Abstract:
A method to passively locate an emitter using two aircraft to form a large baseline interferometer. The basic two element (two aircraft or helicopters) large baseline interferometer includes self-calibration and allows for various configurations for geo-location of ground-based emitters. The two aircraft large baseline interferometer can measure phase difference of arrival (PDOA) to very precisely locate the emitter in angle. Moving emitters can also be located and tracked using the method of the invention with greater accuracy than can be achieved from a single platform.

Description:
RIGHTS OF THE GOVERNMENT 
   The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 

   CROSS-REFERENCE TO RELATED PATENTS 
   The present document is somewhat related to my issued U.S. Patents “SELF-CALIBRATING LARGE BASELINE INTERFEROMETER FOR VERY PRECISE EMITTER LOCATION USING TIME DIFFERENCE OF ARRIVAL” and “TIME DIFFERENCE OF ARRIVAL RATE”, U.S. Pat. No. 6,255,992, issued Jul. 3, 2001; and “MOVING EMITTER PASSIVE LOCATION FROM MOVING PLATFORM”, U.S. Pat. No. 6,577,272, issued Jun. 10, 2003, and both commonly assigned to The United States of America as represented by the Secretary of the Air Force. The contents of these, my somewhat related issued patents, are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   This invention relates to the field of remote energy-emitting source location through passive received signal processing. 
   Although radio locating has been used since the early days of radio, current military apparatus with its limited output signal durations and the availability of computerized signal processing, faster analog to digital conversion apparatus and a need to accomplish rapid, accurate signal locations from a moving vehicle provide opportunity for improvement in this art. 
   Current state of the art for passive geo-location of a ground based emitter using two aircraft requires each aircraft to independently measure time of arrival (TOA) of the radio frequency (RF) signal, exchange the information, so that one or both can then calculate time difference of arrival (TDOA). This will require atomic clocks on both aircraft, or some other method of determining the precise time at each aircraft. The accuracy of this open loop approach is also limited by the sample times of a sampling clock and the linearity of the pulse leading edge. The open loop method is also limited by the differences in the path lengths from the antenna to the measurement receiver. A more precise method of determining TDOA is a differential approach where TDOA is obtained by iteratively adjusting an analog variable delay line until the two signals cancel. This cancellation is very precise because it not only cancels the pulse envelope but also the RF carrier by phase alignment. The method of TDOA measurement by differential delay adjustment requires both RF signals to be available at the two channel measurement receiver and that both channels operate with a common LO and common sampling clock. This method also includes an approach to calibrate the two receive paths of the interferometer. 
   The present invention improves the precision of currently available methods by using two aircraft to form a large baseline interferometer. This avoids problems associated with using a single aircraft with one or more tethered antennas. The present invention allows for various configurations for geo-location of ground-based emitters from moving aircraft and tracking both stationary and moving emitters. 
   SUMMARY OF THE INVENTION 
   A method to passively locate a ground-based emitter using two aircraft to form a large baseline interferometer. The basic two element (two aircraft or helicopters) large baseline interferometer includes self-calibration and allows for various configurations for geo-location of ground-based emitters. The two aircraft large baseline interferometer measurement capability includes phase difference of arrival (PDOA) to very precisely locate the emitter in angle. Moving emitters can also be located and tracked with the method of the invention using multiple aircraft. 
   It is an object of the present invention, therefore, to provide rapid, accurate location of a stationary ground emitter, such as a radar transmitter, from a double moving platform. 
   It is another object of the invention to provide a radio frequency source locating arrangement which is self-calibrating notwithstanding the presence of environmental-sourced and other inaccuracy influences. 
   It is another object of the invention to provide a time based moving platform locating system providing angle and range information relative to a distant moving target through use of received radio frequency signals. 
   It is another object of the invention to provide a stationary emitter locating system that is based on the differing arrival times of a radio frequency signals at two moving aircraft receivers using an iteratively adjusting variable delay line which cancels out two radio frequency signals. 
   It is another object of the invention to provide a moving or stationary emitter location system based on the use of large baseline signal interferometers. 
   It is another object of the invention to provide for the location of a moving emitter, such as an airborne radar, using multiple interferometers each formed from two aircraft. 
   These and other objects of the invention are achieved by the description, claims and accompanying drawings and by a multiple configuration, self-calibrating baseline interferometer radio frequency emitter locating apparatus using two aircraft comprising the combination of:
         a first search aircraft containing a first radio frequency measurement receiver, a first radio frequency antenna and a first variable delay component;   a second aircraft containing a second radio frequency measurement receiver, a second radio frequency antenna and a second variable delay component;   time difference of arrival, phase difference of arrival and phase difference of arrival rate processing apparatus disposed in each of said first and second radio frequency measurement receivers of said first and second aircraft;   selectively operable signal propagation time delay calibration apparatus connectable with signal propagating paths interconnecting said first and second aircraft;   and each of said signals entering each measurement receiver on said first and second aircraft whereby time difference of arrival, phase difference of arrival, phase difference of arrival rate are calculated by said processing apparatus and radio frequency emitter location is determined.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the use of two aircraft to geo-locate a stationary ground-based emitter according to the present invention. 
       FIG. 2  shows geometric relationships applicable to radio frequency signals in a preferred arrangement of the present invention. 
       FIG. 3  shows a family of time and angle error curves relating to the  FIG. 4  geometric relationships and their attending mathematical relationships. 
       FIG. 4  shows the basic parts of a large baseline interferometer according to the invention. 
       FIG. 5  shows a digital measurement receiver used in a preferred arrangement of the invention. 
       FIG. 6  shows a digital measurement receiver that is one approach for the RCVR 1  and RCVR 2  of FIG.  5 . 
       FIG. 7   a  shows a two-aircraft large baseline interferometer determining azimuth angle of an airborne emitter. 
       FIG. 7   b  shows a third aircraft, in addition to those of  FIG. 7   a , determining elevation of an airborne emitter. 
   

