Abstract:
A low-cost interference reduction system reduces interfering and jamming signals in a global positioning system receiver for such applications as rotating weapons and hand-held GPS receivers. A pair of antennas having a spin symmetric gain pattern receive global positioning system signals and interfering signals. An antijam electronics function connected to the antennas processes the global positioning system signals and the interfering signals to produce an output signal with reduced interfering signal levels. The global positioning system receiver is connected to the antijam electronics function for processing the global positioning system signals into position solutions and for providing a power detector output and a clock output to the antijam electronics function.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the global positioning system (GPS), GPS receivers, and interference reduction in GPS receiver systems. The invention may be used in rotating weapons, handheld GPS receivers, and any other GPS system requiring a low-cost interference reduction system. 
     Global positioning systems, such as the US NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Department of Defense is a satellite-based radio navigation system that transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with similar precision. GPS uses eighteen to twenty-four satellites that may be evenly dispersed in three inclined twelve-hour circular orbits chosen to ensure continuous twenty-four hour coverage worldwide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations. Each satellite transmits a pair of L-band signals. The pair of signals includes an L 1  signal at a frequency of 1575.42 MHz and an L 2  signal at a frequency of 1227.6 MHz. The L 1  and L 2  signals are biphase signals modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigation data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite. 
     Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN encoded signal to recover the navigation data, including ephemeris data. The ephemeris data is used in conjunction with a set of Keplerian equations to precisely determine the location of each satellite. The receiver measures a phase difference (i.e., time of arrival) of signals from at least four satellites. The time differences are then used to solve a matrix of four equations. The result is a precise determination of the location of the receiver in three-dimensional space. Velocity of the receiver may be determined by a precise measurement of the L 1  and L 2  frequencies. The measured frequencies are used to determine Doppler frequency shifts caused by differences in velocity. The measured differences are used to solve another set of equations to determine the velocity based upon the Doppler phase shift of the received signal. 
     GPS signals are very low in amplitude and are transmitted using a spread-spectrum signal bandwidth centered at 1575.42 MHz. The GPS signals cover a frequency spread of about 20 MHz. GPS receivers are subject to disruption by jammer signals, which may be transmitted either as narrow band signals or broadband signals. Known GPS receiver systems may reduce the effects of narrow band jamming by using frequency-selective filters, such as notch filters, to attenuate the jamming signal. However, broad band jamming signals are more difficult to reduce or eliminate as the frequency spread of the jamming signals approximates the frequency spread of the GPS signal. Because the frequency spreading sequence of the GPS signal is encrypted according to a pseudo-random noise code, the jammer cannot be precisely synchronized to the GPS signal. This permits the effects of the jamming signal to be reduced by nulling-out the jamming signal. Further, the signal strength of the jamming signal is typically much greater than the signal strength of the GPS signal. 
     Interference and jamming suppression and/or cancellation are a pressing need in military receivers. Cost and size of interference canceling systems such as null steering or beam forming systems have limited their use to GPS systems installed on the most high value platforms such as aircraft and ships. 
     Null steering techniques are known in the art. These nulling techniques are based on determining an angle of arrival of a signal based on the phase shift of the signal observed at different antenna elements of an antenna array. Various weights or weight values are assigned to each antenna element and are used to adjust the phase and level of attenuation of the received signal in an attempt to null-out the jammer signal. Power minimization is a known technique that attempts to adjust the weights so as to reduce the total measured power coming from the antenna array. Power minimization techniques rely on the assumption that the jammer signal is much stronger than the GPS signal and that it emanates from a different single direction of arrival. An array having multiple antenna elements is spatially selective so that a null can be placed in the direction of the jammer by adjusting the weight values. Various known algorithms, such as least mean squares and direct matrix inversion may be used to implement the power minimization technique in digital systems. In power minimization techniques, it is assumed that almost all of the signal power is due to the jammer component, and not due to the GPS signal, because the GPS signal is very weak and therefore provides no significant power contribution. 
     Recent size reductions in GPS receivers have permitted a new capability for them to be use small form factor weapon systems such as bombs, missiles, rockets, and artillery shells. These new platforms are extremely sensitive to cost and sometimes have requirements for an airframe that is continuously rolling and have high launch shock such as artillery shells. Additionally, if they are to be used for military purposes, resistance to interference and jamming is highly desired. 
