Abstract:
A system and technique is described which has the capability to track and identify, in real time, various aircraft and objects including Unmanned Aerial Vehicles (UAVs), Unmanned Combat Aerial Vehicles (UCAVs), and Micro Aerial Vehicles (MAVs). The system uses a combination of techniques including conventional automatic dependent surveillance broadcast (ADS-B), transponder multilateration, broadband emitter multilateration, primary and secondary radar, and passive coherent location. A series of enhancement to conventional passive coherent location are described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a Continuation-In-Part of U.S. patent application Ser. No. 11/688,348, filed on Mar. 20, 2007, and incorporated herein by reference; application Ser. No. 11/688,348 is a Continuation-In-Part of U.S. patent application Ser. No. 11/492,711, filed Jul. 25, 2006, and incorporated herein by reference; application Ser. No. 11/688,348 is a Continuation-In-Part of U.S. patent application Ser. No. 11/343,079, filed on Jan. 30, 2006, and incorporated herein by reference; application Ser. No. 11/688,348 is also a Continuation-In-Part of U.S. patent application Ser. No. 11/342,289 filed Jan. 28, 2006 and incorporated herein by reference; application Ser. No. 11/688,348 is a Continuation-In-Part of U.S. patent application Ser. No. 11/209,030, filed on Aug. 22, 2005, and incorporated herein by reference; application Ser. No. 11/688,348 is a Continuation-In-Part of U.S. patent application Ser. No. 11/257,416, filed on Oct. 24, 2005, and incorporated herein by reference; application Ser. No. 11/688,348 is a Continuation-In-Part of U.S. patent application Ser. No. 11/203,823 filed Aug. 15, 2005 and incorporated herein by reference; application Ser. No. 11/688,348 is a Continuation-In-Part of U.S. patent application Ser. No. 11/145,170 filed on Jun. 6, 2005 and incorporated herein by reference; application Ser. No. 11/145,170 is a Continuation-In-Part of U.S. patent application Ser. No. 10/743,042 filed Dec. 23, 2003 and incorporated herein by reference; application Ser. No. 10/743,042 is a Continuation-In-Part of U.S. patent application Ser. No. 10/638,524 filed Aug. 12, 2003 and incorporated herein by reference; application Ser. No. 10/638,524 is a Continuation of U.S. patent application Ser. No. 09/516,215 filed Feb. 29, 2000 and incorporated herein by reference; application Ser. No. 09/516,215 claims is a Non Prov. of Provisional U.S. Patent Application Ser. No. 60/123,170 filed Mar. 5, 1999 and incorporated herein by reference; application Ser. No. 10/743,042 is a Continuation-In-Part of U.S. patent application Ser. No. 10/319,725 filed Dec. 16, 2002 and incorporated herein by reference. Application Ser. No. 10/743,042 is a Non Prov. of Provisional U.S. Patent Application Ser. No. 60/440,618 filed Jan. 17, 2003 and incorporated herein by reference; The present application is also a Continuation-In-Part of U.S. patent application Ser. No. 11/649,350, filed Jan. 3, 2007, and incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of aircraft tracking. In particular, the present invention is directed toward a method and apparatus using passive coherent techniques for tracking aircraft, aerial vehicles, and other objects. 
   BACKGROUND OF THE INVENTION 
   In a report announced on Oct. 9, 2006, the Teal Group (www.tealgroup.com) predicted that the worldwide UAV market will exceed $54 Billion, and that UAV spending will more than triple over the next decade from current worldwide UAV expenditures of $2.7 billion annually to $8.3 billion within a decade, totaling close to $55 billion in the next ten years. The study concludes that the U.S. will account for 77% of the worldwide research and development spending on UAV technology over the next decade, and about 64% of the procurement. These U.S. expenditures represent higher shares of the aerospace market than for worldwide defense spending in general, with the U.S. accounting for about 67% of total worldwide defense research and development spending and 37% of procurement spending, according to the Teal Group. 
