Patent Publication Number: US-10775472-B2

Title: System and method for enhanced point-to-point direction finding

Description:
RELATED APPLICATIONS 
     The application is a continuation of co-pending U.S. patent application Ser. No. 13/549,215, filed on Jul. 13, 2012, and entitled “SYSTEM AND METHOD FOR ENHANCED POINT-TO-POINT DIRECTION FINDING,” which claims priority of U.S. provisional application, Ser. No. 61/507,495, filed Jul. 13, 2011, and entitled “SYSTEM AND METHOD FOR ENHANCED POINT-TO-POINT DIRECTION FINDING,” by the same inventors, both of which are hereby incorporated in their entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related to a direction finder. More particularly, the present invention is related to a system and method for determining a range and or bearing between two or more units. 
     BACKGROUND OF THE INVENTION 
     Existing technologies allow users to find a location but often do not function correctly under certain circumstances. For example, global positioning satellite (GPS) based systems rely on microwave signals transmitted by Medium Earth Orbit satellites; such microwave signals are affected by multipath propagation and atmospheric conditions. Effects of multipath propagation include data corruption, signal nulling, increased signal amplitude and decreased signal amplitude. Since acquiring and tracking such signals can therefore be difficult or impossible, particularly when used indoors, GPS-based systems may become increasingly inaccurate or stop working. Wideband and ultra wideband signals have been used for some time for locating items in radar, especially radar arrays for close proximity. 
     SUMMARY OF THE INVENTION 
     A system, device and method that enables units (or parts of units) to communicate with each other and point to each other&#39;s location without requiring line-of-sight to satellites (as GPS does) or any other infrastructure. Further, the system, device and method are able to operate outdoors as well as indoors and overcome multipath interference in a deterministic algorithm (vs. statistical), while providing bearings at three dimensions, not only location but actual direction, and pocket-sized implementation. 
     A first aspect of the present application is directed to a system for determining a range between two or more units. The system comprises a first unit including a first transmitter, a first receiver and a first processor, wherein the first unit is configured to transmit a first signal to the second unit with the first transmitter and a second unit including a second transmitter, a second receiver and a second processor, wherein the second unit is configured to receive the first signal with the second receiver and determine the distance between the first unit and the second unit (including a time base error) with the second processor based on the frequency. The second transmitter then transmits a signal that is received by the first receiver. The range (including a time base error) is calculated at the first unit. The range (including a time base error) information from the second unit is then transmitted to the first unit which then nulls the time base error and calculates the range between the units. In some embodiments, the first signal is a chirp signal comprising two or more chirps. In some embodiments, the converting comprises convoluting the first signal such that the first signal becomes a single-sideband signal. In some embodiments, the second receiver comprises one or more mixers and the converting comprises down converting the first signal with the mixers. In some embodiments, the down converting comprises performing a discrete Fourier transform on the first signal. In some embodiments, determining of the distance between the first unit and the second unit is further based on the propagation speed and frequency ramp of the first signal. In some embodiments, determining of the distance between the first unit and the second unit comprises multiplying the frequency of the second signal by the propagation speed of the first signal and dividing by the frequency ramp of the first signal. In some embodiments, the second unit is further configured to synchronize with the first unit. In some embodiments, the first unit comprises a user interface that enables a user to adjust the bandwidth of the first signal and or the number of the first signals transmitted in a sequence for adjusting the resolution of the range determined by the system, the second receiver has an intermediate frequency equal to zero. In some embodiments, the second unit comprises a display and is further configured to use the display to display the calculated range. In some embodiments, the first signal is the sum of a transmitted signal transmitted from a first unit and one or more reflections of the transmitted signal, and the second unit is configured to determine the lowest frequency component or lowest phase component of the second signal and determine the distance between the first unit and the second unit based on the lowest frequency component or the lowest phase component of the second signal. In some embodiments, the second unit comprises three or more antennas and the receiving comprises inputting the first signal with each of the antennas such that the second unit inputs a received signal for each of the antennas, wherein the second unit is configured to determine the lowest frequency component of each of the received signals, calculate the phase of the lowest frequency component of each of the received signals and determine the bearing between the first unit and the second unit based on two or more different pairs of the phases calculated. In some embodiments, the three or more antennas are positioned in an array at the corners of an equilateral triangle. In some embodiments, determining the bearing between the first unit and the second unit based on two or more different pairs of the phases calculated comprises computing a vector sum of the two bearings calculated from each pair of phases. In some embodiments, the second unit comprises a controller coupled to a first switching element, and further wherein the second receiver is selectively coupled to two or more of the antennas with the first switching element. In some embodiments, the second unit is further configured to switch which of the two or more antennas is coupled to the receiver with the first switching element based on commands received from the controller such that the second receiver serially receives the received signals of each of the two or more antennas through the first switching element. In some embodiments, the second receiver is selectively coupled to at least one signal transformer for each of the two or more antennas with a second switching element that is coupled to the controller. In some embodiments, the second unit is further configured to switch which of the signal transformers is coupled to the second receiver with the second switching element based on commands received from the controller such that the second receiver serially transmits the received signals of each of the two or more antennas through the second switching element to a different one of the signal transformers. In some embodiments, the second unit is further configured to use the controller to synchronize the switching of the first switching element with the switching of the second switching element. In some embodiments, one or both of the first and second switching elements are implemented on the second unit with software. In some embodiments, the second unit is further configured to use the controller to adjust the frequency of the switching of the first switching element and the second switching element in order to suppress the switching frequency from affecting the received signals. In some embodiments, the first unit comprises a first altimeter and the second unit comprises a second altimeter, and further wherein the second unit is further configured to use the second processor to calculate a vertical component of the bearing by comparing a first altitude value of the first unit measured by the first altimeter with a second altitude value of the second unit measured by the second altimeter. In some embodiments, the second unit comprises a display and is further configured to use the display to display the calculated bearing. 
     A second aspect of the present application is directed to a unit for determining a range between the unit and one or more other units. The unit comprises a transmitter for transmitting signals to the other units, a receiver for receiving signals from the other units and a processor for processing the received signals, wherein the unit is configured to convert a received signal received from one of the other units to a converted signal with the receiver and determine the distance between the unit and the one of the other units with the processor based on the frequency of the down-converted signal. In some embodiments, determining of the distance between the unit and the one of the other units comprises multiplying the frequency of the converted signal by the propagation speed of the received signal and dividing by the frequency ramp of the received signal. In some embodiments, the unit further comprises a user interface that enables a user to adjust the bandwidth of and or the number of signals to be transmitted in a sequence by the unit to the other units for adjusting the resolution of the range determined by the unit. In some embodiments, the received signal is the sum of a transmitted signal transmitted from the one of the other units and one or more reflections of the transmitted signal, and the unit is configured to determine the lowest frequency component of the converted signal and determine the distance between the unit and the one of the other units based on the lowest frequency component or the lowest phase component of the converted signal. In some embodiments, the unit further comprises three or more antennas coupled to the receiver, wherein the unit is configured to receive the received signal by inputting the received signal with each of the antennas, determine the lowest frequency component of the received signal inputted by each of the antennas, calculate the phase of the lowest frequency component of the received signal inputted by each of the antennas and determine the bearing between the unit and the one of the other units based on two or more different pairs of the phases calculated. In some embodiments, determining the bearing between the unit and the one of the other units based on two or more different pairs of the phases calculated comprises computing a vector sum of the two bearings calculated from each pair of phases. In some embodiments, the unit further comprises a controller coupled to a first switching element, wherein the receiver is selectively coupled to two or more of the antennas via the first switching element, wherein the unit is further configured to switch which of the two or more antennas is coupled to the receiver with the first switching element based on commands received from the controller such that the receiver serially receives the received signals of each of the two or more antennas through the first switching element. In some embodiments, the receiver is selectively coupled to at least one signal transformer for each of the two or more antennas via a second switching element that is coupled to the controller, wherein the unit is further configured to switch which of the signal transformers is coupled to the receiver. In some embodiments, the unit further comprises a display, wherein the unit is further configured to display the calculated bearing and or calculated range with the display. 
