Hi Nitin,
The current implementation returns the ranges to each sonar element in the sonar array. The sonar's are ordered in the same manner as in the robot documentation (thus for p3dx it's 0 on the front left 1-6 on the front and 7 on the front right). When I setup the sonar message it was rather feature poor since all I was trying to do was have the robot not run into things. Going forward I think you would need to setup some frame for the sonar elements using TF. The current SonarArray msg doesn't really work for this, since it only has one header and for your applications you will probably need a separate frame for each of the sonar elements in the array. The current pioneer urdf in p2os_urdf package has a sonar link, but it is not aware of the internal geometry of the separate sonar elements.
Feel free to contact me directly if you need help in working with the sonar code in the p2os_driver.