   An article by Charlotte Adams, published in Avionics Magazine in September 2001, reviewed UAV developments at that time. The article noted that, in wingspan, they range from six inches (15 cm) to 247 feet (75 meters). In flight mode, they embrace fixed-wing, rotary-wing, MV-22 Osprey-like vertical takeoff/landing (VTOL), and birdlike, flapping-wing designs. In altitude, they stretch from 50 feet to 100,000 feet, and in mission, range from intelligence, surveillance and reconnaissance (ISR) to weapons launch. The article goes on to describe avionics and sensor payloads on the various vehicles which are summarized here for reference:
         Uncooled infrared (IR)   “Panospheric” electro-optical (E/O)   Laser radar (ladar)—Jigsaw   Ku-band, over-the-horizon satcom   Collins ARC210 radios (Rockwell Collins ARC210 AM/FM/UHF/satcom HF radio systems)   Synthetic aperture radar sensors   Ultra wide-band (UWB) sensors. These devices generally radiate about 0.25 watt instantaneous peak power and about 750 microwatts (millionths of a watt) average power providing low probability of intercept/detection and high anti-jam immunity. Sensors discussed in the article operate in the 6-to-6.5-GHz range.)       

   While it is generally understood that future UAVs operating in controlled airspace will carry air traffic control compliant avionics, such as transponders, there will be, in general, a broad variety of avionics and sensor payloads used on these aircraft. With the advent of new technologies for tracking aircraft within airspace, it may be necessary to provide means for reliably tracking UAVs and the like to prevent collisions with passenger and other aircraft, as well as to prevent collisions with each other, buildings, and other objects. Additionally, passive coherent techniques offer the capability to detect covert enemy aircraft, at high and low altitudes. It remains a requirement in the art, therefore, to provide a means to track UAVs and other aerial vehicles using passive location techniques. 
   Conventional radar systems comprise a collocated transmitter and receiver, which usually share a common antenna to transmit and receive. A pulsed signal is transmitted and the time taken for the pulse to travel to the object and back allows the range of the object to be determined. Passive Coherent Location (PCL) is a passive radar system, where there is no dedicated transmitter. Instead, the receiver uses third-party transmitters in the environment, and measures the time difference of arrival between the signal arriving directly from the transmitter and the signal arriving via reflection from the object. This allows the bi-static range of the object to be determined. In addition to bi-static range, passive radar will typically also measure the bi-static Doppler shift of the echo and also its direction of arrival. These allow the location, heading and speed of the object to be calculated. In some cases, multiple transmitters and/or receivers can be employed to make several independent measurements of bi-static range, Doppler and bearing and hence significantly improve the final track accuracy. 
   The PCL system includes bi-static radar, which measures the elliptical distance and the Doppler frequency shift. It works with CW transmitters of opportunity, meaning that it uses electromagnetic radiation, primarily assigned for another purpose, for example, radio or television terrestrial broadcasts. It is necessary to detect at least two (in an ideal case three or more) direct signals from transmitters for a proper determination of a target position, as shown in  FIG. 1 .  FIG. 1  is a diagram illustrating a typical Prior Art PCL configuration. 
   From the principles of PCL methodology all targets interacting with the electromagnetic field (having the wavelengths of interest) are detectable. In practice, all flying objects heavier than air are built from some kind of metal materials, carbon composites or at least include some metal parts or wires thereby meeting this requirement. 
   Conventional aircraft stealth technology is not designed for the frequency bands used by PCL. Stealth technology is designed to minimize the radar cross section (RCS) and is based on achieving an anti-reflective coverage of the aircraft surface and the fuselage shaping from a mono-static RCS point of view. These stealth capabilities are lost at the lower frequency bands, such as VHF for FM radio. Compared to conventional primary active radar, PCL relies on a significantly lower EIRP. This handicap is equalized through the use of a long integration interval in the correlation detector in the receiver, because the reference signal is present with a sufficient signal-to-noise ratio. Weather conditions like rain, snow, hailstones, fog and the like, have only a negligible influence in the frequency bands used by PCL. 
