HK1067181B

HK1067181B - Apparatus, method, and system of transferring correction information

Info

Publication number: HK1067181B
Application number: HK04110236.9A
Authority: HK
Inventors: P．阿加西; G．瓦纽西
Original assignee: 高通股份有限公司
Priority date: 2001-07-20
Filing date: 2002-07-19
Publication date: 2008-08-01

Description

Apparatus, method and system for communicating correction information

Background

Technical Field

The present invention relates to information for correcting stored or calculated values and communicating them.

Description of the related Art

Since the development of positioning technologies such as the Global Positioning Satellite (GPS) system, the ease and accuracy of position determination has increased dramatically. An example of a satellite Positioning System is the NAVSTAR GPS System (described in "global Positioning System Standard Positioning Service Signal Specification", 2 nd edition, 6.2.1995, Coast Guard Navigation Center, Alexandria, VA); another example is the GLONASS GPS system maintained by the Russian republic. GPS receivers are currently available in aircraft, ships, ground vehicles, and are carried around by individuals.

The NAVSTAR GPS system currently includes 24 satellites, or "space vehicles" ("SVs"), that orbit the earth in 6 planes (4 satellites in each plane). Every day, SV orbits repeat approximately the same ground track as the earth beneath them. The orbital planes are equally spaced and tilted relative to the equatorial plane to ensure that there is a ray path from any (unobstructed) point on the earth to at least 5 SVs.

Ground-based monitoring stations measure signals from SVs and incorporate these measurements into an orbit model for each satellite. Navigation data and SV clock corrections are calculated for each satellite from these models and uploaded to each SV. The SV then transmits information about its location by modulating a direct sequence spread spectrum signal at a chip rate of 1.023MHz onto a carrier of 1.5 GHz.

A GPS receiver calculates its position by combining the delays or phases of signals received from the SVs, indicating the position of the receiver relative to the SVs from information relating to the position of the SVs. Due to inaccuracies in the receiver's time base oscillator, signals from at least 4 SVs are required to resolve position into three directions, although signals from additional SVs (if available) may be used to provide better accuracy.

It is desirable to augment certain wireless systems of mobile communications by adding the ability to locate the position of a particular mobile unit. One reason is the rules promulgated by the Federal Communications Commission (FCC), abstract number 94-102, Third Report and Order taken 5.9.1999, published 6.10.1999, which requires all cellular operators in the united states to be able to locate cellular telephones making emergency 911 ("E-911") calls 10.2001, 67% to 50 meters and 95% to 150 meters. Other uses of location capabilities in wireless communication systems include value-added user features such as navigation and fleet management support.

Summary of The Invention

In a method according to an embodiment of the invention, the correction factors are delivered in a predetermined order. From this predetermined sequence, the particular stored or calculated values associated with each correction factor may be identified.

Brief Description of Drawings

FIG. 1 shows a flow diagram of a method according to an embodiment of the invention.

Fig. 2 shows a flow diagram of a method according to an embodiment of the invention.

Fig. 3 shows the structure of the GPS data stream.

Fig. 4 shows the format of a GPS data frame.

FIG. 5 shows a block diagram of a system in accordance with an embodiment of the invention.

FIG. 6 shows a block diagram of a system in accordance with an embodiment of the invention.

FIG. 7 shows a flow diagram of a method according to an embodiment of the invention.

Fig. 8 shows a block diagram of an apparatus according to an embodiment of the invention.

Fig. 9 shows a block diagram of an apparatus according to an embodiment of the invention.

FIG. 10 shows a flow diagram of a method according to an embodiment of the invention.

FIG. 11 illustrates a block diagram of an exemplary implementation of task T240 shown in FIG. 9.

FIG. 12 shows a flow diagram of a method according to an embodiment of the invention.

FIG. 13 shows a flow diagram of a method according to an embodiment of the invention.

FIG. 14 illustrates a block diagram of an exemplary implementation of task T242 shown in FIG. 12.

FIG. 15 shows a flow diagram of a method according to an embodiment of the invention.

FIG. 16 shows a flow diagram of a method according to an embodiment of the invention.

