Abstract:
Method and apparatus for creating a compact orbit model is described. Satellite tracking data is obtained having a first set of orbit terms that define a first orbit model. The satellite tracking data is formatted to form formatted data having a second set of orbit terms that define a second orbit model. A number of terms in the first set of orbit terms is greater than a number of terms in the second set of orbit terms.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/915,219, filed Jul. 25, 2001 now U.S. Pat. No. 6,651,000, which incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to generating satellite tracking information for earth orbiting satellites. More specifically, the invention relates to a method and apparatus for generating satellite tracking information in a first format (e.g., a compact ephemeris model) through a network or communications link, then representing the satellite tracking information in a second format (e.g., a standard ephemeris model) at a receiver. 
     2. Description of the Related Art 
     A positioning receiver for the Global Positioning System (GPS) uses measurements from several satellites to compute a position. The process of acquiring the GPS radio signal is enhanced in speed and sensitivity if the GPS receiver has prior access to a model of the satellite orbit and clock. This model is broadcast by the GPS satellites and is known as ephemeris or ephemeris information. Each satellite broadcasts its own ephemeris once every 30 seconds. Once the GPS radio signal has been acquired, the process of computing position requires the use of the ephemeris information. 
     The broadcast ephemeris information is encoded in a 900 bit message within the GPS satellite signal. It is transmitted at a rate of 50 bits per second, taking 18 seconds in all for a complete ephemeris transmission. The broadcast ephemeris information is typically valid for 2 to 4 hours into the future (from the time of broadcast). Before the end of the period of validity the GPS receiver must obtain a fresh broadcast ephemeris to continue operating correctly and produce an accurate position. It is always slow (no faster than 18 seconds), frequently difficult, and sometimes impossible (in environments with very low signal strengths), for a GPS receiver to download an ephemeris from a satellite. For these reasons it has long been known that it is advantageous to send the ephemeris to a GPS receiver by some other means in lieu of awaiting the transmission from the satellite. U.S. Pat. No. 4,445,118, issued Apr. 24, 1984, describes a technique that collects satellite orbit information at a GPS reference station, and transmits the information to the remote GPS receiver via a wireless transmission. This technique of providing the ephemeris, or equivalent data, to a GPS receiver has become known as “Assisted-GPS”. Since the source of ephemeris in Assisted-GPS is the satellite signal, the ephemeris information remains valid for only a few hours. As such, the remote GPS receiver must periodically connect to a source of ephemeris information whether that information is received directly from the satellite or from a wireless transmission. Without such a periodic update, the remote GPS receiver will not accurately determine position. 
     Furthermore, the Assisted-GPS systems typically retransmit the entire ephemeris message to the remote receiver. In many instances, bandwidth or packet size for the transmission of this message is not readily available. 
     Therefore, there is a need for a method and apparatus for providing satellite trajectory and clock information to a remote receiver in a compact form. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for generating satellite tracking data (STD), then transmitting the data to a remote receiver in a compact form. The STD is derived by receiving at one or more satellite tracking stations the signals from at least one satellite and determining satellite tracking information (STI) through signal processing or by extracting the ephemeris message from the received signals. STI contains present satellite orbit trajectory data and satellite clock information. 
     The STD is reformatted into a compact format and provided to a remote satellite signal receiver via a network or communications system. The receiver converts the compact format into a standard format and uses the STD to compute the position of the receiver. The satellite system may include the global positioning system (GPS), GLONASS, GALILEO, or other satellite systems that may use STD to enhance the performance of the receiver. 
    
    
     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 depicts a system for creating and distributing satellite tracking data (STD) to remote GPS receivers; 
     FIG. 2 depicts a flow diagram of a method for forming the STD from the satellite measurements made at satellite tracking stations; 
     FIG. 3 depicts a flow diagram of a method for forming a compact orbit model in accordance with the present invention; and 
     FIG. 4 depicts an example of compacting the orbit model, where two orbit model terms are compacted into a single term. 
    
    
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts a block diagram of a system  100  for creating and distributing satellite tracking data (STD). The satellite system may include the global positioning system (GPS), GLONASS, GALILEO, or other satellite systems that may use STD to enhance the performance of the receiver. The following disclosure uses GPS as an illustrative system within which the invention operates. From the following disclosure, those skilled in the art will be able to practice the invention in conjunction with other satellite based positioning systems. 
     A network of GPS tracking stations  102  is used to collect measurement data from the GPS satellites  104 . Such a network is described in detail in U.S. Pat. No. 6,411,892, issued Jun. 25, 2002, which is incorporated herein by reference. The network could comprise several tracking stations that collect satellite tracking information (STI) from all the satellites in the constellation, or a few tracking stations, or a single tracking station that only collects STI for a particular region of the world. An STD collection and computation server  106  collects and processes the measurement data (this measurement data is referred to herein as satellite tracking information (STI) that includes at least one of: code phase measurements, carrier phase measurements, Doppler measurements, or ephemeris data). The ephemeris data may be the decoded ephemeris message extracted from the GPS signal itself. 
