WO2004027446A1

WO2004027446A1 - Location system

Info

Publication number: WO2004027446A1
Application number: PCT/IT2003/000557
Authority: WO
Inventors: Emanuela Falletti; Letizia Lo Presti; Fabrizio Sellone
Original assignee: Politecnico Di Torino
Priority date: 2002-09-20
Filing date: 2003-09-19
Publication date: 2004-04-01
Also published as: ITTO20020828A1; AU2003274695A1

Abstract

Location system in which a mobile location station LS provided with an aerial reference system LRF is fitted on board an aircraft (3) and is provided with an antenna system (4) designed to detect the direction of arrival DOA of radio waves. An auto-location system (7) determines the position and orientation of the first reference system LRF with respect to a second reference system ERF integral with the earth's surface (9). The auto-location (7) is performed by means of a set of radio beacons (RB) arranged on the earth's surface (9) and designed to transmit position information to the mobile location station LS. The location system (1) determines the position of a user to be located UT provided with a radio signal source (12) operating in the work band of the antenna system (4).

Description

"LOCATION SYSTEM"

TECHNICAL FIELD

The present invention concerns a location system.

DISCLOSURE OF INVENTION

In particular, this invention aims to produce a complete location system that can be employed by users who are unable to auto- locate themselves, for example users not provided with GPS type satellite location systems.

The above aim is achieved by the present invention since it concerns a location system, characterised in that it comprises: a location station LS, in particular a mobile location system fitted on board an aircraft, provided with an antenna system designed to detect the direction of arrival DOA of the radio waves collected by the antenna system; said location system LS being provided with a first reference system LRF; and an auto- location system (7) designed to determine the position and orientation of said first reference system LRF with respect to a second reference system ERF, in particular a reference system integral with the earth's surface; said auto-location system using a set of radio beacons designed to transmit position information to said location station LS; said location system being designed to determine the position r^(E> with respect to the second reference system ERF of a user to be located UT provided with a radio signal source operating in the work band of said antenna system.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be illustrated with particular reference to the attached figure which represents, schematically and as an example, a location system produced according to the dictates of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the figure, a location system is indicated overall by the reference number 1.

The location system 1 consists of the following units: ■ a mobile location station LS fitted on board an aircraft 3 (for example a helicopter) provided with an array of antennae 4 (illustrated schematically) and a processing unit 5 co-operating with the array of antennae 4. The processing unit 5 is designed to detect (in a known way) the direction of arrival DOA of the radio waves collected by the array of antennae 4. The mobile location system LS is provided with a first aerial reference system LRF integral with the array of antennae 4 ; and ^■ an auto-location system 7 designed to determine the position and orientation of the first reference system LRF with respect to a second reference system ERF integral with the earth's surface 9.

According to an alternative form of embodiment, (not illustrated in the figure) , the location station LS could be fitted on a means different from an aircraft, for example on a terrestrial type vehicle. According to another alternative form of embodiment, the location station LS could also be of the fixed type. In the subsequent description reference will be made to the embodiment example according to which the location station is of the mobile type and is fitted on an aircraft.

As will be clarified subsequently, the auto-location system 7 uses a set of radio beacons RB arranged on the earth' s surface 9 and designed to transmit position information to the mobile location station LS.

The location system 1 is designed to determine the position r^(E) with respect to the second terrestrial reference system ERF of a user to be located UT provided with a radio signal source 12 operating in the work band of the array of antennae .

According to an alternative form of embodiment (not illustrated) the reference system ERF does not have to be of the terrestrial type. Also in this case, in the subsequent description, reference will be made to the embodiment example illustrated according to which the reference system ERF is of the terrestrial type. The radio beacons could also be arranged on the surface of the sea or be in flight.

For example, the radio signal source 12 can consist of a mobile phone, in particular a GSM mobile phone or any transmitter, for example a micro-transmitter fitted permanently inside a garment (not illustrated) .

More in particular, the mobile location station LS is designed to obtain measurements that refer to the first reference system LRF. These measurements must be subsequently converted into the second terrestrial type reference system ERF.

In this regard the position of a point P represented by a vector r^(E) referred to the terrestrial reference system ERF can be obtained by means of roto-translation of the vector r^(L) which expresses the position of said point with respect to the aerial reference system LRF, i.e.: _r(E) _{= r}E- _{+ QLE ^ r}( ) ₍₁₎ where QLE is the rotation matrix that expresses the rotation from the aerial reference system LRF to the terrestrial reference system ERF and r^E"L is the translation (or offset) vector that represents the position of the centre of the aerial reference system LRF with respect to the terrestrial reference system ERF.

