Abstract:
Location of an emitter using leaky cables. A two-channel receiver determines the location of an emitter by measuring the phase and/or amplitude difference between emitter signals received by leaky cables. In one embodiment, two leaky cables having different propagation velocities are used. In a second embodiment also suitable for use with fiber optic cables, two cables having the same propagation velocity are used, but have different lengths, the extra length being taken up by serpentine patterns or loops. A single cable in a loop may also be used. The leak points on the cables may be passive, or may be controlled RF switches.

Description:
TECHNICAL FIELD 
   Embodiments in accordance with the present invention relate to signal location. 
   BACKGROUND 
   There are many instances where detecting and locating a source of energy is important. For example, in commercial aircraft there is a concern among pilots and air carriers that personal electronic devices (PEDs) may interfere with aircraft operation and safety. In certain business environments is it important to determine if unauthorized wireless devices are in use in controlled areas. Providing location information on locating transmitters is important in the mining industry. 
   Standard techniques that can be used to locate emitters in free space often do not work well in confined spaces such as buildings, tunnels, or within aircraft. These techniques often reply on direction antennas (angle of arrival), or signal timing between or among a small number of sense antennas (time difference of arrival). 
   SUMMARY OF THE INVENTION 
   A two-channel receiver is used to determine the location of an emitter by measuring the time, phase, and/or amplitude difference between emitter signals received on leaky cables. In one embodiment, a two channel receiver receives signals from two leaky cables having different propagation velocities. In a second embodiment, two leaky cables have the same propagation velocity, but one cable is longer between leak points, for example using serpentine patterns or loops. Other embodiments use a single loop of cable. Leak points may be passive, or may be RF switches. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a diagram of a system using two cables with different propagation velocities, 
       FIG. 2  shows a diagram of a system using two cables with the same propagation velocity, 
       FIG. 3  shows a diagram of a system using a single cable, 
       FIG. 4  shows a diagram of a system using a single cable, 
       FIG. 5  shows an embodiment using switched nodes, 
       FIG. 6  shows a second embodiment using switched nodes, and 
       FIG. 7  shows a third embodiment using switched nodes. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Referring to the embodiment of  FIG. 1 , emitter  100  dates a signal which is received by two-channel receiver  200 . Received signals  202   204  from receiver  200  are processed by signal processor  230 . Signal processor  230  measures differences between signals  202   204  and produces distance estimate  235 . A velocity estimate may also be produced. Signal processor  230  may be a part of receiver  200 , a separate unit, or remote to receiver  200 . The two input channels of receiver  200  are fed by leaky signal cables  210  and  220 . These leaky signal cables  210   220  serve as antennas, and are run generally close to each other. Cables  210  and  220  are considered leaky as they allow energy to pass through to receiver  200 . In the case of coaxial electrical cables, the shielding may be of deliberately low quality, or may be compromised at regular intervals, such as shown by antennas  212  and  222 , by notching or otherwise opening the external conductor(s) of the coaxial cable. Particularly for coaxial cables, it may be beneficial to provide termination for cables  210   220  to reduce reflections. For optical cables, leak points  212  and  222  are introduced by exposing the optical fiber and bending it at the desired points  212   222 . Cables  210   220  are arranged such that corresponding leak points in the two cables are adjacent. 
   In the embodiment of  FIG. 1 , cables  210  and  220  have different propagation velocities. The propagation velocity is the speed at which signals propagate through the medium, usually expressed as a percentage of the speed of light in a vacuum. The propagation velocity of a coaxial cable is related to the type of dielectric material used, and commonly varies from 0.66 to 0.82. Propagation velocities for common twisted-pair networking cables meeting CAT5 or CAT6 specifications are typically in the range of 0.6 to 0.7. 
   Energy from an emitter  100  will teak into the cables through points  212   222  along the length of the cables, but will be strongest at the point in the cable pair closest to emitter  100 . As an example, if the cable pair is run the length of the cabin inside a passenger aircraft fuselage, the largest percentage of energy from a passenger using a cell phone or other wireless device will be at the location where the cable pair passes nearest the offending passenger. The cable pair could also be routed inside the ceiling or walls of a building, in a tunnel, or suspended from poles such as along a border or perimeter. 
   While more predictable results will be obtained when the cable pair and the associated leak points are closely adjacent to each other, small variations in the separation between cables and leak points is not critical. The degree to which variation may be tolerated will be determined primarily by the wavelengths of interest. The cables may be run separately, as an example separated by a fixed distance, run closely together, or run in a larger cable bundle along with other cables. 
