Abstract:
Methods, systems, and computer readable media for mitigation of interference of GPS signals are disclosed providing selective mitigation of in-band interference implementing deterministic phase control. In one embodiment, a system for mitigating interference of GPS signals includes a pair of antennas, each receiving GPS signals that include both a desired signal component and a jammer signal component. The signal from one antenna is phase-shifted as needed to make it anti-phase with the signal from the other antenna, so that when the two signals are combined, the jammer signal components substantially cancel each other, leaving the desired signal components. Determining the phase shift required involves deterministically calculating the phase shift based on the amplitudes of the two input signals and the amplitude of the combined signal instead of the iterative techniques used in conventional systems.

Description:
RELATED APPLICATIONS 
     The presently disclosed subject matter claims the benefit of U.S. Provisional Patent Application Ser. No. 61/604,480, filed Feb. 28, 2012; the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The subject matter described herein relates to methods and systems for enhancing reception of wireless signals. More particularly, the subject matter described herein relates to methods, systems, and computer readable media for mitigation of in-band interference of GPS signals. 
     BACKGROUND 
     GPS signals play an ever increasing role in our commercial, civil and military enterprises, providing everything from cell tower synchronization to delivery truck tracking and unmanned vehicles navigation. This reliance on GPS makes the growing frequency, power and sophistication of interference affecting its reception a substantial threat to our economy, as well as our homeland and national security. 
     Disruptive jamming and spoofing, a particularly insidious form of jamming, apparently were used by Iran to capture a US drone. Spoofing signals are GPS mimics that are broadcast intentionally in the GPS band to mask the actual signals and suborn the drone autopilot to lead the vehicle astray. Furthermore, more powerful signals from easily fabricated transmitters can blind a GPS receiver, potentially causing airline crashes as well as disrupting our national infrastructure. 
     Technology proposed to combat interference affecting GPS or other radio frequency (RF) receivers can be classed generally as digital signal processing, analog filtering, or steered antennas. Digital signal processing methods are sophisticated and diverse but depend on signals with an adequate signal to noise ratio (SNR) and preferably free of distortion before they are digitized. These methods can also require long signal records and substantial computation. Analog bandpass filters improve signal to noise ratio by rejecting frequencies outside a desirable frequency range. While this reduces the risk of distortion, it is ineffective against interference occurring at passband frequencies. Array steering is used to reduce receiver sensitivity in the direction of a jamming source but has several disadvantages: the hardware imposes a substantial burden in terms of size, weight, power, cost, and computational complexity; and array steering equally reduces sensitivity to desirably received signals from the direction of the null and other bearings represented by null side lobes. As a result, null steering is largely restricted to ground stations or large vehicles with the payload capacity and energy to provide the required hardware and power. 
     Conventional systems for defeating interference rely on temporal or spatial diversity. Temporal diversity excises from an antenna signal periods of time when interference is present to avoid its degrading a signal of interest (SI). Spatial diversity relies on differences in direction of propagation between interference and SI, acting to reduce sensitivity of the receiving system to signals from the direction of the interference. Sensitivity can be reduced by mechanically or electrically steering a null in the direction of the interference or by using directional antennas oriented in the direction of the interference and of SI. In this, defeat of interference without degrading SI requires a large number of array antenna elements, computationally intensive calculation of weightings applied to array element signals, and/or steered directional antennas. Such spatial diversity technology, however, also degrades SI propagating from a direction proximate that of the interference. And, the use of fewer array elements and/or less directional antennas broadens the null, resulting in cancellation of SI over wide ranges of bearing and reducing overall system performance. In light of this, technology that can defeat interference at any bearing including those proximate in bearing to SI, and doing so without large numbers of array elements or use of directional antennas, is clearly desirable. 
     In light of the above, we propose GPS receivers comprising selective cancellation of spoofing and other types of in-band interference using omnidirectional receiving antennas providing greater angular operating range and cancellation bandwidth at reduced cost and complexity. 
     OBJECT OF THE INVENTION 
     A first object of the invention is enhanced reception of navigation signals. A second object is enhanced reception of radio frequency signals. A third object is mitigation of signals that degrade GPS reception. A fourth object is mitigation of GPS spoofing. A fifth object is mitigating multiple sources of interference. A sixth object is reducing signal power entering a receiver. 
     SUMMARY 
     Devices and methods are disclosed for phase-controlled filtering of GPS signals to mitigate spoofing and other types of in-band interference. 
