[1] Dept. of Elec. and Electron. Eng., University of Newcastle, Newcastle NE1 7RU, United Kingdom
[2] DERA Def. Res. and Evaluation Agency, Malvern, Worcs. WR14 3PS, United Kingdom
[3] Def. Eval. and Research Agency DERA, Farnborough, Hampshire, GU14 6TD, United Kingdom