Abstract:
A method and apparatus determines the location of a signal transmitter located at an unknown location by using externally-induced intermodulation distortion emitted from the signal transmitter. A transmitter locator emits an interrogation signal having a frequency that is offset from the frequency of the signal emanating from the signal transmitter. The interrogation signal and the carrier signal of the signal transmitter are &#34;mixed&#34; within the signal transmitter to form an intermodulation product signal. The intermodulation product signal having a different frequency is then radiated from the unknown signal transmitter. A receiver tuned to the frequency of the intermodulation signal detects the returned signal and a processor determines the range and direction from the transmitter locator to the location of the signal transmitter.

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus and method for locating a signal transmitter and, in particular, to an apparatus and method for determining the location of a signal transmitter at an unknown location using a single signal receiver. 
     BACKGROUND OF THE INVENTION 
     Locating the position of a signal transmitter located at an unknown location is important for military purposes, as well as for commercial purposes. The most common method for locating a signal transmitter positioned at an unknown location is to use triangulation. Triangulation requires the use of two signal receivers with one receiver positioned at a first location and the other receiver positioned at a second location. Both receivers receive the &#34;true&#34; or desired signal emitted from the signal transmitter at the unknown location and triangulate the position of the source of the signal. The problem with the triangulation technique is that it requires two receivers each positioned at a different location. Further, it requires a communication channel between the two receivers in order for the triangulation to determine the unknown location of the signal transmitter. 
     Accordingly, there exists a need for a transmitter locator that uses externally-induced intermodulation distortion emitted from the unknown transmitter to determine the location of the unknown transmitter. Further, there is needed a transmitter locator positioned at a single point, that uses only one receiving unit. Additionally, there exists a need for a transmitter locator that uses the inherent intermodulation performance (an imperfection) of the power amplifier of a signal transmitter at an unknown location to produce an intermodulation return signal to obtain ranging information. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a transmitter locator and method for determining the location of an unknown transmitter emitting a signal from an unknown location. The transmitter locator includes a transmitter for generating and emitting an interrogation signal to be received by an unknown transmitter emitting a signal from an unknown location. The interrogation signal is offset in frequency from the frequency of the signal emitted from the unknown transmitter A receiver receives an intermodulation return signal emitted from the unknown transmitter in response to the reception by the unknown transmitter of the interrogation signal. A processor connected to the receiver determines from the received intermodulation return signal a distance from the transmitter locator to the unknown transmitter. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 illustrates an overall representation of the transmitter locating scheme in accordance with the present invention; 
     FIG. 2 is a more detailed block diagram illustrating the transmitter locator shown in FIG. 1; 
     FIG. 3 illustrates the intermodulation products emitted from an unknown signal transmitter; and 
     FIG. 4 illustrates a typical transmitter output circuit. 
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, like reference characters designate like or similar parts throughout the drawings. 
     With reference to FIG. 1, there is shown a transmitter locator 10 in accordance with the present invention. The transmitter locator 10 generates and emits an interrogation signal 14 from an antenna 12. Signal transmitter 18 positioned at an unknown location receives the interrogation signal 14 through an antenna 16. The interrogation signal 14 received by the antenna 16 is processed in the signal transmitter 18 to be mixed with a desired transmitted signal as it is emitted from the signal transmitter 18. The mixing of the interrogation signal 14 and the output of the signal transmitter 18 generally occurs in the output amplifier stage of the signal transmitter. The mixing of these signals produces an intermodulation return signal 20 to be emitted from the signal transmitter 18. The transmitter locator 10 receives the intermodulation return signal 20 and determines the range from the transmitter locator 10 to the signal transmitter 18. 
     Now referring to FIG. 2, there is illustrated a more detailed block diagram of the transmitter locator 10 of the present invention. The transmitter locator 10 includes a transmitter 22, a receiver 24 and a processor 26. To identify the location of the signal transmitter 18, the relative bearing and range must be determined. Typically, the signal transmitter 18 emits a signal having a particular frequency. This signal is received and locked onto by the receiver 24 of the transmitter locator 10. The signal received at the antenna 12 is processed to determine the bearing (direction) of the signal transmitter 18 with respect to the transmitter locator 10 using beamforming techniques and/or angle of arrival-techniques. It will be understood by a person skilled in the art that various beamforming techniques and/or angle of arrival techniques can be used. 
     Knowing the frequency of the signal emitted from the signal transmitter 18, the transmitter 22 of the transmitter locator 10 generates and emits the interrogation signal 14. The interrogation signal 14 is offset in frequency from the desired signal emitted from the signal transmitter 18. In the preferred embodiment, the interrogation signal 14 is a short pulsed signal, similar to a radar pulse. Alternatively, a spread spectrum signal may be used that includes different frequencies within the pulse, or a signal may be used that includes pulses each having different frequencies. 
     While the signal transmitter 18 is generating and emitting its own signal, the interrogation signal 14 is received by the antenna 16. The signal transmitter 18 acts like a mixer and heterodynes the interrogation signal 14 with its own carrier signal. This produces an intermodulation signal (i.e. third order, fifth order, seventh order, etc.) transmitted with the desired signal of the signal transmitter 18 and emitted as the intermodulation return signal. 
     By knowing the frequency of the signal emitted from the signal transmitter 18 and the frequency of the interrogation signal 14, the transmitter locator 10 identifies the frequencies of the intermodulation signal emitted from the signal transmitter 18. Generally, the third-order intermodulation signals is used since this has the most energy. A processor 26 connected to the transmitter 22 and the receiver 24 measures the round-trip transit time from the transmission of the interrogation signal 14 to the reception of the intermodulation return signal 20. Distance or range to the unknown signal transmitter 18 is then calculated. Use of a spread spectrum signal could alternatively be used for a timing and clocking function. 
     Intermodulation (return) signals 20 are generated in a final amplifier stage 104 of the signal transmitter 18. The interrogation signal 14 radiated from the transmitter locator (TX B ) 10 is directed toward the antenna 16 of the signal transmitter (TX A ) 18 at a distant location. This results in the interrogation signal 14 entering the final amplifier stage of the signal transmitter 18. Most transmitters operate in either the Class C, D or E mode for maximum efficiency. As such, the final amplifier stage in these transmitters is non-linear and acts as a mixer. The carrier signal frequency (F A ) of the signal transmitter (TX A ) 18 mixes with the interrogating (or locating) signal (F B ) 14 to produce intermodulation signals (F IM ). The intermodulation signals (F IM ) are then re-radiated via the antenna 16 of the signal transmitter (TX A ) 18, along with the desired signal (F A ). These signals are received as the intermodulation return signal 20 by the receiver 24 of the transmitter locator 10 tuned to the intermodulation signal frequency (F IM ). 
     Externally-induced (also known as reverse-intermod or back-intermod) transmitter intermodulation distortion is caused by the mixing of frequencies in the final amplifier stage of a transmitter. New frequencies (intermod products) are generated which are then radiated from the transmitter. The creation of new spurious signals is in accordance with the simple sum and difference mixing formula: 
     