   A PCL system should be able to detect extremely small signal changes scattered by targets, which are produced by an interaction of the transmitted signal of opportunity and the target. These changes in the scattered signals are about 100 dB or so lower than the direct signal. Moreover, ground clutter is present in the received signals at about 10 to 50 dB higher than the reflected signals, depending on the terrain. 
   PCL represents a technological challenge, which must be handled both from a precise antenna and receiver technique point of view, but mainly through the use of sophisticated digital signal processing methods. Consequently, before deploying a system it is important to make detailed simulations to evaluate known effects and processes influencing PCL system functionality. An extremely high sensitivity is allowed by a relatively long signal integration period (0.1-1.0 sec) for coherent processing of CW signals. Detection of these signals, which are lower by 100 dB, is possible due to enhanced antennas and signal processing. 
   As of December 2006, there are several PCL systems in various stages of development or deployment, as follows:
         Silent Sentry is a Lockheed Martin (USA) PCL system that uses FM radio transmissions. Two different antenna variants are believed to be available providing an antenna that provides 360° azimuthal coverage from 4 different beams (an Adcock array), and a variant that provides 100° azimuthal coverage from six different beams (linear array). It has a range of up to around 100 nautical miles depending on the variant employed and a number of receive nodes at different locations can be combined to provide increased coverage. (http://www.dtic.mil/ndia/jaws/sentry.pdf).   Celldar is a British system developed jointly by Roke Manor and BAE Systems. The system is a PCL sensor that can exploit GSM signals, currently in the 900 MHz band, but may also be able to use the 900 MHz and 1800 MHz bands simultaneously in the future. The sensor can track targets in 2D over a 100° sector at ranges of up to around 60 km. Celldar is a low level/surface surveillance system designed to achieve good coverage below 10,000 ft. (http://www.roke.co.uk/skills/radar/).   CORA is a German PCL sensor, developed by FGAN (Die Forschungsgesellschaft für Angewandte Naturwissenschaften e.V.), that exploits Digital Video Broadcast—Terrestrial (DVB-T) and Digital Audio Broadcast (DAB) transmissions.   Cristal is a PCL sensor developed by Thales that exploits FM radio transmissions to track targets. In addition to Cristal, it is believed that Thales has a prototype PCL system that uses analogue TV or DAB transmissions.   One of the PCL systems developed by ERA Systems Corporation, assignee of the present application, (www.rannoch.com) uses an eight-element circular antenna array, as shown in  FIG. 2 . The system uses an analog VF block (receiver and beam forming network circuits) with high dynamic range and linearity. The digitizer block uses 24-bit high quality A/D converters (up to 100 kHz BW). The DSP engine block uses a parallel multiprocessor cluster as shown in  FIG. 3 .       

   The estimated performance of one of ERA&#39;s prototype PCL units, based on 3 FM transmitters, and a target with a radar cross section (RCS) of 0 dBsm (1 m 2 ) for 100 MHz is as follows:
         System latency time, &lt;2 sec (which is dependent on the actual computing power of the DSP engine)   Region of interest, 200×200 km, 360 degree coverage   Position accuracy, H 250 m, Z 550 m   Horizontal velocity accuracy of 2 m/s   Vertical velocity accuracy of 8 m/s   50 simultaneously tracked targets       

   To give a measure of the security appeal of PCL, a recent U.S. Government request for information (RFI) was released to industry (http://www.hsarpabaa.com/Solicitations/MPCL-SOURCES-SOUGHT-NOTICE_final.pdf). That request from the Homeland Security Advanced Research Projects Agency (HSARPA) of the Department of Homeland Security, Science and Technology Directorate (DHS S&amp;T) was issued for information regarding passive coherent localization (PCL) techniques for detection of surface targets on water and land. In the RFI, HSARA stated that they were looking for new and innovative technical approaches to detection and tracking using multi-static localization through PCL, where targets of interest include small and large vessels in coastal waters, ports and waterways and vehicles and persons crossing border areas. 
