GPS satellite signal acquisition assistance system and method in a wireless communications network

Abstract

A system and method for assisting an integrated gps/wireless terminal unit in acquiring one or more gps satellite signals from the gps satellite constellation. The invention includes a method for narrowing the PN-code phase search. That is, by accounting for the variables in geographic location and time delay relative to gps time, the systems and methods of the present invention generate a narrow code-phase search range that enables the terminal unit to more quickly acquire and track the necessary gps satellites, and thereby more quickly provide accurate position information to a requesting entity.

Description

RELATED APPLICATIONS

This application
The term that varies based on the user location is:
ƒ( r)=| r− s|−| r− b|.

The search window, then, is defined by the extreme values of this function for terminal unit 2 anywhere within the uncertainty area. Finding the search window center and size is therefore a two-dimensional function, since the terminal unit elevation) maximization/minimization problem is known withn a reasonably small range of values based on the terrain in the vicinity of the base station.

First Scenario—Base Station at Center of Uncertainty Area

Reference is directed to FIG. 6, which is an illustration of the spatial environment of the case where the base station 8 is at the center of the terminal unit 2 location uncertainty area. Let the plane P be a plane parallel to the earth tangential plane passing through the serving base-station location. In this scenario, it is assumed that the terminal unit 2 uncertainty region is a circular disk 60 of radius R on the plane P, centered at the base-station 8 location. The problem can be resolved analytically if it is assumed that the uncertainty area is limited to a radius R of at most 20 km, which is reasonable given to the typical CDMA ‘cell’ service area. It is further assumed that that the terminal unit 2 is on the same earth tangential plane P as the base-station 8. Note that this will introduce a small amount of error since typically the base-station 8 location is placed at a higher elevation that the terminal unit 2. Because of this assumption, the approximation of the geometrical offset relative to the base-station 8 causes the following pseudo-range difference at the terminal unit: δ_geometry=| r− s|−| b− s|≅( r− b)· l_s. The last term of this equation is the unit vector running from the satellite 18 to the base-station 8:

${\stackrel{\_}{1}}_s=\frac{\stackrel{\_}{b}-\stackrel{\_}{s}}{\left|\stackrel{\_}{b}-\stackrel{\_}{s}\right|}.$

The term that varies in the estimate of the pseudo-range measurement as a function of the terminal unit 2 location is therefore: ƒ( r)−( r− b). l_S−| b− b|.

The distance between the terminal unit 2 location and the base station 8 is defined as d, and φ is defined as the angle between the unit vector l_S and the vector going from the base-station 8 antenna to the terminal unit 2 location. Logically, these two parameters have the following ranges: 0≦d≦R−cos(υ)≦cos(φ)≦cos(υ), where υ is the satellite 18 elevation relative to plane P 60. Using these parameters, the equation of ƒ ( r) can be rewritten as: ƒ (d, φ)=d. (cos(φ)−1). And, it is trivial to show that: −R. (cos(υ)+1)≦ƒ(d,φ)≦0. Hence, the estimated pseudo-range at any point inside the uncertainty region will be in the interval: ρ_BTS−C_ƒ−R.(cos(υ)+1)≦p_user≦ρ_BTS−C_ƒ.

Therefore the search window center and size will be:

${\rho }_{\mathrm{Center}}={\rho }_{\mathrm{BTS}}-c_f-\frac{R·\left(\cos \left(\vartheta \right)+1\right)}{2}$ ${\rho }_{\mathrm{Size}}=R·\left(\cos \left(\vartheta \right)+1\right)$
First Scenario—General Case

Reference is directed to FIG. 7, which is a spatial view of the general case where the uncertainty region is not centered about the base station 8. Let P 66 be a plane parallel to the Earth tangential plane at the base-station 8 antenna location but not necessarily passing through it (typically, it will be below the base station antenna). Also, assume that the satellite 18 is above plane P 66 and that the base station 8 antenna is also above plane P 66. The terminal unit 2 is located within plane P 66 and it is assumed that the terminal unit uncertainty region A, 68 or 70, is a smooth contiguous area on plane P 66. Since EIA IS-801 defines the uncertainty region as an ellipse, in order to make the aforesaid coplanar terminal unit location assumption, A must be restricted to a circle of radius R of at most 50km. This is consistent with the aforementioned 20 km assumption about CDMA ‘cell’ service area dimensions.

