Abstract:
Methods and systems for measuring wireless signals are described. The method includes generating a velocity estimate that includes a speed and a direction of a wireless receiver. A change in the velocity estimate is detected and how frequently the wireless signal is measured is adjusted according to the change detected in the velocity estimate. Systems may include wireless receivers that include an accelerometer that is operable to generate a velocity estimate that includes speed and direction of the wireless receiver. The wireless receivers may also include a processor operable to adjust a measurement period of the wireless signal in the wireless receiver according to a rate of change in the velocity estimate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No: 11/327,036, filed on Jan. 6, 2006 now U.S. Pat. No. 7,973,710, which is a continuation of U.S. patent application Ser. No: 10/912,516 (now U.S. Pat. No. 7,012,564), filed on Aug. 5, 2004. The above-referenced United States patent applications are all hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to position location systems. More particularly, the invention relates to a method for adjusting a measurement cycle in a satellite positioning system signal receiver. 
     2. Description of the Related Art 
     Global Positioning System (GPS) receivers use measurements from several satellites to compute position. GPS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance from the receiver to each of the satellites that are in view of the receiver. 
     More specifically, each GPS signal available for commercial use utilizes a direct sequence spreading signal defined by a unique pseudo-random noise (PN) code (referred to as the coarse acquisition (C/A) code) having a 1.023 MHz spread rate. Each PN code bi-phase modulates a 1575.42 MHz carrier signal (referred to as the L 1  carrier) and uniquely identifies a particular satellite. The PN code sequence length is 1023 chips, corresponding to a one millisecond time period. One cycle of 1023 chips is called a PN frame or epoch. 
     GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences. These measured time delays are referred to as “sub-millisecond pseudoranges”, since they are known modulo the 1 millisecond PN frame boundaries. By resolving the integer number of milliseconds associated with each delay to each satellite, then one has true, unambiguous, pseudoranges. A set of four pseudoranges together with a knowledge of absolute times of transmission of the GPS signals and satellite positions in relation to these absolute times is sufficient to solve for the position of the GPS receiver. The absolute times of transmission (or reception) are needed in order to determine the positions of the GPS satellites at the times of transmission and hence to compute the position of the GPS receiver. 
     Positioning systems, such as GPS, have fostered numerous applications that involve tracking people and assets. Various systems provide periodic location of a fixed asset, notification of proximity to pre-requested services, on-demand location identification, or continuous tracking of the location of a person or asset. Presently, such systems engage in satellite measurements at a device being tracked on a schedule unrelated to the relevance of the tracking information. This results in tracking the device continuously or tracking the device too infrequently to be effective. Continuous tracking directly results in increased power consumption in the device. Conversely, accessing the device too infrequently results in decreased accuracy and tracking performance. 
     Therefore, there exists a need in the art for a method that provides for the automatic adjustment of a measurement cycle in a satellite positioning system signal receiver. 
     SUMMARY OF THE INVENTION 
     A method for adjusting a measurement cycle in a satellite signal receiver is described. In one embodiment, a notification is received at the satellite signal receiver in response to at least one of a route-critical event and a motion-change event. A frequency of the measurement cycle is then adjusted in response to the notification. In one embodiment, the route-critical event comprises the satellite signal receiver being within a threshold distance of a route-critical location along a route. A motion-change event comprises a change in motion of the satellite signal receiver with respect to a threshold value. 
     In another embodiment, a mobile receiver includes a satellite signal receiver and a processor. The satellite signal receiver is configured to measure pseudoranges from the mobile receiver to a plurality of satellites as part of a measurement cycle. The satellite signal receiver is further configured to periodically execute the measurement cycle. The processor is configured to adjust the frequency of the measurement cycle in response to a notification indicative of at least one of a route-critical event and a motion-change event. In one embodiment, the mobile receiver further includes a sequential estimation filter, such as a Kalman filter, and the satellite signal receiver is further configured to apply pseudoranges to the sequential estimation filter as part of the measurement cycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a block diagram depicting an exemplary embodiment of a position location system in which the present invention may be utilized; 
         FIG. 2  is a flow diagram depicting an exemplary embodiment of a method for adjusting a measurement cycle in a satellite signal receiver in accordance with the invention; and 
         FIG. 3  is a flow diagram depicting another exemplary embodiment of a method for adjusting a measurement cycle in a satellite signal receiver in accordance with the invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     A method and apparatus for adjusting a measurement cycle in a satellite positioning system signal receiver is described. Those skilled in the art will appreciate that the invention may be used with various types of mobile or wireless devices that are “location-enabled,” such as cellular telephones, pagers, laptop computers, personal digital assistants (PDAs), and like type mobile devices known in the art. Generally, a location-enabled mobile device is facilitated by including in the device the capability of processing satellite positioning system (SPS) satellite signals, such as Global Positioning System (GPS) signals. 