Detailed description of the preferred embodiments

Methods according to embodiments of the invention disclosed herein may be applied to any situation where an entity is desired to communicate information relating to a set of items (e.g., to a storage medium or device, or by transmission to another entity). The set of items may include physical objects (e.g., satellites, people, etc.) and the information is delivered in a predetermined order, thereby avoiding the need to include tags or similar mechanisms to explicitly specify correspondence between the information and the individual items. If there may be a time-varying ambiguity in the order in which the receiving or retrieving entity applies, then the sending or storing entity may predict the ambiguity and adjust the temporal adaptation of the data it sends accordingly. For example, the sending entity may predict how the receiving entity will apply the predetermined order to the set of items, and select a particular validity time at which no ambiguity in the order applied by the receiving entity is expected.

FIG. 1 shows a flow diagram of a method according to an embodiment of the invention. In task T120, a correction factor is calculated. Each of these factors is associated with one or more of a set of items. In task T210, the correction factors are sent in a predetermined order. The predetermined order may be based on absolute or relative values that indicate a position (or an expected position) of an item, or an identification number of an item, or some other characteristic by which items are distinguishable from one another. The correction factors may be computed and transmitted separately (i.e., in a serial or pipelined fashion), or one or more factors may be computed and/or transmitted in parallel.

Fig. 2 shows a flow diagram of a method according to an embodiment of the invention. In task T120a, a correction factor is calculated for each object in the set of physical objects. The set of objects may be a subset of a larger set of objects: for example, a group may include only those GPS SVs that are determined to be visible from a predetermined location or area. In task T130a, a supplemental location is determined for each object. Such a determination may be based on an observation of the object and/or a signal received from the object or another entity.

In task P140a, a correction factor is calculated for each object that is based on the difference between the supplemental location and the reference location of the object. The reference location may be at a different time than the supplemental location and may have been calculated using a less accurate method and/or from a less accurate basis. Depending on the particular implementation, task T120a may be performed serially or in parallel (i.e., more than one object at a time) for each object in the set of objects.

In the exemplary implementation of the method shown in FIG. 2, one basis for determining the reference and supplemental locations is GPS navigation data transmitted by one or more SVs. This data includes a series of time-stamped data bits that mark the time of transmission from the SV for each subframe. As shown in fig. 3, the GPS data frame 100 consists of 1500 bits sent out over a 30 second period (i.e., at a rate of 50 bits per second). Each data frame is divided into 5 300-bit sub-frames 110, each having a duration of 6 seconds. As shown in fig. 4, the first three sub-frames of a frame contain track and clock data: SV clock corrections are sent in subframe 1 and accurate orbit data sets (or "ephemeris information") for the transmitted SVs are sent in subframes 2 and 3. The ephemeris information is repeated once per frame.

Subframes 4 and 5 are used to send the almanac and other information. Unlike ephemeris information, almanac information includes a set of orbit data that is common to all SVs. Almanac information is less accurate relative to a single SV than almanac information, requiring a full set of 25 frames (also called a superframe and having a transmission period of 12.5 minutes) in order to send a full almanac. (see, for example, "Global Positioning System: the organic and applications (Volume I)", by B.W.Parkinson and J.J.Spilker Jr. 1996, for a more detailed description of the NAVSTAR GPS System).

Ephemeris information provides a very accurate description of the orbits of the SVs. However, entities like mobile units may only access less accurate almanac information (e.g., due to bandwidth constraints). For applications like E-911 location determination, the location calculated from almanac information only may be unacceptably inaccurate. In such a case, it is desirable to provide the mobile unit with a correction factor indicating the difference between a position calculated from the almanac information only and a position calculated from the ephemeris information.

FIG. 5 shows a block diagram of a system in accordance with an embodiment of the invention. The GPS receiver 120 receives almanac and ephemeris information from one or more SVs and forwards this information to the PDE 100. In one implementation, PDE 100 forwards at least a portion of the almanac information to one or more mobile units (not shown) via a base transceiver station ("BTS") 10, which may be part of an existing cellular telephone system. In another implementation, the mobile unit receives almanac information directly from the SVs. PDE 100 calculates correction factors based at least on the almanac and ephemeris information and forwards them to the mobile unit via BTS 10.

