Abstract:
A method and apparatus for locating a mobile communications device such as a cell phone includes apparatus for communicating with base stations and apparatus for communicating with satellites. Signals received from satellites and the base stations are combined to provide the necessary information to calculate the location of the mobile communications device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional application of U.S. patent application Ser. No. 10/827,911, entitled WIRELESS NETWORK ASSISTED GPS SYSTEM, filed Apr. 20, 2004, the entire disclosure of which is incorporated by reference, which claims the benefit of U.S. Provisional Patent Application No. 60/465,823, entitled WIRELESS NETWORK ASSISTED GPS SYSTEM, filed Apr. 25, 2003, the entire disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates to GPS technologies, and more particularly, to a hybrid GPS system in which positioning of a mobile unit is realized with the assistance of a wireless network.  
       BACKGROUND OF THE INVENTION  
       [0003]     To determine the position of a mobile unit equipped with a GPS receiver, measurements of distance between the GPS receiver and GPS satellites are made at the receiver. In addition to the distance measurements, the time at which the GPS signals are transmitted from GPS satellites should be deduced from the distance measurements for correcting timing errors in the receiver.  
         [0004]     Timing errors may be effectively solved by differential GPS (DGPS) technology, in which the correction information of the timing errors is provided from a stationary reference receiver, which uses its known position to calculate timing. This is a result of the facts that, if two receivers are fairly close to each other (compared to the satellites far away in the space), the satellite signals that reach both of the receivers will have traveled through virtually the same slice of atmosphere, and so will have virtually the same errors. There are numerous satellites “in sight” (GPS satellite constellation is designed such that four or more satellites can be seen simultaneously at any location on the surface of the earth), and the reference receiver has no way of knowing which of the many available satellites a moving receiver might be using to calculate its position, thus the reference receiver runs through all the visible satellites and computes each of their errors. Then it transmits the correction information to the moving receiver. The moving receivers get the complete list of errors and apply the corrections for the satellites they&#39;re using.  
         [0005]     There are, however, several problems in current geolocation technologies. The power consumption at a GPS receiver is always one of the major concerns in view of the portability of the mobile unit. The more data processed at the receiver, the more profound the problem. Having a GPS receiver receive plural signals and then calculate its position requires extensive processing power.  
         [0006]     Effective detection of the GPS signals is another issue, since there are numerous satellite signals in space. Furthermore, sometimes there are not enough satellite signals received at the receiver for position calculation purposes, typically when the signals from one or more satellites are blocked from reaching the receiver. This may happen, for example, when the mobile unit is in a subway or in a building. The present invention helps to solve or minimize the above problems.  
       SUMMARY OF THE INVENTION  
       [0007]     In the present invention, the positioning of the GPS receiver in a mobile unit is assisted by a wireless network. In particular, the land based wireless network has its own source of GPS satellite data, which can be provided to the GPS receiver to assist the detection of the GPS signals. Furthermore, the position of the mobile unit is calculated by a location application processor in the wireless network, and the GPS receiver does not need to demodulate the ephemeris information from the received GPS signals, but only needs to decode and transmit a set of parameters (pseudo-ranging measurements and satellite ID) to the base station. Thus, the signal processing and power consumption at the mobile unit is minimized. Furthermore, the position location with partial GPS (p-GPS) is possible as the wireless network can help with timing and/or distance information. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  illustrates of the present invention in which three satellites and two base stations of the wireless network are involved in the positioning of the mobile unit;  
         [0009]      FIG. 2  illustrates of the present invention in which two satellites and three base stations of the wireless network are involved in the positioning of the mobile unit;  
         [0010]      FIG. 3  illustrates an interrogation scenario of the present invention; and  
         [0011]      FIG. 4  illustrates a status report scenario of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Reference is made to  FIG. 1 , in which two base stations  2   a,    2   b  of the wireless network  2  and three of the satellites are involved in positioning of the targeted mobile unit  3  which is equipped with a GPS receiver  3   a . With the base stations  1   a,    1   b,  the wireless network  2  maintains communication with a plurality of available satellites in space and collects information from these satellites. Like a DGPS reference receiver, the wireless network may include timing errors for each satellite.  
         [0013]     The GPS receiver  3   a  of the mobile unit  3  receives ephemeris data from the three satellites  1   a,    1   b  and  1   c,  which are not blocked. According to the present invention, however, the ephemeris data does not need to be extensively processed at the mobile unit  3  because of the fact that the wireless network has its own source of GPS satellite data. The mobile unit  3  only needs to decode and transmit a set of GPS parameters (pseudo-ranging measurements and satellite ID) which is needed for the position calculation to a location application processor  4  provided in the wireless network  2 , e.g., at the base station  2   a.    
         [0014]     The position calculation is carried out at the location application processor  4 . Therefore, the signal processing and the computational power at the mobile unit  3  are minimized.  
         [0015]     In addition to calculating the position of the GPS receiver  3   a , the wireless network  2  can also assist the GPS receiver  3   a  in detection of the GPS signal. In particular, the wireless network  2  can predict what GPS signals the mobile unit  3  will be receiving at any given time and therefore can provide the information such as the Doppler shift experienced by the GPS signal and the PRN sequence that the mobile unit  3  should use to de-spread C/A signal from a particular satellite  1   a,    1   b  or  1   c  as well as the information content carried by the GPS signal, the location and identity of the satellite(s) providing the GPS signal, and any other relevant data. In this way, the detectability of the GPS signal at the mobile unit  3  is significantly enhanced, and the speed of such detection is maximized.  
         [0016]     Furthermore, with the wireless network  2  and its base stations, the position location with partial GPS (p-GPS) is possible, as explained in detail below.  
         [0017]     In order to determine the position in three dimensions and the GPS receiver clock error correction, pseudorange measurements are made to four satellites resulting in the system of equations: 
 