The desired result is, again, to find the minimum and maximum values of the function ƒ( r)=| r− s|−| r− b| for any terminal unit 2 user location within uncertainty region A 68 or 70. Let C (of coordinates c) 69 be the intersection of the line 67 passing through the base station 8 antenna location and the satellite 18 location with the plane P 66. Two postulates are then considered: Case where C∉A:

In this case, the function will take both its minimum and maximum values on the boundary of the uncertainty area A 70. Since it is known that they are on the boundary, the boundary of A 70 is sampled and the value of the function ƒ at each location is taken. Let ƒ_min and ƒ_max be the minimum and maximum values that ƒ can take among all the sample locations chosen. The search window center and size are then given by:
ρ_Center=ρ_BTS−C_j+ƒ_min+ƒ_max2/
ρ_size=ƒ_max−ƒ_min
Case where CεA:

${\rho }_{\mathrm{Center}}={\rho }_{\mathrm{BTS}}-c_f-\frac{f_{\min }+f_{\max }}{2}$ ${\rho }_{\mathrm{Size}}=f_{\max }-f_{\min }$

In this case, the function takes its maximum value at the base-station 8 location and its minimum value somewhere on the uncertainty area A 68 periphery. Therefore ƒ_max=ƒ( c). Since it is known that ƒ_min is on the boundary of A 68, the boundary is sampled and the value of the function ƒ is computed at each location selected. Again, let ƒ_min be the minimum value that ƒ takes among all the sample locations chosen. The search window center and size are then given by:

${\rho }_{\mathrm{Center}}={\rho }_{\mathrm{BTS}}-c_f-\frac{f_{\min }+f_{\max }}{2}$ ${\rho }_{\mathrm{Size}}=f_{\max }-f_{\min }$

The number of sample points taken on the surface Λ will depend on how smooth the uncertainty area is. The smoother the area the fewer the points needed. In the case of an ellipse 20 sample points are enough. Obviously the size selected corresponds to the minimum acceptable guaranty that the terminal unit is going to be within the search window in a virtual noiseless case. When noise is present some margin can be added.

Second Scenario

Reference is directed to FIG. 8, which is a spatial view of the second scenario. In this scenario, there is a more accurate estimation of the terminal unit 2 clock reference. The variable {circumflex over (τ)} is defined as the estimate of the receiver time bias obtained from the network. Based on this estimate of the terminal unit 2 clock bias, the offset in code phase due to the terminal unit 2 clock error will be: δ_clock=−{circumflex over (τ)}. The terminal unit 2, base-station 8 and satellite 18 three-dimensional positions are respectively given by r, b and s. The geometrical offset relative to base station 8 causes the following code-phase offset: δ_geometry=| r− s|−| b− s|. Based on these two values, the best estimate of the pseudo-range measurement at the terminal unit 2 location is:
ρ_user=ρ_BTS+δ_clock+δ_geometry=ρ_BTS+| r− s|−| b− s|−{circumflex over (τ)}

The term that varies based on the terminal unit 2 location is: η( r)=| r− s|. The search window is then defined by the extreme values of this function for terminal unit 2 positions anywhere within the uncertainty area 72. Finding the search window center and size is therefore a two-dimensional function (since the terminal unit 2 elevation is known within a very small, and statistically insignificant range) maximization/minimization problem.

In this scenario the function ƒ( r)−| r− s| is obviously less complex than in the first scenario. Because the satellite 18 distance is so great, the following simplification is reasonable: ƒ( r)=| r− s|≅( r− s)· l_s. Where

${\stackrel{\_}{1}}_s=\frac{\stackrel{\_}{b}-\stackrel{\_}{s}}{\left|\stackrel{\_}{b}-\stackrel{\_}{s}\right|}$
is the unit vector running from the satellite 18 to the base station 8. Therefore, the pseudo-range interval is a projection of the uncertainty area 72 onto the unit vector going from the satellite 18 to the base station 8. In order to illustrate this, take the simple case where the uncertainty area 72 is circular of radius R, within a plane parallel to the Earth 4 tangential plane at the base station 8 and with center 74 at the point with coordinates m. The function ƒ( r) is bounded by the following values: | m− s|−R·cos(υ)≦ƒ( r)≦| m− s|+R·cos(υ). The corresponding search window center and size are:
ρ_Center=ρ_BTS+| m− s|−| b− s|−{circumflex over (τ)}
ρ_Size=2R·cos(υ)
Third Scenario