     According to one aspect, the subject matter described herein includes a device for phase-controlled filtering of GPS signals to mitigate spoofing and other types of in-band interference. The device includes at least one of: an antenna set, a feed-forward type signal modifying circuit, a signal converter circuit, and a receiver circuit. The antenna set includes a first antenna and a second antenna that can receive signals at GPS frequencies and an emitter antenna of any type that can emit a modified second antenna signal. Modifying circuitry may be any type that can modify signals from the second antenna, comprising at least one of: a detector, an amplifier, a phase shifter, a delay element, a combiner, and a controller. The converter circuit may be any type that can convert a signal for digital processing. The receiver may be any type that can process an antenna signal to provide enhanced GPS output. 
     According to another aspect, the subject matter described herein includes a method for phase-controlled filtering of GPS signals to mitigate spoofing and other types of in-band interference. The method includes modifying an antenna signal by amplitude equalizing, phase shifting and/or group delaying, followed by combining modified signals to selectively cancel interference content. Phase shifting is conducted according to a deterministic solution providing anti-phase alignment of desirably mitigated signals at the combiner. In one embodiment, a first antenna comprises a combiner that combines interaction signals induced at the antenna surface; an element of the modifying circuitry is used to combine antenna signals from antennas. The method includes delay compensation to modify cancellation bandwidth. Cancellation, which can be used to reduce more than one source or type of interference, is performed to provide an enhanced antenna signal. The enhanced signal is converted and processed to provide an enhanced GPS output, e.g. signal, data, message or display. 
     In one embodiment, the antenna set includes two adjacent coplanar patch type antennas and an emitter mounted immediately beneath the first patch antenna. The patch antennas are primarily omnidirectional and emitter is primarily directional. The first antenna may be any type that can combine interaction signals induced at the antenna surface by simultaneously arriving RF signals. Modifying circuitry may be any type that can anti-phase align an emitter type interaction signal with an interaction signal induced by a signal including GPS and interference contents. Modifying circuitry may be any type that can combine a plurality of antenna signals. 
     The subject matter described herein can be implemented in software in combination with hardware and/or firmware. For example, the subject matter described herein can be implemented in software executed by a processor. In one exemplary implementation, the subject matter described herein can be implemented using a non-transitory computer readable medium having stored thereon computer executable instructions that when executed by the processor of a computer control the computer to perform steps. Exemplary computer readable media suitable for implementing the subject matter described herein include non-transitory computer-readable media, such as disk memory devices, chip memory devices, programmable logic devices, and application specific integrated circuits. In addition, a computer readable medium that implements the subject matter described herein may be located on a single device or computing platform or may be distributed across multiple devices or computing platforms. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the subject matter described herein will now be explained with reference to the accompanying drawings, wherein like reference numerals represent like parts, of which: 
         FIG. 1  is a graph of the performance of a system according to an embodiment of the subject matter described herein, illustrating a high degree of cancellation of boresight interference; 
         FIG. 2  is a block diagram illustrating an exemplary system for GPS anti-spoofing including boresight cancellation with omnidirectional antennas according to an embodiment of the subject matter described herein; 
         FIG. 3  is a block diagram illustrating exemplary modifying circuitry according to an embodiment of the subject matter described herein; 
         FIG. 4  is a block diagram illustrating an exemplary device for further enhanced GPS reception according to an embodiment of the subject matter described herein; 
         FIG. 5  is a block diagram illustrating exemplary modifying circuitry for further enhanced GPS reception according to another embodiment of the subject matter described herein; 
         FIG. 6  illustrates the design of an antenna set having a coplanar nesting arrangement and a common origin suitable for use in a system for enhanced GPS reception according to an embodiment of the subject matter described herein; and 
         FIG. 7  is a flow chart illustrating an exemplary process for enhanced GPS reception according to an embodiment of the subject matter described herein. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with the subject matter disclosed herein, systems, methods, and computer readable media are provided for providing enhanced GPS reception by selectively cancelling spoofing, jamming and other types of interference (referred to collectively hereinafter as interference) at desirably received frequencies. 