     A third aspect of the present application is directed to a method of determining a distance between two or more units. The method comprises transmitting a first signal from a first unit to a second unit, receiving the first signal at the second unit, down converting the first signal with a receiver of the second unit and determining the distance between the first unit and the second unit based on the frequency of the down-converted signal. In some embodiments, the first signal is a chirp signal comprising two or more chirps. In some embodiments, the converting comprises convoluting the first signal such that the first signal becomes a single-sideband signal. In some embodiments, the converting comprises down converting the first signal with a mixer of the receiver of the second unit. In some embodiments, the converting comprises performing a discrete Fourier transform on the first signal. In some embodiments, determining of the distance between the first unit and the second unit is further based on the propagation speed and frequency ramp of the first signal. In some embodiments, determining of the distance between the first unit and the second unit comprises multiplying the frequency of the second signal by the propagation speed of the first signal and dividing by the frequency ramp of the first signal. In some embodiments, the method further comprises synchronizing the first unit with the second unit. In some embodiments, the method further comprises adjusting the bandwidth of the first signal and or the number of the first signals transmitted in a sequence to create a desired resolution of the determined range. In some embodiments, the receiver of the second unit has an intermediate frequency equal to zero. In some embodiments, the method further comprises displaying the distance between the first unit and the second unit on the first unit or the second unit. In some embodiments, the method further comprises determining a bearing of the location of the first unit relative to the location of the second unit based on the first signal. 
     A fourth aspect of the present application is directed to a method of overcoming multi-path effects. The method comprises receiving a received signal at a second unit, wherein the received signal is the sum of a transmitted signal transmitted from a first unit and one or more reflections of the transmitted signal, converting the received signal to a converted signal with a receiver of the second unit, determining the lowest frequency component of the converted signal and determining the distance between the first unit and the second unit based on the lowest frequency component of the converted signal. 
     A fifth aspect of the present application is directed to a method of synchronizing a two or more units, wherein a first unit is able to generate a first signal and a second unit is able to generate a second signal. The method comprises transmitting the second signal from the second unit to the first unit and measuring a first time based difference with trip delay between the first signal and the second signal at the first unit, transmitting the first signal from the first unit to the second unit and measuring a second time base difference with trip delay between the second signal and the first signal at the second unit, determining the trip delay between the first unit and second unit based on the second time base difference with trip delay and the first time base difference with trip delay, determining the time base difference without trip delay between the first signal and the second signal based on the determined first or second time base difference with trip delay and the determined trip delay and synchronizing the first signal and the second signal based on the determined time base difference without trip delay. In some embodiments, the first signal and the second signal are chirp signals comprising two or more chirps. In some embodiments, the first signal matches the second signal. In some embodiments, the method further comprises transmitting a control signal from a first unit to a second unit, wherein the control signal requests the second unit to begin transmitting the second signal to the first unit. In some embodiments, the method further comprises adding an autocorrelation function to the second signal as a preamble at the second unit and transmitting the second signal to the first unit upon receiving the control signal from the first unit. In some embodiments, determining the time base difference without trip delay comprises summing the first time base difference with trip delay and the second time base difference with trip delay. 
     A sixth aspect of the present application is directed to a method of determining a bearing between two or more units. The method comprises receiving a first signal from a first unit at three or more antennas of a second unit such that the second unit inputs a received signal for each of the antennas, determining the lowest frequency component of each of the received signals, calculating the phase of the lowest frequency component of each of the received signals and determining the bearing between the first unit and the second unit based on two or more different pairs of the phases calculated. In some embodiments, the three or more antennas are positioned in an array at the corners of an equilateral triangle. In some embodiments, the determining the bearing between the first unit and the second unit based on two or more different pairs of the phases calculated comprises computing a vector sum of the two bearings calculated from each pair of phases. In some embodiments, the second unit comprises at least one receiver selectively coupled to two or more of the antennas with a first switching element that is coupled to a controller of the second unit. In some embodiments, the method further comprises switching which of the two or more antennas are coupled to the receiver with the first switching element based on commands received from the controller such that the receiver serially receives the received signals of each of the two or more antennas through the first switching element. In some embodiments, the at least one receiver is selectively coupled to at least one signal transformer for each of the two or more antennas with a second switching element that is coupled to the controller. In some embodiments, the method further comprises switching which of the signal transformers are coupled to the receiver with the second switching element based on commands received from the controller such that the receiver serially transmits the received signals of each of the two or more antennas through the second switching element to a different one of the signal transformers. In some embodiments, the controller synchronizes the switching of the first switching element with the switching of the second switching element. In some embodiments, one or both of the first and second switching elements are implemented with software. In some embodiments, the controller adjusts the frequency of the switching of the first switching element and the second switching element in order to suppress the switching frequency from affecting the received signals. In some embodiments, the method further comprises calculating a vertical component of the bearing by comparing a first altitude value of the first unit measured by an altimeter of the first unit with a second altitude value of the second unit measured by an altimeter of the second unit. In some embodiments, the method further comprises displaying the calculated bearing on a display of the second unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a graph of the instantaneous frequency of a chirp signal according to some embodiments. 
         FIG. 2  illustrates a block diagram of a DF system in an unreflective environment according to some embodiments. 
         FIG. 3  illustrates a block diagram of the receiver as it receives the chirp signal according to some embodiments. 
         FIG. 4  illustrates a block diagram of a DF system in a reflective environment as described above according to some embodiments. 
         FIG. 5  illustrates a second unit configured to determine a bearing according to some embodiments. 
         FIG. 6  illustrates a graph of two chirp signals each having two chirps according to some embodiments. 
         FIG. 7  illustrates a block diagram of a second unit having a switched antenna array according to some embodiments. 
         FIG. 8  illustrates a block diagram of a DF system that is able to be used to perform the functions described herein according to some embodiments. 
         FIG. 9  illustrates a flow chart of a method of determining a trip delay plus time base error between two or more units according to some embodiments. 
         FIG. 10  illustrates a flow chart of a method of overcoming multi-path effects in determining a range and/or bearing between units according to some embodiments. 
         FIG. 11  illustrates a flow chart of a method of determining a bearing between two or more units according to some embodiments. 
         FIG. 12  illustrates a flow chart of a method of synchronizing a two or more units according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, numerous details are set forth for purposes of explanation. However, one of ordinary skill in the art will realize that the invention can be practiced without the use of these specific details. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein or with equivalent alternatives. 
     Direction finding (DF) refers to the establishment of the bearing and range from which a received signal was transmitted. Embodiments of the presently claimed application are directed to a DF system, device and method that enable units (or parts of units) to communicate with each other via signals, and thereby point to each other&#39;s location without requiring line-of-sight to satellites (as GPS does) or any other infrastructure. The DF system, device and method is implemented with a network of two or more units/devices that are able to communicate via radio frequency (RF) or other types of signals and find the bearing and range to each other. The DF system, device and method are able to operate outdoors as well as indoors and overcome multipath interference in a deterministic algorithm (vs. statistical), while providing bearings at three dimensions, not only location but actual direction, and pocket-sized implementation. In particular, some embodiments of the DF system, device and method described herein extend the beneficial use of chirp signals to accomplish the above-described point-to-point range and bearing measurement. 
       FIG. 1  illustrates a graph  100  of the instantaneous frequency of a linear chirp signal  102  according to some embodiments. As shown in  FIG. 1 , the graph of the chirp signal  100  comprises a frequency domain or frequency axis  101  and the time domain or time axis  103 . In particular, the frequency f(t)  104  of the chirp signal  102  is able to be an “up-chirp” and start at the bottom of the frequency range and rise linearly over time to the top of the range. Alternatively, the chirp signal is able to be a “down-chirp” and have the inverse behavior wherein the frequency  104  of the chirp signal  102  starts at the top of the frequency range and drops linearly over time to the bottom of the range. Alternatively, the chirp signal  102  is able to be non-linear, such as an exponential chirp that exponentially rises or drops over time. In some embodiments, the chirp signals  102  described herein are able to be described by the equation:
 