     Handheld GPS receivers are widely used in military applications. Handheld GPS receivers would also benefit from a low-cost small size interference reduction system. 
     What is needed is a very low-cost interference reduction system specifically designed for use in weapons systems having such requirements as small size, a rolling airframe, and high-g acceleration when a shell is fired. It is also desirable to be able to incorporate the low-cost interference reduction system in handheld GPS receivers. 
     SUMMARY OF THE INVENTION 
     A low-cost interference reduction system for a global positioning system receiver is disclosed. The low-cost interference reduction system may be used in rotating weapons such as artillery projectiles and in hand-held GPS receivers. The low-cost interference reduction system comprises a reference antenna and a secondary antenna for receiving global positioning system signals and interfering signals. A complex weight circuit adjusts the phase and gain of the secondary antenna signal by varying the gain of the in-phase and quadrature component of the secondary antenna signal. A summing function adds the reference antenna signal to the complex multiplied secondary antenna signal to reduce the interfering signals. The GPS receiver processes the global positioning system signals into position solutions and provides a power detector output and a clock output. A microprocessor receives and measures the power detector output and uses the measurement along with the clock output from the GPS receiver to generate the in-phase and quadrature gain signals for the complex weight circuit such that the interfering signals are reduced. Amplifier and filter functions are connected to the reference and secondary antennas, the summing function, and the complex weight circuit to filter and amplify the received GPS and jamming signals. A digital attenuator is connected to the output of the summing function and the microprocessor to reduce the level of the received signals so as to maintain them within the range of an AGC circuit in the GPS receiver. A digital-to-analog converter is connected to the microprocessor and the complex weight circuit to convert the in-phase and quadrature gain signal from digital to analog. The GPS receiver further comprises an AGC function for controlling signal levels in the GPS receiver and for providing the power detector output. 
     It is an object of the present invention to provide a low-cost interference reduction system for use with GPS receivers. 
     It is an object of the present invention to provide a low-cost interference reduction system for use in rotating weapons incorporating GPS receivers. 
     It is an object of the present invention to provide a low-cost interference reduction system for use in hand-held GPS receivers. 
     It is an advantage of the present invention to maintain interference cancellation in the presence of a high roll rate by the use spin symmetrical antennas in the interference reduction system. 
     It is an advantage of the present invention to reduce cost by the use of power detector and clock signals from the GPS receiver. 
     It is a feature of the present invention to reduce interference and jamming signals from several directions. 
     It is a feature of the present invention to acquire and receive GPS signals and to direct a null toward a jammer when installed in a continually spinning platform. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiments of the invention in conjunction with the appended drawings wherein: 
     FIG. 1 is a block diagram of a null steering system for use with a GPS receiver in high value platforms; 
     FIG. 2 is a block diagram of a low-cost interference reduction system the present invention; 
     FIG. 3 is a diagram of a slot antenna that may be used for the two antennas of FIG. 2; 
     FIG. 4 is a diagram of an artillery projectile incorporating the present invention; 
     FIG. 5 shows the projectile and a gain pattern of the two antennas with no jamming signal; 
     FIG. 6 shows a gain or radiation pattern of the two antennas with a jammer signal coming from an angle toward the rear of the projectile; 
     FIG. 7 shows another gain pattern with the jammer signal from an angle perpendicular to the projectile with the resulting null toward the jammer; 
     FIG. 8 shows another gain pattern resulting from the jammer signal arriving from an angle off to the front of the projectile; 
     FIG. 9 is a diagram showing a portion of the artillery projectile with a possible position for a circuit board containing the circuit functions of the interference reduction system of the present invention; 
     FIG. 10 is a detailed block diagram of the interference reduction system of FIG. 2; and 
     FIG. 11 is a block diagram of a complex weight circuit of FIG.  10 . 
    
    
     DETAILED DESCRIPTION 
     A block diagram of a null steering system  10  for use in high value platforms is shown in FIG. 1 for use with a GPS receiver  26  as disclosed in U.S. Pat. No. 5,952,968 incorporated herein by reference. The system  10  includes an antenna array  12  having a plurality of antenna elements  14 , where the components comprising each antenna leg portion  15  are substantially identical. In FIG. 1, each antenna element  14  is coupled to a low noise amplifier  16 , the output of which is supplied to a down-converter  18  to convert the signal to an intermediate frequency for processing. Each down-converted signal is supplied to weighting circuits  20  that multiply the signals by complex weight values having real and imaginary portions. The outputs of all of the weighting circuits  20  are coupled to a summing circuit  22 , as shown. 