     This disclosure is in terms of GPS signals but the concepts described herein may be applied to navigation, sensing, and/or communications signals of other types and/or at other frequencies. Examples of other frequencies that are used with or instead of GPS signals to synchronize wireless communications include, but are not limited to, the pilot signal components of long term evolution (LTE) signals. In one embodiment, a null is frequency aligned with the interference and/or the transmission to mitigate interference and provide enhanced synchronization. It is described for canceling one or two sources of interference but the concepts described herein may be applied to cancel more than two sources of interference. Boresight interference is defined as interference propagating on a bearing that is more or less the same as a signal of interest, the latter represented herein by a GPS signal. A non-boresight bearing may be any bearing that is substantially away from boresight, e.g. 5 or more degrees apart. 
     The present disclosure is in terms of interfered signals, defined here as including GPS signal content and jamming and/or spoofing signal content but the concepts described herein may be applied to interfered signals comprising any type of interference. For purposes of the current disclosure, spoofing is define as a type of intentional interference It is in terms of a plurality of antenna signals, such as from a plurality of antennas, although it can also be practiced with one or more polarization signals from one antenna. Antenna signals are characterized by differences in at least one of: amplitude, phase, delay, polarization, and bearing separation. 
     For the purposes of the present disclosure, interaction signals are defined as those induced at an antenna face by interaction of an RF signal with the face of the antenna, and antenna signals are signals from an antenna due to induction of an interaction signal. Distortion filtering is defined as means of preventing with respect to at least one frequency the output of distortion induced by at least one distortion inducing type of circuitry element such as an active type. Distorting, such as by amplifying a signal to induce distortion is defined here as a type of signal modification. 
     Devices and methods for phase-controlled in-band filtering that reduces amplitude of an undesirable signal with respect to at least one frequency of a desirably received signal as a means of providing enhanced reception of position, navigation and timing (“GPS”) signals are disclosed. Devices comprise at least one of: an antenna set, a feed-forward type signal modifying circuitry, a signal converter circuit, and a receiver circuit that can be of any type providing an enhanced GPS output. In one embodiment, an antenna set includes two omnidirectional antennas of any type that can receive GPS signals and an emitter type antenna of any type that can emit a signal that can induce interaction signals at the first antenna surface. An antenna set may be any type that can provide an enhanced first antenna signal. Modifying circuitry may be any type that can modify at least one antenna signal. Modifying circuitry includes at least one of: a detector, an amplifier, a phase shifter, a delay element, a combiner, and a controller. Detector is any type that can detect at least one of; amplitude, phase and delay. A converter circuit may be any type that can convert an enhanced antenna signal for digital processing. A converter includes at least one type of; signal conditioning, digital converting, mixing, down converting, direct sampling, demodulating and intermediate stage filtering. A receiver may be any type that can process an antenna signal to provide an enhanced GPS output. A receiver includes any type of digital signal processor that can provide at least one of: decoding, decrypting, determining of at least one of: position, navigation and timing type signals, and output providing. 
     One embodiment of the method includes phase shifting at least one amplitude modified signal according to a deterministic solution. In other embodiments, computationally less efficient solutions such as error minimization, synthetic annealing and steepest descent may be used. The method includes steps of phase shifting and electrical combining as means of at least partly cancelling interference in an arriving GPS signal to produce an enhanced antenna signal. In some cases, the method includes amplitude equalizing prior to phase shifting. Cancellation can be conducted by combining interaction signals at the surface of an antenna and/or combining antenna signals in circuitry. Cancellation can be provided for more than one source of interference. An enhanced signal is converted by a converter circuit and the converted signal is processed in a receiver circuit to provide enhanced GPS output of any type, e.g. signal, data, message or display. 
     One acceptable antenna set includes two adjacent coplanar patch antennas and an emitter antenna mounted immediately beneath a first patch antenna to provide an emitter signal created by modification of a signal from a second patch antenna, said emitter signal being modified so as to induce an interaction signal at the face of the first antenna which is amplitude equivalent to and anti-phase with respect to the interaction signal induced at the first antenna surface by interference. 
     Modifying circuitry may be any type that can provide at least one of: delaying, amplifying, phase shifting and combining. Antennas are characterized by at least one of: type, frequency range, bandwidth, gain pattern and/or polarization. A converter may be any device that can convert a signal to digital form. A receiver may be any type that can provide a GPS output by processing converted signal. 
     It will be appreciated by those versed in the art that identical signals arriving at an antenna from different locations, once amplitude compensated for antenna gain pattern, will induce exactly the same interaction signal in the face of the antenna. By extension, two simultaneously arriving signals having an anti-phase relationship will induce anti-phase interaction signals that cancel, resulting in no net interaction signal induction. From this, it can be seen that the antenna includes a type of combiner. 
