Method and system for geolocating a terminal of a wireless communication system

1. A method for geolocating a terminal of a wireless communication system, the wireless communication system comprising an access network configured to estimate an approximate geographical position of the terminal, the terminal comprising a positioning device to obtain a precise geographical position of the terminal, a precision obtained by the terminal being better than the precision of the approximate geographical position estimated by the access network, the precise and approximate geographical positions comprising a latitude and a longitude as geographical coordinates, the method comprising:

calculation by the terminal of truncated information on the precise geographical position by breaking down the precise geographical position into a first part and a second part, the first part varying less quickly than the second part when the terminal moves, the truncated information representing the second part of the precise geographical position;

transmission by the terminal of the truncated information in a message to the access network;

estimation of the approximate geographical position of the terminal by the access network;

extraction by the access network of the truncated information from the message received from the terminal;

determination by the access network of the precise geographical position of the terminal from the truncated information and from the approximate geographical position; and

wherein:

the latitude and the longitude, each corresponding to a value encoded respectively in N and M bits;

a latitude Lat 1 and a longitude Lng 1 of the precise geographical position obtained by the terminal being written in a form:

Lat 1 =N 1 ×2 n +n 1

Lng 1 =M 1 ×2 m +m 1

where N 1 , n 1 n, M 1 , m 1 and m are positive integer numbers, n 1 and m 1 corresponding respectively to the n least significant bits of the latitude Lat 1 and to the m least significant bits of the longitude Lng 1 of the precise geographical position obtained by the terminal;

a latitude Lat 2 and a longitude Lng 2 of the approximate geographical position estimated by the access network being written in a form:

Lat 2 =N 2 ×2 n +n 2

Lng 2 =M 2 ×2 m +m 2

where N 2 , n 2 , M 2 and m 2 are positive integer numbers, n 2 and m 2 corresponding respectively to the n least significant bits of the latitude Lat 2 and to the m least significant bits of the longitude Lng 2 of the approximate geographical position estimated by the access network;

the truncated information comprises the n least significant bits of the latitude Lat 1 and the m least significant bits of the longitude Lng 1 of the precise geographical position obtained by the terminal;

the determination by the access network of the precise geographical position of the terminal from the truncated information and from the approximate geographical position comprises:

determination of the latitude Lat 1 as a function of N 2 , n 1 , and a difference between n 1 and n 2 , and

determination of the longitude Lng 1 as a function of M 2 and of m 1 , and a difference between m 1 and m 2 .

2. The method of claim 1 , wherein:

the numbers n and m are chosen so that the precision of the approximate geographical position estimated by the access network, regardless of the geographical position of the terminal, satisfies a following formulae:

|Lat 1 −Lat 2 |<2 (n−1)

|Lng 2 −Lng 2 |<2 (n−1)

the determination by the access network of the latitude Lat 1 of the precise geographical position of the terminal is in accordance with:

Lat

1

=

{

(

N

2

+

1

)

×

2

n

+

n

1

⁢

if

⁢

(

n

1

-

n

2

)

≤

(

-

1

)

×

2

(

n

-

1

)

(

N

2

-

1

)

×

2

n

+

n

1

⁢

if

⁢

(

n

1

-

n

2

)

≥

2

(

n

-

1

)

N

2

×

2

n

+

n

1

⁢

otherwise

the determination by the access network of the longitude Lng 1 of the precise geographical position of the terminal is in accordance with:

Lng

1

=

{

(

M

2

+

1

)

×

2

m

+

m

1

⁢

if

⁢

(

m

1

-

m

2

)

≤

(

-

1

)

×

2

(

m

-

1

)

(

M

2

-

1

)

×

2

m

+

m

1

⁢

if

⁢

(

m

1

-

m

2

)

≥

2

(

m

-

1

)

M

2

×

2

m

+

m

1

⁢

otherwise

.

3. The method of claim 1 , wherein the estimation of the approximate geographical position by the access network is obtained by one of the following methods:

a multilateration method based on a propagation time of a radio signal exchanged between the terminal and the access network;

a multilateration method based on a received power level for a radio signal exchanged between the terminal and the access network;

determination of the geographical position of the terminal from geographical positions of one or more base stations, the terminal being located within a coverage area of said one or more base stations; and

an automatic learning method for associating a radio signal with a geographical position.

4. The method of claim 1 , wherein the access network is a wireless low power wide area network.

5. A wireless communication system comprising a terminal and an access network, wherein the terminal is configured to:

calculate truncated information on a precise geographical position by breaking down the precise geographical position into a first part and a second part, the first part varying less quickly than the second part when the terminal moves, the truncated information representing the second part of the precise geographical position; and

transmit the truncated information in a message to the access network of the wireless communication system;

wherein the access network is configured to:

estimate an approximate geographical position of the terminal;

extract the truncated information from the message received from the terminal;

determine the precise geographical position of the terminal from the truncated information and from the approximate geographical position, the precise and approximate geographical positions comprising a latitude and a longitude as geographical coordinates; and

wherein:

the latitude and the longitude, each corresponding to a value encoded respectively in N and M bits;

a latitude Lat 1 and a longitude Lng 1 of the precise geographical position obtained by the terminal being written in a form:

Lat 1 =N 1 ×2 n +n 1

Lng 1 =M 1 ×2 m +m 1

where N 1 , n 1 , n, M 1 , m 1 and m are positive integer numbers, n 1 and m 1 corresponding respectively to the n least significant bits of the latitude Lat 1 and to the m least significant bits of the longitude Lng 1 of the precise geographical position obtained by the terminal;

a latitude Lat 2 and a longitude Lng 2 of the approximate geographical position estimated by the access network being written in a form:

Lat 2 =N 2 ×2 n +n 2

Lng 2 =M 2 ×2 m +m 2

where N 2 , n 2 , M 2 and m 2 are positive integer numbers, n 2 and m 2 corresponding respectively to the n least significant bits of the latitude Lat 2 and to the m least significant bits of the longitude Lng 2 of the approximate geographical position estimated by the access network;

the truncated information comprises the n least significant bits of the latitude Lat 1 and the m least significant bits of the longitude Lng 1 of the precise geographical position obtained by the terminal;

the determination by the access network of the precise geographical position of the terminal from the truncated information and from the approximate geographical position comprises:

determination of the latitude Lat 1 as a function of N 2 , n 1 , and a difference between n 1 and n 2 ; and

determination of the longitude Lng 1 as a function of M 2 and of m 1 , and a difference between m 1 and m 2 .