Abstract:
A global navigation satellite system configured to operate in a noisy environment receives the same satellite signals in two separate channels. Each channel processes the signals independently according to different filtering constraints; one channel applies narrow filtering constraints while the other channel applies broader filtering constraints. Narrow filtering constraints allow the receiver to acquire a usable signal under certain conditions but not while moving rapidly. Broader filtering constraints allow the receiver to acquire a usable signal during rapid movement, but cannot overcome intense interference. A device implementing both constraint options is usable under a wider range of situations.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally toward global navigation satellite systems, and particularly toward a system and method for overcoming noise. 
     BACKGROUND OF THE INVENTION 
     Global navigation satellite systems (GNSS) such as the Global Positioning System (GPS), the Compass navigation system, the Galileo positioning system and GLONASS all operate on the principle of trilateration based on signals received from satellites in each system. Each system generally functions by measuring some time shift in the signal received from each satellite. The time shift is a measure of the distance each signal traveled to reach the receiver. A computer in the receiver uses the known position of each satellite in orbit and the calculated distance from each satellite to determine the only location where the receiver could be located. 
     Measuring the time shift for each signal requires a high degree of precision; even a very small error can result in a calculated distance hundreds of meters from the receiver&#39;s actual location. Precise time shift measurements require the best possible signal from each satellite. Satellite signals are often degraded by interference. Interference may be caused by active signal jamming or simple noise such as from other electromagnetic devices or quick receiver antenna movement. GNSS signals are generally digital. 
     Interference may be reduced by spreading each signal over a wide frequency range, and by including filters to remove noise. Filters must be tuned for particular situations to be effective. A very narrow filter may provide a high quality signal, but may also filter out the signal completely in a noisy environment such as when the receiver is moving. A broad filter may provide a reasonable quality signal in a dynamic environment, but may not be able to filter out intense interference in a narrow frequency band. 
     Consequently, it would be advantageous if a method and apparatus existed that are suitable for increasing availability of a global navigation satellite system in a dynamic environment and in an environment of intense interference. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel method and apparatus for increasing availability of a global navigation satellite system in a noisy environment and in an environment of intense, narrow frequency interference. 
     One embodiment of the present invention is a GNSS device configured to process the same signal in two or more separate channels. In one channel, the receiver applies a filter having broad constraints to produce a usable signal in a lower interference but dynamic environment. In the other channel, the receiver applies a filter having narrow constraints to produce a usable signal in an environment of higher interference, but low intensity dynamics. 
     Another embodiment of the present invention is a GNSS device configured to dynamically determine separate filtering constraints for a signal, processed in two separate channels, to produce usable signals in a noisy environment. In each channel, the receiver may modify the filtering constraints dynamically to maximize the signal lock of each channel while maintaining a predetermined or dynamically determined divergence of the filtering constraints in each channel. 
     Another embodiment of the present invention is a method for producing a usable GNSS signal in environments of differing noise intensity by receiving one signal in two separate channels, each channel applying different filtering constraints, and determining which channel provides superior signal lock in each environment. The different filtering constraint may be determined dynamically. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a block diagram of a GNSS device; 
         FIG. 2  shows a block diagram of a GNSS constellation and GNSS device; 
         FIG. 3  shows a block diagram of a GNSS encountering interference; 
         FIG. 4  shows a block diagram of a GNSS device processing a signal by two independent filtering constraints; 
         FIG. 5  shows a flowchart of a method for producing a usable signal in a GNSS under different noise conditions; and 
         FIG. 6  shows a flowchart of a method for tracking the performance of different filtering constraints to determine comparative performance of each set of constraints. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a block diagram of a GNSS device  100  is shown. The GNSS device  100  may include a receiver  106  for receiving one or more GNSS signals in two or more channels, the receiver  106  connected to a processor  102  executing computer code to process the one or more GNSS signals to determine the global location of the receiver. Although trilateration is possible with as few as three GNSS signals, GNSS devices  100  generally require four GNSS signals to ensure a desired degree of accuracy. A GNSS device  100  may also include memory  104  to store computer code and data necessary to process the one or more GNSS signals. It will be appreciated that GNSS signals may comprise GNSS like signals, transmitted from a terrestrial source rather than a satellite source. 