     It will also be appreciated that the interference signal content is highly correlated between antenna signals, as is GPS contents between antenna signals, while interference content and GPS content within a signal, being induced by separate sources are not correlated. This difference in correlation between and within signals is exploited here to selectively cancel interference content while retaining GPS content, yielding a GPS signal substantially free of interference content. 
     The interference content in an interfered signal, being stronger than GPS content, dominates measurements of an interfered signal, thereby affecting calculations and modifications based on such measurements. For example, phase shifting of an interfered signal with respect to another to provide anti-phase alignment provides anti-phase alignment of interference contents but not of GPS contents. As a result, antenna combining of anti-phase interfered signals substantially cancels interference content but not GPS content, yielding a GPS signal having an enhanced signal to noise ratio (SNR). 
     The subject matter described herein provides several benefits over conventional systems, including: the ability to cancel interference without requiring a difference in ratio of jamming to desired signals between antenna signals and without requiring temporal or spatial diversity; the ability to provide broadband cancellation instead of the narrow bandwidth resulting from the set-apart antenna configuration of receiving antennas used by conventional systems; the ability to operate in a flush mounted configuration suitable for mounting on vehicles that are desirably aerodynamic and/or free of elements that can be damaged or entangled during operation; and the ability to minimize cosite interaction of its antennas and of nearby antennas. 
       FIG. 1  is a graph of the performance of a system according to an embodiment of the subject matter described herein, illustrating a high degree of cancellation of boresight interference.  FIG. 1  illustrates data recorded during field testing of an embodiment of the present invention having two vertically mounted commercial omnidirectional type whip antennas separated by 0.25 lambda of the test center frequency, which antennas are used to receive signals from sources transmitting interference and desirably received signals at 700 MHz signals from a range of 10 m, which sources are separated by angles up to 90 degree apart, illustrating clearly a high degree of cancellation of boresight interference by an embodiment of the present invention. 
       FIG. 2  is a block diagram illustrating an exemplary system for GPS anti-spoofing including boresight cancellation with omnidirectional antennas according to an embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 2 , a device  100  disclosed herein to mitigate signals from an interfering source  10  as means of enhancing the signal to noise ratio (SNR) of GPS signals from one or more satellite  20  includes at least one of: antenna set  120 , signal modifying circuitry  140  (hereinafter “modifier”), signal converter  160  and receiver  180 . Antenna set  120  includes first antenna  122 , second antenna  124  and emitter antenna  126 , which antennas are of any type that can receive or emit an RF signal. First antenna  122  has phase center  122   a  with respect to at least one frequency and second antenna  124  includes phase center  124   a  with respect to at least one frequency In one embodiment, antenna set  120  includes feedback or cosite interference preventer  128  of any type that can mitigate reception of signals from emitter  126  by second antenna  124 . 
     Modifier  140  may be any type that can modify, for at least one antenna signal, at least one of: amplitude, phase and group delay. Modifier  140  includes any type that can reduce distortion in at least one antenna signal according to the invention disclosed in PCT application serial number PCT/US11/49399, filed on Aug. 26, 2011, and U.S. Provisional Patent Application Ser. No. 61/546,784, filed Oct. 13, 2011, assigned to the assignee of the present invention, herein incorporated by reference in their entireties. Signal converter  160  may be any type that can convert RF signals to digital form. Receiver  180  may be any type that can process converted signal to provide a desirable output, such as signal, data, message or display. 
     In one embodiment, first antenna  122  and second antenna  124  are coplanar and have proximately placed phase centers  122   a ,  124   a , e.g. closely proximate for wide cancellation bandwidth or other placement for other cancellation bandwidth. In one embodiment, emitter  126  is mounted parallel to, directed at, and closely proximate first antenna  122 , although other locations, orientations and/or separations are also acceptable. In one embodiment, first antenna  122  and second antenna  124  are omnidirectional, although other direction types are acceptable. Emitter  126  is directional in direction of first antenna  122  although other direction types are acceptable. In some cases, device  100  includes shielding  128  or other means of mitigating interaction signal induction in second antenna signal or third antenna signal by signal transmitted by emitter  126 . 
