Patent Publication Number: US-8976844-B2

Title: Receiver for detection and time recovery of non-coherent signals and methods of operating same

Description:
BACKGROUND 
     The present disclosure relates to methods and systems for global positioning, and more specifically, to methods and systems for acquiring global positioning system signals from satellites to determine a location of a user receiver. 
     The Global Positioning System (GPS), a world-wide radio navigation system, provides an ability to obtain real time location and position information for mobile platforms and/or individuals. GPS includes a constellation of satellites, known as space vehicles, ground or base stations, and user receivers. 
     Using the locations of satellites as reference points, the position of the user receiver may be calculated accurately to within meters and sometimes even within centimeters. Each of the satellites, the ground stations, and the user receiver has preprogrammed timed signals that initiate at precise times. In order to lock on to the signals broadcasted by the satellites, the ground station and user receiver slew their respective internal generated signals relative to time as predicted by their respective internal clocks. When the signals are locked, each user receiver may calculate ranging measurements to each satellite called pseudo ranges, by determining the delays in the signal transmissions from the satellites. The pseudo range measurements include the actual ranges to satellites, in addition to an error associated with the receiver clock time offset relative to GPS time, plus other smaller errors. The ground stations included in the GPS provide ranging measurements that are used to generate predictions for the satellites, clocks, and orbits. 
     Signal acquisition is an important phase associated with the GPS receiver. Known GPS navigation systems are synchronized when an exact copy of the space vehicle&#39;s specific pseudo-random noise code of the received signal is available to demodulate data properly, a process characterized as coherent. However, if the received GPS signal is distorted or corrupted by factors such as, for example, noise, interfering signals and/or jamming, the signal is non-coherent. GPS receivers are needed that enable non-coherent detection and time recovery for distorted signals. 
     BRIEF SUMMARY 
     In one aspect, a method for processing a signal having a plurality of codes is provided. The method includes receiving the signal at a receiver and removing a carrier signal from the signal. The method further includes isolating a data stream from the carrier signal and determining timing of the plurality of codes. The plurality of codes is filtered to separate from the plurality of codes in the data stream a particular code for each plurality of codes that correlates to the received signal. 
     In another aspect, a receiver for processing a non-coherent signal having a plurality of codes is provided. The receiver includes an antenna configured to receive the non-coherent signal. A demodulator is coupled to the antenna for use in removing a carrier signal from the non-coherent signal. The receiver includes a processor coupled to the demodulator for use in isolating a data stream from the carrier signal and for determining a timing of the plurality of codes. A timed match window filter is coupled to the processor for use in selecting from the plurality of codes a particular code for each plurality of codes that correlates to the received non-coherent signal. 
     In a further aspect, a method for processing a non-coherent signal having a plurality of codes is provided. The method includes receiving the non-coherent signal at a receiver and removing a carrier signal from the non-coherent signal by generating a synchronizing timing signal based on a numerical analysis of the second difference of the non-coherent signal. The method further includes isolating a data stream from the carrier signal and determining the timing of the plurality of codes by determining zero crossings which correspond to a symbol state change of the non-coherent signal. The plurality of codes is filtered to separate from the plurality of codes in the data stream a particular code for each plurality of codes that correlates to the received non-coherent signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary global positioning system (GPS). 
         FIG. 2  is a flowchart illustrating an exemplary method of acquiring a signal. 
         FIG. 3  is a flowchart illustrating another exemplary method of acquiring a signal. 
         FIG. 4  is a schematic block diagram of an exemplary receiver that may be used with the GPS shown in  FIG. 1  for acquiring a signal. 
         FIG. 5  is a constellation chart illustrating exemplary states of an acquired signal. 
         FIG. 6  is a sequence of graphs illustrating exemplary data synchronization of an acquired signal. 
     
    
    
     Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only. Any feature of any drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     DETAILED DESCRIPTION 
     The embodiments described herein relate to space vehicle signal acquisition. More specifically, the embodiments described herein facilitate signal acquisition for rapid, reliable and robust acquisition of the signals, including non-coherent signals. Moreover, the embodiments enable signals to be acquired in challenged environments associated with signal power attenuation or interference, e.g. in-doors, under foliage, and/or under jamming conditions. Furthermore, the embodiments described herein facilitate non-coherent signal detection and time recovery for factory testing of the space vehicle GPS signal quality such as, but not limited to correlation loss, code power ratios, crosstalk, signal coherence and delays. Further, the embodiments facilitate partial correlation of the received signal, such as when only part of the signal is received and a receiver does not have sufficient information for a decision in selecting the proper space vehicle pseudo-random noise code. Moreover, the embodiments facilitate extracting GPS codes for the received signal when a replica of the codes is not available. In addition, the embodiments facilitate developing, and testing the ability to develop and test new pseudo random noise codes. 
