Difference frequency detection with range measurement

An apparatus comprising:

• a transmitter system configured to transmit a first plurality of electromagnetic signals having a first frequency and a second plurality of electromagnetic signals having a second frequency;
• a receiver system configured to detect a delayed difference frequency signal substantially equal to a difference between the first frequency and the second frequency, wherein the delayed difference frequency signal is generated by an object having non-linear electrical characteristics in response to the object receiving the first plurality of electromagnetic signals and the second plurality of electromagnetic signals;
• and
• a processor unit connected to the transmitter system and the receiver system, wherein the processor unit is configured to control the transmitter system and the receiver system to change at least one of the first frequency and the second frequency through a range of frequencies over time during transmission of the first plurality of electromagnetic signals and the second plurality of electromagnetic signals;
• compare the delayed difference frequency signal with an undelayed difference frequency signal substantially equal to the difference between the first frequency and the second frequency to form a comparison, and generate a range measurement for the object from the comparison.

A method for detecting objects, the method comprising:

• transmitting a first plurality of electromagnetic signals having a first frequency and a second plurality of electromagnetic signals having a second frequency;
• changing at least one of the first frequency and the second frequency through a range of frequencies;
• monitoring for a delayed difference frequency signal substantially equal to a difference between the first frequency and the second frequency, wherein the delayed difference frequency signal is generated by an object having non-linear electrical characteristics in response to the object receiving the first plurality of electromagnetic signals and the second plurality of electromagnetic signals;
• and
• generating a range measurement for the object using the delayed difference frequency signal and an undelayed difference frequency signal, wherein the undelayed difference frequency signal is substantially equal to the difference between the first frequency and the second frequency.