ρ i =√{square root over (( x   i   −x   u ) 2 +( y   i   −y   u ) 2 +( z   i   −z   u ) 2 )}+ C·t   u    (1) 
 
 where: 
        ρ i =pseudorange from the receiver to satellite ‘i’;     (x i , y i , z i )=coordinate of satellite ‘i’;     (x u , y u , z u )=coordinate of the receiver to be calculated; and     t u =the receiver clock offset to be calculated.        
 
         [0022]     The satellite clock drift and other delay elements are assumed to be adjusted and therefore not shown in the equation (1) above. The pseudorange is measured using the propagation delay of the signal transmitted from GPS satellites. The coordinates of the satellites are to be decoded from ephemeris data in the signal, and in the present invention, this can be carried out by the wireless network  2  instead of the mobile unit  3 .  
         [0023]     However, as shown in  FIG. 1 , only three satellites  1   a,    1   b  and  1   c  are available to the GPS receiver  3   a  of the mobile unit  3 . This may be caused by the fact that the mobile unit  3  is in a blocked location such as behind a building. With the signals sent from the three satellites  1   a,    1   b  and  1   c,  three “GPS Equations” are as follows: 
 
ρ 1 =√{square root over (( x   1   −x   u ) 2 +( y   1   −y   u ) 2 +( z   1   −z   u ) 2 )}+ C·t   u  
 
ρ 2 =√{square root over (( x   i   −x   u ) 2 +( y   2   −y   u ) 2 +( z   2   −z   u ) 2 )}+ C·t   u  
 
ρ 3 =√{square root over (( x   i   −x   u ) 2 +( y   3   −y   u ) 2 +( z   3   −z   u ) 2 )}+ C·t   u    (2) 
 
 where ρ i =pseudorange, the distance between target satellite ‘i’; and 
        t u =target&#39;s clock offset.        
 
         [0025]     From the two base stations  2   a  and  2   b,  we have one “TDOA (time difference of arrival) Equation”:  
               δ       BS   ⁢           ⁢   2     ,     BS   ⁢           ⁢   1         =                   (       x     B   ⁢           ⁢   2       -     x   u       )     2     +       (       y     B   ⁢           ⁢   2       -     y   u       )     2     +       (       z     B   ⁢           ⁢   2       -     z   u       )     2         -                     (       x     B   ⁢           ⁢   1       -     x   u       )     2     +       (       y     B   ⁢           ⁢   1       -     y   u       )     2     +       (       z     B   ⁢           ⁢   1       -     z   u       )     2                       (   3   )             
 
         [0026]     where δ BS2,BS1 =differences of distances between the target and base stations, which is measured at the target. (Notice that the TDOA equation does not have timing information assuming that the target&#39;s clock is not synchronized with the wireless network.)  
         [0027]     Thus, we have the four non-linear equations with four unknowns, which are sufficient to calculate the position of the GPS receiver  3 . The unknowns can be solved by employing closed form solution, iterative techniques based on linearization, or Kalman filtering.  
         [0028]      FIG. 2  shown another example where only two satellites  1   a,    1   b  are available to the mobile unit  3 , while three base stations  2   a,    2   b,    2   c  are involved in positioning the GPS receiver  3   a . In this example, with the signals sent from the two satellites, two “GPS Equations” are: 
 ρ 1 =√{square root over (( x   S1   −x   u ) 2 +( y   S1   −y   u ) 2 +( z   S1   −z   u ) 2 )}+ C·t   u    ρ 2 =√{square root over (( x   S2   −x   u ) 2 +( y   S2   −y   u ) 2 +( z   S2   −z   u ) 2 )}+ C·t   u    (4)  
         [0029]     where ρ i =pseudorange, the distance between target and satellite ‘i’
            t u =target&#39;s clock offset            
 