       FIG. 3  is a block diagram illustrating exemplary modifying circuitry according to an embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 3 , modifier  140  includes elements of any type that can provide at least one of: time delay, amplification and phase shift. For example, modifier  140  can include an amplifier  142  and a phase shifter  144 . Amplifier  142  of any type that can increase or decrease amplitude of second antenna signal for amplitude equalization respect to first antenna signal amplitude. Amplifier may be any type that can be controlled by a signal from a controller or exogenous source. 
     Phase shifter  144  may be any type that can provide a desirable phase shift of second antenna signal with respect to first antenna signal according to the method described below. Illustrative type of phase shifter  144  includes, but is not limited to, controllable, fixed, continuously variable, selectable, of a combination thereof. Phase shifter may be any type that can be controlled by a controller or exogenous control signal such as a bias voltage. 
     Modifier  140  includes a controller  1400  of any type that can determine and/or provide control signal providing desirable phase shift of at least one signal, e.g. to provide anti-phase alignment between antenna signals. Controller  1400  includes at least one of: analog to digital converter, digital processor, digital to analog converter, exogenous signal source. Controller  1400  includes a signal detector  1500  of any type that can determine at least one of: amplitude, phase and delay of one or more signal at one or more frequency, such as power detector or spectrum analyzer chip. In some cases, detector  1500  is configured to operate with respect to desirable center frequency and/or passband width. 
       FIG. 4  is a block diagram illustrating an exemplary device for further enhanced GPS reception according to an embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 4 , device  100  may be any type that can cancel interference from a first source  10   a  and a second source  10   b , said device  100  comprising an antenna set  120  further comprising a first antenna  122 , second antenna  124 , emitter  126 , cosite blocker  128  and third antenna  130 , and modifier circuitry  140  of any type that can modify signal from second antenna  124  and modify signal from third antenna  130 . 
       FIG. 5  is a block diagram illustrating exemplary modifying circuitry for further enhanced GPS reception according to another embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 5 , modifier  140  may be any type that can modify signal from second antenna  124  to provide desirably modified signal to emitter  126  to be emitted in direction of first antenna  122  as means of creating an anti-phase interaction signal at surface of first antenna  122  that can at least partly cancel interference from a first source at the antenna surface and that can modify the signal from a third antenna  130  using a phase shifter  144   a  and combine said modified third antenna signal with output signal from first antenna  122  using a combiner  146   a  to further cancel interference from first source. 
     Modifier  140  can be modified by including a splitter ( 148 ) to split signal from second antenna  124  to provide two split signals, the first split signal being modified by a phase shifter  144   b  and emitted by emitter  126  to provide interaction signals at surface of first antenna  122  to cancel interference from first source to provide a first output signal from first antenna  122  substantially free of interference from first source. Second split signal is modified by a phase shifter  144   c  and combined with signal from third antenna  130  using a combiner  146   b  to cancel interference from first source to provide a second output signal substantially free of interference from first source. Example phase shifters include, but are not limited to, vector modulators. In some cases, phase shifter ( 144   a ,  144   b ,  144   c ) comprises variable amplifier type. Second output signal is combined with output signal from first antenna  122  using a combiner  146   a  to provide a final output signal that is substantially free of interference from first source and substantially free of interference from said second source. 
     In some cases, modifier  140  includes a delay element  150   a ,  150   b  of any type that can at least partly reduce difference in delay between third antenna signal and first antenna signal, e.g. due to circuitry elements used to modify second antenna signal. Delay element  150   a ,  150   b  may be any type that can provide a difference in group delay between first antenna signal and at least one of second antenna signal and third antenna signal. 
       FIG. 6  illustrates the design of an antenna set having a coplanar nesting arrangement and a common origin suitable for use in a system for enhanced GPS reception according to an embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 6 , antenna set  120  includes spiral type first antenna  122  and spiral type second antenna  124 , which antennas  122 ,  124  comprise a coplanar nesting arrangement having a common origin  123 . First antenna  122  and second antenna  124  have substantially the same shape, which shape provides for a lambda-constant separation of phase centers for at least one of a first frequency and a second frequency. Lambda-constant separation is defined here as phase separation comprising equal number of wavelengths, and/or part thereof, with respect to a plurality of resonant frequency of the antenna. 
