There are many ways to solve trilateration problem. This method is based on solving the equations of circle.The Figure below shows the 3 circles drawn from each of 3 landmarks to the target. r1, r2, r3 are the radius of circles.

Figure

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)

Reference

http://en.wikipedia.org/wiki/Trilateration

http://en.wikipedia.org/wiki/File:3spheres.jpg