Non-cooperative automatic security screening with antennas for high cross-polarization discrimination
Methods and systems for detection of threats in secure areas are disclosed. Microwaves are transmitted into high traffic areas and are reflected off or transmitted through targets within that area. The resulting signals are detected at receiving antennas which are designed to have a high cross-polarization discrimination (XPD) such that co- and cross-polarizations of the resulting signals are separable for further processing. The receiving antennas of the present invention comprise elliptical antennas with a double-ridged waveguide on the interior and a conically-shaped exterior. This particular design for the receiving antennas allows to technologically obtain an XPD of about 30 dB or more for solid angles measured from a receiving antenna's boresight (the main lobe axis), and formed by rotating the corresponding 30-degree planar angle around the main lobe axis, the solid angles measuring approximately 0.84 sr, in a frequency range between 9.5 and 20 GHz.
1. A method for determining a presence of a threat on one or more targets in a security checkpoint, comprising:
transmitting microwave signals at one or more targets within a security checkpoint area, wherein said microwave signals reflect or transmit through one or more objects on said one or more targets, receiving said microwave signals via a means for cross polarization discrimination (XPD), said means for XPD separating orthogonal polarizations of said microwave signals,
calculating a dielectric permittivity value of one or more objects, said dielectric permittivity value corresponding to a variation in a microwave radiation reflected off or transmitted through said one or more objects, and
processing said orthogonal polarizations, thus forming a ratio of co- and cross-polarizations for said one or more objects,
said ratio of co- and cross-polarizations and said dielectric permittivity value corresponding to a presence or an absence of a threat in the security checkpoint.
2. The method of claim 1 , wherein said means for XPD comprises two or more antennas, each antenna comprising an elliptical waveguide and two ridges.
3. The method of claim 2 , said two or more antennas each further comprising a truncated conical exterior.
4. The method of claim 2 , said two or more antennas each further comprising a dielectric ring positioned around an open end of the elliptical waveguide.
5. The method of claim 2 , wherein said two ridges are positioned on a minor axis of the elliptical waveguide.
6. The method of claim 1 , wherein said means for cross-polarization discrimination comprises two or more antennas having a voltage standing wave ratio of less than 2 for a frequency range between 9.5 and 20 GHz.
7. The method of claim 1 , wherein said means for cross-polarization discrimination has a cross-polarization discrimination value of more than 30 dB throughout a range of frequencies between 9.5 and 20 GHz in a solid angle of 0.7-0.9 sr.
8. The method of claim 1 , wherein said means for cross-polarization discrimination has a cross-polarization discrimination value of more than 25 dB throughout the range of frequencies between 9.5 and 20 GHz in a solid angle of 1.4-1.6 sr.
9. The method of claim 2 , wherein said at least 2 antennas are positioned at 90-degree angles relative to each other.
10. The method of claim 1 , wherein the means for XPD comprises two or more antennas, each antenna being surrounded by an array of absorbing material.
11. The method of claim 2 , wherein the two or more antennas each comprises two semi-elliptical halves which are fastened together.
12. A system for determining a presence of a threat on one or more targets in a security checkpoint, comprising:
one or more transmitters for transmitting microwave signals at one or more targets within a security checkpoint area, wherein said microwave signals reflect or transmit through one or more objects on said one or more targets,
two or more receivers for receiving said microwave signals, said two or more receivers comprising a means for cross polarization discrimination (XPD), said means for XPD separating orthogonal polarizations of said microwave signals,
a first processor for calculating a dielectric permittivity value of one or more objects, said dielectric permittivity value corresponding to a variation in a microwave radiation reflected off or transmitted through said one or more objects, and
a second processor for comparing said orthogonal polarizations of said microwave signals, thus forming a ratio of co- and cross-polarizations for said one or more objects,
said ratio of co- and cross-polarizations and said dielectric permittivity value corresponding to a presence or an absence of a threat in the security checkpoint.
13. The system of claim 12 , wherein said means for XPD comprises two or more antennas, each antenna comprising an elliptical waveguide and two ridges.
14. The system of claim 13 , said two or more antennas each further comprising a truncated conical exterior.
15. The system of claim 14 , said two or more antennas each further comprising a dielectric ring positioned around an open end of the elliptical columnar body.
16. The system of claim 12 , wherein said means for cross-polarization discrimination has a cross-polarization discrimination value of more than 30 dB throughout a range of frequencies between 9.5 and 20 GHz in a solid angle of 0.7-0.9 sr.
17. The system of claim 12 , wherein said means for XPD comprises at least 2 antennas, each antenna comprising an elliptical waveguide and two ridges.
18. The system of claim 17 , wherein said at least 2 antennas are positioned at 90-degree angles relative to each other.
19. The system of claim 12 , wherein each receiver comprises an antenna surrounded by an array of absorbing material.
20. The system of claim 12 , wherein said means for cross-polarization discrimination has a cross-polarization discrimination value of more than 25 dB throughout the range of frequencies between 9.5 and 20 GHz in a solid angle of 1.4-1.6 sr.