       FIG. 7  is a flow chart illustrating an exemplary process for enhanced GPS reception according to an embodiment of the subject matter described herein. In the embodiment illustrated in  FIG. 7 , method  1000  includes at least one of: interaction signal cancelling  1200 , antenna signal cancelling  1400 , antenna signal converting  1600  and converted signal processing  1800 . Interaction signal cancelling  1200  includes; receiving first, and second antenna signals  1220 , modifying and transmitting second antenna signal  1240 , inducing anti-phase interaction signal  1260 , canceling interference content of interfered signal at first antenna face to provide an enhanced first antenna signal  1280 . Antenna signal cancelling  1400  includes; receiving third antenna signal  1420 , modifying third antenna signal  1440 , combining modified third antenna signal with first antenna signal to further enhance first antenna signal  1460 . Converting  1600  includes any means of converting first antenna signal for processing. Processing  1800  includes digital processing of converted signal by any means to provide enhanced GPS output in at least one form of; signal, data, message and delay. 
     Modifying  1240  includes altering at least one of: amplitude and phase of second antenna signal and emitting the modified signal in the direction of first antenna to induce an interaction signal which is equal in amplitude and anti-phase with respect an interaction signal induced by an unmodified interfered signal at the surface of the first antenna. 
     Phase shifting is calculated by a method based on those disclosed in U.S. Provisional Patent Application Ser. No. 61/393,157, filed Oct. 14, 2010, and PCT Application Serial No. PCT/US11/49399, filed Aug. 26, 2011, that are assigned to the assignee of the present invention and herein incorporated by reference in their entireties, comprising using results of inducing a test phase shift of any degree, e.g. 90 degrees, into at least one antenna signal and using combined signal amplitude provided by said shift to calculate anti-phase shift providing selective cancellation of interference. 
     Canceling at first antenna face  1260  includes employing antenna as combiner of an interaction signal induced by signals from the emitter and interaction signal induced by interfered GPS signal. Third antenna signal modifying  1440  includes at least one of: amplitude equalizing, delaying, and anti-phase shifting with respect to first antenna signal. Combining  1460  includes using any type of circuit to combine first antenna signal and modified third antenna signal. 
     Converting includes by any means including but not limited to; mixing, down converting, intermediate frequency filtering, demodulating, signal conditioning or digitizing. Processing  1800  includes any means of determining desired GPS values, including but not limited to; dispreading, decoding, digital filtering, location and/or velocity calculating, time extracting, and forming display, although other forms of processing and outputting are also permitted. 
     In one embodiment, the signals from each of two antennas are equalized, a test phase shift is introduced into one of the antenna signals, and the two antenna signals are combined. Anti-phase shift is then calculated deterministically to minimize amplitude of combined signal (β) in equation 1;
 
α(ω t+φ   1 )+α(ω t+φ   2 +δφ)=β(ω t )  (1)
 
     where α is the equalized amplitude of the antenna signals, e.g. using equation 2;
 
δφ= nπ± 2 arccos(β/2α)  (2)
 
The system then determines which of the two values produced by equation (2) is the correct value, and applies that value. For example, the system may select each of the two values in turn, measure β for each value, and choose the phase shift value that resulted in the lowest β. Alternatively, the system could perform a second step of test phase shifting using a second test shift, e.g. 45 degrees, to resolve the ± ambiguity in equation 2, for example by selecting the sum of test and calculated phase shifts that is reported by both the first test shift and the second test shift.
 
     In some cases, modifying further includes mitigating cosite interference by the emitter by any means of preventing induction of emitter content in output of second antenna, e.g. by shielding or by selecting gain pattern of second antenna and/or emitter. Cosite interference by the emitter can also be mitigated by combing a feed signal from the emitter with modified second antenna signal according to the systems and methods described herein. 
     The method includes determining β from time to time and, when β has changed by a predetermined amount, updating at least one of: amplitude and phase shift of modified signal. The method includes determining calculated value of β is optimal by any means, e.g. steepest descent or statistical methods. 
     An embodiment of the present invention can be operated in any manner. For example, at least one portion of the device can be operated more or less continuously as directed by external input. Modifying circuitry operation can be initiated when interference is detected, e.g. by the receiver portion of the device. Operation of modifying circuitry can be continuous without regard to presence of interference, e.g. as means of defeating spoofing type interference that can suborn navigation of a vehicle before such spoofing can be detected by current generation anti-spoofing technology. 
     The subject matter described herein may be applied to mitigate interference of other signals and frequencies. For example, one use of the disclosed technology is to defeat intentional or inadvertent interference that can degrade reception by cell towers of GPS signals which are used to synchronize communications across the system. 
     It will be understood that various details of the subject matter described herein may be changed without departing from the scope of the subject matter described herein. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.