Analyzing a target electromagnetic signal radiating from a computer system

1. A method for characterizing a computer system parameter by analyzing a target electromagnetic signal radiating from the computer system, the method comprising:

monitoring the target electromagnetic signal using a first directional antenna located outside of the computer system, wherein the first directional antenna is directed at a location inside the computer system;

monitoring the target electromagnetic signal using a second directional antenna located outside of the computer system, wherein a receiving axis of the second antenna is oriented non-parallel to a receiving axis of the first antenna, and wherein the second directional antenna is directed at the location inside the computer system;

characterizing the computer system parameter based on the target electromagnetic signal received from the first antenna and the target electromagnetic signal received from the second antenna, wherein characterizing the computer system parameter comprises:

generating a predetermined number of first directional antenna signals based on the target electromagnetic signal monitored using the first directional antenna; and

generating a predetermined number of second directional antenna signals based on the target electromagnetic signal monitored using the second directional antenna; and

generating a request for an action based on the characterization of the computer system.

2. The method of claim 1 wherein:

generating the predetermined number of first directional antenna signals includes dividing the target electromagnetic signal monitored using the first directional antenna into a predetermined number of first directional antenna frequency signals; and

generating the predetermined number of second directional antenna signals includes dividing the target electromagnetic signal monitored using the second directional antenna into a predetermined number of second directional antenna frequency signals.

3. The method of claim 2 wherein:

each first directional antenna frequency signal in the predetermined number of first directional antenna frequency signals is at a different frequency than each other first directional antenna frequency signal in the predetermined number of first directional antenna frequency signals; and

each second directional antenna frequency signal in the predetermined number of second directional antenna frequency signals is at a different frequency than each other second directional antenna frequency signal in the predetermined number of second directional antenna frequency signals, wherein each second directional antenna frequency signal is at the same frequency as a first directional antenna frequency signal.

4. The method of claim 2 , wherein characterizing the computer system parameter includes using a nonlinear, nonparametric regression model.

5. The method of claim 1 wherein:

characterizing the computer system parameter includes processing the target electromagnetic signal monitored using the first directional antenna and the target electromagnetic signal monitored using the second directional antenna using a coincidence detection analysis technique.

6. The method of claim 5 wherein the coincidence detection analysis technique uses an independent component analysis technique.

7. The method of claim 1 , wherein prior to monitoring the target electromagnetic signal using the first directional antenna and prior to monitoring the target electromagnetic signal using the second directional antenna, the method further comprises:

monitoring a reference electromagnetic signal radiating from the computer system;

generating a reference electromagnetic-signal fingerprint from the reference electromagnetic signal; and

building a pattern-recognition model based on the reference electromagnetic-signal fingerprint.

8. The method of claim 1 , wherein characterizing the computer system parameter includes using a sequential probability ratio test (SPRT).

9. The method of claim 1 , wherein the first directional antenna includes a first cantenna and the second directional antenna includes a second cantenna.

10. A computer-readable storage medium storing instructions that when executed by a computer cause the computer to perform a method for characterizing a computer system parameter by analyzing a target electromagnetic signal radiating from the computer system, the method comprising:

monitoring the target electromagnetic signal using a first directional antenna located outside of the computer system, wherein the first directional antenna is directed at a location inside the computer system;

monitoring the target electromagnetic signal using a second directional antenna located outside of the computer system, wherein a receiving axis of the second antenna is oriented non-parallel to a receiving axis of the first antenna, and wherein the second directional antenna is directed at the location inside the computer system;

characterizing the computer system parameter based on the target electromagnetic signal received from the first antenna and the target electromagnetic signal received from the second antenna, wherein characterizing the computer system parameter comprises:

generating a predetermined number of first directional antenna signals based on the target electromagnetic signal monitored using the first directional antenna; and

generating a predetermined number of second directional antenna signals based on the target electromagnetic signal monitored using the second directional antenna; and

generating a request for an action based on the characterization of the computer system.

11. The computer-readable storage medium of claim 10 wherein:

generating the predetermined number of first directional antenna signals includes dividing the target electromagnetic signal monitored using the first directional antenna into a predetermined number of first directional antenna frequency signals, wherein each first directional antenna frequency signal in the predetermined number of first directional antenna frequency signals is at a different frequency than each other first directional antenna frequency signal in the predetermined number of first directional antenna frequency signals; and

generating the predetermined number of second directional antenna signals includes dividing the target electromagnetic signal monitored using the second directional antenna into a predetermined number of second directional antenna frequency signals, wherein each second directional antenna frequency signal in the predetermined number of second directional antenna frequency signals is at a different frequency than each other second directional antenna frequency signal in the predetermined number of second directional antenna frequency signals, wherein each second directional antenna frequency signal is at the same frequency as a first directional antenna frequency signal.

12. The computer-readable storage medium of claim 11 , wherein characterizing the computer system parameter includes using a nonlinear, nonparametric regression model.

13. The computer-readable storage medium of claim 10 wherein:

characterizing the computer system parameter includes processing the target electromagnetic signal monitored using the first directional antenna and the target electromagnetic signal monitored using the second directional antenna using a coincidence detection analysis technique.

14. The computer-readable storage medium of claim 13 wherein the coincidence detection analysis technique uses an independent component analysis technique.

15. The computer-readable storage medium of claim 10 , wherein prior to monitoring the target electromagnetic signal using the first directional antenna and prior to monitoring the target electromagnetic signal using the second directional antenna, the method further comprises:

monitoring a reference electromagnetic signal radiating from the computer system;

generating a reference electromagnetic-signal fingerprint from the reference electromagnetic signal; and

building a pattern-recognition model based on the reference electromagnetic-signal fingerprint.

16. The computer-readable storage medium of claim 10 , wherein characterizing the computer system parameter includes using a sequential probability ratio test (SPRT).

17. The computer-readable storage medium of claim 10 , wherein the first directional antenna includes a first cantenna and the second directional antenna includes a second cantenna.

18. An apparatus that characterizes a computer system parameter by analyzing a target electromagnetic signal radiating from the computer system, the apparatus comprising:

a first directional antenna located outside of the computer system, wherein the first directional antenna is directed at a location inside the computer system;

a second directional antenna located outside of the computer system, wherein the second directional antenna is directed at a location inside the computer system, wherein a receiving axis of the second antenna is oriented non-parallel to a receiving axis of the first antenna;

a monitoring mechanism configured to monitor the target electromagnetic signal received by the first directional antenna and the target electromagnetic signal received by the second directional antenna; and

a characterizing mechanism configured to characterize the computer system parameter based on the target electromagnetic signal, wherein the characterization mechanism includes a mechanism configured to process the target electromagnetic signal monitored using the first directional antenna and the target electromagnetic signal monitored using the second directional antenna using a coincidence detection analysis technique, and wherein the characterizing mechanism is configured to characterize the computer system parameter by:

generating a predetermined number of first directional antenna signals based on the target electromagnetic signal monitored using the first directional antenna; and

generating a predetermined number of second directional antenna signals based on the target electromagnetic signal monitored using the second directional antenna.