Abstract:
A method and apparatus for determining location parameters by processing time and location datagrams made up of data contained in satellite positioning signals. In one embodiment, time and location datagrams are transferred to a GPS processing facility. In one embodiment, a plurality of time and location datagrams are combined. In one embodiment, time and location datagram size is increased, resulting in greater processing gains. In one embodiment, low frequency data overlay data is removed from the time and location datagrams.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to the determination of location parameters of one or more GPS receivers. 
         [0003]    Global positioning systems (GPS) utilize earth-orbiting satellites to transmit GPS signals which contain the necessary information for the determination of location and time coordinates of a GPS receiver. The NAVSTAR system, which has been operational since the early 1990&#39;s, as well as other systems currently being deployed operate on the same basic operation premise; utilize the time delay of signals transmitted by the satellites to the GPS receiver to determine the distance between multiple satellites and the GPS receiver at a precise moment in time. 
         [0004]    The GPS signals from all the satellites are broadcast at the same carrier frequency. However, each satellite has a unique identifier, or pseudorandom noise (PRN) code having 1023 bits or chips, thereby enabling a GPS receiver to distinguish the GPS signal from one GPS satellite from the GPS signal from another GPS Satellite. At any one time, barring obstructions, a GPS receiver will have a direct view of multiple GPS satellites, otherwise known as the satellite constellation. Any obstruction such as buildings and mountains will degrade the incoming satellite signals and impact both sensitivity and accuracy performance. 
         [0005]    A prior art GPS receiver  120  is illustrated in  FIG. 1 , where the incoming GPS signals  122  from the GPS satellite constellation  128  is received by the antenna  121 . The received signal is input into the analog front end module  123 . The main purpose of analog front end module  123  is to remove the 1.57 Ghz carrier frequency of the received signal. The down-converted signal, known as I/F signal  124 , is input to the GPS processor  125 . 
         [0006]      FIG. 2  describes the data structure of the signal that is broadcast by each GPS satellite, where the signal contains a 50 Hz low frequency data overlay signal (low frequency data overlay); 20 mSec data bits modulated by a one millisecond PRN code having 1023 bits or chips. The PRN code is known as a spreading code because it spreads the frequency spectrum of the GPS signal. This spread spectrum signal is known as a direct sequence spread spectrum (DSSS) signal. 
         [0007]    GPS processor  125  determines a one-way range, called a pseudorange because it includes a local time offset, to each GPS satellite from the time-of-arrival of the PRN code, the Zcount and ephemeris parameters in the GPS signal that it receives from that GPS satellite. Normally four or more pseudoranges are used for determining or overdetermining a three dimensional position and GPS time. Once pseudorange output  126  has been determined for at least four GPS satellites, it is a relatively simple process to determine the three dimensional location coordinates via triangulation as well as adding mapping and other application layer functions. 
         [0008]    GPS processor  125  acquires signal power with a search algorithm. In a typical search algorithm, the local frequency is set to a first trial frequency and then correlations are determined between the incoming GPS signal PRN code and all possible code phases of a local replica of the code. In order ensure that the correct code phase is not missed, it is conventional to increment the replica code phase in one-half chip or even smaller steps. A high correlation value indicates that signal power has been found. If no correlations are high enough, the local frequency is set to a second trial frequency and the correlations are repeated. Although no one correlation will take a great deal of time, the great number of correlations that must be performed can result in the time to find signal power to acquire a GPS signal being the largest single component of the time to first fix (TTFF). The search algorithm is used to examine the down converted I/F signal as it streams in from the satellite constellation  128 . The GPS processor  125  uses a sliding window approach for the search, therefore using a relatively small of amount of incoming I/F signal  124  at any one time. 
         [0009]    The time for a GPS receiver to acquire the first location fix is known as the time to first fix (TTFF), generally includes (i) the time to acquire GPS signal power by tuning a local frequency and a local PRN replica code phase in the GPS receiver to match the carrier frequency and the PRN code phase of the incoming GPS signal, (ii) the time to receive data bits in the GPS signal to determine a GPS clock time, (iii) the time to receive ephemeris parameters in the GPS data bits, and (iv) the time to process the code phase timing, GPS clock time and ephemeris for determining a position. 