FIG. 6 shows a block diagram of a system in accordance with an embodiment of the invention. In this example, the PDE 102 (including an integrated GPS receiver) provides position determination support via several BTSs 12a, 12b by a base station controller or mobile switching center 200, which base station controller or mobile switching center 200 may be part of an existing cellular telephone system. In other embodiments, the PDE (with or without an integrated GPS receiver) may be incorporated in the MSC, BSC, and/or BTS.

The almanac correction data is unique for each SV. It does not matter how much data is sent between the base station and the mobile unit if the size of the message sent between the two devices is not limited. An SV Identification (ID) number may be used to tag each satellite's corrections, e.g., SV ID1 indicates that the following correction data applies to satellite 1, SV ID2 indicates that the next correction data applies to satellite 2, and so on.

However, the transmitting tag consumes some bits, which may be a limited number of bits provided within the GPS navigation message. The advantage of avoiding the need for tag bits is particularly desirable in low data rate systems, such as analog Advanced Mobile Phone Systems (AMPS), where the data link capacity may be very limited.

FIG. 7 shows a flow diagram of a method according to an embodiment of the invention. In task T120b, a correction factor is calculated for each of a set of SVs. In task T130b, a supplemental location is determined for each SV, with such determination made based at least in part on ephemeris information received from the SVs.

In task P140b, a correction factor is calculated for each SV based at least in part on a difference between a supplemental position and a reference position of the SV. In an exemplary implementation, the reference portion is determined based at least in part on almanac information received from the SV and/or other SVs. Depending on the particular implementation, task T120b may be performed serially or in parallel for each SV (i.e., more than one SV at a time). Further to the above example, the predetermined order used in task T210 may be relative to an elevation angle, an azimuth angle, or an identification number specified by the SV. All or a portion of the method shown in fig. 7 may begin in response to a request for correction information from a mobile unit (e.g., a request for GPS almanac correction, as described in TIA/EIA provisional standard (IS) IS-801, telecommunications industry alliance, Arlington, VA, 11 months 1999).

FIG. 8 shows a block diagram of a Position Determination Entity (PDE)500 in accordance with an embodiment of the invention. Almanac information received from one or more SVs is stored in almanac information memory 410. Based on the stored almanac information, the reference position calculator 420 calculates the reference position for the selected SV, and gives the position determination to the combiner 430.

Ephemeris information received from several SVs is stored in respective locations of ephemeris information memory 460. Under the control of counter 470, a one-to-one multiplexer 450 passes the stored ephemeris information for the selected SV to the supplemental position calculator 440. Based on the selected ephemeris information, the supplemental position calculator 440 calculates a supplemental position for the selected SV (as shown in FIG. 9, supplemental position calculator 440a may calculate the supplemental position based on both almanac and ephemeris information). The operations of the reference position calculator 420, the supplemental position calculator 440, and the multiplexer 450 are coordinated (either synchronously or asynchronously) such that reference and supplemental positions for the same SV are simultaneously given to the combiner 430.

The combiner 430 outputs a correction factor as the calculated difference between the reference and supplemental positions. In a related implementation, the correction factor may be a truncated, rounded, or scaled version of this calculated difference. The one-to-many demultiplexer 480 is controlled (e.g., with counter 470) to direct the correction factors to corresponding locations within the correction factor storage 490 in a predetermined order as described herein. In a related implementation, different counters (or other coordination mechanisms) may be used to control the multiplexers 450 and demultiplexers 480 to compensate for path delays (e.g., to supplement latency within the position calculator 440 or 440 a). In another example, where demultiplexer 480 and correction factor memory 490 may be omitted, the calculated correction factors are forwarded directly to a transmitter (not shown).

Note that although the same number of storage locations are consistently indicated within the ephemeris information memory 460 and correction factor memory 490 in fig. 8, it is not necessary to calculate a correction factor for each SV storing ephemeris information. However, it may be desirable to construct such a PDE such that each correction factor storage location within correction factor storage 490 corresponds to an ephemeris information storage location within ephemeris information storage 460.

The PDE computes two positions for the SV at a particular time: one position is calculated using only almanac data and one position is calculated using the ephemeris and almanac data. The PDE then generates almanac correction data (i.e., the error between the two calculated positions at a given time) and forwards this data to the mobile unit.