       FIG. 1  illustrates an exemplary global positioning system (GPS)  10 . In the exemplary embodiment, GPS  10  includes a plurality of GPS satellites  12 , a base station  14 , and a user receiver  16 . Satellites  12  are coupled in wireless communication to base station  14  and to user receiver  16 . Additionally, base station  14  is coupled in wireless communication to user receiver  16 . The locations of satellites  12  are used as reference points to assist signal processing and to enable the location of user receiver  16  to be accurately determined. In the exemplary embodiment, satellites  12  include a constellation of “M” number of satellites in the Earth&#39;s orbit that are in view of user receiver  16 . 
     Each satellite  12  transmits GPS satellite signals  18 , including a unique Pseudo-Random Noise (PRN) Code  20  and a Navigation (Nav) message  22 , via a carrier signal  24  such as, for example, two carrier signals, L1 and L2. More particularly, for PRN code  20  and Nav message  22  to travel with carrier signal  24 , PRN code  20  and Nav message  22  are modulated onto carrier signal  24 . In the exemplary embodiment, the L1 carrier signal is 1575.42 MHz and carries both PRN code  20  and Nav message  22  for timing and the L2 carrier is 1227.60 MHz. The L2 signal is normally used for military purposes. Two types of PRN codes  20 , called Coarse Acquisition (C/A) code and Precise (P) code exist. The C/A code, intended for civilian use, modulates the L1 carrier signal at a rate of 1.023 MHz and repeats every 1023 bits. Thus, the length of the C/A code is one millisecond (1023 bits/1.023 MHz). The P code, intended for military use, repeats on a seven-day cycle and modulates both the L1 and L2 carrier signals at a 10.23 MHz rate. When the P code is encrypted, it is called the “Y” code. The (C/A) code and (P) code are coupled in-phase, quadrature of the L1 and L2 carriers arranged orthogonally, and are known as In-phase (I) code and Quadrature phase (Q) code. Satellites  12  also transmit signals  18  on other carrier frequencies such as L3, L4, and L5 carrier frequencies. The L3 carrier frequency is used for monitoring nuclear detonations. The L4 carrier frequency is being studied for additional ionospheric correction, and the L5 carrier frequency is 1176.45 MHz and is used for Safety-Of-Life Pilot signals. The phase for L5 carrier includes In-phase (I) code and Quadrature phase (Q) code. 
     The Nav message  22  includes a low frequency, modulo-two signal coupled to the PRN codes  20  on L1 and L2 carrier signals which carries information about satellites&#39; positions, orbits, clocks corrections and other system status. When GPS satellite ranging signals  18  are broadcast to Earth, signals  18  directly reach the user receiver  16  with a propagation delay that is proportional to the distance between satellite  12  and user receiver  16 . As such, the satellite-user distance can be calculated by multiplying the propagation delay with the speed of light in vacuum. However, along the route to user receiver  16 , GPS satellite signals  18  may encounter additional delay uncertainties, caused by parameters such as, but not limited to, satellite ephemeris errors, clock errors, ionospheric and atmospheric effects, multi-path propagation and/or receiver clock error. 
     At ground level, base station  14  includes a stationary receiver  26  located at an accurately surveyed point. Base station  14  is configured to receive GPS satellite ranging signals  18  from each of satellites  12 . To facilitate reducing or eliminating the delays and errors in GPS satellite ranging signals  18 , other stationary ground stations  28 , called differential GPS ground stations, may be used. Ground stations  28  have known locations from accurate surveyed data and independently calculate their locations from GPS satellites  12 , wherein the error differential is broadcasted to nearby GPS receivers  16  so that receivers  16  may correct their positions by the same differential. The broadcasting of the differential error is performed over a radio frequency (RF) wireless communications link. User receiver  16  may be stationary for the time being, or may be roaming, wherein user receiver  16  applies these corrections to its ranging measurements to reduce position calculation errors. Differential ground station  28  knows its fixed position and calculates an expected propagation delay for each GPS satellite signal  18 , such that the delay calculation is based on a broadcast ephemeris of where each satellite  12  should be positioned in space. Differential ground station  28  compares a calculated propagation delay for satellite ranging signals  18  to an actual propagation delay measured for signals  18 , for all satellites  12  to determine a position error correction information  29  related to the signals  18  for each satellite  12 . Differential ground station  28  then transmits the position error correction information for each satellite  18  to user receiver  16 . 