     The server may create long term STD in accordance with the teachings of U.S. Pat. No. 6,542,820, issued Apr. 1, 2003, or standard ephemeris message data in accordance with the teachings of U.S. Pat. No. 5,365,450, issued Nov. 15, 1994, both of which are incorporated herein by reference. The server  106  may produce one or more of the following: 1) accurate satellite tracking data (STD) (e.g., a trajectory of each satellite and/or a clock offset measurement) during the data collection period, 2) a prediction of the future STD of each satellite, and 3) models that match the future STD of each satellite. 
     The server  106  comprises a central processing unit (CPU)  118 , support circuits  122 , and memory  120 . The CPU  118  may be any one of the many CPUs available on the market to perform general computing. Alternatively, the CPU may be a specific purpose processor such as an application specific integrated circuit (ASIC) that is designed to process satellite tracking information. The support circuits  122  are well known circuits such as clock circuits, cache, power supplies and the like. The memory  120  may be read only memory, random access memory, disk drive storage, removable storage or any combination thereof. The memory  120  stores executable software, e.g., STD software  124 , that, when executed by the CPU  118 , causes the system  100  to operate in accordance with the present invention. 
     The set of satellite trajectory and clock data produced by the STD software  124 . The STD is stored in an STD database  108 . A distribution server  110  accesses the database  108  to gather the most recent set of STD, formats the data using the formatting software  111 , and distributes the formatted data to GPS devices  112  that require satellite orbit information. The software  111  produces a compact format, e.g., a compact ephemeris model, in accordance with the present invention. 
     The distribution process may be implemented using some form of wireless communications system  114 , or over the Internet  116 , or a combination of both, or by some other means of communication. Once the GPS devices  112  have received the compact ephemeris model, they expand the model to a format that is conventional for receiver. The compact ephemeris model distributed to the GPS devices may be in a similar format as the broadcast ephemeris or may be some other model format that is defined by the GPS device. Herein this orbit data is generally referred to as a satellite tracking model (STM). The loading of the STM into the GPS receiver can be accomplished in many ways. Using the cradle for a personal digital assistant (PDA), direct connection to a network, or a wireless technology, such as Bluetooth or a cellular network, are a few examples of how the satellite data can be transferred to the receiver. The transmission is generally accomplished by broadcasting a compact model of the STD (or a compact model representing a portion of the STD) without knowledge of the specific location of the GPS receiver. As such, the distribution server does not require the GPS receiver to send any information through the network to the distribution server. 
     Since GPS is a ranging system in and of itself, the data transmitted by the GPS satellites can be used to determine the range, range-rate and clock offsets to the GPS satellites from a set of tracking stations. This set of observations generated by the tracking stations  102  is used in the orbit determination process, and in the estimation of the satellite clock characteristics. The set of monitoring stations  102  could be a single station, a public network such as the Continuously Operating Reference System (CORS), or a privately owned and/or operated network. 
     FIG. 2 depicts a flow diagram of the process  200  of the present invention. The process  200  begins at step  202 , wherein the satellite measurements are collected at the tracking stations. At step  204 , the satellite trajectory data (STD) is computed or extracted from the satellite signals. The STD is then stored at step  206  in the STD database. At step  208 , the database is accessed and the formatting software is executed to convert the formatting of the accessed STD. The formatted STD is output as the compact model at step  210 . 
     One embodiment of the invention formats the STD as a subset of the standard ephemeris parameters defined in ICD-GPS-200c. Fitting the STD to the desired compact orbit model can be accomplished in a number of mathematical methods. The preferred embodiment is a least-squares fit of the orbit model parameters to the trajectory data. Other methods, such as Kalman filters or other estimators can also be used to obtain the orbit model parameters that best fit the trajectory data. These techniques of fitting data to orbit models are well known to people skilled in the art of orbit determination and orbit modeling. 
     The least squares technique provides an optimal fit of the trajectory data to the model trajectory formed from the compact orbit model parameters. FIG. 3 depicts a flow diagram of a method of generating an orbit model using a least squares estimation technique. 
     At step  302 , the STD for the desired time interval is extracted from the STD database. The orbit model parameters are initialized to the orbit model values obtained by a similar process for the previous interval. This guarantees that the initial orbit model parameters are a good fit at least for the beginning of the desired time interval. The rest of the process  300  will ensure that the parameters are adjusted so that they become a good fit for the entire time interval. 