The location system of the present invention therefore uses the above relation (1) to transform the data detected by the vehicle 3 and measured in the aerial reference system LRF into the terrestrial reference system ERF.

To perform the operations indicated by the relation (1) it is therefore necessary to know:

■ the rotation matrix QLE; and

■ the translation (or offset) vector r^E-

The calculation of said items can be performed by the system of the present invention according to different procedures that can be applied in different operating contexts.

Here, two alternative methods used by the system of the present invention for calculation of the rotation matrix QLE and translation (or offset) vector r^E" will be exemplified.

First method for determining the rotation matrix QLE and the translation (or offset) vector r^E"L

The first method for determining the rotation matrix QLE and the translation (or offset) vector r^E"L is based on the joint estimate of said parameters . This method can be used for operating conditions in which the position estimate of the centre of the reference system LRF with respect to the reference system ERF is not provided with sufficient accuracy and frequency by the instruments available on board the aircraft 3. According to the first method of the present invention, the radio beacons RB are used for determining the rotation matrix QLE and the translation (or offset) vector r^E"L.

In particular, a set number I of radio beacons RB arranged on the earth's surface 9 is used, each having known position r_RB^(E),i (with i=l,2,..I). Location of the positions r_RB^(E),i (referring to the terrestrial reference system ERF) of the radio beacons can be obtained with great accuracy via a known location system, for example a GPS system.

Each radio beacon RB having position r_RB^(E),i emits radio waves which are detected by the array of antennae 4 which measures - or estimates - (in a known way) the angles of arrival Θ^(L> , φ^(L>

(referring to the aerial reference system LRF) of the signals coming from the various radio beacons RB.

Each radio beacon RB sends its own position r_RB^<E),i to the mobile location station LS by means of coded radio message.

The vector that defines the position r_RB^(L) _fi of each radio beacon RB with respect to the aerial reference system LRF is given by the product of the unit vector and the module p_RB^<L),i of the vector, i.e.: r_RB^{( )},i = u_RB^(L),i . p_RB^(L) _/i (2)

The unit vector u_RB^(L) _#i is known as it is calculated on the basis of the angles of arrival Θ^(L) , φ^(L) according to the relation u_RB^(L),i=[sinθ^(L),icosφ^(L),i; sinθ^(L) , isinφ^(L) , i; cosθ^(L),i (2.2)

Re-writing the expression (1) using the notations contained in (2) we have: r_RB ^(E) , i = r^E"L + QLE u_RB ^(L) , i p_RB ^{( )} i . (3 )

In the expression (3) illustrated above the roto-translation matrix QLE, the offset vector r^E"L and the modules p_RB^(L)i are unknown.

The system of the present invention performs differences between the relations (3) for couples of different radio beacons i_#j in order to obtain equations no longer dependent on the offset vector r^E"L i.e.: r_RB^(E),i - r_RB^(E),j = QLE (u_RB^(L) ,i p_RB^(L)i - u_RB^(L),j p_RB^(L)j). (4)

The system of the present invention, taking account of the fact that the matrix QLE is unitary, calculates the norm at the square of both terms of the equality (4) i.e.:

||r_RB^(L),ij||² = ||r_RB^(E),ij||² = ||u_RB^(L),i p_RB^(L,i - u_RB^(L),j p_RB^(L)j||² (5) with

- r_RB^(*',j where * stands for (L) or (E) .

The system of the present invention therefore obtains I(I-l)/2 equations of the type (5) indicated above which are solved by means of an iterative type algorithm (of known type) thus finding the various modules p_RB^(L> i from which, since the unit vectors u_RB^(L),i are known, we can trace back the position r_RB^(L),i of each radio beacon RB with respect to the aerial reference system LRF.

Following determination of the modules p_RB^(L),i of the position of the various radio beacons, the system according to the present invention calculates the rotation matrix QLE. For said purpose the above-mentioned relation (4) is used r_RB ^(E) , i - r_RB ^(E) , j = QLE (u-RB ^<L) , i p_RB ^(L) i - u-RB ^(L) , j p_RB ^{(L) j} ) in which it can be seen that the only unknown factor is the rotation matrix QLE .

