Abstract:
An RF (e.g., GNSS) interference mitigation system and method uses a switchable bank of filters for selectively blocking signals in predetermined bandwidths based on detecting strong, interfering signals with an interference detection circuit including a sniffer antenna. A low-strength RF (e.g., GNSS) system can be combined with a spectrally-close high-strength, telecommunications receiver system for cooperative control. Alternatively, an RF receiver can detect tones, changes in DC bias or level changes to activate a filter selection switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority in U.S. Provisional Patent Application No. 61/566,669, filed Dec. 4, 2011, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to mitigating radio frequency (RF) signal interference in RF/digital receivers, and in particular to a selective filtering system and method for a global navigation satellite system (GNSS) susceptible to interference from signals in spectrally-adjacent bandwidths. 
         [0004]    2. RF (Including GNSS) Background and Description of the Related Art 
         [0005]    Global navigation satellite systems (GNSSs) include the Global Positioning System (GPS), which was established and is operated by the United States government and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 20,200 km. These satellites continuously transmit microwave L-band radio signals in three frequency bands, centered at 1575.42 MHz, 1227.60 MHz and 1176.45 MHz, denoted as L1, L2 and L5 respectively. All GNSS signals include timing patterns relative to the satellite&#39;s onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites. 
         [0006]    GNSS receivers are highly sensitive devices designed to receive very weak signals transmitted by the source satellites. GNSS receivers process the radio signals, computing ranges to the GNSS satellites, and by triangulating these ranges, the GNSS receiver determines its position and its internal clock error. Different levels of accuracies can be achieved depending on the observables used and the correction techniques employed. For example, accuracy within about 2 cm can be achieved using real-time kinematic (RTK) methods with single or dual-frequency (L1 and L2) receivers. 
         [0007]    GNSS also includes Galileo (Europe), the GLObal NAvigation Satellite System (GLONASS, Russia), Beidou (China), Compass (proposed), the Indian Regional Navigational Satellite System (IRNSS) and QZSS (Japan, proposed). Galileo will transmit signals centered at 1575.42 MHz, denoted L1 or E1, 1176.45 denoted E5a, 1207.14 MHz, denoted E5b, 1191.795 MHz, denoted E5 and 1278.75 MHz, denoted E6. GLONASS transmits groups of FDM signals centered approximately at 1602 MHz and 1246 MHz, denoted GL1 and GL2 respectively. QZSS will transmit signals centered at L1, L2, L5 and E6. Groups of GNSS signals are herein grouped into “superbands.” 
         [0008]    The United States&#39; Global Positioning System (GPS) first reached fully operational capability on Jul. 17, 1995. After almost two decades, advances in technology and new demands have prompted efforts to modernize the GPS system. Part of the modernization are new civilian navigation signals to be transmitted on a frequency other than the L1 frequency (1575.42 MHz). This signal became known as the L2C signal because it is a civilian signal broadcast on the L2 frequency (1227.6 MHz). It is transmitted by all block IIR-M and newer generation satellites. 
         [0009]    Whitehead et al. U.S. Pat. No. 6,744,404 shows an Unbiased Code Phase Estimator for Mitigating Multipath in GPS, and is incorporated herein by reference. U.S. Coast Guard Navigation Center, “GPS FAQ,” U.S. Department of Homeland Security; and Navstar Global Positioning System, “Interface Specification-ICD-GPS-200,” Navstar GPS Joint Program Office, are also incorporated herein by reference. Liu and Badke U.S. Provisional Patent Application Ser. No. 61/702,031 for GNSS System and Method Using Unbiased Code Phase Tracking with Interleaved Pseudo-Random Code is also incorporated herein by reference. 
         [0010]    Dual-frequency GNSS receivers are preferably adaptable for use with all present and projected GNSS, transmitting signals which can be grouped into two “superbands” of radio signal frequencies generally in the range of about 1160 MHz to 1250 MHz and 1525 MHz to 1613 MHz. 