chirp( f   c   ,A,t )=cos(2π f   c   t+πAt   2   (1)
 
and the instantaneous frequency f(t) of the chirp signals  102  is able to be described by the equation:
 
                       f   ⁡     (   t   )       ≡       1     2   ⁢   π       ⁢       d   ⁢           ⁢     ϕ   ⁡     (   t   )         dt         =       f   c     +   At             (   2   )               
where f c =The chirp minimal frequency [Hz], A=The chirp ramp [Hz/second] and t=time. Alternatively, the chirp signals  102  described herein are able to be described by other equations as are well known in the art.
 
Range Measurement in a Non-Reflective Environment
 
       FIG. 2  illustrates a block diagram of a DF system  200  in an unreflective environment according to some embodiments. As shown in  FIG. 2 , the DF system  200  comprises a first unit  202  having a signal generator  206  and transmitter/receiver  204 , and a second unit  203  having a signal generator  207  and transmitter/receiver  205 . In particular, the first unit  202  is able to transmit a chirp signal  102  generated by the signal generator  206  to the second unit  203 . In some embodiments, the chirp signal  102  is a radio frequency up-chirp. Alternatively, the chirp signal  102  is able to comprise other frequencies and be any combination of up or down chirps. In some embodiments, the first unit  202  and the second unit  203  comprise transceivers such that they are interchangeable and capable of both transmitting and receiving chirp and other signals at radio and other frequencies to each other. In some embodiments, communications within the DF system  200  are able to be in the radio frequency (RF) or in other frequency ranges. Each unit  202 ,  203  is able to be identified by a unique ID to enable communication between multiple units. The communications are able to be ensured by using, for example, half duplex, including cyclic redundancy checks (CRC) and acknowledgments (Ack) for every transferred message, however other suitable protocols are able to be used or adapted for ensuring communication as are well known in the art. Additionally, although the DF system  200  of  FIG. 2  only shows two units  202  and  203 , any number of receiving and/or first units are able to be included within the system  200 . 
     As shown in  FIG. 2 , the transmitted chirp signal  102  travels distance L  201 , wherein travel time or propagation delay τ is equal to L/C, where C is the propagation speed of the signal  102 , which in this case is able to be the speed of light corrected for the medium (e.g. air). The chirp signal  102  as transmitted by the first unit  202  is T(t)=chirp(f c , A, t), where t is time, f c  is the initial frequency, and A is the frequency ramp. However, due to the propagation delay τ, the chirp signal  102  as received by the second unit  203  is R(t)=T(t−τ)=chirp (f c , A, t−τ). For simplicity, these equations assume that the receiver  203  and transmitter  202  are fully synchronized and that the receiver unit  203  comprises a single-sideband (SSB) chirp down converting receiver or other type of down converting receiver as will be discussed in detail below. 
       FIG. 3  illustrates a block diagram of the receiver  205  as it receives the chirp signal  102  according to some embodiments. As shown in  FIG. 3 , the receiver  205  comprises an input node  305  for inputting signals detected by an antenna, a Hilbert filter element  304 , one or more signal mixers  301 ,  302 , a summing element  303  and an output node  306 . Specifically, the input node  305  is coupled to the input of one of the mixers  301  directly and to the input of the other mixer  302  via the Hilbert filter  304 . The mixers  301 ,  302  each have inputs coupled to the signal generator  207  for receiving locally generated signals and an output coupled to the summing element  303 , which is coupled to the output node  306 . In some embodiments, the receiver  205  is a SSB down converting receiver. In some embodiments, the receiver  205  is configured such that it has an intermediate frequency f IF  equal to zero (see for example http://en.wikipedia.org/wiki/Direct-conversion_receiver for information about zero IF receivers). Alternatively, the receiver  205  is able to be other types of down converting receivers having other intermediate frequencies. For example, the receiver  205  is able to comprise other components and configurations capable of down converting a received chirp signal  102  as are well known in the art. In some embodiments, the filter element  304  is able to be a different transform filter. Alternatively, a non-SSB receiver is able to be implemented. 
     As shown in  FIG. 3 , the chirp signal  102  enters at input node  305  and is input by the filter element  304  which filters the signal  102  (using a Hilbert transform) such that after summation at summing element  303  it becomes a SSB signal. In particular, as described above, if the chirp signal  102  as transmitted by the first unit  202  is T(t)=chirp(f c , A, t), the chirp signal  102  as received at input node  305  is R(t)=T(t−τ)=chirp(f c , A, t−τ). As a result, assuming that the chirp frequency is approximately equal to f c  (e.g. if the modulating signal&lt;&lt;f c ), it is able to be shown that: Hilbert(R(t))≅sin [2π(f c −f IF )(t−τ)+πA(t−τ) 2 ]. A down conversion is then able to be performed on the signal  102  as input directly from the input node  305  and as output by the filtering element  304 . Specifically, the mixers  301 ,  302  are able to down convert the received signals with the quadrate components of a local chirp signal generated by the signal generator  207 . The resulting output signals are then output to the summing element  303  which sums the received signals and outputs the receiver output signal to the output node  306 . Specifically, after the down converting the output signal is given by: 
                     out   ⁡     (   t   )       =     cos   ⁡     [       2   ⁢     π   ⁡     (       f   IF     -     A   ⁢           ⁢   τ       )       ⁢   t     +     2   ⁢     π   ⁡     (         A   2     ⁢     τ   2       -       f   c     ⁢   τ       )           ]               (   3   )               
where f IF  is the intermediate frequency of the receiver  205 . It should be noted that for the sake of brevity the amplitude of the signals was neglected during the above calculation and it was assumed that the receiver  205  and transmitter  204  are fully synchronized.
 
