In an unassisted GPS-type position determination system, subscriber stations determine their own positions from satellite transmissions originating from the GPS-type position determination system, without requesting significant acquisition or calculation assistance from other network entities, for example, dedicated servers. That places significant processing demands on the subscriber stations because of the uncertainty in the timing, position, and frequency of these transmissions, requiring the subscriber stations to expend significant processing resources in searching for and locating these transmissions by, for example, testing large numbers of hypotheses varying the assumed timing, position and frequency of the transmissions. Since the number of hypotheses that must be tested is often staggering, the time required to search for the transmissions can be inordinately long and consume an excessive amount of processing resources, even for subscriber stations with dedicated receiver chains.
The uncertainty experienced by the subscriber stations stems from several sources. Assuming GPS positioning, there is first the uncertainty in knowing which of the 32 GPS satellites are visible to the subscriber station. That uncertainty is present because a subscriber station, upon power up or before a position fix is available, has no basis for identifying which signals of these 32 satellites can be usefully received. The useful reception of satellite signals is referred to as an ability of the subscriber station to “see” the satellite emitting the signal, or, in other contexts, as the satellite being “visible” to the subscriber station.
This leads to inefficient searching because the subscriber station may waste considerable resources in searching for transmissions from satellites that are not visible to it, and which are therefore not useful for position determination purposes. For example, referring to FIG. 1, while satellites 54a, 54b, and 54c are visible to subscriber station 50 located at position 51 on the earth's surface 52, satellites 56a, 56b, and 56c are invisible to subscriber station 50, as they are located on the other side of the earth. Therefore, it would be wasteful for subscriber station 50 to search for the transmissions from satellites 56a, 56b, and 56c during a position fix attempt.
In addition, there is an uncertainty in knowing the timing or phase of the 32 chip PN “gold” codes that are embedded within the individual satellite transmissions. As these codes are circularly shifted versions of one another, the phase of a code uniquely identifies which of the satellites originated the transmission. The phase also reflects the propagation delay caused from transmission from the satellite to the subscriber station. To account for the possible variations in phase, the subscriber station must expend resources in searching over the full range of possible PN codes within a code phase searching window that is large enough to encompass the possible variations.
Moreover, there is an uncertainty in knowing the relative movement between the subscriber station and the GPS satellites, which typically introduces a Doppler shift of approximately ±4 kHz in the frequency of transmission. To account for the possible variation of frequency introduced by the Doppler shift, the subscriber station must expend resources is searching over the full range of possible transmission frequencies within a frequency searching window that is large enough to encompass the possible variations caused by the Doppler shift.
Finally, there is the uncertainty in knowing the degree to which the local oscillator (LO) of the subscriber station is out of tune with the GPS carrier frequency. Upon power-up, for example, it is not uncommon for the LO frequency to differ from the GPS carrier frequency by as much as ±5 ppm. Until synchronization between the LO frequency and GPS carrier frequency is achieved, the subscriber station must account for this uncertainty by increasing the size of the frequency search window that is employed.
Even if the host wireless communications system or GPS-type position determination system eliminating some of this uncertainty by providing timing, positional information, or synchronization to the subscriber station, the processing demands on the subscriber station are often still substantial. For example, a synchronous system, such as a CDMA system, provides the subscriber station with time, and also synchronizes the LO frequency of the subscriber station to the GPS carrier frequency. Although the synchronization substantially reduces the LO frequency uncertainty, for example, from ±5 ppm to ±0.2 ppm, and the timing information allows the subscriber station to determine the position of the GPS satellites (using the GPS almanac or ephemeris data provided by the satellites), the subscriber station is still unable to determine which of the GPS satellites are visible to it, and it is still subject to the frequency uncertainty caused by Doppler shift.