1. Helios, Ltd., 29 Hercules Way, Aerospace Boulevard, AeroPark, Farn-borough, Hampshire GU14 6UU, United Kingdom.
2. Department of Civil and Environmental Engineering, Imperial College Road, London SW7 2BU, United Kingdom.
3. Alan M. Voorhees Transportation Center, Edward J. Bloustein School of Planning and Public Policy, Rutgers University, New Brunswick, NJ 08901.
4. Space and Atmospheric Physics Group, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom.