Abstract:
A satellite-based positioning method includes: obtaining predicted satellite data for at least one satellite vehicles (SVs) in a global navigation satellite system (GNSS); obtaining reference satellite data for the at least one SV; calculating satellite prediction error data for each of the at least one SV according to the predicted satellite data and the reference satellite data; and utilizing a processing unit to calculate a parameter for each of the at least one SV based on the satellite prediction error data. An associated satellite-based positioning apparatus is also provided.

Description:
BACKGROUND 
       [0001]    The disclosed embodiments of the present invention relate to satellite positioning, and more particularly, to a satellite-based positioning method and an associated apparatus. 
         [0002]    A Global Navigation Satellite System (GNSS) receiver determines its position by computing arrival times of signals transmitted simultaneously from multiple satellites. Current fully-operational GNSS include the US GPS (Global Positioning System) and the Russian GLONASS (GLObalnaja NAvigatsionnaja Sputnikovaja Sistema). As part of their messages, these satellites transmit both satellite positioning data and satellite clock timing data. The satellite positions and clock timing are represented by almanac or ephemeris data, wherein the ephemeris data provides an extremely accurate estimate ( ˜ 1 meter error) of satellite positions and clock bias. The process of searching for and acquiring satellite signals, reading the ephemeris data transmitted by the satellites, and computing the location of the receiver from this data is time consuming, however, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and furthermore, greatly limits battery life in miniaturized portable applications. 
         [0003]    For example, a GNSS receiver determines position based on the measurement of the arrival times at a GNSS receiver antenna of the GNSS signals broadcast from orbiting satellites. As stated, one disadvantage of such a system is the relatively long time needed to perform signal acquisition under certain conditions. Satellite signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and observed Doppler frequency shift. The process of the GNSS receiver searching for, acquiring, and demodulating satellite signals is sometimes referred to as a “standalone” mode of operation, which can be contrasted with an “assisted” mode of operation. 
         [0004]    In order to reduce the delay associated with a stand-alone mode of operation, information may be provided to aid a GNSS receiver in acquiring a particular signal. Such assistance information permits a receiver to narrow the search space that must be searched in order to locate a signal, by providing bounds on the code and frequency dimensions. A system that employs a GPS receiver augmented with externally sourced GPS assistance data is commonly referred to as an “assisted global positioning system” (AGPS). 
         [0005]    AGPS is a system that predicts satellite orbit information and, in many cases, greatly improves the startup performance or time-to-first-fix (TTFF) of a GNSS satellite-based positioning system. Extended Prediction Orbit (EPO) is a prior art example of an AGPS with off-line server based AGPS technology, which is a swift way to obtain specific information of the satellite, such as position and clock, before the receiving of broadcast ephemeris. The specific information aids the GNSS positioning engine (PE) to quickly locate the satellite, and thus obtain a First Fix with a faster speed. Refer to US Patent No. 20110273329 for more details. 
         [0006]    EPO supports up to 30 days satellite trajectory prediction using numerical fitting analysis with some models (mathematical, force, etc.), and greatly enhances a user experience by improving TTFF of a GNSS receiver. While providing a prediction of the extended satellite orbit and clock up to many days ahead, however, errors between the predicted and the actual orbit with respect to some satellites may be enormous. The First Fix may be poor since the GNSS PE cannot foretell quality of the orbit and clock prediction for respective satellites in advance. 
         [0007]    Therefore, a novel satellite positioning method is desired for aiding a GNSS receiver to derive a First Fix. 
       SUMMARY 
       [0008]    One of the objectives of the present invention is to provide a satellite-based positioning method for aiding a GNSS receiver to derive a First Fix, and an apparatus utilizing the same. 
         [0009]    According to a first aspect of the present invention, a satellite-based positioning method is disclosed. The satellite-based positioning method comprises: obtaining predicted satellite data for at least one satellite vehicle (SV) in a global navigation satellite system (GNSS); obtaining reference satellite data for the at least one SV; deriving calculating satellite prediction error data for each of the at least one SV according to the predicted satellite data and the reference satellite data; and utilizing a processing unit to calculate a parameter for each of the at least one SV based on the satellite prediction error data. 
         [0010]    According to a second aspect of the present invention, a satellite-based positioning apparatus is disclosed. The satellite-based positioning apparatus comprises a prediction unit and a processing unit. The prediction unit is arranged for generating predicted satellite data for at least one satellite vehicles (SVs) in a global navigation satellite system (GNSS). The processing unit is arranged for calculating satellite prediction error data for the at least one SV according to the predicted satellite data and the reference satellite data, and calculating a parameter for each of the at least one SV based on the satellite prediction error data. 