         [0010]    In order to improve cold start sensitivity and TTFF, many GPS receivers are equipped to receive assistance from an external source. One well known technique is providing timing information to the GPS receiver from an external source via a communication link such as a cellular based data network. The timing information is used to set the initial frequency of the GPS receiver. This is commonly referred to as assisted GPS, or A-GPS. Aside from receiving timing information from an external source, the GPS signal processing functions of conventional GPS and A-GPS receivers are identical. Once again referring to  FIG. 1 , a GPS assistance device  129  has a clear view of satellite constellation  128  and determines the timing information via a communication link, it provides GPS processor  125  with the timing information  130  useful to setting the local oscillator frequency. Other assistance that the GPS receiver may receive from an external source may include constellation and almanac information, mapping, and wide area augmentation (WAAS) assistance. 
         [0011]    However, the GPS processing techniques common to both GPS and A-GPS receivers are unable to provide reliable indoor-outdoor sensitivity and accuracy. Furthermore emerging location-based applications, such as E-911, mobile Yellow pages, and asset management require reliable indoor and outdoor operation. 
         [0012]    There is a need in the art for a method of processing received C;PS signals that provides reliable operation at indoor and urban canyon locations. 
       SUMMARY OF INVENTION 
       [0013]    In general, the object of the present invention is to increase the receive sensitivity and accuracy of GPS receivers in order to provide for the reliable determination of location parameters, thus providing improved outdoor and indoor/urban canyon operation. GPS-based location devices have become the de facto standard for navigation assistance. 
         [0014]    The fact that these devices are unreliable indoors is, for the most part, irrelevant. Emerging location-based applications, such as E-911, mobile Yellow pages, and asset management, on the other hand, require reliable indoor and outdoor operation. However, the GPS processing techniques employed in today&#39;s GPS devices are unable to provide the reliable indoor-outdoor sensitivity and accuracy that is required. Even with the improvements of A-GPS, it has been shown that GPS-only based clients cannot meet the acquisition reliability and accuracy required by emerging indoor applications. 
         [0015]    It is a common belief that for most indoor and urban canyon environments, the received GPS satellite signals are too weak to reliably acquire a fix and What accuracy suffers by signal multi-path. However, there are a number of factors that limit the effectiveness of processing the received GPS signals. According to the present invention, there are advances in system architecture and signal processing methods that would extend GPS reliability and accuracy beyond prior art performance. 
         [0016]    First, prior art GPS receivers do not take full advantage in the highly repetitive nature of incoming satellite signals. As mentioned above, prior art GPS receivers perform a search of the incoming I/F signal looking for peaks in the power caused by correlation of a specific PRN code. Furthermore, as described by  FIG. 2 , the data structure of the signal that is broadcast by each GPS satellite contains a low frequency data overlay; 20 mSec data bits modulated by a one millisecond PRN code having 1023 bits or chips. Due to the low frequency overlay, there is a bit boundary that occurs after twenty PRN code cycles. Therefore prior art GPS search techniques use very small number of PRN code cycles to obtain a correlation with a particular PRN code sequence; typically 10 PRN cycles. 
         [0017]    Secondly, prior art GPS receivers do not take advantage of other GPS receivers within close proximity to each other. As with A-GPS, an A-GPS receiver will be provided assistance with timing information from an external source, but the processing of the incoming I/F signal is not enhanced by the combination processing of received signals from multiple GPS receivers that are in view of the same satellite constellation. 
         [0018]    Instead of searching the received GPS signals as a stream of data using a sliding window approach as described above, one key that allows the current invention to remove these shortcomings is that incoming GPS data is placed in time and location specific datagrams of varying lengths by the GPS receiver. This is not a dissimilar concept of data packets used in data networking technologies. This allows the time and location datagrams to be post processed, allowing for a number of distinct advantages. 