       FIG. 1  is a block diagram depicting an exemplary embodiment of a position location system  100  in which the present invention may be utilized. The system  100  comprises a mobile receiver  102  in communication with a server  108  via a wireless communication network  110  (e.g., a cellular communication network). For example, the server  108  may be disposed in a serving mobile location center (SMLC) of the wireless communication network  110 . The mobile receiver  102  obtains satellite measurement data (e.g., pseudoranges, Doppler measurements) with respect to a plurality of satellites  112 . The server  108  obtains satellite navigation data (e.g., orbit trajectory information, such as ephemeris) for at least the satellites  112  in view. Position information for the mobile receiver  102  is computed using the satellite measurement data and the satellite navigation data. 
     Satellite navigation data, such as ephemeris for at least the satellites  112 , may be collected by a network of tracking stations (“reference network  114 ”). The reference network  114  may include several tracking stations that collect satellite navigation data from all the satellites in the constellation, or a few tracking stations, or a single tracking station that only collects satellite navigation data for a particular region of the world. An exemplary system for collecting and distributing ephemeris is described in commonly-assigned U.S. Pat. No. 6,411,892, issued Jun. 25, 2002, which is incorporated by reference herein in its entirety. The reference network  114  may provide the collected satellite navigation data to the server  108 . 
     The mobile receiver  102  is configured to receive assistance data from the server  108 . In one embodiment, the assistance data comprises acquisition assistance data. The acquisition assistance data may comprise expected pseudoranges or pseudorange models, expected Doppler data, and like type acquisition aiding information known in the art. Exemplary pseudorange models and details of their formation are described in commonly-assigned U.S. Pat. No. 6,453,237, issued Sep. 17, 2002, which is incorporated by reference herein in its entirety. For example, the mobile receiver  102  may request and receive acquisition assistance data from the server  108  and send satellite measurement data to the server  108  along with a time-tag. The server  108  then locates position of the mobile receiver  102  (referred to as the mobile station assisted or “MS-assisted” configuration). Acquisition assistance data may be computed by the server  108  using satellite trajectory data (e.g., ephemeris or other satellite trajectory model) and an approximate position of the mobile receiver  102 . An approximate position of the mobile receiver  102  may be obtained using various position estimation techniques known in the art, including use of transitions between base stations of the wireless communication network  110 , use of a last known location of the mobile receiver  102 , use of a location of a base station of the wireless communication network  110  in communication with the mobile receiver  102 , use of a location of the wireless communication network  110  as identified by a network ID, or use of a location of a cell site of the wireless communication network  110  in which the mobile receiver  102  is operating as identified by a cell ID. 
     In another embodiment, the assistance data comprises satellite trajectory data (e.g., ephemeris, Almanac, or some other orbit model). Upon request, the server  108  may transmit satellite trajectory data to the mobile receiver  102  via the wireless communication network  110 . Alternatively, the mobile receiver  102  may receive satellite trajectory data via a communications network  142  (e.g., a computer network, such as the Internet). Notably, the satellite trajectory data may comprise a long term satellite trajectory model, as described in commonly-assigned U.S. Pat. No. 6,560,534, issued May 6, 2003, which is incorporated by reference herein in its entirety. Having received the satellite trajectory data, the mobile receiver  102  may locate its own position using the satellite measurement data (referred to as the “MS-Based” configuration). In yet another embodiment, the mobile receiver  102  may locate its own position by obtaining ephemeris directly from the satellites  112 , rather than from the server  108 . That is, the mobile receiver  102  locates its own position without assistance from the server  108  (referred to as the “autonomous” configuration). 
     The server  108  illustratively comprises an input/output (I/O) interface  128 , a central processing unit (CPU)  126 , support circuits  130 , and a memory  134 . The CPU  126  is coupled to the memory  134  and the support circuits  130 . The memory  134  may be random access memory, read only memory, removable storage, hard disc storage, or any combination of such memory devices. The support circuits  130  include conventional cache, power supplies, clock circuits, data registers, I/O interfaces, and the like to facilitate operation of the server  108 . The I/O interface  128  is configured to receive satellite navigation data from the reference network  114  and is configured for communication with the wireless communication network  110 . In addition, the I/O interface  128  may be in communication with the network  142 . 
