The present invention relates generally to radio communication systems.  More particularly, the present invention relates to method and apparatus for mobile station location determination in such a radio communication system. 
Radio communication systems generally provide two-way voice and data communication between remote locations.  Examples of such systems are cellular and personal communication system (PCS) radio systems, trunked radio systems, dispatch radio networks, and global mobile personal communication systems (GMPCS) such as satellite-based systems.  Communuication in these systems is conducted according to a pre-defined standard.  Mobile stations, also known as handsets, portables or radiotelephones, conform to the system standard to communicate with one or more fixed base stations. 
It is desirable to obtain and communicate physical locations of mobile stations within the system.  In the future, such location information will be required.  The United States Federal Communications Commission (FCC) has required that radio systems provide physical location information for emergency ("E911") purposes.  Further, with location information available for individual mobile stations, position-dependent services and messaging including advertising can be tailored to the user of the mobile station responsive to location of the mobile station. 
Current generations of radio communication have only limited location determination capability.  In one technique, the position of the mobile station is determined by monitoring mobile station transmissions at several base stations.  From time of arrival measurements, the mobile's position can be calculated.  The precision of this technique at times may not be sufficient to meet requirements, though. 
In another technique, each mobile station is equipped with a receiver suitable for use with a global satellite navigation system such as the Global Positioning System (GPS).  The GPS receiver detects transmissions from a constellation of GPS satellites orbiting the Earth. 
Using data and timing from the transmissions, the GPS receiver calculates the positions of the satellites and from those positions, its own position.  A GPS satellite in orbit moves at about 4,000 meters per second.  The satellite has location data defined by a parameter X(t) and velocity data defined by a parameter V(t).  The parameters X(t) and V(t) are three dimensional position and velocity vectors for this satellite and are referenced to an earth-centered-earth-fixed Cartesian coordinate system.  The GPS system includes 24 satellites, several of which may be in view of the mobile station at any one time.  Each satellite broadcasts data according to pre-defined standard format and timing. 
Traditionally, the satellite coordinates and velocity have been computed inside the GPS receiver.  The receiver obtains satellite ephemeris and clock correction data by demodulating the satellite broadcast message stream.  The satellite transmission contains 576 bits of data transmitted at 50 bits per second.  The constants contained in the ephemeris data coincide with Kepler orbit constants requiring many mathematical operations to turn the data into position and velocity data.  In one implementation, this conversion requires 90 multiplies, 58 adds and 21 transcendental function calls (sin, cos, tan) in order to translate the ephemeris into a satellite position and velocity vector at a single point, for one satellite.  Most of the computations require double precision, floating point processing.  A receiver must perform this computation every second for every satellite, for up to twelve satellites. 
Thus, the computational load for performing the traditional calculation is significant.  The mobile must include a high-level processor capable of the necessary calculations.  Such processors are relatively expensive and consume large amounts of power.  As a portable device for consumer use, a mobile station is preferably inexpensive and operates at very low power.  These design goals are inconsistent with the high computational load required for GPS processing. 
Further, the slow data rate from the GPS satellites is a limitation.  GPS acquisition at a receiver may take many seconds or several minutes, during which time the receiver circuit and processor of the mobile station must be continuously energized.  Preferably, to maintain battery life in a portable radio, circuits are de-energized as much as possible.  The long GPS acquisition time can rapidly deplete the battery of a mobile station.  In any situation and particularly in emergency situations, the long GPS acquisition time is inconvenient and possibly dangerous for the user. 
One proposal includes receiving the ephemeris and clock correction data at a base station of the radio communication system and transmitting this data over a conventional communication link to the mobile station.  FIG.  1 shows a system incorporating this base-to-mobile communication link.  A base station 102 receives the 50 bits per second (bps) transmission from a GPS satellite 104.  The base station 102 acts as a repeater, gathering the data from the GPS satellite 104 and re-transmitting it at a higher data rate to a mobile station 106.  The ephemeris and clock correction data are received at the mobile station and used for calculation of satellite position.  From satellite position, mobile station position can in turn be determined.  FIG.  2 illustrates the computational requirements for determining the position vector X(t), the velocity vector Y(t) and the clock correction C(t) at the mobile station using the ephemeris data and clock correction data received from the base station. 
The illustrated system has some advantages.  A greater data rate is possible when using the base-to-mobile communication link.  As illustrated in FIG.  1, a conventional cellular system allows data transmission over this link at a typical rate of 9600 bits per second.  This allows the mobile receiver circuit to be turned off a greater amount of time, reducing power consumption in the mobile station.  However, the high computational load associated with the raw ephemeris data remains. 
Another proposed solution stores a GPS almanac at the mobile station.  The almanac data are a truncated, reduced precision subset of the ephemeris data.  A base station computes location and clock correction information for the almanac and transmits this correction over the communication link to the mobile station.  The mobile station determines that it has the proper correction data for its almanac and, if so, computes satellite location and clock data using the almanac. 
This system reduces slightly the computational load required of the mobile station.  However, the mobile station receiver must still remain energized during transmission of its almanac data and subsequently during all possible transmission times to receive correction data for its almanac.  Also, the almanac data must be stored at the mobile station which can increase the size and cost of the mobile station. 
Accordingly, there is a need for an improved method and apparatus for location determination in a radio communication system.