         [0019]    In one embodiment of the current invention, the low frequency data overlay embedded in transmitted satellite signal, as described by  FIG. 2 , is removed from the data contained in the time and location datagram. This results in a contiguous string of repetitive PRN code cycles that can be significantly larger than 20 PRN cycles. This allows for a processing gain that results in a much greater receive sensitivity characteristic. 
         [0020]    In one embodiment of the current invention, a GPS signal processor processes two or more time and location datagrams using signal combining techniques, resulting in a much greater receive sensitivity characteristic. Furthermore, the combining effect will also aid in the resolution of multi-path. This allows for improved accuracy of location parameters for each individual GPS receiver. The greatest improvement results are achieved when the GPS signal processor  200  combines time and location specific datagrams from GPS receivers in view of the same satellite constellation at the same time. 
         [0021]    In one embodiment of the current invention, the GPS signal processor aligns the timing of multiple datagrams in order to insure maximum signal combining effect. 
         [0022]    In one embodiment of the current invention, the GPS signal processor aligns multiple datagrams of varying sizes. 
         [0023]    In one embodiment of the current invention, the low frequency data overlay is removed from two or more time and location datagrams and are processed using signal combining resulting in improved sensitivity of the-satellite receivers. 
         [0024]    In one embodiment of the current invention, input from a wide area augmentation system (WAAS) is used by the GPS processing facility to improve the location accuracy of all GPS receivers. 
         [0025]    In one embodiment of the current invention, the time and location datagrams are transferred to the GPS processing facility via a wireless communication link. Examples include Wifi, 3G, GSM Edge, GPRS-Edge, Zigbee, and Bluetooth. 
         [0026]    In one embodiment of the current invention, the time and location datagrams are transferred to the GPS processing facility via a wired communication link. Examples include Ethernet. 
         [0027]    In one embodiment of the current invention, the time and location datagrams are transferred to the GPS processing facility via memory download. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0028]      FIG. 1  shows the system diagram of a prior art GPS receiver 
           [0029]      FIG. 2  describes the data structure transmitted by a GPS satellite 
           [0030]      FIG. 3  shows the system diagram of the current invention 
           [0031]      FIG. 4  describes the resulting data structure of the current invention 
           [0032]      FIG. 5  shows the system diagram of the current invention 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 3  shows a non-limiting embodiment of the current invention. Incoming GPS satellite signal  201  composed of transmitted signals from GPS satellite constellation  215  is received by a plurality of GPS receivers  202 ,  203 ,  204 . Each GPS receiver contains an analog front end module (FEM) with primary responsibility to down convert the 1.57 GHz incoming GPS satellite signal  201 . Each GPS receiver  202 ,  203 ,  204  forwards time and location datagrams, which contains the I/F data of the satellite constellation  215 , to GPS processing facility  200 . The time and location datagrams can be transferred to the GPS processing facility  200  via communication links  211 ,  212 ,  213  in real-time via a wireless or wired data communications link, or be stored in memory by the GPS receivers  202 ,  203 ,  204  and transferred to the GPS processing facility  200  at a later time. The time and location datagrams are processed by GPS signal processor  207 . The GPS signal processor  207  removes the low frequency data overlay. Removing the low frequency data overlay results in a contiguous string of repetitive PRN code cycles that can be significantly larger 20 PRN code cycles. The increased data sire provides for increased processing gain, resulting in improved GPS receiver sensitivity.  FIGS. 4   a  and  4   b  illustrate the resulting data structure of the incoming satellites signals resulting from the removal of the low frequency data overlay, where  4 a describes the unprocessed satellite signals, and  FIG. 4   b  described post processed data. The removal of the low frequency data overlay is performed on incoming time and location datagrams for each GPS receiver  202 ,  203 ,  204 . GPS signal processor  207  determines the pseudorange for each GPS receiver  202 ,  203 ,  204  respectively. Once pseudorange information has been determined for at least four GPS satellites for each GPS receiver, it is a relatively simple process to determine the three dimensional location coordinates via triangulation as well as adding mapping and other application layer functions. 