   HSARA went on to state that an envisioned system should be able to meet the following requirements:
         Be capable of providing coverage over either a major port or harbor area (such as Miami, New York, Hampton Roads, or Puget Sound) or along extended coastlines (such as the Florida Keys, Gulf Coast, and Long Island Sound).   Be capable of detecting a 25-foot boat (estimated to be −5 to 0 dBsm radar cross section) at 3 knots or a jet ski at 10 knots.   Be capable of distinguishing between surface targets and low flying fixed and rotary winged aircraft.   Be capable of tracking a target with an accuracy of better than 100 meters.   Be capable of separating and distinguishing targets 50 meters apart.   Be capable of developing and maintaining a complete surface track picture in the area of concern.   Be capable of an update rate of less than 10 seconds and a latency of less than 5 seconds.       

   Many militaries and security agencies around the world are exploring the utility of PCL. In a report on Chinese developments from the Taipei Times http://www.taipeitimes.com/News/archives/1999/11/30/0000013001/wiki, incorporated herein by reference, it was reported back in 1999 that China was close to deploying an anti-aircraft defence system that uses technology so advanced it can track even stealth-type warplanes. According to a recent Newsweek article, US analysts expressed worries that the new early-warning defence system could defeat current US Air Force tactics against enemy air defences. 
   The Newsweek article went on to state that current defences use radar to track incoming aircraft, but outgoing signals can be found and jammed or destroyed. The “passive” technology the Chinese are believed to have, detects aircraft by monitoring disruptions in commercial radio and TV signals, and are essentially undetectable, Newsweek reported. The technology, which could detect US stealth aircraft, including the F-117 bomber and perhaps even the futuristic F-22 fighter, has so alarmed the defense community that top military and industry experts have been called to discuss the strategic implications. At the time, the military in Taiwan, however, said it was unlikely the development of PCL technology by China has been taken beyond the theoretical stage. 
   The patents and published patent applications listed below and incorporated herein by reference, describe various methods and enhancements to perform passive coherent location using a receiver subsystem which receives reference signals from an uncontrolled transmitter and scattered transmissions originating from the uncontrolled transmitter and scattered by an object. 
   U.S. Pat. No. 7,012,552, incorporated herein by reference, describes a passive coherent location system which enhances object state awareness to track several approaching airborne objects. The system including a receiver subsystem to receive reference signals from an uncontrolled transmitter and scattered transmissions originating from the uncontrolled transmitter and scattered by approaching airborne objects. A front-end processing subsystem determines a radial velocity of the object, based on the received transmissions, and buffers digitized transmission replicas of the received transmissions. A back-end processing subsystem receives the digitized transmission replicas of the received transmissions and determines object state estimates based on the determined radial velocity. The front-end processing subsystem and back-end processing subsystem are remotely located relative to one another. U.S. Pat. No. 7,012,552 goes on to describe a method for determining when the object is within a predetermined distance from a ground location. 
   U.S. Pat. No. 7,019,692, incorporated herein by reference, describes a system for narrowband pre-detection signal processing for passive coherent location applications. The apparatus receives an input signal including a target signal and a reference signal. The reference signal is received on a direct path from an uncontrolled transmitter along with the target signal, which comprises the reference signal reflected from a target. The received signals are passed in two paths for parallel processing. Each of the paths includes a target channel and a reference channel for the target signal and the reference signal, respectively, and one of the paths is for correlation signal processing. U.S. Pat. No. 7,019,692 claims a method for narrowband pre-detection signal processing for passive coherent location applications. According to that method, an input signal is received, including a target signal reflected from a target. A coherent processing interval is selected. The system performs motion compensation by accepting tracker feedback for a target. Motion compensation is performed over the coherent processing interval. 
