Communications: radio wave antennas – Antennas – With polarization filter or converter
Reexamination Certificate
1999-11-12
2001-08-21
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
With polarization filter or converter
C343S705000, C342S029000, C342S188000
Reexamination Certificate
active
06278409
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority from Israel Patent Application No. 123626 filed on Mar. 11, 1998 in Israel and entitled “Wire detection system and method” and from the PCT patent application No. PCT/IL99/00139 filed on Mar. 11, 1999 with the same title and claiming priority from the abovementioned Israel application.
FIELD OF THE INVENTION
The present invention relates to systems for detection of wires, and more particularly to such systems for detecting wires using polarized radio waves.
Heretofore, various systems were devised to detect suspended wires, which form an obstacle for helicopters and for low flying light aircraft. Various wires include high voltage power cables, medium voltage cables, telephone cables and more.
Helicopters may collide with these wires, with fatal consequences. The problem is that it is difficult to see wires from the air, on the dark background of the ground. This is difficult at daytime in a good weather. It is impossible to see wires at night or in bad weather.
Suspended wires are more dangerous to helicopters than other ground obstacles. Ground obstacles usually have a relatively small width and height, whereas wires are located higher and span a large width, so the danger of collision with wires is much higher. Therefore, it is important to distinguish suspended wires from other ground reflectors and to warn the pilot accordingly.
Prior art sensor systems apparently do not detect wires effectively. These include, for example, millimeter wave radar, laser radar, FLIR and more. These prior art systems are complex, heavy and costly and only achieve a limited success in detecting wires. There is a need for a light weight, simple structure system for wire detection and pilot warning.
Following is a description of prior art systems for wire detection.
Thurlow, U.S. Pat. No. 5,264,856, discloses a system and method for detecting radiant energy reflected by a length of wire. The system has two antennas that transmit and receive at two fixed polarizations that are orthogonal to each other. The system measures three values of echo return: Svv, Svh and Shh, that correspond to all the combinations of transmit and receive at a horizontal (h) and vertical (v) polarization. The system then computes the polarization of the target based on the above three values. The result is presented on a Poincare Sphere, wherein circular polarization nulls lay on the poles of the sphere, whereas linear polarization nulls lay on the equator of the Sphere. Where two nulls coincide and are located on the equator of the Sphere, this is an indication of a wire target.
The system in Thurlow does not actually measure the echoes in a polarization parallel to a wire and normal to the wire; rather, it uses two fixed polarizations for transmit and receive, and uses a rather complex calculus to evaluate the polarization of the wire (if indeed there is a wire).
A possible problem in Thurlow is the effect of ground clutter. Ground clutter, like a radar target, has a specific scattering matrix that may also be represented as (Svv, Svh, Shh). Actually, the system in Thurlow measures the sum of clutter and wire in each dimension of the matrix. As each component of the matrix is corrupted with clutter, the location of the resulting nulls may move randomly over the surface of the Poincare Sphere. The ground return (clutter) has a complex value (I, R) for each of the three components of the scattering matrix; each component is unrelated to the scattering from a wire located in that area. Therefore, a gross error in the null location may result.
Furthermore, the clutter return is not fixed. The scattering matrix for ground clutter is a statistic value with one or more parameters. Thus, the values in the scattering matrix (Svv, Svh, Shh) may vary from pulse to pulse, with corresponding variations in the computed nulls.
It may be difficult, under the circumstances, to have a null on the desired equator of the Sphere; to have two nulls may be more difficult still.
One way to reduce the influence of clutter is to increase the resolution of the radar. Indeed, Thurlow indicates using range intervals of 10-30 meters that are further divided into 32-128 range gates. An interval of 10 meter divided to 128 will result in a range gate of about 8 cm (centimeter), that would require a radar of a wavelength of 0.8 cm or less. These waves may not be capable of detecting real world wires using the polarization effect.
It is known that wires only reflect a linear polarization when the wavelength of the waves is significantly longer than the diameter of the wires. Preferably, the wavelength should be more than about 3 times the wire diameter. For this case, the “thin wire” approximation applies, that is the wire does not reflect waves at a polarization normal to the wire.
Electrical wires now in use may have a diameter of about 1 inch (about 2.5 cm). The radar in Thurlow, with a wavelength of about 0.8 cm or less, will receive echoes in both polarizations, so it may not detect a linear polarization indicative of wires.
Thurlow gives no indication nor suggestion regarding the frequency of operation of a radar that has to detect wires based on polarization discrimination. There is no suggestion on how to concurrently achieve two opposite goals, that is a high radar resolution and long wavelength for stimulating the linear polarization characteristics in real world wires.
The clutter problem is further aggravated in Thurlow by the fact that the wire orientation is usually different than that of the transmitted and received waves. Thus, whereas the system transmits horizontally- and vertically polarized waves, a wire may have an oblique orientation. This may be a result of the geography of the place, or of a maneuvering aircraft.
For example, with a wire at an angle of 20 degrees to the horizontal antenna of the system, for a horizontally polarized transmit wave E, the component parallel to the wire will be E*cos(20). The wave reflected off the wire will be at 20 degrees, so it will again be attenuated when received at the horizontal antenna by the factor cos(20). Thus, the wave received at the radar for a horizontal/horizontal case, is attenuated by cos(20){circumflex over ( )}2. This is the electrical component of the wave; the received power is attenuated by the square of that value, that is by a factor of cos(20){circumflex over ( )}4. Here, “{circumflex over ( )}4” denotes the fourth power of the value it succeeds.
It is known that cos(20)=0.94 (all the values here are approximations). Therefore, cos(20){circumflex over ( )}4=0.78 or 1.1 dB.
Thus, the component Shh is attenuated by a factor of 1.1 dB relative to that of a horizontal wire.
Similarly, the component Svv will be attenuated by cos(70){circumflex over ( )}4 relative to a vertical wire, and the component Svh is attenuated by (cos(20)*cos(20)) 2.
Thus, in this example, the loss or attenuation in the matrix components will be:
Shh is reduced by 1.1 dB
Svv is reduced by 18.6 dB
Shv is reduced by 9.9 dB
To detect a target in clutter (a Yes/No decision) a S/C ratio of about 14 dB is required. To compute spatial orientation of vectors, a better ratio may be required, otherwise the direction may be in gross error. Assuming a S/C ratio of 20 dB, to reliably measure the Shv component the radar has to achieve a S/C of 30 dB or better; for the Svv component, an S/C ratio of 38 dB or better is required.
Therefore, to measure these components, an exceptionally high signal/clutter ratio may be required. This may be difficult to achieve because of the abovedetailed bandwidth limitation or other practical limitations. In a practical implementation, therefore, the computation in Thurlow may be largely based on clutter rather than the wire returns, so that the location of the computed nulls may be in gross error.
Wires may not be detected, or clutter may erroneously give false alarms.
A complex data processing may be required, to map radar echoes into a tri-dimensional Poincare Sphere. Moreover, tri-dimensional
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