   DETAILED DESCRIPTION 
   Traditionally interferometers operate by way of measuring signal phase characteristics. In large baseline interferometers, however, phase measurement provides ambiguous results because of the cyclic or periodic nature of the signal phase data. Time difference of arrival measurements are, however, not of this ambiguous nature since they are not cyclic or periodic in character. The time difference of arrival measurement arrangement is therefore the theoretical basis for the present invention as is disclosed in the paragraphs following. 
   Traditionally large baseline interferometers (LBI) are located on one aircraft. This limits the LBI length to the physical size of the aircraft. Examples are the side of the aircraft, the wing tips of the aircraft, or the nose to wing tip of the aircraft. U.S. Pat. No. 6,255,992 taught how to extend the LBI by using a tethered antenna and also taught how to calibrate the LBI to improve accuracy. (This calibration scheme can also be applied to a LBI installed on a single aircraft.) In U.S. Pat. No. 6,225,992 the geo-location accuracy is improved due to both the self-calibration scheme and the increased LBI length due to the ability to use a tethered antenna. This invention teaches how to form a phase interferometer from two separate aircraft. 
     FIG. 1  in the drawings shows a preferred arrangement of the invention for geo-locating a ground-base emitter  102  by aircraft  104  and  106 . Phase difference of arrival (PDOA) from the two aircraft interferometer locates the line of bearing  112  to the emitter very precisely but this measurement is ambiguous. Time difference of arrival (TDOA) is then used to resolve the ambiguities. The two aircraft determine the azimuth angle to the emitter. The elevation angle is determined by the aircraft altitude and the fact that the emitter is located on the ground. 
     FIG. 2  shows geometric relationships applicable to radio frequency signals in a preferred arrangement of the present invention. The two LBI antenna are  202  and  204 . The points  202  and  204  represent the two aircraft of FIG.  1 . The stationary emitter is located at  200 . The emitter is located relative to  206  the center of the LBI. The bearing  208  and range R of the emitter relative to the LBI center  206  can be determined by equations 10 and 11 of U.S. Pat. No. 6,225,992. 
     FIG. 3  shows the results of applying equations 10 and 11 of U.S. Pat. No. 6,255,992. In the Cross Range Down Range diagram the emitter is located at  300 . An exact measurement of TDOA would result in a line of bearing from the origin  301  to the emitter  300 . Lines of bearing  302  and  304  represent the results of TDOA measurements with negative and positive error respectively. Likewise the error contours  306  and  308  represent measurement of TDOA rate with positive and negative errors respectively. The region bound by the four contours  302 ,  304 ,  306 ,  308  then bound the location of the emitter. 
     FIG. 4  of the drawings shows the basic parts of a LBI. The purpose of the LBI is to locate the emitter relative to the antennas  402  and  404 . This is accomplished by measuring PDOA, TDOA, and PDOA rate. (PDOA rate is also frequency difference of arrival (FDOA)). Primarily what is needed is PDOA and TDOA at the antennas. However the measurement is made at the receiver  400  not at the antennas. To determine the values at the antennas the paths P 1   406  and P 2   408  must be known. If the LBI is installed on a single aircraft then P 1  and P 2  can be measured before they are installed on the aircraft. However because of differential heating as the aircraft flies the paths may change. The self-calibration scheme of U.S. Pat. No. 6,255,992 can be used to calibrate for these changes. In the invention the two antennas are located on separate aircraft. As the aircraft fly both L and the paths too the receiver changes. Therefore self-calibration is essential for accurate operation. 
   In the present invention, self-calibration is achieved primarily by sending the signal received at each aircraft through the same paths. Therefore, the paths are not measured as such by calibration but are instead common to both signals.  FIG. 5  shows a digital measurement receiver used in a preferred arrangement of the invention. The purpose of the  FIG. 5  diagram is to measure the TDOA, PDOA and PDOA rate of the emitter. The TDOA is given by equation (1):
 