Reference is directed to FIG. 9, which is a spatial view of the third scenario. In this scenario, the base station 8 has a more accurate estimation of both time (as in scenario II) and terminal unit 2 location. Define {circumflex over (p)}=[{circumflex over (x)} ŷ {circumflex over (z)}] 78 and {circumflex over (τ)} respectively the estimate of the user position in the ECEF frame (Earth Centered, Earth Fixed) and the estimate of the receiver time bias both obtained beforehand (most likely from network measurements). In addition to these estimates, it is possible based on information obtained beforehand concerning measurement statistics to obtain an estimated covariance matrix. One of the rows (and the same column) in the matrix corresponds to the time bias estimate. The matrix will be expressed in a specific frame but it is trivial to rotate it so as to bring the x axis to be parallel to the unit vector going from the base station 8 to the satellite 18. After this is done, the variance ν_G of the positioning error along the LOS direction to the satellite G is known. In addition to this, the covariance matrix provides the variance ν_T of the time bias estimate error T and the cross correlation K_GT between T and G.

The terminal unit 2, base station 8 and satellite 18 positions are respectively given by {circumflex over (r)}, {circumflex over (b)} and ŝ. Based on these definitions and assumptions, the offset in code phase due to the receiver clock bias and the position offset are: δ_clock=−{circumflex over (τ)}−T and δ_geometry=|{circumflex over (p)}− s−| b− s|+G. Based on these two values the best estimate of the pseudo-range measurement at the terminal unit 2 location is:
ρ_user=ρ_BTS+δ_clock+δ_geometry=ρ_BTS−{circumflex over (τ)}−T+|{circumflex over (p)}− s|−| b− s|+G

The term that varies with specific statistics is: ƒ(G,T)=G−T.

The function ƒ(G,T)=G−T is handled as a random variable with mean 0 and variance:
E[ƒ(G,T)]=E└(G−T)²┘=E/[G²]−2·E[G·T]+E[T²]=υ_G−2·K_GT+υ_T.

The corresponding standard deviation is: σ=√{square root over (υ_G−2·K_GT+υ_T)}. Based on a trade-off between probability of miss and size of the search window the factor α is selected as the number of standard deviations that should be included in the search window. The final search window center and size are:
ρ_Center=ρ_BTS+|{circumflex over (p)}− s|−| b− s−{circumflex over (τ)}
ρ_Size=2·α·√{square root over (υ_G−2·K_GT+υ_T)}.

Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. For example, while the present invention is described herein with respect to CDMA, those skilled in the art will appreciate that other technologies may be used. In addition, the satellite may be pseudo-lites or other mobile platforms operating in low orbit or high altitude without departing from the scope of the present teachings.

It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.

Accordingly,

Claims

1. A system for transmitting a gps receiver code-phase search range to a integrated gps/wireless terminal unit operating in a wireless network, said system comprising:

a receiver operable to generate a gps time reference;

a controller operable to calculate a gps code-phase search range with reference to a base station geographic location, the wireless coverage area, an angle between a vector extending from the base station to a gps satellite and a vector extending from the base station to the gps/wireless terminal unit, said gps time reference and the estimated wireless signal propagation delay within said coverage area, and

a transmitter coupled to said controller and operable to transmit said calculated gps code search range.

2. The invention of claim 1 wherein said gps code-phase search range is defined by a center value and a size value.

3. A system for transmitting a gps receiver code-phase search range to a integrated gps/wireless terminal unit operating in a wireless network, comprising:

a gps receiver operable to generate a gps time reference;

means for obtaining a time offset for the gps/wireless terminal unit relative to said gps time reference;

a controller operable to calculate a gps code-phase search range with reference to a base station geographic location, a radius of the wireless coverage area served by the base station, an elevation angle of a gps satellite, the time offset, and said time reference; and

a transmitter coupled to said controller and operable to transmit said calculated gps code search range.

4. The invention of claim 3 wherein said gps code-phase search range is defined by a center value and a size value.

5. The invention of claim 3 wherein said means for obtaining a time offset utilizes the round-trip wireless signal propagation time between said base station and the terminal unit to establish said time offset.

6. A system for transmitting a gps receiver code-phase search range to a integrated gps/wireless terminal unit operating in a wireless network, comprising:

a gps receiver operable to generate a gps time reference;

means for obtaining a time offset for the gps/wireless terminal unit relative to said gps time reference;

means for obtaining a location reference for the gps/wireless terminal unit;

a controller operable to calculate a gps code-phase search range with reference to a variance of a positioning error of said location reference, and said time reference; and

a transmitter coupled to said controller and operable to transmit said calculated gps code search range.

7. The invention of claim 6 wherein said gps code-phase search range is defined by a center value and a size value.

8. The invention of claim 6 wherein said means for obtaining a location reference utilizes means for providing terrestrial based trilateration to establish said location reference.

