Algorithm 1 Improved recursive DV-Hop algorithm.

Input: WSN; Anchor nodes with their positions ($x_i$, $y_i$) where $i=1\dots n$, n is the total number of anchors; Output: Estimated position X of unknown node 1: Begin 2: $X_0=0$; /* initialization 3: $P_0=\alpha *I$; /* initialize the covariance matrix P 4: S= set of reachable anchor nodes, where unknown node can communicate with them 5: For ($k=0$ to $N{b}_{iter}$) Do       5.1: Selection of randomly three different anchors from S       5.2: Computation of RSSI values between unknown node and its neighbors       5.3: Estimation of distance between anchors and unknown node using       the polynomial approximation and the RSSI technique Equation (8)       5.4: Computation of $A_k$ and $B_k$ based on estimated distance using Trilateration, and the covariance matrix $P_k$ Equation (11)       5.5: Estimation of unknown node position $X_k$ using Recursive Least Square method Equation (12) 6: end For; 7: $X=X_{N{b}_{iter}}$ /* Final estimated position of unknown node 8: end;