     Alternately, any user receiver  16  within a connected network of user receivers  16  (not shown) may substitute for base station  14 , performing the same functions as base station  14 , and provide the same type of initialization information to aid operation of a second user receiver  16 . The transmission medium connecting the two user receivers can be either radio frequency (RF) wireless or a direct line cable connection. Thus, it will be understood that while certain embodiments may be described with reference to base station  14  (and information supplied from base station  14 ) that the use of base station  14  is not necessary in implementing all of the embodiments and methods described herein. 
       FIG. 2  is a flow chart illustrating an exemplary method  30  of acquiring satellite signal  18 . For signal processing, when user receiver  16  is first turned ON or activated to begin processing GPS signals  18 , receiver  16  searches for, acquires and locks on to GPS satellite ranging signals  18  from multiple satellites  12  in view. User receiver  16  also make distance measurements (called pseudo ranges) for each satellite PRN code  20  in view of user receiver  16 , demodulates the Nav message  22  superimposed on the PRN code  20 , applies any error corrections sent to it from base station  14 , if operating in the differential GPS mode, and uses this information to solve for user receiver&#39;s position and user receiver clock offset relative to GPS time. Additionally, in order to determine the distance between any satellite  12  in view and user receiver  16 , user receiver  16  determines the actual propagation delay for signal  18  and applies the error correction information received from base station  14  to calculate corrected propagation delay. The corrected propagation delay is then multiplied by the speed of light to determine the distance to signal sending satellite  12 . After acquiring GPS satellite ranging signals  18  of at least four satellites  12 , user receiver  16  solves for its position and time error relative to GPS time. 
     As each GPS satellite signal  18  is received by base station  14  and user receiver  16 , satellite signals  18  may be adversely affected by ionosphere or atmospheric conditions, error in GPS broadcast ephemeris and clock data, by multipath conditions, or other factors that may cause errors in the propagation delay  18  reaching base station  14  and/or user receiver  16 . Base station  14  transmits  32  additional aiding data such as, but not limited to, a unique set of initialization data, base-station-location data and size and/or boundary data. The set of initialization data includes GPS almanac or satellite ephemeris and clock data, and PRN ranging data including base station ranging measurements as received at base station  14 . The boundary data includes predetermined geographic dimensions to limit a search space grid in which user receiver  16  is located. 
     Through the wireless assist link, user receiver  16  receives from base station  14  the set of initialization data to aid acquisition along with the base-station-location data, and the boundary data to assist in the acquisition search. Additionally, over the wireless link, user receiver  16  may be provided GPS time synchronization (time synch) function  34  from base station  14  to allow user receiver  16  to synchronize its internal clock and to reduce a search space grid associated with an unknown user clock error. User receiver  16  also receives data on the accuracy of the time synch for use in the search from base station  14 , or time synch accuracy is inferred by user receiver  16  based on the boundary data. Additionally, user receiver  16  includes a time aiding function and a frequency aiding function to reduce user receiver clock error in order to reduce time search space. 
     Using the almanac or ephemeris data, user receiver  16  determines the number of GPS-satellite-ranging signals  18  in view for use to determine its location within the search space grid. The search space grid includes a two-dimensional search grid. The search space grid alternately may include a three-dimensional search grid. For example, in order to determine a location for the two-dimensional search space grid, at least three satellites  12  in view of user receiver  16  will be needed. Alternatively, at least four satellites  12  are needed in order to determine a location of user receiver  16  within a three-dimensional search grid. Additionally, user receiver  16  adjusts PRN codes  20  to known predictable offsets, to correlate simultaneously GPS satellite signals  18  received from each of the satellites  12  in view in order to simultaneously detect the presence of all of the signals  18  at some probable grid point location. 
