Abstract:
A receiver and method for acquiring a large pseudorandom (PN) sequence such as a GPS P(Y) code signal with poor or no knowledge of an external time reference are disclosed. A reference PN sequence is sectioned into a predetermined number of subsequences. The large PN sequence to be acquired is received from a remote source. The received PN sequence is correlated with each of the predetermined number of subsequences simultaneously. The large PN sequence is acquired, and a GPS solution may be provided, in the event the correlation produces a correlation between the received PN sequence and one of the predetermined number of subsequences. The large PN sequence may acquired without knowledge of an external time reference in a reasonable amount of time (e.g., less than ten minutes).

Description:
RELATED APPLICATIONS 
     This application is a continuation of prior application Ser. No. 09/321,966 filed on May 28, 1999, now abanndoned and claims priority thereto. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of GPS systems and particularly to acquisition of PN sequences in GPS systems. 
     BACKGROUND OF THE INVENTION 
     In GPS P(Y) code acquisition, the PN spread spectrum sequence is a one week long sequence having a rate of 10.23 MHz (6,187,104,000,000 bits). With no knowledge of an external time reference and with present digital correlator technology, it is impractical to accomplish a direct acquisition of a very large PN sequence such as the GPS P(Y) code in a reasonable amount of time (e.g., less than ten minutes). 
     Typical P(Y) code acquisition techniques involve correlating a reference PN signal with the transmitted PN sequence at all possible code positions. This correlation generally involves sequentially advancing the reference PN sequence with respect to the transmitted PN sequence until correlation is detected (indicating identity of the sequences). However, with very large length PN sequences and no knowledge of an external time reference, it is often impractical to sequentially correlate at all code positions in a reasonable amount of time due to the finite number of correlation processors available. Thus, there lies a need to provide a system and method for acquiring a relatively large, finite PN sequence in a reasonable period of time without knowledge or with poor knowledge of an external time reference. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for acquiring a pseudorandom (PN) sequence. In one embodiment, the method includes steps for sectioning a reference PN sequence into a predetermined number of subsequences, (with possible gaps between the subsequences) receiving the PN sequence from a remote source, correlating the received PN sequence with each of the predetermined number of subsequences simultaneously, and in the event the correlating step produces correlation between the received PN sequence and one of the predetermined number of subsequences, acquiring the received PN sequence. 
     The present invention is further directed to a GPS receiver for acquiring a GPS P(Y) code signal without knowledge or with poor knowledge of an external time reference. In one embodiment, the receiver includes a first filter and amplifier for selecting a predetermined frequency band of a received GPS signal and for amplifying the filtered signal, a first converter for converting the received GPS signal into an intermediate frequency signal, a second filter and amplifier for filtering and amplifying the intermediate frequency signal, a second converter for converting the filtered and amplified intermediate frequency signal into a digital signal representative of the GPS signal, and a processor for receiving the digital signal, the processor being configured to simultaneously correlate the digital signal with a plurality of subsequences of a reference signal such that the GPS signal is acquired upon correlation of the digital signal with one of the plurality of subsequences of the reference signal. 
     It is an object of the present invention to provide a method and appartus for acquiring large PN sequences in a short period of time. 
     It is a feature of the present invention to acquire a large PN sequence in six minutes or less. 
     It is a feature of the present invention to acquire a large PN sequence by holding the reference PN sequence fixed and waiting until the received PN sequence generates a match between the sequences. 
     It is an advantage of the present invention to acquire a large PN sequence without a time reference. 
     It is an advantage of the present invention to acquire a large PN sequence with a poor time reference. 
     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 as 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 of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The numerous 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 is a block diagram of a GPS receiver and processor operable to tangibly embody the present invention; 
     FIG. 2 is a block diagram illustrating the preprocessing of a reference PN sequence in accordance with the present invention; 
     FIG. 3 is a block diagram of a correlator operable with the present invention; and 
     FIG. 4 is a flow diagram of a method for acquiring a PN sequence in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the presently preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. 
