Multistatic ISAR autofocus with an image entropy-based technique

A key issue in radar research for many years has been the noncooperative identification (NCI) of moving targets. Such targets can be aircraft, ships, ground vehicles, satellites, or ballistic missiles. Nowadays, identification is mostly based on cooperative methods like the identification friend-foe (IFF) systems in the military sector or automatic dependent surveillance-broadcast (ADS-B) for civil aircraft. However, a number of situations exist in which a cooperative identification is not possible. In conflict situations, hostile targets do not identify themselves. Knowledge of their type by NCI yields a tactical advantage. History has also shown a significant number of cases in which neutral or friendly targets have been destroyed due to their inability to identify themselves. Neutral targets usually do not share IFF codes of conflict parties, and friendly targets may turn off their IFF when performing covert operations. Operations between allied nations have also been a source of possible IFF failure. Furthermore, the desire for NCI is not limited to conflict situations. Unidentified aircraft or ships often are suspected to be terrorists. In all of the above situations, a decision on a possible response is extremely time critical, and possible consequences can be serious. For this reason, NCI remains an important research topic.