         [0031]     From the three base stations, we have two “TDOA Equations”: 
 
ρ BS2,BS1 =√{square root over (( x   B2   −x   u ) 2 +( y   B2   −y   u ) 2 +( z   B2   −z   u ) 2 )}−√{square root over (( x   B1   x   u ) 2 +( y   B1   −y   u ) 2 +( z   B1   −x   u ) 2 )}
 
ρ BS3,BS1 =√{square root over (( x   B3   −x   u ) 2 +( y   B3   −y   u ) 2 +( z   B3   −z   u ) 2 )}−√{square root over (( x   B1   x   u ) 2 +( y   B1   −y   u ) 2 +( z   B1   −x   u ) 2 )}  (5) 
 
 where δ BSi,BSj =difference of distance between target and base station. 
 
         [0032]     Thus, we have four equations for the four unknowns, which are sufficient for the positional calculation of the GPS receiver  3   a.    
         [0033]     According to the present invention, the insufficiency in GPS satellite signals can be supported by the information provided from one or more base stations of the wireless network  2 , which may include timing information and distance information.  
         [0034]     Basically, there are four degrees in terms of the assistance from the wireless network  3 :  
         [0035]     First degree: support is timing information from a base station of the wireless network  2 . For this degree, three satellites are needed for acquiring a full set of four equations.  
         [0036]     Second degree: support is timing information with distance information from a base station (distance from one base station to the target mobile unit). For this degree, two satellites are needed for acquiring a full set of four equations.  
         [0037]     Third degree: support is timing information with distance information from two base stations (distance from each base station to the target mobile unit). For this degree, one satellite is needed for acquiring a full set of four equations.  
         [0038]     Fourth degree: support is timing information with distance information from three base stations (distance from each base station to the target mobile unit). For this degree, no satellite is needed, and the positioning of the mobile unit  3  can be carried out solely by the wireless network  3 .  
         [0039]      FIG. 3  illustrates a scenario of interrogation where the wireless network  2  initiates a query to locate the mobile unit  3 . To start, the wireless network  2  sends a query message to the mobile unit  3 , asking for the mobile&#39;s current status and the pseudo-range measurements from GPS signals. Importantly, the query message contains the information for assisting in the detection of the signals from GPS satellites such as Doppler shift and PRN sequence. In response, the GPS receiver  3   a  of the mobile unit  3  detects the proper satellites for the GPS signals and sends a set of GPS parameters, as well as status information of the mobile unit  3 , back to the base stations of the wireless network  2  for the position calculation processor  4  to determine the position of the mobile unit  3 .  
         [0040]      FIG. 4  illustrates a scenario of an assistance request initiated by the mobile unit  3 . To start, the mobile unit  3  that needs to determine its position using GPS sends an assistance request message to a nearby base station, asking for the information for assisting in the detection of the signals from GPS satellites. Upon receiving the assistance request from the mobile unit  3 , the wireless network  3  provides the information assisting the detection of the signals from GPS satellites. With the detection assistance information provided by the wireless network, the mobile unit  3  will be able to more easily detect the GPS signals from the proper satellites. Again, the pseudo-measurements from the GPS receiver along with the status information of the mobile unit  3  are transmitted to base stations.  
         [0041]     Notably, the assistance request message may itself reach plural base stations, each of which provides information. The difference in arrival times of the information can itself be utilized as yet further information in detecting the position of the GPS receiver, which further information may be combined with the use of the satellite data that is at least partially specified in the information.  
         [0042]     A status report signal received at multiple base stations may also be used to determine the position of the mobile unit  3 , using TOA, TDOA and/or AOA. Base stations determine the received signal properties (e.g., signal propagation delay in case of TOA) that are relevant in determining the position of the mobile unit  3 , and forward these signal properties along with the GPS pseudo-ranging measurement data contained in the status report message to the location application processor where the final position calculation is carried out.  
         [0043]     It is also possible that the mobile unit  3  calculates the position by itself, but more power will be consumed at the mobile unit due to more signal processing requirement.  
         [0044]     In general, there are in fact at least three types of information that may be utilized to assist in locating the GPS receiver. First, the difference in path lengths of plural signals sent from different based stations to the GPS receiver. Second, the difference in lengths of signals sent to/from satellites from/to the GPS receiver. Third, the difference in path length of one or more signals sent from the GPS receiver to plural base stations. These three signals can be combined using known mathematical equations to locate the GPS receiver with in a more accurate fashion than was previously possible.