In a method according to an embodiment of the present invention, almanac correction data is sent to the mobile unit without sending SV IDs. The PDE forwards the almanac data to the mobile unit, which then forwards the almanac correction data for all satellites in view to the mobile unit, excluding the satellite IDs. The base station transmits a reference time and a reference position, wherein the correction data is valid. The satellites are arranged in a predetermined order (in ascending or descending order according to elevation or azimuth, or some other order). The PDE knows which almanac data is in the mobile unit's memory and the reference times and reference locations for the visible satellite list that will be computed in the mobile unit. Using this information, the base station can predict the actual order that will be computed within the mobile unit and transmit correction data for the satellites in view in that order.

As an example of the implementation of the method according to an embodiment of the invention, corrections are arranged in ascending order of elevation so that by informing the mobile unit of the arrangement, the mobile unit can learn which almanac correction data belongs to which SV because it has a list of satellites calculated using ascending order of elevation. That is, the base station arranges the SVs in order of elevation and then transmits the parameters to the mobile unit in the same order as the order of elevation. Other embodiments include ascending or descending SV IDs within the line of sight in some pre-ordered manner that may be calculated at the base station and the mobile unit.

Ambiguity in position determination may arise in having a transmitting entity (e.g., a PDE) associate correction factors with objects (e.g., SVs) in a different manner than a receiving entity (e.g., a mobile unit). For example, where the predetermined order relates to the elevation angles of the SVs, the use of different algorithms, different accuracy data, and/or arithmetic and rounding such factors may cause the mobile unit to interpret the locations of two closely located SVs in a different order than determined by the PDE. This confusion may result in the mobile unit applying a correction factor to the wrong reference location.

FIG. 10 shows a flow diagram of a method according to an embodiment of the invention. In task T110, the set of reference locations is filtered to identify a set of locations having an acceptably low ambiguity potential and validity times corresponding to the set.

In task T220, a validity time T is selected for the set of reference locations to be calculated_g. This time is sufficient in the future to allow for delays in transmission and processing. In task T230, a calculation is made regarding T_gA set of reference positions. In task T240, the reference locations within the group are compared to determine whether there is a possible ambiguity. For example, all possible pairs of locations may be tested to ensure that no two locations are closer than a predetermined threshold (e.g., a threshold for distance or angle). Alternatively, if the locations are ranked, the comparison task may be reduced by testing only adjacent locations. FIG. 11 (discussed below) shows an exemplary structure for performing task T240 in this manner.

If a possible ambiguity is indicated, the validity time T is adjusted in a task T260_gAnd tasks T230, T240, and T250 are repeated. For example, task T260 may include T being a predetermined fixed time step_gAnd (4) increasing. For GPS applications, such time steps may be measured in milliseconds. In another embodiment, the parallel tasks of task T110 may be limited to a maximum number of iterations. In a related embodiment, each set of reference locations may be characterized by a degree of ambiguity, such that after a maximum number of iterations is reached, the least ambiguous set may be selected.

In task T120, a correction factor is calculated as described above. In task T212, the correction factors are sent in a predetermined order, along with a validity time T_g(or sufficient to indicate t_gInformation of (d). In another embodiment, perhaps in accordance with t_gThe transmission of the correction factor is delayed to maintain a fixed relationship between the transmission time and the validity time. In this case, it is possible to omit the AND-t_gAnd (4) related information transmission.

In a method according to a further embodiment of the invention, a sending entity (e.g., a PDE) learns that a receiving entity (e.g., a mobile unit) may use one of several different sets of information (e.g., almanac information) to calculate a reference position, where the different sets of information are sufficiently known to the sending entity. In this approach, the sending entity selects a validity time (e.g., by a task similar to that described herein) at which any group will not produce the expected possible ambiguity (e.g., in elevation order).

FIG. 11 illustrates a block diagram of an exemplary implementation of task T240 shown in FIG. 10. The set of reference locations output by task T230 are arranged and available in a block (e.g., storage element) 310 i. Adjacent pairs of these locations (e.g., of differences) are combined in combiner 320i and the results of such comparisons are tested against thresholds in comparator 330 i. If it is determined that any pair of positions is closer than the threshold, OR gate 340 indicates that a possible ambiguity is detected.

FIG. 12 shows a flow diagram of a method according to an embodiment of the invention. In this example, the set of reference positions calculated in task T230a is based on almanac data received from one or more SVs.