     In FIG. 1, the sum of the signals from the summing circuit  22  is then coupled to a power detector circuit  24  and is also supplied to the GPS receiver  26 . The output of the power detector circuit  24  is then converted to a digital signal by an analog to digital converter  28 . The digital signal is then processed by an adaptive algorithm under control of a microprocessor or other suitable computer or processor  30 . Alternately, dedicated or discrete hardware circuitry may be used in place of the microprocessor  30 . In FIG. 1, the microprocessor  30  adjusts the values of the weight values for each antenna element  14  and iteratively processes the digital signal attempting to constantly improve the signal received by the GPS receiver  26 . Additionally, navigational data, such as data provided by an inertial navigation system (INS or NAV system)  32  may be utilized by the adaptive algorithm under control of the microprocessor  30 . 
     In FIG. 1, only a single set of weights  20  is required. One weight  20  corresponds to one antenna element  14 . Therefore, the single set of weights corresponds to the array  12  of antenna elements  14 , which is optimized for all of the GPS satellites. The NAV system  32  provides information with respect to the position and/or orientation of the receiver (the antenna array of the receiver) that permits the system  10  to solve for the gain of the received GPS signal in the direction of each satellite. In FIG. 1, one set of weights  20  in combination with the NAV data is used to optimize the average signal to noise ratio (SNR). Accordingly, the gain of the received GPS signal is increased. 
     To perform this optimization of the SNR in the embodiment of FIG. 1, the total power received by the antenna array  12  is divided by the measurement bandwidth to obtain a noise floor. Using the positional information provided by the NAV system  32 , the gain in the direction of the satellite may be obtained. This value can then be used as the criteria for increasing the SNR by adjusting the value of the weights  20  via an adaptive algorithm, such as a “hill climbing” algorithm. Therefore, instead of merely reducing the total power received, as is done in known power minimization techniques, the system adjusts the weights  20  to maximize the SNR, meaning that the signal strength of the received GPS signal is boosted, while simultaneously minimizing the jamming signal. 
     Because each antenna element  14  is part of the antenna array  12 , each element has a different carrier phase due to its relative position with respect to each satellite. GPS receiver systems utilizing an antenna array  12  are useful for nulling the effects of broad band jamming signals. In some situations, multiple jammer signals may be present. To null-out N number of different jammer signals, N+1 individual antenna elements  14  are utilized. For example, four antenna elements  14  are used to effectively null-out three jammer signals. 
     The GPS signal may be below the noise level. One aspect of the system in FIG. 1 utilizes the fact that the GPS signal is extremely weak relative to the strength of the jammer signal. Additionally, the jamming signal typically emanates from a different direction than that of the GPS signal. Often, the GPS signal is transmitted from a satellite that is relatively high in sky, while the jamming signal is transmitted from a tower or mobile vehicle located on the ground. Hence, the signals arrive from different directions. If the jamming signal emanates from an airborne platform and is at the same relative position as that of the satellite, no difference in the angle of arrival between the GPS signal and the jammer signal would be seen, and nulling would be very difficult. However, this is a rare occurrence and would only be temporary. 
     As described above, the illustrated embodiment of the system  10  of FIG. 1 utilizes a single set of weights  20  for the entire antenna array  12 . For each antenna element  14 , processing the signal in accordance with the weight  20  causes the phase and/or amplitude of the received signal to be modified in an attempt to “steer” a null in the direction of the jamming signal to reduce its received signal strength, while simultaneously increasing the gain of the received signal in the direction of the GPS satellite. 
     The system of FIG. 1 offers high performance advantages in jammer nulling for high value platforms such as ships and aircraft. However the system of FIG. 1 is quite large and expensive and not suitable for such applications as weapon systems and compact low-cost hand held GPS receivers. 
     FIG. 2 is a block diagram of a low-cost interference reduction system  100  the present invention. The low-cost interference reduction system  100  comprises an antijam (A/J) electronics unit  110 , a GPS receiver  126 , and two antennas  114  and  115 . The two antennas  114  and  115  have a roll symmetric or omnidirectional radiation pattern. The antennas may be slot antennas, circumferential band antennas, or dual dipole antennas that are oriented along the roll axis of the weapon (or whichever axis is spinning). Because the antennas are spin-symmetric, the platform may be continually spinning without adversely affect the GPS signal reception or nulling capability of the interference reduction system  100 . 