 TDOA =( R 1 =R 2 /c    Eq 1 
 
where c is the velocity of light.
 
   The PDOA is given by:
 
 PDOA =( 2π/λ)*(   R 1 −R 2)   Eq 2 
 
where λis the wavelength.
 
 PDOA rate=[PDOA ( t 2)− PDOA ( t 1)]/( t 2 −t 1)   Eq 3 
 
   The invention depicted in  FIG. 5  consists of two similar circuits on each aircraft. Aircraft 1 is illustrated at  500  and aircraft 2 is illustrated at  501 . The two main parts are the loop,  502  for aircraft 1 and  504  for aircraft 2 and the measurement receiver, illustrated at  503  for aircraft 1 and  505  for aircraft 2. Each loop contains a variable delay component, illustrated at  511  and  510 . This could be a digital RF memory. An alternative would be a multiple port switch to switch between fixed delays such as multiple lengths of fiber optic lines.  FIG. 5  is not to scale. The emitter is far from both aircraft so that paths R 1  at  506  and R 2  at  513  are approximately parallel. 
   The basic operation of the two aircraft interferometer is illustrated in  FIG. 5  where signal R 1  enters antenna 1 illustrated at  508  travels through loop1 at  502  then crosses over  509  to aircraft 2 and through loop 2  at  504 . Signal R 1  then travels into measurement receiver 2 (MR2),  505 , at input port 1 and back to aircraft 1 and into measurement receiver 1 (MR1),  503 , at input port 1. Likewise signal R 2  enters antenna 2 at  507  and travels through loop 2  at  504  then crosses over to aircraft 1 and through loop 1  at  502 . Signal R 2  then travels into MR1 at  503  at input port 2 and back to aircraft 2 and into MR2 at  505  at input port 2. Thus both signals enter each measurement receiver on each aircraft. The two signals R 1  and R 2  are prevented from overlapping in the amplifiers by timing. That is, the signal arriving from antenna 2 at  507  in loop  2  at  504  is held until after the signal arriving from antenna 1 is also stored in variable delay  2  at  510 . Then the signal from antenna 2 is transferred to variable delay  1  at  511 . Both signals enter each measurement receiver on each aircraft where TDOA, PDOA, and PDOA rate are measured. However, the values at each antenna are required not the values at the measurement receivers. That is, the paths from antenna 1 and antenna 2 to MR1 and MR2 are needed. However, since all the measurements are differential measurements the path absolute lengths are not required—only the difference between the two path lengths. 
   The following equations show how the required values are determined. There are four equations. Each equation starts at the emitter  512  in FIG.  5  and ends at the input to the measurement receiver. The equation sums up each path from the emitter to the input to the measurement receiver. 
   Path 1 is the path from the emitter through antenna 1 to the input of MR1.
 
 R 1+ P 11+ P 1L+P12+ P 1 g+Pc+P 2 g+P 22+ P 2 L+P 22+ P 2 g+Pc=T 11 m    Eq 4 
 
T11m is the measured arrival time of the signal through antenna 1 to MR1
 
Path 2 is the path from the emitter through antenna 2 to the input of MR1
 
 R 2+ P 21+ P 2 L+P 22+ P 2 g+Pc+P 1 g+P 12+ P 1 L+P 12 =T 12 m    Eq 5 
 
T12m is the measured arrival time of the signal through antenna 2 MR1
 
Path 3 is the path from the emitter through antenna 1 to the input of MR2
 
 R 1 +P 11 +P 1 L+P 12 +P 1 g+Pc+P 2 g+P 22 P 2 L+P 22 =T 21 m    Eq 6 
 
T21m is the measured arrival time of the signal through antenna 1 to MR2
 
Path 4 is the path from the emitter through antenna 2 to the input of MR2
 
 R 2+ P 21 +P 2 L+P 22 P 2 g+Pc+P 1 g+P 12 P 1 L+P 12 +P 1 g+Pc=T 22 m    Eq 7 
 
T22m is the measured arrival time of the signal through antenna 2 to MR2
 
 Now    TDOA*c=R 1 =R 2.   Eq 8 
 
   Solving equation 4 and 5 for R1 and R2 respectively and substituting into equation 8 gives:
 