The above possible ambiguity also arises with respect to the elevation mask. Only a few SVs of the GPS system are visible at any one time and the elevation of the cutoff angle may be used to indicate which SVs are visible. For example, if the cutoff elevation angle is set to 5 degrees, a satellite with an elevation angle below 5 degrees is considered "invisible". Due to possible differences in the elevation determinations made by the PDE and the mobile unit described above, one entity may consider SVs with elevation close to cutoff as "visible" and another entity may consider them as "invisible". Thus, two entities may choose to have different correspondences between correction factors and reference positions.

FIG. 13 shows a flow diagram of a method according to an embodiment of the invention. In task T242, the reference locations in each group are compared to a cutoff elevation to determine whether any of the locations are closer to cutoff than a predetermined threshold. FIG. 14 illustrates a block diagram of an exemplary implementation of task T242 shown in FIG. 13. In this arrangement, the reference positions available within box 310i are combined (e.g., differentiated) with the cut-off angle in combiner 320i, the results are compared to a predetermined threshold in comparator 330i, and the results of the comparison are combined in OR gate 340. In another implementation, the reference locations are aligned (e.g., by rising or falling elevation angles) such that it is sufficient to compare the cut-off angle to only one (e.g., lowest) or two reference locations (e.g., closest to the eye-level) of each group.

Fig. 15 shows a flow diagram of a method that may be performed by a receiving entity, such as a mobile unit, in accordance with an embodiment of the invention. In task T520, the correction factors are received in a predetermined order. In task T610, at least one of these factors is applied (e.g., to correct the reference position).

Fig. 16 shows a flow diagram of a method according to another embodiment of the invention. In task T510, almanac information is received (e.g., from the PDE or directly through an integrated GPS receiver). In task T530, the reference position of the selected SV is determined based on the almanac information. At the same time or at different times, the correction factors are received in a predetermined order in task T520. Depending on its position within the predetermined sequence, a correction factor corresponding to the reference position is selected and applied in task T612.

In a method according to another embodiment of the invention, if the receiving entity receives a correction factor with a future validity time, but determines that the predetermined order may be applied unambiguously to a set of reference positions, the receiving entity may immediately utilize this information. For example, by determining what the elevation angle will be in the next half second, the mobile unit can rank the elevation angles accordingly and fit the correction factors according to the order of the elevation angles at that time. From this information, the calculated elevation angles can be adjusted back to the second half of the second to obtain their present values.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments are possible, and the generic principles presented herein may be applied to other embodiments as well. For example, the present invention may be implemented partially or wholly as follows: hard-wired circuitry, firmware programs fabricated in application-specific integrated circuits, or as firmware programs loaded into non-volatile memory or software programs loaded into or onto data storage as machine-readable code, which are instructions executable by an array of logic elements, such as a microprocessor or other digital signal processing unit. Thus, the present invention is not limited to the embodiments described above, any particular sequence of instructions, and/or any particular configuration of hardware, but is to be accorded the widest scope consistent with the principles and novel features disclosed in any fashion herein.

Claims

1. A method, comprising:

calculating a plurality of correction factors, each correction factor relating to a position of at least a respective one of a set of physical objects; and

transmitting the plurality of correction factors in a predetermined order, wherein the correspondence between each of the plurality of correction factors and at least one of the set of physical objects is indicated at least in part by the predetermined order;

wherein said calculating a plurality of correction factors comprises:

calculating a reference position for each of the set of physical objects; and

calculating a supplemental location for each of the set of physical objects, wherein each of the correction factors is based at least in part on a difference between the respective reference and supplemental locations.

2. The method of claim 1, wherein the method further comprises: determining the presence of potential ambiguity between at least two of said reference positions.

3. The method of claim 2, wherein the potential ambiguity relates to a relationship between elevation angles of at least two of the set of physical objects.

4. The method of claim 2, wherein the potential ambiguity relates to a relationship between an elevation mask angle and an elevation angle of at least one of the set of physical objects.

5. The method of claim 1, wherein the first and second light sources are selected from the group consisting of a red light source, a green light source, and a blue light source,

characterized in that each of said reference positions is based at least in part on almanac information, an

Wherein each of the supplemental locations is based at least in part on ephemeris information.