     As a result, the output signal out(t) is then able to be then analyzed using Fast Fourier Transform (FFT) methods or other types of signal analysis (e.g. spectral estimation) methods as are well known in the art in order to determine the range of the first unit  202  from the second unit  203 . In particular using these methods it is able to be determined that the output signal of the receiver is a sinusoidal signal at a frequency f out  given by: 
                     f   out     =       2   ⁢     π   ⁡     (       f   IF     -     A   ⁢           ⁢   τ       )         =     2   ⁢     π   ⁡     (       f   IF     -     A   ⁢     L   C         )                   (   4   )               
and at a phase of 2π((A/2)τ 2 −f c τ), assuming (A/2)τ 2 &lt;&lt;f c τ, the phase P(τ) of the output signal is given by:
 
                     P   ⁡     (   τ   )       =         -   2     ⁢   π   ⁢           ⁢     f   c     ⁢   τ     =       -   2     ⁢   π   ⁢           ⁢     f   c     ⁢     L   C                 (   5   )               
Therefore, it is apparent that the difference between the receiver&#39;s output signal frequency f out  and intermediate frequency of the receiver f IF  is proportional to the distance L between the second unit  203  and first unit  202 . Accordingly, as described above, with the intermediate frequency f IF  of the receiver  205  chosen to be equal to 0, the output frequency f ouf  is linear with the distance L. Similarly, the signal&#39;s phase P(τ) is practically linear to the distance L.
 
       FIG. 9  illustrates a flow chart of a method of determining a distance between two or more units according to some embodiments. As shown in  FIG. 9 , the first unit  202  transmits a chirp signal  102  to the second unit  203  at the step  902 . In some embodiments, the chirp signal  102  comprises two or more chirps. The second unit  203  receives the chirp signal  102  at the step  904 . The receiver  205  of the second unit  203  converts the chirp signal  102  to a second signal at the step  906 . In some embodiments, the converting comprises convoluting the chirp signal  102  such that the chirp signal  102  becomes a single-sideband signal. In some embodiments, the converting comprises down converting the chirp signal  102  with a mixer  301 ,  302  of the receiver  205  of the second unit  203 . In some embodiments, the converting comprises performing a discrete Fourier transform on the chirp signal  102 . The second unit  203  determines the distance between the first unit  202  and the second unit  203  based on the frequency of the second signal at the step  908 . In some embodiments, determining of the distance between the first unit  202  and the second unit  203  is further based on the propagation speed and frequency ramp of the chirp signal  102 . In some embodiments, determining of the distance between the first unit  202  and the second unit  203  comprises multiplying the frequency of the second signal by the propagation speed of the chirp signal  102  and dividing by the frequency ramp of the chirp signal  102 . In some embodiments, the receiver  205  of the second unit  203  has an intermediate frequency equal to zero. In some embodiments, the method further comprises displaying the distance between the first unit  202  and the second unit  203  on the first unit  202  or the second unit  203 . Thus, the method provides the advantage of enabling the range between the units to be accurately determined using the chirp signal  102 . 
     Range Measurement in a Reflective Environment 
     Unlike the DF system  200  shown in  FIG. 2  wherein a single signal takes a single path to the receiver unit  203 , if a signal  102  is transmitted in a reflective environment (e.g., indoors), the received signal is able to be the sum of the unreflected or line-of-sight (LOS) signal and the reflections of the signal that each take different paths. This results in a multi-path problem in which it must be determined which component of the received signal is the unreflected signal and thus both points to and indicates the range to the first unit  202 . 
       FIG. 4  illustrates a block diagram of a DF system  400  in a reflective environment as described above according to some embodiments. Specifically, the DF system  400  shown in  FIG. 4  is substantially similar to the DF system  200  shown in  FIG. 2  except for the differences described herein. As shown in  FIG. 4 , the DF system  400  comprises a first unit  402 , a second unit  403  and a reflecting element  401 . The reflecting element  401  is able to be any element or group of element able to reflect the signal  102 . Although only a single reflecting element  401  creating a single reflection and reflection path  404  is shown, it is understood that any number of reflecting elements  401  and paths  404  are able to be present. 
     Thus, as described above, because the reflections of transmitted signals have different path lengths than the unreflected signal, said reflections each have different propagation delays τ when received by the receiver  203 . As a result, the DF system  400  is able to distinguish between these different components of the received signal and determine which component relates to the unreflected or LOS signal. Further, by being able to discern the correct component having the LOS path, the bearing of the LOS path is also able to be determined by the system  400 . Thus, the system provides the benefit of enabling corrections for both bearing and distance to be made, as opposed to using other signals, where a mix of the two would often occur. 
     As shown in  FIG. 4 , the transmitter  402  sends a chirp signal  102  via antenna, which is received by receiver  403  via antenna. The signal  102  has a direct or unreflected path  407  having a path length L, and the reflected path  404 , reflected off of object  401 , having a path length L 1 . Thus, the signal received by the second unit  403  is given by:
 
 R ( t )= G×T ( t −τ)+ G   1   ×T ( t−τ   1 )= G ×cos [2π f   c ( t −τ)+π A ( t −τ) 2 ]+ G   1 ×cos [2π f   c ( t −τ)+π A ( t −τ) 2 ]  (6)
 
where G, G 1  are the received amplitudes for each path L, L 1 . As a result, after processing the received signal as described in  FIG. 3 , the output signal out(t) of the receiver is given by:
 
                     out   ⁡     (   t   )       =       D   ⨯     cos   ⁡     [       2   ⁢     π   ⁡     (       f   IF     -     A   ⁢           ⁢   τ       )       ⁢   t     +     2   ⁢     π   ⁡     (         A   2     ⁢     τ   2       -       f   c     ⁢   τ       )           ]         +       D   1     ⨯     cos   ⁡     [       2   ⁢     π   ⁡     (       f   IF     -     A   ⁢           ⁢     τ   1         )       ⁢   t     +     2   ⁢     π   ⁡     (         A   2     ⁢     τ   2       -       f   c     ⁢     τ   1         )           ]                   (   7   )               
where D and D 1  are amplitude constants. Then, assuming the intermediate frequency f IF  of the receiver  403  is equal to zero, the output signal out(t) becomes:
 
                     out   ⁡     (   t   )       =       D   ⨯     cos   ⁡     [       2   ⁢   π   ⁢           ⁢   A   ⁢           ⁢   τ   ⁢           ⁢   t     +     2   ⁢     π   ⁡     (         A   2     ⁢     τ   2       -       f   c     ⁢   τ       )           ]         +       ∑     i   =   1     N     ⁢     [       D   1     ⨯     cos   ⁡     (       2   ⁢   π   ⁢           ⁢   A   ⁢           ⁢     τ   i     ⁢   t     +     2   ⁢     π   ⁡     (         A   2     ⁢     τ   i   2       -       f   c     ⁢     τ   i         )           )         ]                 (   8   )               
where τ=L/C and τ1=L 1 /C. Consequently, it is able to easily be seen that for the more general case of N reflecting element  401  and N reflections having N reflected paths  404  the output signal out(t) is given by:
 