     In the preferred embodiment there are 15 orbital parameters to be adjusted: 
     Square root of semi-major axis (meterŝ½) 
     Eccentricity (dimensionless) 
     Amplitude of sine harmonic correction term to the orbit radius (meters) 
     Amplitude of cosine harmonic correction term to the orbit radius (meters) 
     Mean motion difference from computed value (radians/sec) 
     Mean anomaly at reference time (radians) Amplitude of cosine harmonic correction term to the argument of latitude (radians) 
     Amplitude of sine harmonic correction term to the argument of latitude (radians) 
     Amplitude of cosine harmonic correction term to the angle of inclination (radians) 
     Amplitude of sine harmonic correction term to the angle of inclination (radians) 
     Longitude of ascending node of orbit plane at weekly epoch (radians) 
     Inclination angle at reference time (radians) 
     Rate of inclination angle (radians/sec) 
     Argument of perigee (radians) 
     Rate of right ascension (radians/sec) 
     At step  303 , some of the terms in the 15 term set are set to zero. The terms that are selected are the 6 harmonic terms such that there are 9 remaining parameters. This approach is particularly useful when bandwidth and/or packet size is limited in the communication link that will be used to convey the orbit model to the satellite signal receiver, e.g., the remote GPS receiver. The subset of 9 parameters, by setting all harmonic terms in the model to zero, is: 
     Square root of semi-major axis (meterŝ½) 
     Eccentricity (dimensionless) 
     Mean motion difference from computed value (radians/sec) 
     Mean anomaly at reference time (radians) 
     Longitude of ascending node of orbit plane at weekly epoch (radians) 
     Inclination angle at reference time (radians) 
     Rate of inclination angle (radians/sec) 
     Argument of perigee (radians) 
     Rate of right ascension (radians/sec) 
     The receiver can then reconstruct a standard ephemeris model by setting the “missing” harmonic terms to zero. In essence, the receiver reformats the STD for processing by the receiver circuits. 
     As an example of the method of generating the compact model, consider FIG. 4, which shows, for simplicity, just two terms of an orbit  400 : an orbital radius (A), and a radial harmonic term (r). For this simple example, these two terms form the non-compact model, wherein the orbit is described by a circle of radius (A) plus a harmonic perturbation (r). To produce a more compact model that fits the actual orbit over an interval  402 , the method of the invention removes the harmonic term (i.e., sets the term (r) to zero) and increases the orbital radius (A) to a larger value (A 1 ). The compact model is an orbit described by a circle with radius A 1 . If an application requires a non-compact orbit model, then the compact model (A 1 ) can be represented as a non-compact model by specifying a harmonic term (r 1 ) equal to zero. This compact model will fit the original orbit, over an interval  402 , with a small error. 
     In the preferred embodiment, 6 harmonic terms are removed from the 15-parameter model, and the other 9 terms are adjusted by process  300  that is analogous to the example 400 to provide a compact model that is accurate over a pre-defined interval. By adjusting the 9 remaining terms of an orbit model, while “zeroing” 6 harmonic terms, the compact model can be made accurate over a period of time such that a GPS receiver that relies on a compact model to compute position would compute a location that is no more than 2 meters less accurate than if the receiver used a full orbit model to compute position. 
     There are many alternative embodiments that will be readily apparent to those skilled in the art, such as removing more or fewer terms before adjusting the remaining terms, setting removed terms to some value other than zero, and defining new terms that model the orbit. 
     Returning to FIG. 3, at step  304 , the orbit model is used to predict what the trajectory would be, the predicted data is denoted the “Model Trajectory Data” (MTD). If the model were perfect, the MTD would coincide exactly with the STD. At step  306 , the MTD and STD are compared to see how closely the orbit model fits the orbit data. In the preferred embodiment, the comparison step  306  is performed by summing the squares of the differences between each trajectory point in the STD and the corresponding point in the MTD, and comparing the resulting sum to a threshold. If the fit is “good”, the model parameters are deemed “good” and the process stops at step  310 . If the fit is not good then the model parameters are adjusted at step  308 . There are many techniques well known in the art for adjusting model parameters to fit data. Steps  304 ,  306  and  308  are repeated until the model parameters are found that fit the STD well. 
     There are a large number of alternative embodiments to reduce the size of the data, i.e., compacting the STD, while still providing a model that fits the STD, including: 
     Removing parameters from the model, and replacing them with a constant, such as zero—as done above—or some other predetermined value, which is either stored in the Remote GPS Receiver, or occasionally sent to the receiver. The predetermined value may be determined by a GPS almanac stored at both the receiver, and the distribution server. 
     The resolution of the parameters may be restricted in the process  300 , this too reduces the amount of data that must be sent to the mobile GPS receiver. 
     Parameters, which are similar among two or more satellites, may be represented as a master value plus a delta, where the delta requires fewer bits to encode; an example of this is the parameter Eccentricity, which changes very little among different GPS satellites. 
     Some of these approaches reduce the ability of the model to fit the data over a period of time (e.g., four hours). In this case, the fit interval may be reduced (e.g. to two hours) to compensate. The accuracy of fit of the model can be traded off against the period of time over which the model is valid. 
     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.