 TDOA*c=T 11 m−T 12 m−Pc+ ( P 21 −P 11)+( P 12 P 22)− P 2 g    Eq 9 
 
Solving equation 6 and 7 for R 1  and R 2  respectively and substituting into equation 8 gives:
 
 TDOA*c=T 21 m−T 22 m+Pc+ ( P 21 −P 11)+( P 12 −P 22)+ P 1 g    Eq 10 
 
Now equating equations 9 and 10 and solving for Pc gives:
 
 Pc= 0.5*[( T 11 m−T 12 m )−( T 21 m−T 22 m )+ P 1 g+P 2 g]   Eq 11 
 
   Pc is the path length between input port 1 of MR1 and input port 2 of MR2 and is one parameter needed to determine TDOA. Pc cannot be pre-calibrated since it varies with aircraft position. (T11m−T12m) is a TDOA measurement made on aircraft #1. Note that T11m and T12m did not have to be measured separately. Only TDOA1=(T11m−T12m) needed to be measured. Likewise only TDOA2 =(T21m−T22m) needed to be measured. The terms P 1 g and P 2 g refer to the gap between the two inputs of MR1 and MR2, respectively. They are very small and can be calibrated in the factory so they are known values. Now that Pc has been determined by the measurements, TDOA*c can be determined from either equation 10 or 11 if the terms (P 21 −P 11 )+(P 12 −P 22 ) are known. Again, these terms can be made small and calibrated in the factory. A thermocouple could also be added to these small rigid paths to compensate for length variations as a function of temperature. 
     FIG. 6  in the drawings is a digital measurement receiver. An analog measurement receiver could be used as well. An analog receiver could measure amplitude difference of arrival (ADOA) and phase difference of arrival (PDOA). The digital measurement receiver depicted in  FIG. 6  will measure ADOA, PDOA, TDOA and PDOA rate. The measurement receiver depicted in  FIG. 6  is one of the two receivers in  FIG. 5  shown as RCVR  1  at  503  and RCVR  2  at  505 . The local oscillator frequency (LO) in  FIG. 6  is from the same source. The same sampling clock is also used for the two analog-to-digital (A/D) converters. The LO on each aircraft are similar but do not have to be the same. Likewise the sample clocks on the two aircraft are similar but do not have to be the same. The receiver in  FIG. 6  is a receiver as depicted as  400  in  FIG. 4  plus the addition of a variable delay line in each path. The basic receive function is that two radio frequency (RF) signals are received at ports  1  and  2  and converted to digital signals as they travel down paths Pr 1  and Pr 2  respectively. This part is typical of state of the art two channel digital receivers. One of these paths (Pr 1  for example) is typical of state of the art single channel digital receivers. A single receive channel (input port  1  to the digital processor) works as follows. The switch (SW) is set to receive the signal from port  1 . The attenuator (α 1 ) is set to prevent the Low Noise Amplifier (LNA) from saturating. The variable delay (τ 1 ) would not be in a typical receiver and will be explained later. The mixer  604  converts the signal from the high RF frequency to a low intermediate frequency (IF). The filter is to prevent frequencies other than the IF from passing. The IF is then converted to a digital signal by the analog-to-digital (A/D) converter. Most two channel digital receivers fabricate paths Pr 1  and Pr 2  to be equal. Some may also include calibration as shown in FIG.  6 . Note that if path Pr 1  at  601  and Pr 2  at  600  are the same them the TDOA measured by the digital processor is the same as the TDOA at the input ports. 
   To calibrate paths Pr 1  and Pr 2  in  FIG. 6 , set α 1  and α 2  (at  602  and  607  respectively) and τ 1  and τ 2,  (at  603  and  608  respectively) to zero. Then inject a signal down path Pt1. Since Pa and Pb are set equal at the factory, the measured TDOA through Pr 1  and Pr 2  should be zero. If it is not then the difference is used as the calibration correction factor. In equation form we can write:
 