     In one embodiment, the position location system  100  includes a travel information server  144 . The travel information server  144  is configured to provide map information and the like for providing travel instructions from an origin to a destination (a “route”). The mobile receiver  102  may request and receive routing information from the travel information server  144  through the network  142  or through the wireless communication network  110  via the server  108 . Such travel information servers are well known in the art. 
     The mobile receiver  102  illustratively comprises a GPS receiver  104 , a wireless transceiver  106 , a processor  122 , an I/O interface  150 , and a memory  120 . In one embodiment, the mobile receiver  102  includes a sequential estimation filter, such as a Kalman filter  138 . In one embodiment, the mobile receiver  102  also includes a motion measurement device  152 . The GPS receiver  104  receives satellite signals from the satellites  112  using an antenna  116 . The GPS receiver  104  may comprise a conventional GPS receiver. The wireless transceiver  106  receives a wireless signal from the wireless communication network  110  via an antenna  118 . The GPS receiver  104  and the wireless transceiver  106  are controlled by the processor  122 . The I/O interface  150  may comprise a modem or like-type communication interface for communicating with the network  142 . 
     The processor  122  may comprise a microprocessor, instruction-set processor (e.g., a microcontroller), or like type processing element known in the art. The processor  122  is coupled to the memory  120 . The memory  120  may be random access memory, read only memory, removable storage, hard disc storage, or any combination of such memory devices. Various processes and methods described herein may be implemented using software  140  stored in the memory  120  for execution by the processor  122 . Notably, the Kalman filter  138  may be implemented via the software  140 . Alternatively, the mobile receiver  102  may implement such processes and methods in hardware or a combination of software and hardware, including any number of processors independently executing various programs and dedicated hardware, such as application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and the like. Notably, the Kalman filter  138  may be implemented using hardware or a combination of hardware and software. 
     Position of the mobile receiver  102  may be located using a navigation model in a well-known manner. Notably, in the general satellite navigation problem, there are nine unknowns:
         Three position unknowns: x, y, z   Three velocity unknowns: {dot over (x)}, {dot over (y)}, ż{dot over ( )}   Three clock unknowns: t c , t s , f c  
 
where t c  is the common mode timing error (usually a sub-millisecond value in GPS), t s  is the absolute time tag error, and f c  is the frequency error in a local oscillator within the mobile receiver  102 . One or more of the variables may be known or estimated based on a-priori information (e.g., t s  may known if the mobile receiver  102  is calibrated to precise GPS time). One or more of the unknown variables may be solved for using satellite measurement data from the mobile receiver  102  in a well-known manner.
       

     In another embodiment, a history of information may be used to continuously produce a filtered position result. The incorporation of history relies upon a formal model or an informal set of assumptions regarding the tendency of the mobile receiver  102  to move from position to position. By placing bounds on the motion of the mobile receiver  102  (and the behavior of a clock in the mobile receiver  102 ), filtering time constants may be selected that adequately track receiver dynamics, yet allow improved accuracy through the averaging process. Another advantage of filtering techniques is that the mobile receiver  102  may continue to operate when insufficient satellite measurements exist to create independent solutions. For purposes of clarity by example, an aspect of the invention is described with respect to a Kalman filter. It is to be understood, however, that other types of sequential estimation filters may be employed that are known in the art, such as Batch Filters. 
     Notably, position of the mobile receiver  102  may be located using the Kalman filter  138 . The Kalman filter  138  includes a plurality of states, such as position states, velocity states, clock states, and frequency states. The satellite measurements are applied to the Kalman filter  138 , which is configured to provide position upon request. Multiple measurement sets may be used to update the states of the Kalman filter  138 . The update weighs both the current state information and the measurements to produce new state information. For further details regarding operation of the Kalman filter  138 , the reader is referred to commonly-assigned U.S. patent application Ser. No. 10/790,614, filed Mar. 1, 2004, which is incorporated by reference herein in its entirety. 
