In this paper, we use Levenberg-Marquardt [14], which is a standard algorithm for solving unconstrained nonlinear optimization. Alternative solvers for SBA include preconditioned conjugate gradient =-=[2]-=- or Powell’s dog-leg solver [12]. The LM implementation of SBA repeatedly solves a large linear subproblem whose LHS matrix is positive definite. Recent work in direct sparse Cholesky solvers [4] has