9. A method for defining a gps receiver code-phase search range for an integrated gps/wireless terminal unit operating in a wireless network having a base station comprising the steps of:

calculating a gps code-phase search range with reference to the base station geographic location plus the wireless coverage area, an angle between a vector extending from the base station to a gps satellite and a vector extending from the base station to the gps/wireless terminal unit, and with reference to a base station gps time reference plus the estimated wireless signal propagation delay within said coverage area and

transmitting said calculated gps code-phase search range.

10. The invention of claim 9 wherein said gps code-phase search range is defined by a center value and a size value.

11. A method for defining a gps receiver code-phase search range for an integrated gps/wireless terminal unit operating in a wireless network having a base station, comprising the steps of:

obtaining a time reference for the gps/wireless terminal unit establishing the time offset relative to the base station gps time;

calculating a gps code-phase search range with reference to the base station geographic location plus a radius of the wireless coverage area served by the base station, an elevation angle of a gps satellite, and said time reference; and

transmitting said calculated gps code-phase search range.

12. The invention of claim 11 wherein said gps code-phase search range is defined by a center value and a size value.

13. The invention of claim 11 wherein said obtaining step utilizes the round-trip wireless signal propagation time between said base station and the terminal unit to establish the time offset.

14. A method for defining a gps receiver code-phase search range for an integrated gps/wireless terminal unit operating in a wireless network having a base station, comprising the steps of:

obtaining a time reference for the gps/wireless terminal unit establishing the time offset relative to the base station gps time;

obtaining a location reference for the gps/wireless terminal unit;

calculating a gps code-phase search range with reference to a variance of a positioning error of said location reference, and said time reference; and

transmitting said calculated gps code-phase search range by the base station.

15. The invention of claim 14 wherein said gps code-phase search range is defined by a center value and a size value.

16. The invention of claim 14 wherein said obtaining a location reference step utilizes terrestrial based trilateration techniques to establish said location reference.

17. A system for transmitting a gps receiver code-phase search range to an integrated gps/wireless terminal unit operating in a wireless network, said system comprising:

a receiver operable to generate a gps time reference;

a controller operable to calculate the gps code-phase search range with reference to a base station geographic location, a position estimate of the integrated gps/wireless terminal unit having an uncertainty area with a center distinct from the base station geographic location, and said gps time reference; and

a transmitter coupled to said controller and operable to transmit said calculated gps code-phase search range.

18. The system of claim 17, wherein the controller is further operable to determining a propagation delay from the base station to the gps/wireless terminal unit and correct the gps time reference based on the propagation delay, and wherein the controller calculates the gps code-phase search range based on the corrected gps time reference.

19. The system of claim 18, wherein the controller is further operable to determine a signal delay at an antenna of the base station, and wherein the controller corrects the gps time reference based on the signal delay at the antenna of the base station.

20. The system of claim 17, wherein the controller obtains the position estimate of the gps/wireless terminal unit based on network measurements.

21. The system of claim 17, wherein the controller obtains the position estimate of the gps/wireless terminal unit based on a terrestrial based trilateration system.

22. The system of claim 17, wherein the controller calculates the gps code-phase search range by calculating a search window center and a search window size.

23. A method for defining a gps receiver code-phase search range for an integrated gps/wireless terminal unit operating in a wireless network having a base station, the method comprising:

determining, with a gps receiver, a gps time reference;

determining a geographic location of a base station serving the gps/wireless terminal unit;

obtaining a position estimate of the gps/wireless terminal unit having a center distinct from the base station geographic location;

calculating a gps code-phase search range based on the gps time reference, the geographic location of the base station and the position estimate of the gps/wireless terminal unit; and transmitting said calculated gps code-phase search range.

24. The method of claim 23, further comprising:

determining a propagation delay from the base station to the gps/wireless terminal unit; and

correcting the gps time reference based on the propagation delay,

wherein the calculating the gps code-phase search range is based on the corrected gps time reference.

25. The method of claim 24, further comprising:

determining a signal delay at an antenna of the base station, and

wherein the correcting the gps time reference is based on the signal delay at the antenna of the base station.

26. The method of claim 23, wherein the obtaining the position estimate of the gps/wireless terminal unit comprises obtaining the position estimate based on network measurements.

27. The method of claim 23, wherein the obtaining the position estimate of the gps/wireless terminal unit comprises obtaining the position estimate based on a terrestrial based trilateration system.

28. The method of claim 23, wherein the calculating the gps code-phase search range comprises calculating a search window center and a search window size.