   U.S. Pat. No. 6,839,026, incorporated herein by reference, describes a system and method for narrowband pre-detection signal processing for passive coherent location applications. The method comprises the steps of receiving a first reference signal and a first target signal, filtering the first reference signal with respect to the first target signal using a filter to form a first output reference signal, and combining the first output reference signal with the first target signal to form a first output target signal, receiving a second reference signal and a second target signal, updating the filter with respect to a difference between the first target signal and the second target signal, filtering the second reference signal using the updated filter to form a second output reference signal, and combining the second output reference signal with the second target signal to form a second output target signal. 
   U.S. Pat. No. 6,798,381, incorporated herein by reference, describes a system for measurement of domain data association in passive coherent location applications. The patent describes a method for associating a detection report having measurements to a line track, where the line track correlates to a signal transmitted from a source of opportunity, reflected from a target, and detected by a passive coherent location system. The method comprises the steps of estimating a line track state vector from measurements using a set of Kalman filters wherein the measurements are at least partially derived by comparing the signal to a reference transmission from a source of opportunity, and initializing the line track. U.S. Pat. No. 6,798,381 describes a system for measurement data association of detection reports to a line track within a passive coherent location system. The detection reports correlate to target signals transmitted from a source of opportunity, reflected from a target, and received at the passive coherent location system. The system comprises a state estimation means to associate new detection reports with existing line tracks and to estimate update states for extending line tracks, wherein the new detection reports are at least partially derived by comparing the target signal to a reference transmission from a source of opportunity, a line track merge means to merge line tracks, a line track termination means to terminate line tracks according to a specified criteria, and a line track initialization means to initiate new line tracks for unassociated detection reports. 
   U.S. Pat. No. 6,738,021, incorporated herein by reference, describes a system for detection and feature extraction in passive coherent location applications which includes a method for detecting and extracting target information during a coherent processing interval within a passive coherent location system. The patent describes a method including the steps of forming an ambiguity surface having ambiguity surface data for the coherent processing interval, identifying bins from a previous ambiguity surface, associating bins from the previous ambiguity surface with ambiguity surface data, and identifying new bins for new target echoes within the ambiguity surface. U.S. Pat. No. 6,738,021 also describes a method for detecting and extracting target data for targets within a coherent processing interval in a passive coherent location system. The method includes the steps of generating an ambiguity surface, associating bins with the ambiguity surface, forming peak detections from the bins and a plurality of old bins from a previous ambiguity surface, wherein the peak detections correlate to target echoes within the coherent processing interval, and estimating target parameters from peak detections. 
   U.S. Pat. No. 6,710,743, incorporated herein by reference, describes a system for central association and tracking in passive coherent location applications that uses a method for associating a line track with a target for a passive coherent location system, wherein the passive coherent location system uses comparison data from a direct and a reflected signal emitted from one or more transmitters geographically separated from a receiver. The system receives a detection report at least partly derived from comparison data, a detection report having a line track corresponding to the target. A target state is computed using measurements of the line track; computing a state covariance using a measurement of the line track. The line track Is scored according to the target state covariance and the line track is assigned to a target track according to the scoring. 
   U.S. Pat. No. 6,710,743 also describes a method for associating and tracking target data within a passive coherent location system, where the target data include measurements from a direct and a reflected signal emitted from one or more transmitters geographically separated from a receiver. A target state and a state covariance are computed from the measurements, where the measurements at least partly comprise the comparison data from direct and reflected signals. A line track is assigned correlating the target data to a target track according to the target state and state covariance. The target track is initialized and a filter is initialized according to the target state and state covariance. The target track is tracked with the filter and the target data is extrapolated from the target track. 
   U.S. Pat. No. 6,703,968, incorporated herein by reference, describes a system and method for mitigating co-channel interference in passive coherent location applications. The system identifies a primary illuminator signal from a primary illuminator, where the primary illuminator signal comprises a frequency-modulated carrier at a given frequency. The primary illuminator signal is regenerated and cancelled from the co-channel signals. A secondary illuminator signal is identified from a secondary illuminator, where the secondary illuminator signal comprises a frequency-modulated carrier at a given frequency. The secondary illuminator signal is regenerated and cancelled from the co-channel signals. 
