A new way related to the performance analysis of Surveillance and Reconnaissance Systems is proposed to evaluate the situation awareness. At first, NIIRS and GIQE are used to find the relationship between the remote images and targets' detection probability, so the probability distribution of the targets in the surveillance area is gotten. Secondly, considering the real distribution of the targets as the users' need for information, we use Kullback-Leibler distance in the information theory to measure the difference between the probability distribution of the targets and the users' need for information. Finally, a situation awareness curve of different S&R systems is derived and the impact analysis of the transmission and processing delay on the curve follows.