Threat detection system

1. A first system for detecting a structural change to a structure, comprising:

an exciter positioned at a first location of the structure to transmit an excitation signal that is based on a reference signal and to apply the excitation signal to the structure;

an accelerometer positioned at a second location of the structure to sense the excitation signal after it has propagated to the second location and outputs an electrical signal based on seismic activity applied to the structure by the excitation signal;

a signal processing component that is synchronized with the reference signal and receives the electrical signal, the signal processing component configured to use the reference signal to extract portions of the electrical signal that are synchronized with the reference signal, and to output phase and amplitude values of the extracted portion; and a computer that is operatively connected to the signal processing component to evaluate the output phase and amplitude values using pre-determined threshold values that indicate a change in the structure.

2. The first system of claim 1 , wherein the first system communicates with a second system for detecting a structural change by at least one of:

sensing at the second location an excitation signal transmitted by the second system from a third location that is remote from the first system, and

transmitting an excitation signal at the second location for receipt by the second system at a fourth location that is remote from the first system.

3. The first system of claim 2 , wherein at least one of the first system and the second system exchange data with a host computer 150 by a communication link, wherein at least one of:

the first system receives the data from the second system and transmits it to the host computer 150 , and

the first system receives the data from the host computer 150 and transmits it to the second system.

4. The first system of claim 1 , further comprising a modulator to modulate the exciter signal with a first modulation scheme and the modulated exciter signal is applied to the structure, and a demodulator to demodulate the phase and amplitude values using the first modulation scheme.

5. The first system of claim 1 , wherein the signal processing component includes a lock-in amplifier.

6. The first system of claim 1 , wherein the excitation signal is a subliminal signal.

7. The first system of claim 1 , wherein detected change in structure is associated with at least one of structural damage or deterioration from use.

8. A method for detecting a structural change to a structure, comprising:

applying at a first location of a structure an excitation signal that is based on a reference signal to the structure;

sensing the excitation signal at a second location of the structure;

outputting a modulated signal based on seismic activity applied to the structure by the excitation signal;

extracting from the modulated signal using the reference signal portions of the modulated signal that are not synchronized with the reference signal;

outputting phase and amplitude values of the extracted portion; and

evaluating the output phase and amplitude values to a plurality of structural damage signals indicative of structural damage stored in a database; and

determining, based on the comparison, whether the modulated signal is indicative of a change in the structure.

9. The method of claim 8 , further comprising:

modulating the exciter signal with a first modulation scheme and the modulated exciter signal is applied to the structure; and

demodulating the phase and amplitude values using the first modulation scheme.

10. The method of claim 8 , further comprising:

comparing the determination whether the modulated signal is indicative of a change in the structure with a second determination whether the modulated signal is indicative of a change in the structure that is based on a second excitation signal; and

determining whether to activate an alarm indicating an occurrence of change in the structure based on the comparison with the second determination.

11. The method of claim 8 , further comprising:

categorizing the phase and amplitude values with an indication of whether the phase and amplitude are indicative of a structural change; and

storing the phase and amplitude values in the database in association with the categorization.