         F.sub.IM =+/-nF.sub.A +/-mF.sub.B 
    
     where 
     F IM  =frequency of the intermodulation signal 
     F A  =frequency of the unknown signal transmitter 
     F B  =frequency of the transmitter locator interrogation signal 
     n, m=1,2,3 . . . integers 
     If more than two frequencies are involved, the number of combinations rises rapidly. The order of the intermodulation signal is equal to the sum of the integers n plus m. The most important intermodulation signals are those that are closest to the carrier frequency with low integers, because these signals are both the strongest and the most difficult to filter. The third-order signals (2F A  -F B  and 2F B  -F A ) and the fifth order signals (3F A  -2F B  and 3F B  -2F A ) are shown in FIG. 3. The amplitude of each signal is shown relative to the output signal carrier level. The intermodulation signals are spaced at the difference frequency (Fb-Fa). 
     FIG. 3 illustrates the intermodulation signals for a specific example where the signal transmitter (TX A ) 18 is transmitting a signal having a frequency equal to 152 MHz. Accordingly, the transmitter locator (TX B ) 10 generates and emits an interrogation signal 14 having a frequency offset from the frequency of the signal transmitter (TX A ) 18. In this specific example, the frequency of the interrogation signal 14 is 153 MHz. Therefore, the delta frequency is 1 MHz. This produces two third-order intermodulation signals--one at 151 MHz and the other at 154 MHz, and two fifth-order intermodulation products--one at 150 MHz and the other at 155 MHz, and so on. 
     As will be appreciated, the third-order intermodulation signal is generally selected as the signal of interest to the transmitter locator (TX B ) 10 since it usually has the most energy. 
     It will be understood that the frequency of the interrogation signal 14 chosen to be emitted from the transmitter locator (TX B ) 10 will depend on the frequency of the signal emitted from the signal transmitter (TX A ) 18. Generally, the frequency of the interrogating signal 14 should be close to the frequency of the signal transmitter (TX B ) 18. 
     Now referring to FIG. 4, there is illustrated a typical transmitter output circuit. The interrogation signal 14 is received by the antenna 16 of the signal transmitter 18 mixes with the second harmonic of the operating frequency (2F A ) in the collector or drain of the final amplifier stage 104. A low pass filter 100 blocks the third-order, sum intermodulation signal (2F A  +F B ) output from the amplifier 104. The low pass filter 100 is normally a part of a signal transmitter 18 to filter harmonic energy (i.e., 2F A , 3F A , 4F A ). The third-order, difference intermodulation signal (2F A  -F B ) from the amplifier 104 will, however, pass through the wideband matching network 102 and the low pass filter 100 and be radiated by the antenna 16. If the frequency of the interrogation signal 14 is close to the frequency of the signal transmitter 18, even a high-Q cavity filter at the output of the signal transmitter 18 will not filter the interrogation signal 14 from reaching the final amplifier stage 104. 
     Although one embodiment of the present invention has been described in the foregoing detailed description and illustrated in the accompanying drawings, it will be understood by those skilled in the art that the invention is not limited to the embodiments disclosed but is capable of numerous rearrangements, substitutions and modifications without departing from the spirit of the invention.