     In operation, the mobile receiver  102  periodically executes a measurement cycle, where pseudoranges from the mobile receiver  102  to the satellites  112  are measured by the GPS receiver  104 . In one embodiment, the measurement cycle further includes application of the pseudoranges to the Kalman filter  138 . The measurements are used to periodically locate position of the mobile receiver  102 . Position may be located using the Kalman filter  138  or using a navigation model. For example, the mobile receiver  102  may be traveling along a route as set forth by the travel information server  144 . Progress of the mobile receiver  102  along a route may be tracked by periodically locating its position. Notably, the frequency of the measurement cycle and the frequency of the position location cycle (position fix cycle) may be the same or may be different (e.g., measurements may be obtained more of less often than position computations). In another example, the mobile receiver  102  may be tracked (i.e., location of the mobile receiver  102  may be periodically located) without having a route designated by the travel information server  144 . 
     As discussed below, the frequency of the measurement cycle may be automatically adjusted in response to a course-change event, such as a route-critical event, a motion-change event (heading and/or speed), or combination of such events. Note that the term “course” is used in a general sense to include heading and/or speed. The GPS receiver  104  may receive a notification of such an event from the processor  122 . Notably, a route-critical event may be triggered when the mobile receiver  102  is within a threshold distance of a route-critical location along a route (e.g., a route designated by the travel information server  144 ). A route-critical location may be any location or group of locations relevant to travel along a route such as, for example, an approaching intersection, an approaching fork in the road, an approaching maneuver (e.g., a required turn to stay on the established route), an approaching off-ramp, an approaching freeway exit, and like-type travel events. 
     A motion-change event may be triggered in response to a change in motion of the mobile receiver  102  with respect to a threshold value. In one embodiment, a motion change event is detected by the Kalman filter  138 . The Kalman filter  138  may be configured with states that continuously estimate velocity and heading of the mobile receiver  102 . A change in such states beyond a threshold may be used to indicate a change in velocity and/or heading of the mobile receiver  102 . If the Kalman filter  138  detects changes in one or more of such velocity and heading states, the Kalman filter  138  may trigger a motion-change event. In another embodiment, a change in motion of the mobile receiver  102  may be detected using one or more motion measurement devices  152 . The motion measurement devices  152  may comprise an accelerometer, a speedometer, compass, flux-gate compass, and like-type motion measurement, motion detection, and direction measurement devices known in the art, as well as combinations of such devices. For a given type of motion measurement, a threshold may be established in accordance with a given metric to delineate whether the mobile receiver  102  has transitioned from one motion state to another. In yet another embodiment, a combination of the Kalman filter  138  and motion measurement devices  152  may be used to trigger the motion-change event. 
     The frequency of the measurement cycle may be increased or decreased in response to a triggered event. For example, the frequency of the measurement cycle may be increased in response to a route-critical event. By increasing the frequency of the measurement cycle, the mobile receiver  102  obtains measurements more often to achieve greater tracking accuracy. This may assist the user of the mobile receiver  102  to navigate through the route-critical location. In another example, the frequency of the measurement cycle may be decreased in response to a motion-change event indicative of a stationary condition. If the mobile receiver  102  is in a stationary condition, the mobile receiver  102  may conserve power by performing less measurements. 
       FIG. 2  is a flow diagram depicting an exemplary embodiment of a method  200  for adjusting a measurement cycle in a satellite signal receiver in accordance with the invention. The method begins at step  202 , where a nominal frequency is designated for the measurement cycle of the satellite signal receiver. That is, the satellite signal receiver executes the measurement cycle at a predefined, nominal frequency. The nominal frequency for execution of the measurement cycle is a design parameter based on desired tracking accuracy versus power consumption. The nominal frequency may be set such that sufficient measurements exist for a desired frequency of position computations. For a given position fix frequency, less measurements are required if the Kalman filter  138  is employed, since the Kalman filter  138  is capable of producing a continuously filtered position result based on previous measurements. For example, the measurement cycle may be performed once every five seconds nominally. 
     At step  204 , a determination is made as to whether a route-critical event has occurred. If not, the frequency of the measurement cycle is maintained at the nominal frequency and step  204  is repeated. If a route-critical event has occurred, the method  200  proceeds to step  206 . At step  206 , the frequency of the measurement cycle is increased. In one embodiment, the frequency of the measurement cycle may be increased from the nominal frequency to an increased frequency value. In another embodiment, an increased frequency value may be selected from a plurality of increased frequency values, and the frequency of the measurement cycle may be increased to the selected value. Selection of an increased frequency value may be based on the type of route-critical event (e.g., an approaching intersection may engender less of an increase in frequency than an approaching sequence of required turns). 