     Referring to  FIG. 2 , every GNSS consists of a constellation of satellites  202 , each satellite transmitting a signal  204 . A GNSS device  100  configured to operate with the particular GNSS receives signals  204  from available satellites  202  and determines its own location based on information contained in and derived from the signals  204 . Each signal  204  from each satellite  202  may contain at least the position of the satellite  202  and the intrinsic clock bias of the satellite  202 . With that information, the GNSS device  100  may derive the transmission time of each signal  204 , and thereby calculate the location of the GNSS device relative to each satellite  202 . It will be appreciated that GNSS signals may comprise signals from multiple GNSS systems. 
     Referring to  FIG. 3 , an interference source  300  may broadcast an interfering signal  302  in a frequency channel used by one or more of the satellites  202 . Satellites  202  may broadcast signals  204  using some multiple access mechanism such as code division multiple access (CDMA) or frequency division multiple access (FDMA) to multiplex multiple signals over the same physical channel. In systems using CDMA for multiple access, interference in a frequency channel used by more than one satellite  202  in the GNSS can be especially problematic. To combat potential interference, signals  204  are broadcast over a wide frequency spectrum. Whether or not interfering signals  302  just degrade or fully deny the ability of a GNSS device  100  to acquire and track depends on several factors, including strength of the interfering signal  302 , strength of the received signal  204 , antenna gain of the interfering signal  302 , antenna gain of the GNSS device  100 , distance between the GNSS device  100  and the interfering signal source  300 , etc. Assuming the strength of the interfering signals  302  is enough to degrade the desired signals (but not totally deny operation), a GNSS device  100  according to the present invention can mitigate the interference. 
     Where an interfering signal  302  is intense, a GNSS device  100  may impose narrow filtering constraints (narrow bandwidth) on the channel receiving the GNSS signals  204 . Narrow filtering constraints may provide a tracking signal in interference levels up to, for example, 51 dB J/s, but the signal may only be usable for low user dynamics (such as near-stationary conditions). 
     Where an interfering signal  302  is a low intensity signal, the GNSS device  100  may impose broad filtering constraints (broad bandwidth) on the channel receiving the GNSS signals  204 . Broad filtering constraints may provide a tracking signal in interference levels up to, for example, 41 dB J/s, but the signal may be usable for extensive user dynamics (such as fast movement). Broad filtering constraints are not useful for a high intensity interfering signal  302  because the interfering signal  302  may not be filtered out by the broad bandwidth filtering constraints. 
     Referring to  FIG. 4 , a GNSS device  100  may include two separate channels  402 ,  404  for processing the same GNSS signals  204  with two different filtering constraints. The GNSS device  100  may receive one or more GNSS signals  204  in a first channel  402 . The first channel  402  may filter the GNSS signals according to narrow filtering constraints. The GNSS device  100  may also receive the same GNSS signals  204  in a second channel  404 . The second channel may filter the GNSS signals  204  according to broad filtering constraints. The processor  102  may then determine which channel  402 ,  404  is producing usable GNSS signals  204 . The processor  102  may then use the usable GNSS signals  204  to determine its location. 
     For example, a first channel  402  may filter the GNSS signal  204  on very narrow bandwidths. Very narrow bandwidth filtering would be expected to overcome interference up to 50 dB J/s while the GNSS device  100  is stationary or moving only very slowly, but would be expected to produce an unusable signal when the GNSS device  100  is moving rapidly, such as in a vehicle. A second channel  404  may filter the GNSS signal  204  on wider bandwidths. Wider bandwidth filtering would be expected to overcome interference up to about 41 dB J/s, even when the GNSS device is moving quickly. 