   U.S. Pat. No. 6,522,295, incorporated herein by reference, describes a passive coherent location system for enhancing object state awareness. A receiver subsystem receives a reference signal from an uncontrolled transmitter and scattered transmissions originating from the uncontrolled transmitter and scattered by an object. A front-end processing subsystem determines a radial velocity of the object based on the received transmissions and buffers digitized transmission replicas of the received transmissions. A back-end processing subsystem receives the digitized transmission replicas and determines object state estimates based on the determined radial velocity. 
   U.S. Pat. No. 6,522,295 goes on to describe a method for determining an updated state estimate for an object. A reference transmission is received from an uncontrolled transmitter, along with a scattered transmission that originated from the uncontrolled transmitter and scattered by the object. The received transmissions are compared to determine a frequency-difference-of-arrival and a previous state estimate is updated based on the determined frequency-difference-of-arrival. Digitized transmission replicas of the received transmissions are buffered, where the digitized replicas are received by a back-end processing subsystem. 
   Published U.S. Patent Application 2003/0001778, incorporated herein by reference, describes a system for detection and feature extraction in passive coherent location applications. The system includes generating an ambiguity surface and associating bins with the ambiguity surface. The method also includes forming peak detections from the bins where the peak detections correlate to target echoes within the coherent processing interval. The method also includes estimating targets parameters from the peak detections. 
   U.S. Pat. No. 6,930,638, incorporated herein by reference, describes a passive method of detecting an object. A receiver receives a first signal transmitted by a mobile phone base station, along with a second signal comprising the first signal transmitted by a mobile phone base station after it has been reflected off an object. The first and second signals are compared to derive data relating to position or speed of movement of the object. The method further comprising providing a plurality of mobile phone base stations which transmit a signal, such that as the object moves out of range of one base station, it comes into range for another, whereby the distance of the object from a base station being used to determine a position of the object remains substantially constant and distance of the object from the receiver changes, thereby improving the received power and range. 
   SUMMARY OF THE INVENTION 
   A system and technique is described which has the capability to track and identify, in real time, various aircraft and objects including Unmanned Aerial Vehicles (UAVs), Unmanned Combat Aerial Vehicles (UCAVs), and Micro Aerial Vehicles (MAVs). The system uses a combination of techniques including conventional automatic dependent surveillance broadcast (ADS-B), transponder multilateration, broadband emitter multilateration, primary and secondary radar, and passive coherent location. A series of enhancements to conventional passive coherent location are described. The use of an enhanced PCL system to track and categorize these and other targets is described, along with the integration of tracking and identification data from other sources. 
   The present invention includes a system for tracking an object by receiving a reference transmission from a controlled or uncontrolled transmitter, and a scattered transmission that originated from a controlled or uncontrolled transmitter and scattered by the object, comparing the received transmissions to determine a frequency-difference-of-arrival thereby establishing a track or tracks for the object or objects, and updating each track on a predetermined periodic basis, and forwarding each track to the system&#39;s processing unit for filtering and display. The transmissions may include one or more of FM radio, Digital Video Broadcast Terrestrial (DVB-T), Digital Audio Broadcast (DAB), and the Global System for Mobile Communications (GSM). 
   The present invention also includes a system for narrowband pre-detection signal processing for passive coherent location applications. Input signals include a target signal and a reference signal, the reference signal being received on a direct path from a controlled or uncontrolled transmitter and the target signal comprising the reference signal reflected from a target. The target signal and the reference signal are correlated in a signal processing unit using a database of a priori illuminator signals to discriminate between reference signals and target signals. 
   The present invention also includes a method for mitigating co-channel interference in passive coherent location applications. A database is used to store a priori knowledge of the proximate illuminator signals, wherein the illuminator signals comprise one or more of FM radio, Digital Video Broadcast Terrestrial (DVB-T), Digital Audio Broadcast (DAB), and the Global System for Mobile Communications (GSM). Illuminators are then generated from the database and used to cancel the illuminator signals from the co-channel signals to remove co-channel interference. 