6. The method of claim 5, wherein the almanac information is received from at least one of the spacecraft.

7. An apparatus that, when coupled to a processor, controls the processor, the apparatus comprising:

means for controlling the processor to calculate a plurality of correction factors, each correction factor being related to a position of at least a respective one of the set of physical objects; and

means for controlling the processor to transmit the plurality of correction factors in a predetermined order without transmitting information relating to the identity of the physical object,

wherein the correspondence between each of the plurality of correction factors and at least one of the set of physical objects is indicated at least in part by the predetermined order, an

Wherein the means for controlling the processor to calculate a plurality of correction factors comprises:

means for controlling the processor to calculate a reference position for each of a set of physical objects; and

means for controlling the processor to calculate a supplemental location for each of a set of physical objects,

wherein each of the correction factors is based at least in part on a difference between the respective reference and supplemental locations.

8. An apparatus, comprising:

a reference position calculator configured and arranged to calculate a reference position for each of the plurality of physical objects;

a replenishment position calculator configured and arranged to calculate a replenishment position for each of the plurality of physical objects;

a correction factor calculator configured and arranged to receive the reference position and the supplemental position and to output a plurality of correction factors in a predetermined order,

characterized in that each correction factor is related to the position of at least a respective one of the plurality of physical objects, an

Wherein the correspondence between each of the plurality of correction factors and at least one of a plurality of physical objects is indicated at least in part by the predetermined order.

9. The apparatus of claim 8, wherein at least one of the set of physical objects is a space vehicle.

10. A system, comprising:

a receiver for receiving a signal from at least one of a plurality of physical objects;

a position determination entity comprising

a correction factor calculator configured and arranged to receive the reference position and the supplemental position and to output a plurality of correction factors, an

A transmitter configured and arranged to transmit a plurality of correction factors,

characterized in that the plurality of correction factors are transmitted in a predetermined order, an

Wherein each correction factor is related to a position of at least a respective one of the plurality of physical objects, an

11. The system of claim 10, wherein at least one of the set of physical objects is a space vehicle.

12. A method, comprising:

receiving information related to a location of a corresponding physical object;

determining a reference location of the physical object, the determining based at least in part on the information;

receiving a plurality of correction factors in a predetermined order without receiving information related to an identification of the physical object; and

correlating said correction factors with respective ones of said reference positions using said predetermined order to identify respective ones of said physical objects, an

A corresponding correction factor is applied to the reference position.

13. The method of claim 12, wherein at least one of the set of physical objects is a space vehicle.

14. A method, comprising:

transmitting the plurality of correction factors in a predetermined order without transmitting information related to the identity of the physical object;

wherein the correspondence between each of the plurality of correction factors and at least one of the set of physical objects is indicated at least in part by the predetermined order;

wherein said calculating a plurality of correction factors comprises:

calculating a reference position for each of the set of physical objects; and

15. The method of claim 14, further comprising determining the presence of potential ambiguity between at least two of the reference locations.

16. The method of claim 15, wherein the potential ambiguity relates to a relationship between elevation angles of at least two of the set of physical objects.

17. The method of claim 15, wherein the potential ambiguity relates to a relationship between an elevation mask angle and an elevation angle of at least one of the set of physical objects.

18. The method of claim 14, wherein the first and second light sources are selected from the group consisting of,

19. The method of claim 18, wherein the almanac information is received from at least one of the space vehicles.

20. A method, comprising:

calculating a plurality of correction factors, each correction factor being related to the position of at least a respective one of the set of physical objects, the correction factors being determined at least by the difference between the reference and supplemental positions of each object in the set of physical objects; and

transmitting the plurality of correction factors in a predetermined order;

wherein the correspondence between each of the plurality of correction factors and at least one of the set of physical objects is indicated at least in part by the predetermined order.

21. The method of claim 20, wherein at least one of the plurality of correction factors relates to a correction to a position determination.

22. The method of claim 20, wherein at least one of the plurality of correction factors relates to a correction to a position determination for a predetermined future time.

23. The method of claim 20, wherein the predetermined order relates to a relative arrangement of the physical objects.

24. The method of claim 23, wherein the relative arrangement is valid at a future time.

25. The method of claim 23, wherein the relative arrangement is related to an elevation angle of the physical object.

26. The method of claim 20, wherein the predetermined order is determined at least in part by a relative order of elevation of the physical objects.