                     out   ⁡     (   t   )       =       D   ⨯     cos   ⁡     (       2   ⁢   π   ⁢           ⁢   A   ⁢     L   C     ⁢   t     -     2   ⁢   π   ⁢           ⁢     f   c     ⁢     L   C         )         +       ∑     i   =   1     N     ⁢     [       D   i     ⨯     cos   ⁡     (       2   ⁢   π   ⁢           ⁢   A   ⁢       L   i     C     ⁢   t     -       f   c     ⁢       L   i     C         )         ]                 (   9   )               
Then, assuming (A/2)τ 2 &lt;&lt;f c τ and substituting τ with L/C returns the following equation for the receiver output signal at reflective environment:
 
                     out   ⁡     (   t   )       =       D   ⨯     cos   ⁡     [       2   ⁢   π   ⁢           ⁢   A   ⁢           ⁢   τ   ⁢           ⁢   t     +     2   ⁢     π   ⁡     (         A   2     ⁢     τ   2       -       f   c     ⁢   τ       )           ]         +       D   1     ⨯     cos   ⁡     [       2   ⁢   π   ⁢           ⁢   A   ⁢           ⁢   τ1   ⁢           ⁢   t     +     2   ⁢     π   ⁡     (         A   2     ⁢     τ   2       -       f   c     ⁢     τ   1         )           ]                   (   10   )               
Thus it is able to be seen that the signal is composed of N+1 sinusoids, with each sinusoid&#39;s frequency proportional to a path length, and each phase also practically linear to the path length. Thus, according to the system and method disclosed herein, the correct length/range and bearing is able to be calculated for each path based on the frequencies of the output signal f out  and phases of the output signal, wherein the shortest path is the unreflected or LOS path. Further, it is able to be seen that the shortest path generates the lowest frequency f out  if the intermediate frequency of the receiver is equal to zero.
 
     Consequently, a simple spectral decomposition (e.g., FFT) enables measuring the frequency of the lowest frequency component of the received signal and deducing the range (distance from transmitter  402  to receiver  403 ). In particular, if the lowest component frequency is f, then L=(f*C)/A defines the range measurement. Phase measurement is able to be done on the lowest frequency component enabling bearing measurement with multipath suppression. Accordingly, the DF system  400  provides the benefit of enabling multipath suppression as well as range determination by simply examining the lowest frequency component of the receiver output signal. 
       FIG. 10  illustrates a flow chart of a method of overcoming multi-path effects in determining a range and/or bearing between units according to some embodiments. A second unit  203  inputs a received signal that is the sum of an unreflected chirp signal  102  transmitted from the first unit  202  and one or more reflections of the chirp signal  102  at the step  1002 . In some embodiments, the unreflected chirp signal  102  comprises two or more chirps. The receiver  205  of the second unit  203  converts the received signal to a converted signal at the step  1004 . In some embodiments, the converting comprises convoluting the received signal such that it becomes a single-sideband signal. In some embodiments, the converting comprises down converting the received signal with a mixer  301 ,  302  of the receiver  205  of the second unit  203 . In some embodiments, the converting comprises performing a discrete Fourier transform on the received signal. The second unit  203  determines the lowest frequency component of the converted signal at the step  1006 . The second unit  203  determines the distance between the first unit  202  and the second unit  203  based on the lowest frequency component of the converted signal. In some embodiments, determining of the distance between the first unit  202  and the second unit  203  is further based on the propagation speed and frequency ramp of the unreflected chirp signal  102 . In some embodiments, determining of the distance between the first unit  202  and the second unit  203  comprises multiplying the lowest frequency component of the converted signal by the propagation speed of the unreflected chirp signal  102  and dividing by the frequency ramp of the unreflected chirp signal  102 . In some embodiments, the receiver  205  of the second unit  203  has an intermediate frequency equal to zero. In some embodiments, the method further comprises displaying the determined distance between the first unit  202  and the second unit  203  on the first unit  202  or the second unit  203 . Thus, the method provides the advantage of enabling the range between the units to be accurately determined using the chirp signal  102  in a reflective environment wherein only the distance of the unreflected signal needs to be calculated. 
     Bearing Measurement 
       FIG. 5  illustrates a second unit  500  configured to determine a bearing according to some embodiments. In some embodiments, the second unit  500  is substantially similar to the second unit  203  described in  FIGS. 2 and 3  except for the differences described herein. As shown in  FIG. 5 , the second unit  500  comprises a plurality of antenna  502 ,  503  each coupled to one or more receivers (not shown) for receiving transmitted signals  505  from a transmitting antenna or source  501 . In some embodiments, the plurality of antenna  502 ,  503  form an array and are positioned in a set formation such as at the corners of an equilateral triangle. Alternatively, the plurality of antenna  502 ,  503  are able to be positioned in other set formations such as along the perimeter of a circle or in other formations as are well known in the art. In some embodiments, the second unit  500  comprises two antennas  502 ,  503  wherein by detecting the angle and the side, the correct signal  505  is able to be defined, but a phantom signal  504  is also identified. Specifically, if the receiver  500  comprises only two antennas  502 ,  503 , the transmitted signal  505  and the phantom signal  504  are not able to be distinguished because the sign of angle θ  506  is not given by the formula to determine the angle θ  506  using a single pair of antennas. Thus, alternatively the second unit  500  is able to comprise three or more antenna wherein the third and more antennas are able to be used to define another antenna pair in order to distinguish a phantom signal angle from the correct signal angle. In either case the bearing is able to be calculated as described below. 
     Specifically, the second unit  500  enables the bearing (θ) of a received signal (or to the transmitter) to be determined by 1) performing a spectral estimation (FFT or other type of spectral estimation) for the signal received at each antenna (by one or more receivers coupled to the antennas) and 2) calculating the phases P 1 , P 2 , . . . of the lowest frequency component of the signal received at each antenna, wherein the bearing is able to be calculated using any pair of the phases with the following equation: 
                   θ   =       sin     -   1       ⁢       λ   ⨯     (       p   1     -     p   2       )         2   ⁢   π   ⁢           ⁢   d                 (   11   )               
where λ is the average wave length of the transmitted signal and d is the distance between the two antennas where d&lt;λ/2. In other words, using a single pair of phases P 1 , P 2  from a single pair of antenna  502 ,  503 , an ambiguous bearing of plus or minus θ is able to be determined. To eliminate the ambiguity, a second pair of phases (e.g. P 1 , P 3  or P 2 , P 3 ) from a second pair of antenna is able to be used to determine a second ambiguous bearing of plus or minus θ. In some embodiments, only a single ambiguous bearing is calculated and other means are used to determine the correct bearing of the two results. In some embodiments, two ambiguous bearings are calculated to determine the correct bearing from the four results.
 