     Referring now to FIG. 1, a block diagram of a GPS receiver operable to tangibly embody the present invention will be discussed. The GPS receiver  100  includes an antenna  110  for receiving a pseudorandom (PN) sequence signal such as the P(Y) code transmitted from a space vehicle in a global positioning system (GPS). The received signal is applied to the input of a preselect filter  112  (e.g., a band pass filter) for selecting the frequency of the received signal. The filtered signal is amplified by an amplifier  114  that is preferably a low noise amplifier (LNA). The signal is then converted from a radio-frequency (RF) signal to an intermediate frequency (IF) signal by RF to IF converter  116 . The intermediate frequency signal is then further filtered by band pass filter  118  and further amplified by amplifier  120 . The signal is then down converted into in-phase (I) and quadrature (Q) components by I, Q down converter  122 . The I and Q components are converted into a digital signal by A/D converter  126  and then provided to GPS processor  128 . GPS processor  128  preferably comprises or includes a signal processor (digital, optical, analog, etc.) for implementing faster processing of signal processing algorithms (e.g., correlations, convolutions, etc.). A frequency synthesizer  124  provides a reference signal of a predetermined frequency to RF to IF converter  116 , I, Q down converter  122 , and to A/D converter  126 . A real-time clock  130 , random-access memory (RAM)  132  and non-volatile memory (NVM)  134  couple with GPS processor  138  via bus  136 . GPS processor may receive an auxiliary signal  138  from an auxiliary signal source, for example as a reference signal for providing a Doppler correction of a received GPS signal. The result of the acquisition of the PN signal, such as a coarse/acquisition (C/A) solution of a GPS signal may be provided at an output  140  of GPS processor  128  according to known GPS processing techniques. 
     Referring now to FIG. 2, the preprocessing of a reference PN sequence in accordance with the present invention will be discussed. The reference PN sequence  210 , the sequence against which an incoming PN sequence received by receiver  100  is correlated, is sectioned into a plurality of subsequences: subsequence  1  ( 212 ), subsequence  2  ( 214 ), up to subsequence N ( 224 ) where N is the number of subsequences. Gaps can exist between the subsequences as shown in FIG. 2 with gap  1  ( 232 ) through gap M ( 244 ) where M is the number of gaps. The subsequences and gaps can be of differing lengths. In the particular case of GPS P(Y) code acquisition, the PN spread spectrum sequence is a one-week long sequence having a rate of 10.23 MHz (6,187,104,000,000 bits). The entire reference P(Y) code is sectioned into many (N) spaced fixed reference PN subsequences  212 - 224  with many (M) gaps  232 - 244 . The GPS P(Y) code can be segmented into approximately 15000 subsequences of 204,600 bits (20 milliseconds) each, which comprises only a small portion of the entire sequence. Since the phase of the incoming signal is correlated with an in-phase and quadrature reference, 7500 in-phase and quadrature subsequence pairs are used in a preferred embodiment of the present invention. In this case N=7500. Different numbers of subsequences and numbers of bits in the subsequences can also be used. The reference PN sequence, either as a whole sequence  210  or as individual subsequences  212 - 224 , may be stored in non-volatile memory  134  until required by processor  128  (e.g., to perform a correlation with an incoming PN sequence). The reference sequence may be stored in memory in a digitized format. 
     Referring now to FIG. 3, a block diagram of a correlator operable with the present invention will be discussed. The acquisition of an incoming PN sequence  310  involves near continuous correlation of the fixed reference PN subsequences  212 - 224  simultaneously with a received, incoming PN sequence  310 . The subsequences used with a typical correlator  312  are arranged in groups such that group  1  includes I and Q subsequence  1  ( 212 ) followed by I and Q subsequence  2  ( 214 ), I and Q subsequence  3  ( 216 ) and I and Q subsequence  4  ( 218 ). Subsequence  2  ( 214 ) is selected to be one bit later than subsequence  1  ( 212 ) in the reference PN sequence. I and Q subsequence  3  ( 216 ) is selected to be one bit later than subsequence  2  ( 214 ) and I and Q subsequence  4  ( 218 ) is one bit later than subsequence  3  ( 216 ). The groups of four I and Q subsequences are evenly spaced over the incoming PN sequence  310 . With correlator response time of approximately 396 nanoseconds or four bits of the incoming PN sequence  310 , the four subsequences ( 212 - 218 ) are overlaid with their one bit delay with respect to each other. A correlation is made between the 204,600 bits of incoming PN sequence  310  and the 15,000 sequences  212  through  224  simultaneously. The overlay of four bits for each I and Q subsequence in a group allows for the correlation of these four I and Q subsequences during a correlator response period resulting in 1875 subsequence groups to be correlated simultaneously. The correlator  312  can consist of 15,000 parallel correlation channels or 15,000 individual correlators to perform the correlation operation simultaneously between the incoming PN code and the 1875 subsequence groups in the same fashion as described for the first group of I and Q subsequences and the corresponding correlator  312  channel. This correlation operation is performed in a near continuous fashion on the incoming PN sequence limited by the correlator response time of 396 nanoseconds in this example. The output of the I and Q correlations can be combined mathematically in an envelope or power detector. 