   According to the invention, the location of an emitter is determined by observing the time difference between signals arriving at two-channel receiver  200 . Received signals  202   204  are analyzed by signal processor  230  and the distance  235  determined. 
   As an example, in the case of  FIG. 1 , assume the distance from receiver  200  to the maximal signal entry location along leaky cables  210  and  220  is l, the velocity factor of cable  210  is v 1  and the velocity factor of cable  220  is v 2 . Transit times t 1  and t 2  from l to receiver  200  are therefore: 
   
     
       
         
           
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   The delay between the two signals can be determined by computing the time difference using cross-correlation techniques, phase differences such as cross-spectrum, combinations, or other analog and digital signal processing techniques known to the art for estimating the delay between two signals. If phase is estimated, then it must be converted to delay (delay is equal to the derivative of phase with respect to frequency). 
   Once the time delay between the two signals is calculated, it can be converted to distance using the velocity factors of the cables, and to a location, using the known routing of the cables. 
   Such measurements assume that the characteristics of receiver  200  are known. In some embodiments, it may be desirable to use a coherent receiver design, or a design incorporating two identical channels. In other embodiments it may be adequate that differences between channel performance are sufficiently characterized that they can be corrected by signal processor  230 . 
   For the embodiment of  FIG. 2 , cables  210   250  with the same propagation velocity are used, but the effective length of cable  250  between leak points  252  is increased by looping  254  the cable or running it in a serpentine fashion. As an example, assume leaky cable  210  has leak points  212  at 1 meter intervals. Leaky cable  250  could have leak points at 1.2 meter intervals, with loops or serpentine features  254  used so that corresponding leak points  252  and leak points  212  are adjacent at 1 meter intervals, resulting in a delay difference in signals reaching receiver  200 . 
   It is also possible to use amplitude ratios in determining or improving location estimates. Coaxial cables have a loss factor expressed in dB which is frequency dependent the attenuation increasing with frequency. As an example, a popular coaxial cable, Belden 8237 has approximately 2.35 dB of attenuation per 100 feet at 144 MHz, rising to 8.73 dB at 1296 MHz. Times Microwave LMR-400, a similarly sized cable, has 1.46 dB of attenuation per 100 feet at 144 MHz, rising to 4.72 dB at 1296 MHz. Given that the amplitude response of receiver  200  is well characterized, the amplitude difference of signals between channels may be used to improve location estimates. In an embodiment such as shown in  FIG. 2 , the difference in amplitude information may be used to improve location estimates. An amplitude-only approach may also be used, determining location based on amplitude ratios at the receiver. This technique is likely to be less accurate than time-based approaches, but may be less expensive to implement; one approach would be to use power detectors in place of two-channel receiver  200 . Where timing information is not critical, it may be possible to use a quasi-two channel design using for example a high-speed PIN-diode switch to connect leaky cables  210   220  to a single power detector or a single channel receiver  200 , simulating a two-channel design. 
   As an example, assume a system according to  FIG. 1  where propagation velocity v 1 =0.7 (times the speed of light in a vacuum) for leaky cable  210  and propagation velocity v 2 =0.6 for leaky cable  220 . Assume an attenuation factor for cable  210  of 1 dB/meter, and a loss factor for cable  220  of 1.2 dB/meter at the frequency of interest. Given a CDMA emitter  100  located 10 meters from receiver  200  along leaky signal cables  210   220 , the signal from emitter  100  will take 48 nanoseconds (10 meters/(3e8 m/sec*0.7) to reach receiver  200  on cable  210 , and 56 nanoseconds (10 meters/(3e8 m/sec*0.6) to reach receiver  200  on cable  220 . This results in a time delay difference  202   204  of 8 nanoseconds to signal processor  230 . For a modulated signal, the cross-spectrum phase would have an observed slope at processor  230  of 2.88 udeg/Hz, or 3.6 Hz over the 1.25 MHz bandwidth of a CDMA signal. The observed amplitude difference at processor  230  would be 2 dB (10 m*(1.2 dB/m−1 dB/m)). 
   The embodiment of  FIG. 3  uses a single cable connected to both receiver channels, with only one side  220  of the loop having leak points  222 . Difference information is obtained due to the different distance from the entry point to each channel of the receiver in each direction. 
