Method for determining a position deviation, a positioning system and an underwater vehicle

A positioning system (2000) for determining a position deviation for a first node (N1), wherein the positioning system (2000) comprises:

• a positioning unit for estimating a position of the first node,
• means for receiving an acoustic signal and for measuring the frequency thereof,
• means for determining a first velocity vector of the first node, and
• processing means configured for:
• obtaining input data (1110), at a first position (Pt1) of the first node at a first point in time (t1), and at a second position (Pt2) of the first node at a second point in time (t2), wherein the second position of the first node (p1t2) is a different position than the first position of the first node (p1t1), and the second point in time (t2) follows the first point in time (t1), wherein said input data comprises:
• an estimated position of the first node (p1t1,p1t2), and a first velocity vector (v1t1,v1t2) of the first node (N1),
• obtaining (1120) the exact position of the second node (p2*);
• generating an acoustic signal(S) from a source at the first node (N1);
• obtaining (1125) the emitted frequency (fe) of the acoustic signal at the first node (N1);
• receiving (1130) a reflection of the acoustic signal(S) from a second node (N2) and measuring the observed frequency (fo) of the reflected acoustic signal(S);
• calculating (1140) on basis of said observed frequency (fo) and said emitted frequency (fe), a second velocity vector (v12t1, v12t2) which defines the velocity of the first node in relation to the second node;
• calculating (1150), an angle (αt1,αt2), wherein the angle (αt1,αt2) is the angle between the first velocity vector (v1t1,v1t2) and the second velocity vector (v12t1, v12t2);
• and
• determining (1160), based on the angle (αt1,αt2), the first velocity vector (v1t1, v1t2), and the estimated position of the first node (p1t1,p1t2), a line of direction (Lt1,Lt2), wherein the line of direction (Lt1,Lt2) indicates the direction from the estimated position of the first node (p1,Pt1,Pt2) towards an estimated position of the second node (p2*);
• and
• based on the obtained measurements (1100) further:
• determining (1300), based on a first line of direction (Lt1) and a second line of direction (Lt2), an intersection point defining the estimated position of the second node (p2);
• determining (1400) a deviation vector (Vd) which corresponds to the difference between the estimated position of the second node (p2) and the exact position of the second node (p2*), and
• determining (1500) the position deviation of the first node, wherein the position deviation of the first node corresponds to the deviation vector (Vd);
• and
• based upon said determined position deviation of the first node and using only the second node as reference, updating the estimated position of the first node (p1,Pt1,Pt2) to an actual position of the first node.

An underwater vehicle comprising a positioning system (2000) for determining a position deviation for a first node (N1), wherein the positioning system (2000) comprises:

• a positioning unit for estimating a position of the first node,
• means for receiving an acoustic signal and for measuring the frequency thereof,
• means for determining a first velocity vector of the first node, and
• processing means configured for:
• obtaining input data (1110), at a first position (Pt1) of the first node at a first point in time (t1), and at a second position (Pt2) of the first node at a second point in time (t2), wherein the second position of the first node (p1t2) is a different position than the first position of the first node (p1t1), and the second point in time (t2) follows the first point in time (t1), wherein said input data comprises:
• an estimated position of the first node (p1t1,p1t2), and a first velocity vector (v1t1,v1t2) of the first node (N1),
• obtaining (1120) the exact position of the second node (p2*);
• generating an acoustic signal(S) from a source at the first node (N1);
• obtaining (1125) the emitted frequency (fe) of the acoustic signal at the first node (N1);
• receiving (1130) a reflection of the acoustic signal(S) from a second node (N2) and measuring the observed frequency (fo) of the reflected acoustic signal(S);
• calculating (1140) on basis of said observed frequency (fo) and said emitted frequency (fe), a second velocity vector (v12t1, v12t2) which defines the velocity of the first node in relation to the second node;
• calculating (1150), an angle (αt1,αt2), wherein the angle (αt1,αt2) is the angle between the first velocity vector (v1t1,v1t2) and the second velocity vector (v12t1, v12t2);
• and
• determining (1160), based on the angle (αt1, 012), the first velocity vector (v1t1, v1t2), and the estimated position of the first node (p1t1,p1t2), a line of direction (Lt1,Lt2), wherein the line of direction (Lt1,Lt2) indicates the direction from the estimated position of the first node (p1,Pt1,Pt2) towards an estimated position of the second node (p2*);
• and
• based on the obtained measurements (1100) further:
• determining (1300), based on a first line of direction (Lt1) and a second line of direction (Lt2), an intersection point defining the estimated position of the second node (p2);
• determining (1400) a deviation vector (Vd) which corresponds to the difference between the estimated position of the second node (p2) and the exact position of the second node (p2*), and
• determining (1500) the position deviation of the first node, wherein the position deviation of the first node corresponds to the deviation vector (Vd);
• and
• based upon said determined position deviation of the first node and using only the second node as reference, updating the estimated position of the first node (p1,Pt1,Pt2) to an actual position of the first node.