Method and apparatus for through-the-wall motion detection using CW radar
View Patent ↗A CW radar ( 10 ) is used to detect motion of objects ( 22 ) behind a wall ( 20 ) by projecting a radar beam through the wall and by measuring the returns from the objects behind the wall, with a change in the phase difference between the transmitted and received CW signals providing an indication of motion behind the wall and thus the presence of an individual.
1. A method for detecting the presence of an individual behind a wail, comprising the steps of:
projecting CW radar signal through a wall;
detecting portions of the CW radar signal returned by an object behind the wall;
determining the phase difference between projected and returned CW radar signals to generate a phase difference signal;
sampling the phase difference signal;
thresholding the sampled signal; and,
detecting when the sampled signal exceeds the threshold, thus to indicate the presence of a moving individual, whereby individuals moving behind the wall can be detected.
2. The method of claim 1 , wherein the frequency of the projected signal is constant.
3. The method of claim 2 , wherein the frequency is in a 900 MHz band.
4. The method of claim 1 , wherein the threshold is an adaptive threshold.
5. The method of claim 1 , and further including the step of determining the location of a moving individual behind the wall from peaks in a graph of phase difference versus distance.
6. Apparatus for the detection of a moving individual behind a wall, comprising:
a frequency source;
a power divider coupled to said frequency source for outputting as a first output a CW signal of one predetermined magnitude for forming a radar beam and for outputting as a second output a CW signal of a diminished magnitude;
a circulator coupled to said first output;
an antenna coupled to said circulator for transmitting a CW radar beam towards said wall and for detecting radar returns from objects behind said wall;
a mixer coupled to said second output and said circulator for deriving a phase difference signal representing the phase difference between transmitted and returned signals at said antenna;
a sampling circuit for sampling the said phase difference signal;
a thresholding circuit for thresholding said sampled signal;
a detector for detecting when said sampled signal exceeds said threshold, thus to indicate the presence of a moving object behind said wall; and,
a transmitter for transmitting the fact of the detection of a moving object to a remote location.
7. The apparatus of claim 6 , wherein said detector includes a processor for sampling the output of said mixer, a threshold detector for ascertaining when said change in phase difference of the output of said mixer exceeds an adaptive threshold, and a motion indicator responsive to the output of said threshold detector for the presence of a moving object behind said wall.
8. The apparatus of claim 7 , wherein the frequency of said transmitted signal is constant, thus to provide a single-frequency CW radar.
9. The apparatus of claim 8 , wherein said single frequency is in a 900 MHz band.
10. The apparatus of claim 6 , wherein said detector includes a threshold detector having an adaptive threshold detector.
11. The apparatus of claim 6 , wherein said detector detects a moving object behind said wall based on a change in the phase difference between the transmitted and returned signals.
12. The apparatus of claim 11 , wherein said phase difference is sensed as a change in a graph of phase difference versus time.
13. The apparatus of claim 6 , wherein said antenna is a directional antenna having minimal back lobes to prevent any motion behind said antenna from affecting said phase difference.
14. The apparatus of claim 13 , wherein said antenna is a YAGI antenna.
15. The apparatus of claim 13 , wherein said antenna is a planar antenna having conductive elements spaced from a ground plane.
16. A system of determining the presence of an individual behind a wall, comprising:
a CW radar having a directional antenna adapted to project CW radar energy through said wall, and detecting returns from objects behind said wall;
a phase difference detector for detecting when the phase difference between CW energy directed through said wall and energy returned from objects behind said wall exceeds a predetermined threshold indicating the presence of a moving object behind said wall,
a transmitter for transmitting the fact of threshold exceedance to a remote location.
17. The system of claim 16 , wherein said CW radar operates in a 900 MHz band.
18. The system of claim 16 , wherein said CW radar is a single-frequency radar, whereby no clutter rejection is necessary and no long integration times are required.
19. The system of claim 16 , wherein said directional antenna has minimal back lobes to reject motion behind said antenna so that behind-the-antenna motion is not detected.