The system of the present invention rearranges the vectors measured r_RB^(E),ij and the vectors calculated r_RB^<L),ij on columns of matrixes R_E and R_L so as to rewrite the system (4) in the compact form: R_E =QLE R_L (6)

The system of the present invention therefore solves the system rewritten in compact form according to quaternion algebra in order to obtain the rotation matrix that best solves the equation (6) .

For this purpose the system (6) is rewritten as: R_E =q* R_L q (7) where q is the quaternion corresponding to the rotation from the system LRF to the system ERF and the apex * represents the conjugation operation.

The system (7) is solved by an iterative procedure that provides at output the estimated value of the rotation matrix QLE.

The system of the present invention now proceeds with calculation of the offset vector r^E" .

For said purpose consider the above equation (1) _r(E) _{= r}E-L _{+ QLE r}(_L) rewritten for the radio beacons : r_RB,i^(E) = r^E" + QLE r_RB,i^(L> in which at this point the only unknown factor is the translation vector r^E"L . In particular, the translation vector r^"L is obtained as a mean of the I vectors that can be obtained from inversion with respect to r^E"L of the I equations according to the above-illustrated expression, i.e.:

^L = -∑ r_RB^(E),i-QLEr_RB^w,i) (8)

The roto-translation parameters from the reference system LRF to the reference system ERF are thus completely determined.

The second method of estimating the roto-translation parameters can be used for operating conditions in which estimation of the position of the centre of the reference system LRF with respect to the reference system ERF is provided with sufficient accuracy and frequency by the instruments available on board the aircraft 3.

In particular, the expression (3) r_RB^(E),i= r^E"L + QLE u-RB^{( >},i p_RB^(L)i. (3) can be rewritten as: r_RB^(E),i - r^E"L = QLE u-RB^(L),i p_RB^(L)i.

Furthermore, by invoking the unitariness of the rotation matrix we have :

Moreover by defining r RBi^(E) - r^E~L

( u RBiU^E) (10) Bi^w£) - r E-L

and writing the rotation in the form of its quaternion q the expression (7 ) becomes : u _ RBiL ^E) = q * u_RBi^{L)q ( 11 )

where the unit vectors u_RBi^(L) are obtained from the estimates of the directions of arrival DOA of the signals coming from the various radio beacons .

By organising the unit vectors u_RBil ^E) and u _RBi ^L) respectively on the columns of matrixes U_E and U_L the system can be rewritten in the form:

U_E=q*U_Lq

Since r^L-^E is known with sufficient accuracy by the instruments on board, only the quaternion q is unknown. In this case, the iterative procedure is applied to solution of the system U_E=q*U_Lq leading to the estimation of q from which the rotation matrix QLE is obtained.

As previously said, the system of the present invention is designed to define the position of the user UT with respect to the terrestrial reference system ERF.

Said estimate of the position of the user UT can be obtained using one of two methods .

In particular, a first estimation method (crossing of straight lines) is used when the aircraft 3 flies over the area involved at a relatively low altitude so that, at different moments, it is in spatially different positions with respect to the position of the radio beacons and with respect to the position of the user UT to be located. For example, the aircraft 3 can perform a circular path above the radio beacons RB.

The system of the present invention therefore detects a number I of signals coming from the radio beacons RB and determines for said signals the angles of arrival Θ_RB^<L) (i) , φ_RB^(L> (i) (with i=l,2,..I) of the signals coming from the radio beacons RB with known positions r_RB^(E),i.

The system also detects the angles of arrival Θ_UT^(L> (k) , φ__UT^{( )} (k) (with k=l,2,..K) of the signals coming from the user UT with unknown position.

The angles of arrival θ_RB^(L)(i), φ_RB^(L) (i) are used, according to one of the two procedures illustrated above, to define the rotation matrix QLE and the offset vector r^E" so as to define the relation (1) that links the aerial LRF and terrestrial ERF reference systems.

The angles θ_UT^(L)(k)_/ φ_UT^(L> (k) are used to define a number of straight lines converging in the same area, which coincides with the position of the user UT if the user does not move (or moves very little) with respect to the station LS.

The system of the present invention therefore defines, for each pair of angles Θ_UT^(L) (k) , φ_UT^(L) (k) , a respective straight line.

The equation of each straight line is then converted by the reference system LRF to the reference system ERF thanks to the relation (1) .