         [0011]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a flowchart illustrating a First Fix generation method applicable to a GNSS receiver according to an embodiment of the present invention. 
           [0013]      FIG. 2A  is a diagram illustrating errors of First Fixes derived by a conventional approach. 
           [0014]      FIG. 2B  is a diagram illustrating errors of First Fixes derived by the present approach. 
           [0015]      FIG. 3  is a diagram illustrating the concept of generation of confidence indexes. 
           [0016]      FIG. 4  is a diagram illustrating the groups of computed errors from past EPO for an SV. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is electrically connected to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
         [0018]      FIG. 1  is a flowchart illustrating a First Fix generation method applicable to a GNSS receiver according to an embodiment of the present invention. Provided that substantially the same result is achieved, the steps of the flowchart shown in  FIG. 1  need not be in the exact order shown and need not be contiguous; that is, other steps can be intermediate. Some steps in  FIG. 1  may be omitted according to various embodiments or requirements. Flow  102  is a conventional EPO method. Orbits and clocks of satellite vehicles (SVs) can be predicted by the flow  102  based on a prediction source. The prediction source directs to [the International GNSS Service] (IGS) in the embodiment, but this is not a limitation of the invention. In detail, initial data obtained from the EPO prediction ( 1022 ) is utilized for numerical fitting modeling ( 1024 ), and numerical model parameters ( 1026 ). The numerical model parameters are subsequently referenced by a [central ephemeris prediction machine] to produce predictions of the orbit and clock of each SV ( 1028 ). It should be noted that the flow  102  is not limited to the EPO method as described, and may be replaced by any other mechanism that achieves the same objective. Specifically, the flow  102  may be implemented by any other orbit/clock prediction method capable of providing an orbit/clock prediction result many days in advance. 
         [0019]    Flow  104  is a process to generate at least a confidence index. In  1044 , the confidence index is produced based on prediction errors generated by  1042 . The prediction errors represent how much the prediction results deviate from the actual orbits and clocks. Specifically, each SV may have a plurality of confidence index sets, which will be described later. By incorporating the confidence index sets obtained by the flow  104  (represented by the dotted line) into the orbit/clock prediction result produced by the flow  102 , the GNSS receiver can derive a weight of respective SV ( 1068 ). Please note that the processes  1062 ,  1064 ,  1066 , and  1070  may be replaced by any other First Fix computation mechanism to achieve the same objective. Compared with a conventional flow (bypass  1068 ), the disclosed method provides a more reliable and precise First Fix by referring to the weight of respective SV while the GNSS receiver computes for the First Fix. This is the spirit of the invention. 
         [0020]      FIG. 2A  and  FIG. 2B  are diagrams illustrating positioning errors of First Fixes derived by different approaches.  FIG. 2A  stands for performance of the conventional First Fix calculation approach represented by percentile vs. positioning error in meters.  FIG. 2B  stands for performance of the disclosed approach represented by the same format. In  FIG. 2A  and  FIG. 2B , First Fixes are derived by referring to satellite information (satellite orbit data and/or satellite clock data) predicted at 5-8 days ago, and errors of the results are shown in the curve diagrams. As described above, the conventional approach cannot determine the quality of respective SVs. In contrast, the disclosed approach is capable of selecting the best weighted SVs with the best statistical performances.  FIG. 2B  is enhanced from approximately 82% to 97% compared with  FIG. 2A  as the positioning error is 30 m. 
         [0021]      FIG. 3  is a diagram illustrating the concept of generation of confidence indexes. The basic concept to generate the confidence index is to group the errors from past information provided by the EPO. The invention is not limited to a specific grouping pattern. A more detailed descriptions regarding grouping errors will be shown in  FIG. 4 .  FIG. 4  is a diagram illustrating the groups of computed errors from past EPO for an SV. To a certain extent,  FIG. 4  reveals prediction of EPO error trend for a satellite. From the distribution of the error trend, the EPO prediction quality for each of the satellite can be computed. Various analyses can also be performed on the data of  FIG. 4  to obtain confidence indexes with a desired format. The confidence index is a dimensionless index which to predict the accuracy of the EPO. Please note that the clock prediction errors may also be used to make a similar diagram as  FIG. 4  for analysis. In other words, the confidence index may include derived information based on either or both of the orbit prediction errors and the clock prediction errors. 
         [0022]    The confidence index may be incorporated into the conventional EPO file, or into other types of orbit/clock prediction file. The PE therefore may check the confidence index and appropriately weight a respective SV according to the confidence index, as mentioned in  FIG. 1 . The SVs may then be arranged by weight, so that each SV has a priority when referenced for computing a First Fix. In this way, a more reliable and precise First Fix may be obtained based on the SVs with higher priorities. 
         [0023]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.