   The present invention includes a system and method for continuously detecting and extracting target information using a series of samples within a passive coherent location system. An ambiguity surface is formed having ambiguity surface data for the series of samples. This ambiguity surface then forwards the extracted target information to the central processing system for comparison with a database of predetermined target type characteristics. 
   The present invention also includes a method for associating detections to a line track, where the line track correlates to a signal transmitted from a controlled or uncontrolled source, reflected from a target, and detected by a passive coherent location system. A line track vector is determined from a series of measurements using a set of adaptive or other filters wherein the measurements are at least partially derived by comparing the signal to a reference transmission from the controlled or uncontrolled source. A track comparable to conventional air traffic control radar systems is generated, including the two air traffic control tracker states of track and coast. 
   In another embodiment of the present invention, combined portable or fixed multilateration, ADS-B, PET, and PCL receiver systems share the same mast, power, communications, and infrastructure for ease of deployment. 
   In another embodiment of the present invention, a mobile or fixed based system is employed which combines PET and PCL to provide real time tracking and characterization or identification of cooperative and non cooperative targets including land, sea, and airborne targets. 
   In another embodiment of the present invention, the PET includes direction finding using the phase of the received target emissions and Time Difference of Arrival (TDOA) processing of the received target emissions. 
   In another embodiment of the present invention, the PCL includes TDOA processing of signals reflected by the cooperative and non-cooperative targets. 
   In another embodiment of the present invention, the characterization or identification may include electronic intelligence or fingerprinting of target emissions, as well as target classification based on modulation of the reflected signals. 
   In another embodiment of the present invention, a mobile or fixed based system combines PET and PCL to share the same system infrastructure. 
   In another embodiment of the present invention, the PET and the PCL are co-mounted on the same mast and use the same communications channels from the sensors to the central server. 
   In another embodiment of the present invention, the PET may be mounted on a plurality of masts having all or a subset equipped with PCL sensors, and each PCL sensor may track based on the same or different frequency bands, including terrestrial and satellite based radio, television, and other CW sources. 
   In another embodiment of the present invention, correlation of aircraft tracking and aircraft identification is made using a number of sources including ADS-B, multilateration, broadband passive emitter tracking, SSR, primary radar, and PCL. 
   In another embodiment of the present invention, prioritization of data sources is used to provide comprehensive surveillance and a back up or validation to the preferred sources used for air traffic control. 
   In another embodiment of the present invention, electronic intelligence from broadband passive emitter tracking coupled with PCL extracted target characteristics is used to provide high confidence in target identification and classification. 
   In another embodiment of the present invention, a look up table or database of PET and or PCL features is used for vehicles, vessels, and aircraft, including UAVs, UCAVs, MAVs and other objects. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating a typical Prior Art PCL configuration. 
       FIG. 2  illustrates an eight-element circular antenna array used in one of the PCL systems developed by ERA Corporation, assignee of the present application. 
       FIG. 3  is a block diagram of an RF, Analog and Digitizer unit used in a Prior Art PCL system of ERA Systems Corporation, assignee of the present application. 
       FIG. 4  is a block diagram of a first embodiment of the present invention. 
       FIG. 5  is a block diagram of a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 4 , in a first embodiment of the present invention, a PCL tracking and identification system is provided as a standalone system. Referring to  FIG. 4 , reference signal antenna  1000  passes the various reference signals to the signal conditioner  1010 . The beam forming antenna  1015  passes the reflected signals through beam former  1010  to the signal conditioner  1030 . Like most passive radar systems that use simple antenna arrays with several antenna elements and element-level digitization, this allows the direction of arrival of echoes to be calculated using standard radar beam forming techniques, such as amplitude monopulse using a series of fixed, overlapping beams or more sophisticated adaptive beam forming. 