     Alternatively, the processor  102  may determine filtering constraints for each channel  402 ,  404  dynamically based on user input or other criteria such as a persistent background noise level. The processor  102  may maintain statistical data in memory  104  concerning which channel  402 ,  404  produces a usable GNSS signal  204  more often and modify the filtering constraints of the other channel accordingly while continuing to maintain some minimum differentiation in filtering constraints for each channel. For example, if the processor  102  determines that the second channel  404  produces a usable GNSS signal  204  roughly 75% of the time with wider bandwidth, and the first channel  402  never produces a usable GNSS signal  204  with very narrow bandwidth constraints, the process may modify the filtering constraints of the first channel  402  to a narrow bandwidth somewhere between the very narrow and wider bandwidth constraints. One skilled in the art will appreciate that specific bandwidth filtering constraints may be particular to each situation. 
     One skilled in the art will appreciate that, while the GNSS device  100  described herein may process a single GNSS signal  204  in separate channels to apply two separate filtering constraints, a GNSS device  100  requires more than one GNSS signal  204  to function. One embodiment of the present invention may include multiple methods, substantially as set forth herein, to produce a plurality of GNSS signals  204  for use by a GNSS device  100 . 
     Referring to  FIG. 5 , a flowchart is shown for filtering the same GNSS signals  204  in two or more different channels  402 ,  404 , according to different filtering constraints. A GNSS device  100  may receive  500  a GNSS signal  204  in a first channel  402 . The GNSS device  100  may simultaneously receive  502  the same GNSS signal  204  in a second channel  404 . The GNSS device  100  may then determine  504  a first set of filtering constraints for the first channel. The first set of filtering constraints may be predefined, based on user input, or based on some analysis of the GNSS signal  204 . Likewise, the GNSS device  100  may determine  506  a second set of filtering constraints for the second channel. The second set of filtering constraints may be predefined, based on user input, or based on some analysis of the GNSS signal  204 . The first set of filtering constraints and the second set of filtering constraints may be tuned to produce a usable GNSS signal  204  in environments of different background noise and interference. The GNSS device  100  may then apply  508  the first set of filtering constraints to the first channel, and apply  510  the second set of filtering constraints to the second channel. Each channel may produce a filtered GNSS signal  204 . The GNSS device  100  may then compare  512  the signal produced by the first set of filtering constraints to the signal produced by the second set of filtering constraints to determine which set of filtering constraints produced a usable GNSS signal  204 . A usable GNSS signal  204  is a signal that the GNSS device  100  can use to determine its location. The GNSS device  100  may then utilize whichever signal the GNSS device  100  determines is usable. 
     One skilled in the art will appreciate that, while the methods set forth herein describe processing a single GNSS signal  204  in separate channels to apply two separate filtering constraints, a GNSS device  100  requires more than one GNSS signal  204  to function. One embodiment of the present invention may include multiple methods, substantially as set forth herein, to produce a plurality of GNSS signals  204  for use by a GNSS device  100 . 
     Referring to  FIG. 6 , a GNSS device  100  implementing the method of the present invention may record  600  statistical data pertaining to the usability of filtered signals produced by the first set of filtering constraints, and record  602  statistical data pertaining to the usability of filtered signals produced by the second set of filtering constraints. The GNSS device may then modify  604  either the first set of filtering constraints or the second set of filtering constraints based on the statistical data to improve the overall ability of the GNSS device  100  to determine its own location. The first set of filtering constraints and the second set of filtering constraints should always be tuned to produce a usable signal in different noise environments. Ideally, the first set of filtering constraints should be tuned to produce a usable signal whenever the second set of filtering constraints fails to produce a usable signal. It will be appreciated that whenever the second set of filtering constraints fails to produce a usable signal, the statistical data and signal tracking phase pertaining to the first set of filtering constants can assist with the rapid reestablishment of track using a retuned second set of filtering constraints. 
     It is believed that 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.