As the name suggests, trilateration technique uses three RTT values to estimate the coordinates of target node. In trilateration technique, when several landmarks ping the specified target, three landmarks with smallest RTT values (probably nearest located landmarks) are selected.The algorithm then tries to estimate the location on the basis of overlapping region of the drawn circles around these selected landmarks. Figure 1 depicts this technique. In trilateration technique, the center of the overlapping region is considered as the location of the target node.
There are many ways to implement trilateration technique such as
- Linear Least Square Method
- Nonlinear Least Square Method
- Circles intersection with Clustering
- Trilateration in 2D and 3D
Previously we were implementing Linear Least Square Method to implement trilateration. With this method 50% of targets showed distance error above 1000 km. We went through a paper on ?TOA-based trilateration method which discusses different positioning algorithms. In this paper it is mentioned that Linear Least Square Method is not very accurate and it just provides initial position which can be used in other positioning algorithms (i.e. Nonlinear Least Squares and Independent Time GPS Least Squares) as initialization value for further iterations.
We went through trilateration method as described on wikipedia and found that method to be easy and straight forward. But the algorithm mentioned was solving trilateration in 3D plane. We implemented it in 2D plane.
This method is based on solving equations of circle. Figure 2 below shows three circles drawn from each of three landmarks to the target. r1, r2, r3 are the radii of circles.
P1=1st Landmark position in x,y coordinates
P2=2nd Landmark position in x,y coordinates
P3=3rd Landmark position in x,y coordinates
d1= distance from 1st Landmark to target (Calculated using distance=(MinRTT/2)*alpha)
d2=distance from 2nd Landmark to target (Calculated using distance=(MinRTT/2)*alpha)
d3=distance from 3rd Landmark to target (Calculated using distance=(MinRTT/2)*alpha)
i1=P1.x,
i2=P2.x,
i3=P3.x
j1=P1.y,
j2=P2.y,
j3=P3.y
x=target x coordinate
y=target y coordinate
Using Equation for circles and solving the above figure
x =
{ ( [ (d1^2-d2^2) + (i2^2-i1^2) + (j2^2-j1^2) ] * (2*j3-2*j2) - [ (d2^2-d3^2) + (i3^2-i2^2) + (j3^2-j2^2) ] *(2*j2-2*j1) ) /
[ (2*i2-2*i3)(2*j2-2*j1)-(2*i1-2*i2)(2*j3-2*j2 ] }
y = [ (d1^2-d2^2) + (i2^2-i1^2) + (j2^2-j1^2) + x*(2*i1-2*i2)] / (2*j2-2*j1)
x,y are the required coordinates of a Point (Target)
Results
The performance of trilateration has dramatically improved as shown in Figure 3. The spreadsheet can be found here.
