1. Engineer, Southwest Research Institute, 201 Randolph Hall, Blacksburg, VA 24061 (corresponding author).
2. Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Illinois at Urbana-Champaign, 205 N. Mathews Ave., Urbana, IL, 61801.
3. Associate Professor, Unmanned Systems Laboratory, Dept. of Mechanical Engineering, Virginia Tech, 201 Randolph Hall, Blacksburg, VA 24061.