   Signal conditioners  1010  and  1030  perform some transmitter-specific conditioning of the signal before cross-correlation processing. Conditioning includes analogue band-pass filtering of the signal, equalization to improve the quality of the reference signal, and removal of unwanted structures in digital signals to improve the radar ambiguity function. A major metric in the detection range for most passive radar systems is the signal-to-interference ratio, due to the large and constant direct signal received from the transmitter. An adaptive filter  1040  using the reference emitter  1100  removes the direct signal ensuring that the range/Doppler sidelobes of the direct signal do not mask the smaller echoes in the cross-correlation stage  1050 . 
   The cross-correlation function  1050  acts as the matched filter and also provides the estimates of the bi-static range and bi-static Doppler shift of each target echo. Since analog and digital broadcast signals are noise-like in nature, and only correlate with themselves, the cross-correlation processing  1050  uses a bank of matched filters, each matched to a different target Doppler shift. This may use an efficient implementation of the cross-correlation processing based on the discrete Fourier transform, as described in the paper  Using Reconfigurable HW for High Dimensional CAF Computation , by A. Hermanek, M. Kunes, and M. Kvasnicka, at the Institute of Information Theory and Automation, Academy of Sciences of the Czech Republic, Prague, Czech Republic, 2003, incorporated herein by reference. 
   Targets are detected on the cross-correlation surface by applying an adaptive threshold, and declaring all returns above this surface to be targets. A standard averaging constant false alarm rate (CFAR) algorithm or more advanced algorithms may be used as illustrated in block  1060 . The line-tracking function  1070  tracks target returns from individual targets, over time, in the range-Doppler space produced by the cross-correlation processing. A standard Kalman filter may be used to reject most false alarms during this stage of the processing. 
   For the tracking function  1080 , a simple bi-static configuration using one transmitter and one receiver may be employed or the target state (position, heading, velocity) may be derived from the measurement set of bi-static range, bearing and Doppler using a non-linear filter, such as a extended Kalman filter. When multiple transmitters are used from the database, a target may be detected by each transmitter and the returns from the target will appear at a different bi-static range and Doppler shift from each transmitter and so it is necessary to associate the returns from each of the transmitters. The target is located accurately with an approach that combines the measurements from each transmitter using a non-linear filter, such as an extended Kalman filter. 
   The feature extraction element  2000  uses a processing function to detect targets and determine target parameters from ambiguity surface data. The system may use a peak/noise discriminator to compare previous ambiguity surface data to the ambiguity surface data and to update bins correlating to the previous ambiguity surface data. Target classification  2010  is performed through comparing the extracted features, e.g., rotorcraft information, to a database of information  2000 . 
   Referring to  FIG. 5 , in a second embodiment of the present invention, PCL tracking and identification is combined with other sources. This embodiment shows the utility of PCL when integrated with other sources of surveillance that include ADS-B, multilateration, broadband emitter tracking, or Passive Emitter Tracking (PET), as well as identification information from all sources including electronic intelligence (ELINT). Aircraft  100  is flying with the range of multilateration/ADS-B receivers and broadband PET receivers, shown as combined receivers for this embodiment  150 . Multilateration/ADS-B/PET sensor data is sent to central server  250  using a variety of communications media, including either of fiber, telephone line, wireless, or satellite communications  200 . 
   The PCL channel consists of a reference antenna and a beam-forming network and is shown separately as  450  but in practice may be co-located with the other sensors  150 . The PCL information is processed to provide tracks and target classification  450  and the data sent to the central server. Finally, the ELINT channel, which provides electronic “fingerprinting” associated with the PET tracking provides data on the types of emitter payload of each target. Therefore, at central server  300  the tracking consists of the fusion of the following sources, where available:
         ADS-B   Multilateration   Broadband PET   PCL   SSR   Primary Radar       

   The target identification consists of the fusion of the following data where available:
         ADS-B/Mode S identification   Mode A/C squawk   PET ELINT fingerprinting and emitter classification   PCL target extraction and classification       

   While the preferred embodiment and various alternative embodiments of the invention have been disclosed and described in detail herein, it may be apparent to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.