     At step  208 , a determination is made as to whether the route-critical event is complete. If not, the frequency of the measurement cycle is maintained at the increased frequency and step  208  is repeated. If the route-critical event has completed, the method  200  proceeds to step  210 . At step  210 , the frequency of the measurement cycle reverts back to the nominal frequency value. The route-critical event may be deemed complete, for example, if the mobile receiver  102  is outside a route-critical location by a threshold distance. Alternatively, a route-critical event may be deemed complete after a predetermined time period has elapsed. The method  200  may be repeated for various route-critical events. 
     For purposes of clarity by example, the method  200  has been described with respect to adjustment of the measurement cycle from a nominal value. It is to be understood, however, that the frequency may be adjusted from a current value (whether the nominal value, or not) to any other increased value. 
       FIG. 3  is a flow diagram depicting another exemplary embodiment of a method  300  for adjusting a measurement cycle in a satellite signal receiver in accordance with the invention. The method  300  begins at step  302 , where a nominal frequency is designated for the measurement cycle of the satellite signal receiver. At step  304 , a determination is made as to whether a motion-change event has occurred. If not, the frequency of the measurement cycle is maintained at the nominal frequency and step  304  is repeated. If a motion-change event has occurred, the method  300  proceeds to step  306 . 
     At step  306 , the frequency of the measurement cycle is adjusted in response to the motion-change event. The adjustment of the measurement cycle may be based on the type of motion-change event. For example, if the motion-change event indicates that the mobile receiver  102  has transitioned into a stationary state, the frequency of the measurement cycle may be decreased from a nominal value. In another example, if the motion-change event indicates that the mobile receiver  102  has changed direction, the frequency of the measurement cycle may be increased from a nominal value. By increasing the frequency of the measurement cycle, the mobile receiver  102  obtains measurements more often to achieve greater tracking accuracy. This may assist the user of the mobile receiver  102  to navigate through a critical location as indicated by the change in direction. Those skilled in the art will appreciate that various other types of motion changes may be used to trigger increases or decreases in the frequency of the measurement cycle at step  306 . 
     At step  308 , a determination is made as to whether the motion-change event is complete. If not, the frequency of the measurement cycle is maintained at the increased frequency and step  308  is repeated. If the motion-change event has completed, the method  300  proceeds to step  310 . At step  310 , the frequency of the measurement cycle reverts back to the nominal frequency value. The motion-change event may be deemed complete, for example, if the mobile receiver  102  after a predetermined time has elapsed or after a predetermined time has elapsed without the occurrence of another motion-change event. The method  300  may be repeated for various motion-change events. 
     For purposes of clarity by example, the method  300  has been described with respect to adjustment of the measurement cycle from a nominal value. It is to be understood, however, that the frequency may be adjusted from a current value (whether the nominal value or not) to any other increased value. Furthermore, those skilled in the art will appreciate that a combination of the method  200  of  FIG. 2  and the method  300  of  FIG. 3  may be performed to adjust the frequency of the measurement cycle in response to either a route-critical event or a motion-change event. 
     Method and apparatus for adjusting the measurement cycle of a satellite signal receiver has been described. In one embodiment, the measurement cycle of the satellite signal receiver is adjusted in response to an external event (referred to as a course-change event), such as a route-critical event, a motion-change event, or a combination of such events. The frequency of the measurement cycle may be increased or decreased depending on the type of event that triggered the adjustment. The invention may be used to automatically adjust the measurement cycle frequency according to the user&#39;s current need. For example, a user&#39;s need of greater tracking accuracy through a route-critical location is met by automatically increasing the frequency of the measurement cycle, which provides more measurements. A user&#39;s need of less tracking accuracy while in a stationary state is met by automatically decreasing the frequency of the measurement cycle, which conserves more power. 
     In the preceding discussion, the invention has been described with reference to application upon the United States Global Positioning System (GPS). It should be evident, however, that these methods are equally applicable to similar satellite systems, and in particular, the Russian GLONASS system, the European GALILEO system, combinations of these systems with one another, and combinations of these systems and other satellites providing similar signals, such as the wide area augmentation system (WAAS) and SBAS that provide GPS-like signals. The term “GPS” used herein includes such alternative satellite positioning systems, including the Russian GLONASS system, the European GALILEO system, the WAAS system, and the SBAS system, as well as combinations thereof. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.