One embodiment of the present invention provides a system that characterizes a computer system parameter by analyzing a target electromagnetic signal radiating from the computer system. First, the system monitors 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. The system also monitors 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. Next, the system characterizes 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. Then, the system generates a request for an action based on the characterization of the computer system.