     More particularly, after GPS time synch function and other transmitted data are received by user receiver  16 , user receiver  16  starts a search  36  within a search space grid based on the set of initialization data. Referring again to  FIG. 2 , user receiver  16  acquires simultaneously PRN codes  20  from each of the plurality of satellites  12  in view of user receiver  16 . While user receiver  16  is searching grid points in the search space grid, user receiver  16  demodulates  38  a power output of each received satellite PRN code  20  to determine a probable location of user receiver  16 . More specifically, user receiver  16  searches the search space grid, sums the power output related to each received satellite code for a plurality of grid point locations within the search space grid, and determines which grid point provides a maximum combined power output for the received satellite codes. The maximum combined power output indicates a highest combined power output at a particular grid point location to identify the probable location of user receiver  16 . 
       FIG. 3  is a flowchart illustrating an exemplary method  40  of detecting signal  18 , which is non-coherent, by receiver  16 .  FIG. 4  is a schematic block diagram of receiver  16  that is configured to acquire and demodulate non-coherent signal  18 . Referring to  FIG. 3 , in the exemplary embodiment, satellite  12  transmits  44  signal  18  having PRN codes  20  and Nav message  22  modulated on carrier signal  24 . More particularly, satellite  12  transmits L-based carrier signals such as, but not limited to, carrier frequencies L1, L2, L3, and L5. Moreover, in the exemplary embodiment, PRN codes  20  used on L1 and L2 carrier signals are C/A, P(Y) and M. PRN codes  20  used on L3 carrier is C/A. For L5 carrier, satellite  12  is configured to modulate at least two codes: an in-phase (I5) code and a quadrature phase (Q5) code of L5 carrier. 
     A particular code carrying combination is configured to form signal  18 . In the exemplary embodiment, signal  18  includes, but is not limited to, L1 C/A, L1 P(Y), L1M, L2 P(Y), L2M, L3 C/A, L5I5 and L5Q5. Moreover, signal  18  includes data chips  42  such as, for example, timing data chips and range data chips as shown in  FIG. 1 . 
     For transmission, PRN codes  20  and Nav message  22  are modulated  46  onto signal  18  by a phase shift keying modulation scheme that conveys data by changing or modulating a phase of signal  18 . In the exemplary embodiment, satellite  12  includes a quadrature phase-shift keying (QPSK) scheme. QPSK modulation scheme includes a method of transmitting digital information across a medium, by which an RF carrier signal is passed through a three-port device (one input and two outputs) (not shown), emerging as two signals of the same frequency having half of the original power and having a phase difference of 90 degrees between them (in quadrature). One signal is called the In-phase channel (I-channel) and the other the Quadrature-phase channel (Q-channel). Each one of these channels is further divided thus obtaining two I-channels and two Q-channels, the different code, such as, for example, C/A, P, M and T/A, bi-phase modulate each channel (one code per channel), wherein amplitude is independently controlled. 
     Channels are combined to form a composite QPSK signal which is then transmitted to stations  14  and user receivers  16 . Data chips  42  are grouped into pairs, and each pair is represented by a particular waveform, known as a symbol, to be sent across the medium after modulating signal  18 . As described herein, receiver  16  demodulates signal  18  and analyzes chips  42  to determine which pair of chips  42  was transmitted by satellite  12 . The QPSK scheme requires having a unique symbol for each possible combination of data bits in a pair. Because there are four possible combinations of data chips in a pair, QPSK scheme creates four different symbols, one for each pair, by changing the in phase (I gain) and quadrature phase (Q gain) for the cosine and sine modulators. A QPSK transmitter (not shown) uses both the sine and cosine at carrier frequency to transmit two separate message signals, known as the in-phase and quadrature signals. 
     Referring to  FIGS. 4 and 3 , receiver  16  includes at least an antenna  48 , a demodulator  50 , a processor  52 , an integrate/dump filter  54 , a time matched window filter  56 , a code generator  58 , a clock  60  and a mixer  62 . Receiver  16  is configured to receive  64  transmitted signal  18 . As noted, signal  18  may be distorted or corrupted during transmission. Receiver  16  is configured to lock onto signal  18  in the carrier frequency range, wherein receiver  16  is configured to remove  66  carrier signal  24  from transmitted signal  18  to determine chip timing sequence of signal  18 . Because signals  18  were modulated onto carrier signal  24 , receiver  16  is configured to separate signals  24 ,  18  after demodulation. Carrier signal  24  is removed to isolate  68  a data stream of a plurality of codes of signal  18  from carrier signal. 