         [0011]    RF signal frequency spectra allocations are highly regulated by the Federal Communications Commission (FCC) in the United States and by other agencies worldwide. As the airwaves become more crowded as a consequence of demand for RF signal spectra allocations, reception problems arise from signal interference. 
         [0012]    For example, the telecommunications industry has experienced significant growth and increasing wireless traffic levels. Wireless telecommunications via RF signals are becoming increasingly popular among telecommunications service subscribers. To accommodate such demand, telecommunications service providers, through their industry associations, commonly seek FCC allocations of more frequency spectra. 
         [0013]    The interests of the telecommunications industry are sometimes adverse to the other RF service providers. For example, GNSS service providers, including the U.S. Department of Defense with its Global Positioning System (GPS), are increasingly likely to encounter interference problems associated with nearby or spectrally-adjacent telecommunications bandwidth usage. 
         [0014]    The present invention addresses the RF-digital signal interference problems with previous RF receivers. Heretofore, there has not been available an interference mitigating RF system and method with the advantages and features of the present invention. 
       SUMMARY OF THE INVENTION 
       [0015]    In the practice of the present invention an interference mitigating system and method are provided for detecting interfering signals and automatically switching among a bank of filters, or by selective filtering based on signals&#39; bandwidths not being used by a receiver. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The accompanying drawings illustrate the principles of the present invention and an exemplary embodiment thereof. 
           [0017]      FIG. 1  is a diagram of an RF antenna and receiver system adapted for maximizing LNA gain while not saturating on spectrally-nearby, high-strength communications signals, which system is adapted for utilizing an embodiment of the present invention, e.g., in the LNA. 
           [0018]      FIG. 2  is a diagram of a combined GNSS and communication system receiver, which is adapted for use with an interference avoidance LNA switchable filter system embodying an aspect of the present invention. 
           [0019]      FIG. 3  is a diagram of an interference avoidance circuit within the LNA. 
           [0020]      FIG. 4  shows an interference avoidance LNA comprising an alternative embodiment or aspect of the present invention including a tone decoder or level detector for bandwidth selection in the switchable filter. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    I. Introduction and Environment 
         [0022]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. 
         [0023]    Certain terminology will be used in the following description for convenience in reference only and will not be limiting. For example, up, down, front, back, right and left refer to the invention as oriented in the view being referred to. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the embodiment being described and designated parts thereof Said terminology will include the words specifically mentioned, derivatives thereof and words of similar meaning The invention can be fabricated in various sizes and configurations from a wide variety of suitable materials and components, which are chosen for their characteristics according to the intended use and the operation of the invention. 
         [0024]    II. Interference Avoidance System and Method 
         [0025]      FIG. 1  shows an RF antenna and anti-saturation filtering system  2  embodying an aspect of the present invention including an antenna element  4  connected to a low loss pre-filter  6 , which is connected to an LNA  8 . The LNA  8  is connected to an interstage SAW filter  10 , which in turn connects to a block amplifier  12  with an output to a receiver. This configuration provides a solution to the problem of designing a GNSS receiver that is highly sensitive to receive the relatively weak GNSS satellite signals and not be de-sensitized by communication signals that are spectrally close to the GNSS signals. The resulting high-precision GNSS receiver configuration can thus be compatible with high-strength communication system signals in the nearby frequency spectrum. 
         [0026]    It is highly important to maximize the gain of the LNA  8  and minimize its noise figure. These factors determine the sensitivity of the entire GNSS antenna and receiver system based on the cascaded amplification function, which can be represented by the Friis equation: 
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         [0027]    By carefully designing the LNA  8  to maximize its gain, maximize it saturation level, and minimize its noise figure so that the GNSS receiver has maximum sensitivity, the interstage filter  10  can be aggressively designed to eliminate interference from spectrally nearby high-strength signals. The spectral proximity of the high-strength (e.g., telecommunications) signals to the GNSS signals of interest tends to compromise the effectiveness of analog filtering techniques. Additional digital filtering is thus required, along with increased sample rates to avoid aliasing of the interfering signals and increase the number of bits in the analog-to-digital converter (ADC) to improve the dynamic range of the GNSS receiver so that the required digital filtering can be effectively implemented. 