Bearing Measurement Using a Switched Antenna Array
 
       FIG. 7  illustrates a block diagram of a second unit  700  having a switched antenna array according to some embodiments. The second unit  700  is able to be substantially similar to the second units described in reference to  FIGS. 2-5  described above. As shown in  FIG. 7 , the second unit  700  comprises an antenna array  704  having a plurality of antennas, one or more switching elements  703 ,  705 , a receiver  706 , a controller  702  and one or more spectral analysis or signal transformers  701   a - c . Specifically, each antenna of the antenna array  704  is selectively coupled with the input of the receiver  706  via a first switching element  705 . The output of the receiver  706  is then selectively coupled with the one or more transformers  701   a - c  via a second switching element  703 . The controller  702  is also coupled to each of the switching elements  703 ,  705 . In some embodiments, the plurality of antenna of the antenna array are positioned in a set formation such as at the corners of an equilateral triangle. Alternatively, the plurality of antenna are able to be positioned in other set formations such as along the perimeter of a circle or in other formations as are well known in the art. In some embodiments, the receiver  706  is able to be substantially similar to the receiver  205  described above with reference to  FIGS. 2-5 . Alternatively, other types of receivers  706  are able to be used. 
     In some embodiments, the switching elements  703 ,  705  are switching circuitry or switches that are able to controllably physically couple a primary connection to each of a plurality of secondary connections. For example, the switching elements  703 ,  705  are able to comprise radio frequency or other types of switches. Alternatively, one or more of the switching elements  703 ,  705  are able to implemented with switching software such as they are able to controllably virtually couple a primary connection to each of a plurality of secondary connections. In some embodiments, the transformers  701   a - c  are FFT elements that perform FFT spectral analysis on received signals. Alternatively, one or more of the transformers  701   a - c  are able to be other types of elements capable of performing spectral analysis on a received signal as are well known in the art. Although, as shown in  FIG. 7 , the second unit  700  comprises three antennas, a single receiver  706  and three transformers  701   a - c , it is understood that the second unit  700  is able to comprise any number of antennas, receivers  706  and transformers  701   a - c . For example, in some embodiments there is a separate receiver  706  and/or transformer  701  for each antenna within the array  704 . Alternatively, one or more antennas are able to share or couple to a single receiver  706  and/or transformer  701  such there is at least one less receiver  706  and/or transformer  701  then the number or antennas within the array  704 . 
     In operation, the controller  702  controls the switching of the first switching element  705  such that the received signal from each of the antennas of the antenna array  704  is sequentially coupled to the receiver  706  one at a time. The received signals are processed by the receiver  706  and are then individually/serially sent from the receiver  706  to one of the transformers  701   a - c  via the second switching element  703 . In some embodiments, the received signals are processed by the receiver  706  in the same manner as described in reference to  FIGS. 3-5 . The controller  702  is able to control the switching of the second switching element  703  such that each signal received by the receiver  706  is coupled and output to a different transformer  701   a - c . For example, the controller  702  is able to synchronize the switching of the first switching element  705  with the switching of the second switching element  703  such that a selected transformer  701   a - c  receives all signals input by a specified antenna of the array  704 . In some embodiments, to suppress the switching frequency of the switching elements  703 ,  705  from affecting the resulting received signal spectrum for each of the antennas, the controller  702  is able to configure the switching between the antennas in a gradually changing frequency, in a pseudo random orthogonal sequence or in other manners that are able to suppress or minimize the switching frequency from affecting the received signals. In particular, these switching approaches are able to yield a spectrum of the receiver that is highly similar to an “unswitched” approach. Alternatively, the second switching element  703  is able to be omitted and all the received signals are able to be sequentially transmitted from the receiver to a single transformer  701 . By comparing the results of these transformers  701   a - c , as shown earlier in  FIG. 3  and following discussions, both the bearing and the range of a transmitted signal/first unit are able to be calculated. 
     Vertical Bearings 
     In some embodiments, the calculated bearing is able to include a vertical or altitudinal aspect. Specifically, in some embodiments both the first unit  202  and the second unit  203  are able to comprise a barometric or other type of altimeter such as a micro digital altimeter. In such embodiments, the units  202 ,  203  are able to display/present the altitude difference with or separately from a calculated bearing and/or range by transmitting an altitude request signal to the other unit. In response to receiving the request signal, the other unit is configured to detect its current altitude with the local altimeter and transmit a response signal including the altitude data back to the first unit  202 ,  203 . The first unit  202 ,  203  then determines its own altitude using its altimeter and computes the altitude difference between its value and the value received from the other unit. This determined difference equals the vertical component of the bearing and is then able to be displayed with or separately from the range and/or bearing on the first unit  202 ,  203 . In some embodiments, if the other unit is out of range such that the request signal or response signal is not received, the first unit is able to use the last valid altitude data received from the other unit in order to calculate the vertical bearing component. This approach is very beneficial, especially for static targets (like parking cars), because it does not require the use of GPS or infrastructure and is in many cases able to extend the effective range of the unit  202 ,  203  beyond the radio frequency range. 
       FIG. 11  illustrates a flow chart of a method of determining a bearing between two or more units according to some embodiments. A plurality of antennas of a second unit  203  receive a chirp signal  102  from a first unit  202  such that the second unit  203  inputs a received signal for each of the antennas at the step  1102 . In some embodiments, the plurality of antennas comprises three or more antennas. In some embodiments, the three or more antennas are positioned in an array at the corners of an equilateral triangle. The second unit  203  determines the lowest frequency component of the received signals at the step  1104 . The second unit  203  calculates the phase of the lowest frequency component of each of the received signals at the step  1106 . The second unit  203  determines the bearing between the first unit  202  and the second unit  203  based on the phases calculated. In some embodiments, the bearing is determined based on two or more different pairs of the phases calculated. In some embodiments, the determining the bearing comprises computing a vector sum of the three bearings calculated from each of the three pair of phases. In some embodiments, the second unit  203  comprises at least one receiver  205  selectively coupled to two or more of the antennas with a first switching element  705  that is coupled to a controller  702  of the second unit  203 . In some embodiments, the method further comprises switching which of the two or more antennas are coupled to the receiver  205  with the first switching element  705  based on commands received from the controller  702  such that the receiver  706  serially receives the received signals of each of the two or more antennas through the first switching element  705 . In some embodiments, the at least one receiver  706  is selectively coupled to at least one signal transformer  701  for each of the two or more antennas with a second switching element  703  that is coupled to the controller  702 . In some embodiments, the method further comprises switching which of the signal transformers  701  are coupled to the receiver  705  with the second switching element  703  based on commands received from the controller  702  such that the receiver  706  serially transmits the received signals of each of the two or more antennas through the second switching element  703  to a different one of the signal transformers  701 . In some embodiments, the controller  702  synchronizes the switching of the first switching element  705  with the switching of the second switching element  703 . In some embodiments, one or both of the first and second switching elements  703 ,  705  are implemented with software. In some embodiments, the controller  702  adjusts the frequency of the switching of the first switching element  705  and the second switching element  703  in order to suppress the switching frequency from affecting the received signals. In some embodiments, the method further comprises calculating a vertical component of the bearing by comparing a first altitude value of the first unit  202  measured by an altimeter of the first unit  202  with a second altitude value of the second unit  203  measured by an altimeter of the second unit  203 . In some embodiments, the method further comprises displaying the calculated bearing on a display  807  of the second unit  203 . Accordingly, the method of determining a bearing provides the benefit of enabling an unambiguous bearing to be determined for only the direct or unreflected signal. 
     Chirp Signal Bandwidth Considerations 
     Although the chirp signal  102  described in the previous sections is theoretically able to be neither time limited nor bandwidth limited, such a signal would be impractical. Accordingly, in some embodiments the 2.4 GHz Industrial, Scientific and Medical (ISM) radio band (wherein the bandwidth (BW) is limited to 83.5 MHz) and a chirp signal  102  length (T) of 1 ms are able to be selected resulting in a chirp ramp A that is equal to BW/T=83.5 MHz/1 ms=83.5e9 [Hz/sec]. Alternatively, other pulse lengths T and bandwidths are able to be chosen resulting in differing chirp ramps A. 
     DF System Resolution 
     The ability to distinguish between two paths (e.g. reflected and direct) as well as the range measurement resolution is dependent on the frequency difference per distance (frequency separation) of the chirp signal  102  and on the resolution of the spectrum estimator/transformer  701 . The frequency separation is given by df=A×(dl/C), wherein a separation of dl path length produces a df frequency offset at the receiver  205  output signal out(t). Assuming the use of FFT as a spectral estimator, the frequency resolution of the estimator is given by Δf=1/T window  where T window  is the sampling window length. As a result, the range resolution of the DF system  200  is able to be calculated by comparing df to Δf, where if df=Δf, then:
 