     In the exemplary embodiment, receiver  16  is configured to implement a numerical analysis  70  to generate a synchronizing timing signal of data chips  42 . In response to satellite implementing QPSK modulation scheme for signal  18 , receiver  16  is configured to determine chip timing by a numerical analysis such as, but not limited to, numerical analysis of the second difference signal, for example, the second derivative equal to zero. In the exemplary embodiment, signal timing is determined by locating  72  the zero crossings or inflection points while using numerical analysis of the second difference to correlate zero crossings to signal states. 
       FIG. 5  illustrates constellation charts  74  for exemplary states of acquired signal  18 . More particularly, a legacy constellation chart  76  and an interplex constellation chart  78  are shown. Legacy constellation chart  76  includes two codes, C code and P code, with one code per channel. Interplex constellation chart  78  includes two codes per channel, such as C code, P code, T code and M code. Once timing signal is synchronized, codes are still combined such as for example, P+M codes and/or C/A+T codes. Constellation codes are generated by receiver  16  and data chips  42  are parsed into respective signal states. Because signal  18  is non-coherent, codes are combined such that the receiver  16  does not have a replica of signals for comparison to demodulate data and timing chips. Receiver  16  is configured to process signal  18  to recover codes without having knowledge of codes such as, for example, locally generated codes or pre-loaded codes within receiver  16 . In response, receiver  16  is configured to generate a new timing signal using the numerical analysis of the second difference. 
       FIG. 6  is a sequence of graphs  82  illustrating data synchronization and state selection using the exemplary numerical analysis of the second difference at coincident, timed data transitions  84 . During synchronization, receiver  16  is configured to determine the zero crossings, based on the numerical analysis of the second difference, which correlate to a chip state change. For example, the zero crossings may correspond to a symbol state change of a non-coherent signal. Receiver  16  is configured to determine timing of the plurality of codes to generate a new timing signal  86  using the numerical analysis of the second difference. More particularly, receiver  16  is configured to identify chip timing based on each zero crossing. Receiver  16  integrates each chip timing sequence to facilitate state identifying codes with data stream. In the exemplary embodiment, receiver  16  includes integrate and dump filter  54  to integrate each symbol state change of signal  18  to facilitate state identification of codes, wherein the identified codes are then grouped into states. 
     In the exemplary embodiment, the plurality of codes can be multiplied by one specific code of interest, for example one code per satellite  12 , wherein the resulting product is low-pass filtered to eliminate high frequency signals resulting from the multiplication process and the output of the low pass filter is routed to the processor  52  in order to extract the data associated with a specific satellite  12  that utilizes a specific code of interest. This process can be performed in parallel by using multiple specific codes applied to several multipliers, for example one specific satellite code per multiplier, to enable extracting location and time data from multiple satellites  12  at the same time by parallel processing. 
     Receiver  16  is configured to separate combined codes. In the exemplary embodiment, P-code  20  is separated from M-code and C/A code is separated from T/A code. Since satellite information such as code assignment and signal structure are known for each satellite  12 , receiver  16  is configured to separate codes. Receiver separates  16  the plurality of codes in data stream into an individual code using another filter process. In the exemplary embodiment, receiver  16  includes matched time window filter  56  to filter codes and separate codes  88  from the data stream a particular code for each plurality of codes that correlate to received signal  18 . Matched time window filter  56  is configured for a specific chip rate of each of the plurality of codes of signal  18 . More particularly, matched time window filter  56  is configured to filter, select, and/or separate data from each code a particular code for grouping into states. 
     Receiver  16  converts  90  filtered data for each code into a digitized, binary data stream. In the exemplary embodiment, binary stream of codes is correlated to the received PRN codes of particular satellite  12 . Receiver  16  verifies a signal quality based on the correlated binary data. In the exemplary embodiment, signal  18  is verified for quality parameters such as, but not limited to, code power ratios, correlation loss, crosstalk, signal coherence and delays. 
     Exemplary embodiments of systems and methods for a satellite receiver are described above in detail. The systems and methods are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the method may be utilized independently and separately from other components and/or steps described herein. Each component and each method step may also be used in combination with other components and/or method steps. Although specific features of various embodiments may be shown in some drawings and not in others, this is for convenience only. Any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to describe the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.