     With 1875 subsequence groups evenly spaced over the entire incoming PN sequence  310  of one week or 604,800 seconds, one subsequence group is present in each 320 second period of the code or one uniform gap length. In the current embodiment with a correlator response time of 396 nanoseconds or four PN code bits, the incoming PN code shifts four bits relative to the 1875 subsequence groups while a correlator operation is performed. To search the entire window of uncertainty, approximately 320 seconds would be required. In this example, the worst case acquisition time of a desired correlation is approximately 320 seconds or under six minutes. Other correlation intervals can be used depending on the correlator and other numbers of subsequences can be used resulting in different acquisition times. 
     Instead of sequentially advancing reference PN sequence  210  with respect to incoming PN sequence  310 , the incoming PN sequence propagates through the receiver  100  and at times determined by the correlator response time, the subsequences  212 - 224  are correlated with the incoming sequence  310 . As one of the reference PN subsequences  212 - 224  and incoming PN sequence  310  align mathematically, correlation is detected at the correlation point and provided at the output  314  of correlator  312 . This means the receiver doesn&#39;t need a predetermined time reference but just needs to wait for a correlation match. Correlator  312  may be optimized for correlating a large, fixed sequence such as incoming PN sequence  310 . Correlator  312  may be implemented as a hardware embodiment as a component of GPS processor  128  or as a program of instructions or algorithm executed by a digital signal processor or microprocessor embodied as processor  128  or preferably may be implemented as an optical correlation embodied as processor  128 . 
     Referring now to FIG. 4, a method for acquiring a PN sequence in accordance with the present invention will be discussed. The method  400  includes a step  410  for sectioning a reference PN sequence  210  into a plurality of subsequences  212 - 224 . An incoming PN sequence  310  to be acquired is received by receiver  100  at step  412 . Incoming PN sequence  310  continuously propagates through the receiver  100  and shifts with respect to reference subsequences  416  such that incoming PN subsequence  310  is correlated with all reference subsequences simultaneously at step  416 . A determination is made at step  418  whether correlator  312  detects correlation between incoming PN sequence  310  and one of the reference subsequences  212 - 224 . Incoming PN sequence  310  continues to shift with respect to reference subsequences  212 - 224  and in the event correlation is not detected, the correlation step  416  continues at a different point in the incoming PN subsequence  310 . In the event correlation is detected at the correlation point step  418 , incoming PN sequence is acquired at step  420  such that a GPS solution may be provided by processor  128 . 
     Although the invent ion has been described with a certain degree of particularity, it should be recognized that elements thereof may be altered by persons skilled in the art without departing from th e spirit and scope of the invention. One of the embodiments of the invention can be implemented as sets of instructions resident in a memory  132  or  134  of one or more processors such as processor  128  generally as described in FIG.  1 . Such a processor may be considered generally as part of a computer system. Until required by the computer system or processor, the set of instructions may be stored in a computer readable memory. Further, the set of instructions can be stored in the memory of another computer and transmitted over a local area network or a wide area network, such as the Internet, as desired. Additionally, the instructions may be transmitted over a network in the form of an applet (a program executed from within another application) or a servlet (an applet executed by a server) that is interpreted or compiled after transmission to the computer system rather than prior to transmission. One skilled in the art would appreciate that the physical storage of the sets of instructions or applets physically changes the medium upon which it is stored electrically, magnetically, chemically, physically, optically or holographically so that the medium carries computer readable information. 
     It is believed that the direct acquisition of very large PN sequences in GPS systems 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.