         [0034]    It is important to note that other GPS systems currently being deployed will have a similar low data rate overlay used in the NAVSTAR system. It is the object of the current invention that time and location datagrams from these alternate GPS systems be processed in a similar manner as to take advantage of the highly repetitive nature of GPS data. 
         [0035]      FIG. 3  describes a second non-limiting embodiment of the current invention. Incoming GPS satellite signal  201  composed of transmitted signals from GPS satellite constellation  215  is received by a plurality of GPS receivers  20 w,  203 ,  204 . Each GPS receiver contains an analog front end module (FEM) with primary responsibility to down convert the 1.57 GHz incoming GPS satellite signal  201 . Each GPS receiver  202 ,  203 ,  204  forwards time and location datagrams, which contains I/F data of the satellite constellation  215 , to GPS processing facility  200 . The time and location datagrams can be transferred to the GPS processing facility  200  via communication links  211 ,  212 ,  213  in real-time via a wireless or wired data communications link, or be stored in memory by the GPS receivers  202 ,  203 ,  204  and later transferred to the GPS facility  200  at a later time. Using two or more time and location datagrams, GPS signal processor  207  uses signal combining techniques, resulting in improved sensitivity and accuracy of each individual GPS receiver  202 ,  203 ,  204 . GPS receivers in different locations, but in view of the same satellite constellation, contain significantly similar PNR code profiles, differing only by the time-of-arrival offset due to the location difference. Signal combining of two or more time and location datagrams increases the signal to noise ratio of each individual signal. Furthermore, GPS signal processor  207  can use time and location datagrams which have had the low frequency data overlay removed. This further increases the processing gain and results in even greater receive sensitivity and accuracy. GPS signal processor  207  determines the pseudorange for each GPS receiver  202 ,  203 ,  204 . Once the pseudorange information has been determined for at least four GPS satellites per GPS receiver  202 ,  203 ,  204 , it is a relatively simple process to determine the three dimensional location coordinates via triangulation as well as adding mapping and other application layer functions. 
         [0036]    Furthermore, GPS signal processor  207  is capable of time aligning two or more time and location datagrams. 
         [0037]    Furthermore, GPS signal processor  207  can use signal combining techniques using two or more time and location datagrams varying in size. 
         [0038]      FIG. 3  describes yet another non-limiting embodiment of the current invention where GPS receivers receive external requests to obtain and/or forward time and location datagrams to a GPS processing facility. Non limiting examples include a request from an end user of an application seeking location coordinates of a particular GPS receiver. It could be a predetermined request by a GPS processing facility or location based application. This feature can be especially useful to extend battery life in GPS clients used in such applications as asset management tags. The ability to send a request to a particular GPS receiver is represented by location request signal  206 . 
         [0039]      FIG. 5  shows a another non-limiting embodiment of the current invention illustrating a GPS processing facility, where a GPS processing facility can-be comprised of the following elements; a plurality of GPS processing centers  230 ,  231 , an external wide area augmentation system (WAAS)  240 , an auxiliary location technology  250 , a GPS database  232 . Furthermore, all of these elements are connected via a GPS communication bus  260 . GPS communication bus  260  may be comprised of well known WAN and LAN technologies. 
         [0040]    This allows for the processing of time and location datagrams from GPS receivers connected to distributed GPS processing centers  230 ,  231 . A GPS database  232  can be employed to assist in the storage, search, and synchronization Of GPS receiver time and location datagrams. 
         [0041]    An external wide area augmentation system (WAAS)  240 , irk communication with the GPS processing facility can be used to improve accuracy of individual GPS receivers. An auxiliary location technology  250  is employed by the GPS processing facility to augment location coordinates obtained via the GPS system. This allows for improve location reliability and accuracy of individual GPS receivers. This is especially useful in cases where GPS receivers will be in service at locations where GPS signal levels are not present. Non-limiting examples of these types of technologies are wifi (802.11) based and television signal based location systems.