27. The method of claim 20, wherein at least one of the plurality of correction factors is based at least in part on a signal received from at least one of the set of physical objects.

28. The method of claim 20, wherein at least one of the set of physical objects is a space vehicle.

29. The method of claim 20, wherein each of the set of physical objects is a space vehicle, each space vehicle having an identification number associated with a global positioning system, and

wherein the predetermined order is determined at least in part by the relative order of the identification numbers of the spacecraft.

30. The method of claim 20, wherein the method further comprises: transmitting information related to validity times of the plurality of correction factors.

31. The method of claim 20, wherein the method further comprises: determining the presence of potential ambiguity between at least two of said reference positions.

32. The method of claim 31, wherein the potential ambiguity relates to a relationship between elevation angles of at least two of the set of physical objects.

33. The method of claim 31, wherein the potential ambiguity relates to a relationship between a cutoff elevation angle and an elevation angle of at least one of the set of physical objects.

34. The method of claim 20, wherein the step of selecting the target,

35. The method of claim 34, wherein the almanac information is received from at least one of the space vehicles.

36. A method, comprising:

calculating a plurality of correction factors, each correction factor being associated with a respective one of a set of satellites; and

transmitting the plurality of correction factors from the base station to the mobile station in a predetermined order;

wherein the predetermined order is known to both the base station and the mobile station; and

the correspondence between each correction factor and each satellite is indicated at least in part by the predetermined order.

37. The method of claim 36, wherein at least one of the plurality of correction factors relates to a correction to a position determination.

38. The method of claim 36, wherein at least one of the plurality of correction factors relates to a correction to a position determination for a predetermined future time.

39. The method of claim 36, wherein the predetermined order relates to a relative arrangement of the physical objects.

40. The method of claim 39, wherein the relative arrangement is valid at a future time.

41. The method of claim 39, wherein the relative arrangement is related to an elevation angle of the physical object.

42. The method of claim 36, wherein the predetermined order is determined at least in part by a relative order of elevation of the physical objects.

43. The method of claim 36, wherein at least one of the plurality of correction factors is based at least in part on a signal received from at least one of the set of physical objects.

44. The method of claim 36, wherein at least one of the set of physical objects is a space vehicle.

45. The method of claim 36, wherein each of the set of physical objects is a space vehicle, each space vehicle having an identification number associated with a global positioning system, and

46. The method of claim 36, wherein the method further comprises: transmitting information related to validity times of the plurality of correction factors.

47. The method of claim 36, wherein the method further comprises: determining the presence of potential ambiguity between at least two of said reference positions.

48. The method of claim 47, wherein the potential ambiguity relates to a relationship between elevation angles of at least two of the set of physical objects.

49. The method of claim 47, wherein the potential ambiguity relates to a relationship between a cutoff elevation angle and an elevation angle of at least one of the set of physical objects.

50. The method of claim 36, wherein said step of selecting said target,

51. The method of claim 50, wherein said almanac information is received from at least one of said spacecraft.

52. A system for: transmitting a plurality of correction factors to a mobile station, wherein the mobile station is configured to receive the correction factors; for associating the correction factors with particular satellite signals according to a predetermined order in which the correction factors are caused to be received; using the correction factor in determining the position of the mobile station, the system comprising:

a base station at a fixed location for receiving signals from a plurality of satellites;

a position determination entity associated with the base station, comprising:

a reference position calculator for calculating a reference position from the signals for each of the satellites;

a supplemental position calculator for calculating a supplemental position from said signals for each of said satellites;

a correction factor calculator for generating a respective plurality of correction factors for each of said satellites from said reference and supplemental locations; and

means for arranging said correction factors in a predetermined order relative to the respective identities of said satellites; and

a transmitter associated with the base station for transmitting the plurality of correction factors.

53. The system of claim 52, wherein said means for ordering said correction factors in a predetermined order relative to the respective identities of said satellites orders said correction factors in an ascending order of azimuth of said satellites.

54. The system of claim 52, wherein said means for ordering said correction factors in a predetermined order relative to the respective identities of said satellites orders said correction factors in descending order of azimuth of said satellites.