   The embodiment of  FIG. 4  also shows leaky cables  240   260  connected as a loop. In this embodiment leak points  242   262  alternate rather than being adjacent as in the embodiments of  FIGS. 1 and 2 . This embodiment has limited application when emitter  100  is stationary, as the phase, delay, and amplitude do not change linearly with respect to distance. For moving emitters, this embodiment does offer the advantage that the amplitude and phase seen by receiver  200  and signal processor  230  will be modulated at a rate proportional to the relative motion between emitter  100  and leaky cables  240   260 . 
   The embodiment of  FIG. 5  uses RF switching diodes such as PIN diodes to provide switched nodes  300   310   320   330 . Each switched node has an RF diode switch  302   312   322   332  which presents a high impedance to RF when no current is flowing through the diode, and a low impedance to RF when current is flowing through the diode. One end of diode  302   312   322   332  connects to cable  220 . The other end of diode  302   312   322   332  connects to RF choke  304   314   324   334  and capacitor  308   318   328   338 . Capacitor  308   318   328   338  provides a low impedance path for RF energy from antennas  310   320   330   340 . Choke  304   314   324   334  blocks RF energy but allows DC to pass, from control line  350  tough chokes  304   314 , diodes  302 ,  322 , through cables  220  and  210  to RF choke  342  and current limiting resistor  344 . This provides a low impedance path for RF from antennas  310   330 , and a high impedance path from antennas  320   340 . Blocking capacitors  404   406  prevent the flow of DC into receiver  200  while allowing the flow of RF signals; these capacitors may be internal to receiver  200 . Similarly, when control line  360  is energized, switched antennas  320   340  provide a low impedance to RF, and switched antennas  310   330  provide a high impedance. Thus switching between control lines  350  and  360  selects different sets of switched nodes for RF detection. Switched nodes may be switched in groups as shown in  FIG. 5 , alternating or may be enabled in groups according to distance, or individually addressed, depending on the complexity of the driving circuitry. Other RF switching means such as relays may also be used in place of PIN diodes  302   312   322   332 . 
   In the embodiment of  FIG. 6 , individual diode current is controlled by introducing constant current source  306  in series with choke  304 . Current source may be a constant current diode, a field effect transistor ET) connected as a constant current source, or similar device. By introducing a constant current source at each switched node. the current through active diodes such as diode  302  is controlled by constant current source  306 , independent of the resistance of lines  210   220 , and the voltage drop resulting from the line resistance and the current draw of other active nodes. If the DC drop along lines  210   220  is easily characterized, a simple resistor may be used in place of constant current source  306 . While  FIG. 6  shows capacitor  308  between antenna  310  and diode  302 , capacitor  308  may be eliminated at the risk of exposing a DC control point; capacitor  308  provides DC isolation while providing a low impedance path for RF energy. 
   Where the embodiment of  FIGS. 5 and 6  use control lines external to cables  210   220 , the embodiment of  FIG. 7  runs the DC control voltage along cables  210  and  220 . Switch  400  selects between a positive and a negative voltage which is placed on cables  220   210  through choke  402 , which passes DC but blocks RF. DC return is through the shield  250  of cables  210   220 . Receiver  200  now connects to cables  210   220  through blocking capacitors  404   406  which block the DC control voltage, but pass RF. Switched nodes  410   420  connect between the central conductor of cable  220  and the outer shield  430   230 . The polarity of PIN diodes  412   422  and current sources  416   426  is reversed between nodes  410  and  420 , such that node  410  is on, providing a low impedance path to RF, when node  420  is off, and vice versa. As the DC path is between the center conductor of cable  220  and its shield  230 , the need for components completing the DC path,  342   344  of  FIGS. 5 and 6  is eliminated. Again, simple resistors may also be used in place of constant current sources  416   426 . 
   It should be noted that receiver  200  may perform other functions. For example, a micro-cellular base station could incorporate this location technology as part of the normal receiver chain. This would allow cellular signals to be precisely located within tunnels and buildings. 
   Similarly, the cables used may perform other functions. As an example, a in a network cable having multiple twisted pairs, individual conductors or pairs could be used to determine the location of unwanted interference. One or more optical fibers in a wiring bundle could be periodically exposed through the cable jacket, providing leak points  222  of  FIG. 3 . In network environments such as standard deployments of twisted-pair cabling for 100 Base-T Ethernet only two of the four twisted-pairs are used for signaling. One or more of the unused wires could be periodically teased out of the cable jacket and used with the present invention. 
   While the embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to these embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.