     The output of antenna  114  serves as a reference and is processed by amplifying and filtering in block  116 . Antenna  115  functions as a secondary antenna  115  and its output is also amplified and filtered in an identical block  116 . The secondary antenna  115  output after amplifying and filtering is adjusted in phase and gain with a complex weight circuit  120  so that any interference is equal in magnitude but opposite in phase with respect to the reference antenna  114 . The output of the complex weight circuit  120  is added to the reference antenna  114  amplified and filtered output signal in a summing element  122 . The summing element  122  output signal has the desired GPS signals with reduced or cancelled interfering or jamming signals that were present on the reference antenna  114 . 
     The complex weight circuit  120  receives in-phase (I) and quadrature (Q) gain signals from a digital signal processing circuit  130  to adjust the phase and again of the secondary antenna  115  signals. The digital signal processing circuit  130  receives a power detector feedback signal from an AGC circuit (not shown) that functions as a power detector in the GPS receiver  126  to detect the level of cancellation achieved. The digital signal processing circuit  130  measures this power detector signal level and then continuously adjust the gain and phase of the secondary antenna  115  signal by varying the I and Q gain signals to the complex weight circuit  120  to produce a minimum interfering signal level in the power detector. A digital attenuator  135  following the summing element  122  keeps the summing element output signal level within the operating range of the AGC circuit of the GPS receiver  126 . The digital signal processing circuit  130  provides a gain control signal to the digital attenuator  135 . The interference cancellation system  100  also uses a reference clock from the GPS receiver  126  to reduce the parts count, size, and cost. 
     An antenna such as that disclosed in U.S. Pat. No. 6,098,547 incorporated herein by reference in is entirety may be used in the present invention. FIG. 3 is a diagram of a circumferential slot antenna  210  disclosed in the referenced patent that may be used for both the antennas  114  and  115  of FIG.  2 . The slot antenna  210  may have a radiation pattern symmetric about the center feed point  324 . The antenna  210  consists of two conducting discs  312  and  314  that act as ground planes spaced apart by a dielectric disk  310 . Apertures  316 - 322  are used to tune the antenna  210 . The diameter of the antenna may be also varied to adjust the frequency and bandwidth of the antenna. 
     A circumferential band antenna may also be used for antennas  114  and  115  as shown in FIG. 4. A circumferential band antenna may be a conducting band wrapped around the outer circumference of artillery projectile  400 . 
     Antennas  114  and  115  on the projectile  400  in FIG. 4 may also be the slot antenna  210  shown in FIG.  3 . The antennas  114  and  115  are positioned with the edge of the slot at the outer surface of the projectile  400 . The positions of antennas  114  and  115  may be interchanged from that shown in FIG.  4 . Also, note that the size of the antennas may be different to fit the projectile  400  as shown in FIG.  4 . The antennas  114  and  115  are positioned in the projectile  400  such that they are roll symmetric with the projectile. The antennas  114  and  115  are spaced at approximately one half wavelength at the GPS frequencies. Both antennas  114  and  115  require a common ground and the projectile structure around the antennas must be grounded for proper antenna functioning. 
     FIG. 5 shows the projectile  400  and a gain pattern  410  of the two antennas  114  and  115  with no jamming signal. FIG. 6 shows a gain or radiation pattern  420  of two antennas  114  and  115  with a jammer signal indicated by arrow  425  coming from an angle toward the rear of the projectile  400 . Note that a null in the gain pattern is directed toward the jammer signal  425  as well as in a complete ring around the projectile. FIG. 7 shows another gain pattern  430  with the jammer signal  425  from an angle perpendicular to the projectile  400  with the resulting null toward the jammer. FIG. 8 shows another gain pattern  440  resulting from the jammer signal  425  arriving from an angle off to the front of the projectile  400 . The antennas  114  and  115  have a roll symmetric pattern as shown in FIG.  5 . With this roll symmetric pattern, the cancellation pattern is also roll symmetric as shown in FIGS. 6,  7 , and  8 . 
     FIG. 9 is a diagram showing a portion of the artillery projectile  400  with a possible position for a circuit board  910  containing the circuit functions of the interference reduction system  100 . The antennas  114  and  115  are located on an assembly with the circuit board  910  as shown in FIG.  9 . With this embodiment, the outer skin of the projectile may be a radome made of material transparent to the GPS signals. 