 S+Pt 1 +Pa+Pr 1 =Tm 1   Eq 12 
 
 S+Pt 1 +Pb+Pr 2 =Tm 2   Eq 13 
 
Subtracting equation 12 from equation 13 gives
 
 Pr 2 −Pr 1=( Tm 2 −Tm 1)−( Pb−Pc )   Eq 14 
 
   Since Pb−Pa is known or zero then the measure values Tm1 and Tm2 determine the difference in Pr 2  and Pr 1 . Therefore the measurement receiver self calibrates. The reason for the analog variable delays (τ 1  and τ 2  ) in the  FIG. 6  TDOA receiver is to allow measurement accuracy greater than one clock cycle of the A/D clock. The digital processor can determine whether the signal arrived by measuring if it crossed a threshold. However this measurement is limited by the clock step. By varying the analog delay  603  or  608  such that the signal just crosses the threshold will allow a more precise result. It could be assumed that the pulse leading edge is linear and linear interpolation performed between the pulse amplitude samples just before and just after crossing a threshold. The two analog delays allow for an accurate result even if the pulse leading edge is not linear. Note that the two attenuators  602  and  607  need to also be adjusted so the received pulses average amplitudes are equal. Therefore they both cross a threshold at the same level below their amplitude. Another method that could be used to determine TDOA is cross correlation. Cross correlation is possible with this invention because both signals from the LBI antennas are available on both aircrafts receivers. 
   The invention discloses how to form a LBI from two separate aircraft. These two aircraft now form a two element LBI as depicted in FIG.  2 .  FIG. 2  shows the geometric relationships of a two aircraft-disposed large baseline interferometer system having receiver antennas located orthogonal to the aircraft velocity vectors. In the  FIG. 2  drawing the emitter source is presumed located at the point  200  and each aircraft associated radio frequency measurement receivers are located at  202  and  204 . The straight line paths between each interferometer antenna and the emission source at point  200  are indicated to have lengths R 1  and R 2 . 
     FIG. 3  in the drawings shows a family of time and angle error curves relating to the  FIG. 2  geometric relationships and their attending mathematical relationships. In  FIG. 3 , a measurement system with no errors may measure a time difference of arrival of 50 nanoseconds and a time difference of arrival rate of 217 picoseconds/second and thereby the emitter would be correctly located at a point  300  in the  FIG. 3  drawing, (wherein from  FIG. 2 , R=50 kilometers and Θ=30 degrees). If instead, the measurement system incorrectly measured time difference of arrival as  49  nanoseconds, it would incorrectly place the emitter angle at line  302  in FIG.  3 . Similarly, if the measurement system incorrectly measured time difference of arrival as 51 nanoseconds, it would incorrectly place the emitter at line  304  in FIG.  3 . The angular wedge bounded by lines  302  and  304  in  FIG. 3  therefore represents the location bound of a system with ±1 nanosecond measurement accuracy. Similarly the curved boundaries  306  and  308  represent the error boundaries for the measurement accuracies of ±10 picoseconds/second for the time difference of arrival rate. The rectilinear wedge defined by the intersection of lines  302  and  304  and the error contours  306  and  308  in  FIG. 3  therefore show the possible location of the point  300 .  FIG. 3  is thus an example of results that may be obtained using the present invention. 
   The proceeding describes how a two element interferometer can be used to geo-locate a stationary emitter on the earth&#39;s surface. The TDOA gives the azimuth angle to the emitter. The intersection of this with the earth (aircraft altitude is known) then gives the emitter elevation. The TDOA rate or PDOA rate then determines range. When the emitter is airborne more than one LBI is required. For example as depicted in  FIG. 7  three aircraft would be required to determine azimuth and elevation. In  FIG. 7   a  aircraft  702  and  704  form a two aircraft LBI to determine the azimuth to the airborne emitter. To determine elevation a third aircraft depicted in  FIG. 7   b  is needed to determine elevation. In  FIG. 7   b  aircraft  702  and  706  form a vertical two element LBI to determine elevation. These three aircraft still have not determined range. Since the emitter is airborne, radar on any one of the three aircraft could be used to determine range. This, however, would not be a completely passive approach since one of the aircraft would have to radiate. For a completely passive approach a forth aircraft would be required to obtain a second azimuth line of bearing. Where these two azimuth line of bearings cross would then determine range. For even more accuracy two additional aircraft widely separated from the three aircraft in  FIG. 7  could be used to provide a second line of bearing. As the two lines of bearing become more and more orthogonal then the range accuracy would increase. The basic invention of the patent is how to form a self-calibrating LBI from two aircraft. The application of one LBI as depicted in FIG.  1  and of multiple LBIs as depicted in  FIG. 7  for emitter location are some examples of how this self-calibrating two aircraft LBI can be used. 
   The foregoing description of the preferred embodiment has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modification or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable on of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.