The system then crosses the straight lines to find the position of the user UT with respect to the terrestrial reference system ERF. The system of the present invention therefore determines the position of the user UT with respect to the terrestrial reference system ERF, thus permitting location of the user.

In practice, the system of the present invention operates with estimated values and therefore the converging straight lines identified may not meet in the same point. In this regard the system of the present invention can conveniently calculate the position of the user UT as a solution according to the algorithm of the minimum squares of the intersection of the straight lines .

The method of crossing the straight lines can also be used for measurements taken by several different location stations.

According to an alternative method (integration with the DTM) for calculating the user position, the angles of arrival Θ_UT^{( )} (k) , φ_UT^(L> (k) (with i=l,2,..I) of the signals coming from the user UT with unknown position are identified. In this case, it may be sufficient to know only one direction of arrival DOA.

Also in this case, the angles of arrival Θ_RB^(L) (i) , φ_RB^(L) (i) (with i=l,2,..I) of the signals coming from the radio beacons RB with positions r_RB^(E) _#i are identified to define, according to one of the two procedures described above, the rotation matrix QLE and the offset vector r^E"L.

The angles θ_UT^(I,)(k), φ_UT^(I,) (k) are used to define at least one straight line referring to the aerial reference system LRF. The position of said straight line is converted (via (1) ) into the terrestrial reference system thus expressing a straight line, referring to the terrestrial reference system ERF, stretching between the user UT and the location station LS. In this case, the position of the user UT is obtained by the intersection between the straight line in the terrestrial reference system and a three-dimensional digital map DTM, referring to the same terrestrial reference system ERF, which expresses the trend of the earth's surface 9 on which the radio beacons RB and the user UT are found.

Claims

1.- Location system, characterised in that it comprises:

■ a location station LS, in particular a mobile location system fitted on board an aircraft (3) , provided with an antenna system (4) designed to identify the direction of arrival DOA of the radio waves collected by the antenna system (4) ; said location station LS being provided with a first reference system LRF; and

■ an auto-location system (7) designed to determine the position and orientation of said first reference system LRF with respect to a second reference system ERF, in particular a reference system integral with the earth's surface (9) ; said auto-location system (7) using a set of radio beacons (RB) designed to transmit position information to said location station LS; said location system (1) being designed to determine the position r^(E> with respect to the second reference system ERF of a user to be located UT provided with a radio signal source (12) operating in the work band of said antenna system(4) .

2.- System according to claim 1, in which said location system (LS) obtains measurements that refer to said first reference system (LRF) ; said system converting said measurements into the second reference system ERF.

3.- System according to claim 2, in which said conversion of said measurements is obtained via the relation: _r(E) _{= r}E-L _{+ QLE r}(L) ₍₁₎ where : - the vector r^(E) represents the position of a point P with respect to the second reference system ERF,

- the vector r^(L) expresses the position of said point with respect to the first reference system LRF; - QLE is the rotation matrix that expresses the rotation of the first reference system LRF to the second reference system ERF and

- r^E"L is the translation vector that represents the position of the centre of the first reference system LRF with respect to the second reference system ERF.

4. - System according to claim 3 , providing for means of estimation designed to produce an estimate of said rotation matrix QLE and said translation vector r^E_L on the basis of the information coming from said radio beacons .

5.- System according to claim 4, in which said means of estimation are designed to identify the angles of arrival Θ^(L) , φ^(L) of the signals coming from the radio beacons RB and are designed to calculate the modules of the vectors that define the positions r_RB^{( )},i of the radio beacons RB with respect to the first reference system LRF; said means of estimation being designed to calculate said rotation matrix QLE by processing the information associated with said angles of arrival and said modules .