     FIG. 10 is a detailed block diagram of the interference reduction system  100  of FIG.  2 . GPS and jammer signals are received by reference antenna  114 . A burnout protection diode  113  may be connected to the antenna input to prevent damage to the system  100  from strong jamming and other signals. The output of the reference antenna  114  is filtered in preselect filter  117  that performs a bandpass function to pass the GPS signals and any in-band interfering and jamming signals. Low noise amplifier (LNA)  118  amplifies the input GPS signals as well as jamming signals. The output of the LNA  118  is connected to a summing element  122  that may be a 6-dB combiner. The secondary antenna  115  also receives GPS and jamming signals. Connected to the secondary antenna  115  is a burnout protection diode  113  and a preselect filter  117  as with antenna  114 . An another LNA  118  is used to amplify the secondary antenna  115  signals. 
     The secondary antenna  115  signals from LNA  118  are processed in the complex weight circuit  120 . A block diagram of the complex weight circuit is shown in FIG.  11 . Splitter  141  divides the signal into in-phase (I) and quadrature (Q) signals. The quadrature signal is delayed 90 degrees in carrier phase by phase shift  142 . Variable gain amplifiers  143  and  144  apply a varying gain (possibly zero or negative) to each signal component. The I and Q gain signals from the microprocessor  130  vary the gain of the variable gain amplifiers  143  and  144 . The signal components are recombined in summer  145  and output. The complex weight circuit  120  affects a complex multiply between an in coming RF signal and the real and imaginary parts of the I and Q gain signal. 
     The output of the complex weight circuit  120  may be attenuated in a pad  121  and then passed to the summing element  122 . In the summing element  122 , the GPS signals from antennas  114  and  115  are added while any jamming signal from reference antenna  114  are added to phase and amplitude adjusted jamming signals from complex weight circuit  120  such that the jamming signals subtract to reduce or eliminate the jamming signals. 
     The output of the summing element  122  may be amplified in amplifier  123  before being passed to the digital attenuator  135  that keeps the signal level within the AGC range of the GPS receiver  126 . The signals from the attenuator  135  are received by the GPS receiver  126  where they are filtered by a post selector  131  bandpass filter, amplified in amplifier  132 , and down-converted to a first IF in mixer  133 . Mixer  133  receives a first local oscillator (LO) signal from a frequency synthesizer  135 . The first IF signal is amplified and processed in automatic gain control (AGC) circuit  134 . 
     The AGC  134  functions as a AGC known in the art to provide a constant signal level to the I and Q circuit functions that follow. The I and Q outputs are then passed to a digital section (not shown) of the GPS receiver  126  for further processing. The AGC circuit  134  also provides a power detector function to detect the power level of any jamming signal. A power detector (not shown) in the AGC circuit  134  provides a power detector control signal proportional to the input level to control the gain of a voltage controlled amplifier (not shown) in the AGC circuit  134  as is know in the art. The power detector control output signal is fed back to the single chip microprocessor  130 . In addition, a frequency standard  136  that provides a reference frequency to the synthesizer  135  also provides a clock for the microprocessor  130 . 
     The microprocessor  130  digitizes the power detector signal from the AGC circuit  134  and processes it to generate a digital word on a serial data port. The digital word represents an I and Q signal needed to minimize the power detector output level. The digital word is converted to analog I and Q gain signals in serial digital to analog converter  119 . The analog I and Q gain signals are used to control the I and Q levels of the secondary antenna  115  signal in the complex weight circuit  120  as discussed above. The microprocessor  130  examines the power detector signal and generates I and Q gain signals to null out or reduce the jammer signals as indicated by the power detector  133  signal level. 
     By using power detector signals and clock signals from the GPS receiver  126  and spin symmetrical antennas, the interference reduction system  100  of the present invention is able to achieve jamming and interference reduction at low cost. The interference reduction system  100  may also be implemented with a hand-held GPS receiver by using similar spin symmetrical antennas as used in artillery projectiles as described above. The antennas  114  and  115  may be oriented with their spin axis vertical and suppression of jamming signal will be optimized to signals received perpendicular to the spin axis or from the horizon. This is an advantage in hand held applications since most jammers are on the ground and hence aligned with the horizon in elevation. 
     It is believed that the low cost interference reduction system of the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages, the form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.