization of antenna gain variation as a function of angle of arrival. Such a system makes its angle of arrival determination by comparing the intercepted amplitude at each of the plurality of antennas disposed around the aircraft, and deducing the angle that must have caused the amplitude ratios therebetween. The problem with the amplitude-difference approach is that it is not very accurate because of calibration difficulties. These systems need to operate on a wideband basis because the radar that they are intended to intercept can be operating at any frequency across a wide range of possibilities. Although normally the processing is independent of carrier frequency, since the amplitude measurements are usually based on the signals (video) envelope (the carrier having been stripped off), calibration issues create an indirect frequency dependence. The antenna patterns vary considerably with frequency, and although calibration lookup tables are helpful, they are difficult to determine utilizing ground measurements and vary from individual aircraft to aircraft and may even vary based on the varying weapons configuration of each aircraft. Gain variations as a function of the unknown elevation angle also introduces significant errors.
The interferometer approach, while providing angle of arrival measurements that have high resolution and accuracy, are expensive and usually impractical to mount on a fighter aircraft. Such systems are usually limited to a larger intelligence gathering aircraft. The multiple antennas of the system are physically separated, and for long-baseline interferometers, the sensors are especially spaced far apart. Due to the physical separation of the antennas, the propagating wave's carrier will generally have a unique phase angle for each individual antenna location. The receiver measures this carrier phase for the received pulse and passes the phase data to a central processing unit. The central processing unit takes the difference of the phase, multiplied by a constant, and uses trigonometry to solve for the angle of arrival. The constant utilized in the calculation depends on the distance between the sensors, the speed of propagation of the wavefront and the frequency of the carrier. For the interferometer approach, the angle of arrival of resolution is proportional to the antenna separation. However, because the carrier phase angles have a modulo 360° characteristic, an antenna system with a long baseline, operating in the frequency bands of interest, will not provide a unique solution. For example, such a system that measure 10° difference between the signals intercepted by the two antennas, would not know whether the true additional propagation delay corresponded to 10°, 370°, 730° or any other combination of 10° plus a multiple of 360°. Thus, to get a unique solution, the interferometer approach requires additional antennas at precise locations between the two furthest-apart antennas. The antenna locations are precisely selected so that, for any frequency in the band of interest, the set of phase differences provides a unique solution for the angle of arrival. A typical interferometer system will have from three to five antennas to cover the field of view, depending on the operating bandwidth and center frequency, the greater the bandwidth, the more antennas needed to resolve the ambiguities. Further, the multiple antennas are usually pointed in the same direction, and the overlapping beam widths are made as wide as possible, sometimes approaching 180°. The number of antennas required for the interferometer approach creates many difficulties for application on fighter aircraft. Whereas a quadrant amplitude-difference approach would use just four 90° beamwidth antennas, the interferometer approach might need fifteen antennas (five 120° beamwidth antennas for each of three fields of view). The number of antennas and cabling required therefor also adds to the complexity and difficulty in calibrating such a system.
The time difference of arrival approach determines the angle of arrival by measuring the time difference between when the RF wavefront strikes two antennas. The approach is generally implemented with a high-speed counter that measures the time between threshold crossings of the envelope of the signals intercepted by the two antennas. The envelopes, or video signals, are generated by wideband detectors which rectify the RF carrier voltage. The angle of arrival is calculated utilizing conventional mathematical and trigonometric processing. Since the time difference of arrival approach uses the envelope, stripping off the carrier from the signal, the approach has the advantage that the processing normally does not depend on the carrier frequency, unlike the interferometer approach. A fixed envelope-delay error can generally be calibrated out easily at any one carrier frequency, with the calibration result being good for the whole band. However, like the amplitude-difference approach, there is an indirect-frequency dependence via frequency-dependent gain differences in the antenna and other front-end components. While the time difference of arrival approach would have good resolution for acoustic or vibrational waves, the propagation speed of electromagnetic waves are so fast that it is difficult to count correspondingly fast to get adequate resolution. The antenna spacing limitations on military aircraft requires a resolution better than one n
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