6.- System according to claim 5, in which the vector that defines the position r_RB^(L),i of each radio beacon RB with respect to the first reference system LRF is given by the product of the module p_RB^(L),i of the vector and the unit vector u_RB^(I,),i, of the vector the value of which is identified by said antenna system via said angles of arrival : r_RB^(L),i = u_RB^(L),i . p_RB^(L),i (2) the system of the present invention performing differences between relations of the type: r_RB^(E),i = r^E"L + QLE u-RB^(L),i p_RB^(L)i. (3) referring to different radio beacons i,j; said relations expressing the link between the two reference systems for the vectors that express the positions of said radio beacons ; said system performing by means of said differences equations not depending on the offset vector r^E"L: r_RB^(E),i - r_RB^(E),j = QLE(u-RB^(L) ,i p_RB^(L)i - u-RB^(L),j p_RB^(L)j). (4) said system calculating the norm at the square of both terms of said expressions not depending on the offset vector ||r_RB^(L),ij||² = ||r_RB^(E),ij||² = ||u-RB^(L),i p_RB^(L)i - u-RB^(L),j p_RB^(L)j||² (5) with r_RB^(*},ij = i^RB^,! - r_RB^(*',j where * stands for (L) or (E) . in order to obtain I(I-l)/2 equations that are solved by means of an iterative type algorithm to find said modules p_RB^(L)i.

7. - Method according to claim 6 , in which said system rearranges the vectors measured r_RB^(E),ij and the vectors calculated r_RB^(L),ij on columns of matrixes R_E and R_L so as to rewrite the system (4) which expresses said differences in the compact form: R_E =QLE R_L (5) the system solving the system rewritten in compact form according to the algebra of quaternions and via an iterative procedure that provides at output the estimated value of the rotation matrix

QLE.

8.- System according to claim 4, in which the translation vector r^E_ is obtained on the basis of the mean:

of the I vectors that can be obtained from the inversion, with respect to r^E"L, of the I equations _r(E) _{B r}E-L _{+ QLE r}(_L) rewritten for the position of the radio beacons RB referring to the second reference system ERF and to the first reference system LRF.

9. - System according to claim 4 in which said rotation matrix QLE is calculated, with said translation vector known, solving by means of iterative procedure the system:

U_E=q*U_Lq where :

- q represents the quaternion of the rotation; - the matrix U_E comprises unit vectors u_RBil}^E) organised by columns and the matrix U_L comprises unit vectors u __RBi^(L) organised by columns; r RBi^iE) - r^E-^L

RBi (^wE) - r E-L = u RBiU^E)

r_RBi^(E) represent the positions of the radio beacons with respect to the second reference system;

- r^E"L represents the translation vector and is known; and

- u_RBi^(L) represent the unit vectors obtained from the estimates of the angles of arrival of the signals coming from the radio beacons .

10.- System according to claim 4, providing for means of estimation of the user position UT with respect to the second reference system ERF; said means of estimation of the user position detecting a number I of signals coming from the radio beacons RB having different positions r_RB^(E) _#i and determining the angles of arrival θ_RB^(L)(i), φ_RB^(L> (i) (with i=l,2,..I) of said signals referring to the first reference system LRF; said means of estimation furthermore identifying the angles of arrival Θ_UT^(L) (k) , φ_UT^(L) (k) (with k=l,2,..K) of the signals coming from the user UT with unknown position referring to the first reference system LRF; said angles of arrival Θ_RB^(L> (i) , φ_RB^(L) (i) being used to define said rotation matrix QLE and said offset vector r^E"L; said angles Θ_UT^(L) (k) , φ_UT^(L) (k) being used to define a number of converging straight lines; said system converting the equation of said converging straight lines from the first reference system LRF to the second reference system ERF via said relation (1) ; said system calculating, for example by means of the algorithm of the minimum squares, the crossing of said converging straight lines to give the estimated position of the user UT with respect to the second reference system ERF.

11. - System according to claim 4 , providing for means of estimation of the user position UT with respect to the second reference system ERF; said means of estimating the user position identifying a number I of signals coming from the radio beacons RB with different positions r_RB^(E),i and determining the angles of arrival θ_RB^(L)(i), φ_RB^{( )}(i) (with i=l,2,..I) of said signals; said means of estimation identifying furthermore the angles of arrival Θ_UT^{( )} (k) , φ_UT^L) (k) (with k=l,2,..K) of the signals coming from the user UT with unknown position; said angles of arrival Θ_RB^(L) (i) , φ_RB^(L) (i) being used to define said rotation matrix QLE and said offset vector r^E- ; said angles Θ_UT^(L> (k) , φ__uτ^<L) (k) being used to define at least one straight line γ_UT^(E , referring to the second reference system ERF, stretching between the user UT and the location station LS; said means of estimation of the position obtaining the user position from the intersection between said straight line γ_ϋT^(E) and a three-dimensional digital map DTM, referring to the second reference system ERF, which expresses the trend of the earth' s surface (9) on which said radio beacons (RB) and said user (UT) are found.