 A ×( dl/C )=1/ T   window   (12)
 
Assuming a single chirp is transmitted per signal  102 , then:
 
 A=BW/T   window   (13)
 
And thus, the DF system  200  range resolution per single chirp pulse (as well as the multipath mitigation ability) is given by:
 
Δ l=C/BW.   (14)
 
Thus, for BW=83.5 MHz, the range resolution is 3.59 m, resulting in a maximal estimated error of approximately 1.8 m. For many applications this is not a sufficiently low error. To improve that, a sequence of two or more chirp signals are able to be transmitted per signal  102 , resulting in a longer sampling window T window  and lower range error. Specifically, for a sequence of two 1 ms length chirp signals the maximal range estimation error reduces to 0.9 m (with an average error of about 0.45 m).
 
     Accordingly, in some embodiments, the chirp signal  102  is able to comprise two or more chirps as shown in the chart  600  illustrated in  FIG. 6 . In particular,  FIG. 6  illustrates two chirp signals  601 ,  611  each having two chirps according to some embodiments. As shown in  FIG. 6 , curve  602  shows multiple, repeated up-chirps going up and then starting again for each chirp, whereas curve  612  shows an up-chirp ramp-up and then a down-chirp down slope. Specifically, the first chirp of each signal  602 ,  612  is shown by T chirp    605 ,  615  compared to the entire signal  602 ,  612  of both chirps shown by T window    606 ,  616 . Both curves  602 ,  612  are set on axes of a chirp frequency  604 ,  614  and a window time  603 ,  613 . Additionally, it should be noted that the sequence of two or more up-chirps, down-chirps and/or a combination of up and down chirps per signal  102  is possible. In some embodiments, to further improve the DF system  200  range resolution, other signal spectral estimators/transformers are able to be used such as MUSIC, ESPRIT, SAGE, and other types of transformers as are well known in the art (For more information see http://www.dtic.mil/dtic/tr/fulltext/u2/a514411.pdf). 
     Time Synchronization Between Units 
     In the previous discussions, a theoretical assumption was made that the first and second units  202 ,  203  are fully synchronized. This approach is obviously impractical. To overcome this problem, the signal generators  206 ,  207  of the units  202 ,  203  are able to generate a local repetitive chirp signal. As a result, assuming that the time base difference between the units is T 0 , the local chirp signal at the first unit  202  is able to be represented as X first =chirp(f o , A, t), and the local chirp signal at the second unit  203  is able to be represented as X second =chirp(f o , A, t+T 0 ). 
     The synchronization is then able to be performed in three levels:
     1. Control-level synchronization. The units  202 ,  203  communicate by sending control signals to each other (e.g., via FSK or other types of communication). Specifically, a control signal is able to be sent from the first unit to the second unit (or vice versa) that requests the second unit to begin sending the local chirp signal of the second unit to the first unit. This control signal method crudely synchronizes the units  202 ,  203  and limits T 0  to a typical range of hundreds of μs.   2. Sampling time synchronization level. The second unit  203  adds a preamble to its local chirp signal comprising a narrow autocorrelation function. In some embodiments, the autocorrelation function comprises a Zadoff-Chu (ZC) sequence. Alternatively, other types of autocorrelation functions or orthogonal signals are able to be used. Upon receiving the preamble to the local chirp signal of the second unit, the first unit  202  shifts its local chirp signal based on a cross correlator that recognizes the received timing of the autocorrelation function of the preamble. This approach further synchronizes the units  202 ,  203  and limits T 0  to a typical range of sampling time period (e.g., for sampling rate of 1 MHz: ¦T 0 ¦&lt;1 μs). However, for most applications, this still does not allow accurate enough measurement, as 1 μs at the speed of light equals a range error of about 300 m.   3. Fine synchronization. The local chirp signal sequence is transmitted from the second unit  203  to the first unit  202 , the first unit  202  receives the chirp signal of the device  203  shifted by the trip delay (τ). Thus, the Xfreceiving signal as received by the first unit  202  is given by:
 
 X   freceiving =chirp( f   o   ,A,t+T   0 −τ).  (15)
 
Therefore, the first device is able to measure (e.g. using the method described above in the Range measurement sections) the time difference between the received signal X freceiving  and its own local chirp sequence X first . Given the above, it is able to be seen that the measured time base difference result at the first unit  202  will be t+T 0 −τ−t, which is equal to the value of T 0 −τ.
   