         [0028]    Moreover, in addition to mitigating their impact on the GNSS positioning functions, an objective of the system design is to utilize these spectrally-close, high-strength communication signals by: 1) receiving differential correctors; 2) remote programming; 3) remote configuration; and 4) remote data collections. 
         [0029]      FIG. 2  shows a combined GNSS and communication system receiver  20  including an antenna assembly  22  connected to a GNSS receiver assembly  24 . An optional LTE modem can be provided for connecting an LNA in the antenna assembly  22  with a GNSS solution processor (ARM) in the receiver assembly  24 . 
         [0030]      FIG. 3  shows an interference avoidance LNA  30  embodying an aspect of the present invention and including a switchable bank of filters  32 . GNSS signals and correctional services currently occupy to wide bandwidths of 1167-1253 MHz and 1525-1610 MHz. Most high-end GNSS LNAs cover these entire bandwidths for use with multiple receiver architectures. A disadvantage is that the LNA can be susceptible to saturation from any strong signal in these bands, even if those bands are not being used. The present invention provides a switchable bank of filters for selecting respective bandwidths based on either: 1) interfering signal detection; or 2) selection from the receiver when some signals&#39; bandwidths are not required. 
         [0031]    An RF (e.g., GNSS) antenna  34  is connected to the filter bank  32  by a filter-selection switch  36 , which selectively inputs signals received by the antenna  34  to first, second and third bandwidth filters  38 ,  40 ,  42  respectively. The bandwidth filters can be chosen based on any desired performance characteristics, e.g., high, medium and low frequency bands. The switching function among the bandwidth filters  38 ,  40 ,  42  is performed automatically by the interference detection circuit  44 , which includes a sniffer antenna  46  adapted to detect interference in conjunction with filters  48 ,  50 ,  52 , communicating through respective diodes  54  to a logic circuit  56  adapted to select a respective bandwidth based on the interference power of the detected interfering signal. An output selection switch  58  can ask a respective bandwidth filter  38 ,  40  or  42  to a GNSS receiver, e.g., via a block amplifier  12 . 
         [0032]    If interference is detected in one of these bands by the detection circuit  44 , the filter bank  32  will be switched accordingly and the receiver will not operate in the interfering bandwidth, although other signals will still be usable. In a normal LNA the presence of a strong jamming interference signal would block all of these signals by saturation of the LNA. 
         [0033]    III. Alternative Embodiment Interference Avoidance LNA  60   
         [0034]    An interference avoidance LNA  60  comprising an alternative aspect or embodiment of the present invention is shown in  FIG. 4  and includes a receiver  66  coupled to (or including) a tone decoder or level detector  62  adapted for detecting a tone, a level change or a change in DC, which can be communicated through the same coaxial cable  68  to an LNA  64  with a change in DC bias voltage or a low-frequency tone, for example. A filter bank  72  includes first, second and third bandpass filters  74 ,  76 ,  78  respectively. A frequency selection switch  80  and an output selection switch  82  are connected to the filter bank  72  and are similar to the switches  36 ,  58  described above. The switches  80 ,  82  can be operated by a logic circuit similar to the logic circuit  56  described above. 
         [0035]    It will be appreciated that the components of the system  2  can be used for various other applications. Moreover, the subsystems, units and components of the system  2  can be combined in various configurations within the scope of the present invention. For example, the various units could be combined or subdivided as appropriate for particular applications. The system  2  is scalable as necessary for applications of various complexities. It is to be understood that while certain aspects of the disclosed subject matter have been shown and described, the disclosed subject matter is not limited thereto and encompasses various other embodiments and aspects.