     The first unit  202  transmits its local chirp signal X first  to the second unit  203 . Therefore, the second unit  203  receives the chirp signal of the first unit  202  shifted by the trip delay (τ), which is given by X rsecond =chirp(f o , A, t−τ). The second unit  203  is then able to similarly measure the time difference between the received signal X rsecond  and its own local chirp signal X second . In this case, it is able to be seen that the measured time base difference result at the second unit  203  will be t−τ−(t+T 0 ), which is equal to the value of −τ−T 0 . 
     Accordingly, this measured time base difference value at the second unit  203  is able to be sent to the first unit  202  and the measured time base difference value at the first unit  202  is able to be summed with the measured time difference value at the second unit  203 . This summed value will be equal to T 0 −τ−τ−T 0  which is equal to −2τ (because the synchronization error T 0  is canceled out), meaning that the trip delay τ is equal to the summed value divided by −2. The range is straightforwardly calculated by multiplying the trip delay with the signal propagation speed. In addition for other purposes than range measurement, once the value of the trip delay τ is determined, the first unit  202  is able to plug that value back into the time base difference equation, wherein the measured time base difference result at the first unit  202  is equal to the value of T 0 −τ in order to solve for T 0  (e.g. the synchronization error or time base difference between the first unit  202  and the second unit  203 ) The first unit  202  is able to adjust the local chirp signal and/or other signals such that they are synchronized with the second unit  203  based on the determined value T 0 . It should be noted that synchronization is needed for range measurement, whereas bearing measurement are able to be performed as described above even for an unsynchronized system. Note that the order of transmission is able to be exchanged so that the first to transmit will be the first unit  202  followed by a transmission from the second unit  203 . 
       FIG. 12  illustrates a flow chart of a method of synchronizing a two or more units according to some embodiments. A first unit  202  transmits its local chirp signal to a second unit  203  and the second unit  203  measures a first time base difference with trip delay between the local chirp signal and the received chirp signal at the step  1202 . The second unit  203  transmits its local chirp signal to the first unit  202  and the first unit  202  measures a second time base difference with trip delay between the local chirp signal and the received chirp signal at the step  1204 , wherein the second time base difference has an opposite sign as the first time base difference. The second unit  203  transmits its calculated time base difference with trip delay to the first unit  202  at the step  1206 . The first unit  202  determines the trip delay based on the received information and its local calculation at the step  1208 . In some embodiments, the first unit  202  synchronizes the local first unit chirp signal with the local second unit chirp signal based on the determined time base difference without trip delay. In some embodiments, the local chirp signals each comprise two or more chirps. In some embodiments, the local first unit chirp signal matches the local second unit chirp signal. In some embodiments, the method further comprises transmitting a control signal from the first unit  202  to the second unit  203 , wherein the control signal requests the second unit  203  to begin transmitting the local second unit chirp signal to the first unit  202 . In some embodiments, the method further comprises adding an autocorrelation function to the local second unit chirp signal as a preamble at the second unit  203  and transmitting the local second unit chirp signal to the first unit  202  upon receiving the control signal from the first unit  202 . In some embodiments, the method further comprises receiving the local second unit chirp signal from the second unit  203  and shifting the local first unit chirp signal based on the autocorrelation function to increase synchronization between the local signals. In some embodiments, determining the time base difference without trip delay comprises summing the first time base difference with trip delay and the second time base difference with trip delay. 
     Signal Sampling Rate at the Receiver 
     It is able to be seen from the equation (10), described above for the receiver output signal out(t), that the path-related frequencies are given by A(L i /C), where L i  is a specific path length. As the path is able to be a result of a multi-order reflections (e.g. the signal bounces from the source to a reflector and then to other reflectors) the maximal path length L i  is theoretically infinite and so is the output frequency of the output signal out(t). The signal strength, however, fades with path length. As a result, the receiver  205  is able to be configured/adjusted such that unwanted signals below a signal strength threshold are able to be ignored. For example, assuming interest only in signals that are 10 dB below the maximum LOS signal strength (e.g. a 10 dB signal strength threshold), and assuming that the DF system  200  is built for a maximum range of L max , the range that generates a 10 dB lower signal is 3.16*L max  (assuming free space signal fading). Alternatively, the signal strength threshold is able to be greater or less than 10 dB below the maximum LOS signal strength. The maximum resulting frequency is given by the following equation for the maximum output frequency: 
                     f   max     =     A   ⁢       3.16   ⁢           ⁢     L   max       C               (   16   )               
The sampling frequency is able to satisfy Nyquist theorem and be thus more than double f max . If de-chirping is performed prior to sampling, then, f max =A(L max /C), as higher frequencies are able to be filtered out. In this example, for a mile L max  and BW=83.5 MHz, a sampling rate of 1 mega-sample per second (MSPS) is able to suffice. Accordingly, in some embodiments, the signal strength threshold and/or the maximum range L max  of the receiver  205  are able to be adjusted by a user. If only the direct path is of interest (e.g. for range calculation) then filtering is able to be implemented assuming L max  as the maximum range of the direct path.
 
Hardware Implementations
 
       FIG. 8  illustrates a block diagram of a DF system  800  that is able to be used to perform the functions described herein according to some embodiments. Specifically, the DF system  800  is able to be substantially similar to the first and/or second units  202 ,  203  except for the differences described herein. As shown in  FIG. 8 , the system  800  comprises a transceiver  816 , a central processing unit (CPU)  801 , a bus  802 , memory  803 , nonvolatile memory  804 , a display  807 , an I/O unit  808 , and a network interface card (NIC)  813  all coupled together via the bus  802 . The transceiver  816  is able to comprise the transmitting and/or receiving components described above in relation to  FIGS. 2-7 . In some embodiments, the transceiver  816  is integrated into a device of the system  800 . Alternatively, one or more of the components of the transceiver  816  are able to be added as one or more NICs to the system  800  and/or coupled to the device via the network  814 . In some embodiments, all or some of the components of the system  800  are able to be further integrated into one or more chips or integrated devices, reducing the component count and the cost of the system  800 . The I/O unit  808  is able to, typically, be coupled to a keyboard  809 , a pointing device  810 , a hard disk  812 , a real-time clock  811  and/or other peripheral devices. The NIC  813  is able to couple with a network  814 , which is able to be the Internet, a local network, or other types of wired or wireless networks as are well known in the art. Also shown as part of system  800  is power supply unit  805  which is coupled to an ac power supply  806 . Alternatively, or in addition to the system  800  is able to comprise batteries or other types of power sources. In some embodiments, the system  800  is able to comprise a graphical user interface (GUI) that enables a user to select one or more units to target, command the device to determine a bearing and/or range to the targeted units and/or perform the other adjustments and or commands described herein. In addition, the system  800  is able to comprise other devices and modifications that are well known in the art but have been omitted for the sake of brevity. It is understood that various modifications and changes are able to be made to the system  800  without departing from the broader spirit and scope of the system and method disclosed herein. 
     SUMMARY OF ADVANTAGES 
     The DF system, device and method described herein enables devices (or parts of devices) to communicate with each other and point to each other&#39;s location including the following advantages:
         The system does not require line of site to satellites (like GPS) nor other infrastructure.   The technology enables point-to-point direction finding with no need for any infrastructure.   The system operates outdoors as well as indoors and is able to overcome multipath interference in a deterministic algorithm (vs. statistical).   The technology is able to provide bearings in three dimensions.   Pocket size implementation.   Provides not only location but actual direction (i.e. bearing).       

     The algorithm/methods described herein are based on chirp signal transmission between the nodes and are unique in the following areas:
         The chirp signals are able to be used in a way that enables the “disassembly” of the received signal to components that are separated to “LOS” components and multipath generated components.   After the decomposition as described above, the range is able to be calculated only on the LOS signal component.   Bearing is able to be found using three antennas at the locating device on the LOS component only. No triangulation or other infrastructure is needed.   The accuracy of the measurement is able to be highly improved by lengthening the transmitted signal with no effect on the bandwidth of the transmitted signal.   Trip delay measurement—In many other systems, trip delay between units is measured by sending a “time stamp” or equivalent from one unit to the other and then replying within a period of time which must be very accurate (with low variance). Such a method requires very accurate time measurement and very accurate response time. As described herein, the synchronization is based on simple measurements and cancel-out of the time-based differences.   In the algorithm/methods described herein, the trip delay between the units is able to be measured using an algorithm that eliminates the difference of time bases between the units/devices. In that way, the response time start accuracy is not needed and the time measurement accuracy is simply achieved by the de-chirping.
 
Accordingly, the DF system, device and method described herein has numerous advantages.
       

     The present application has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. For example, the amount of implementation using hardware and software is able to be changed, without departing from the spirit of the inventions. Further, such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. A person skilled in the art would appreciate that various modifications and revisions to system and method for locating items and places. Consequently, the claims should be broadly construed, consistent with the spirit and scope of the invention, and should not be limited to their exact, literal meaning.