  Consider a sliding camera that travels back and forth along an orthogonal
line segment $s$ inside an orthogonal polygon $P$ with $n$ vertices. The camera
can see a point $p$ inside $P$ if and only if there exists a line segment
containing $p$ that crosses $s$ at a right angle and is completely contained in
$P$. In the minimum sliding cameras (MSC) problem, the objective is to guard
$P$ with the minimum number of sliding cameras. In this paper, we give an
$O(n^{5/2})$-time $(7/2)$-approximation algorithm to the MSC problem on any
simple orthogonal polygon with $n$ vertices, answering a question posed by Katz
and Morgenstern (2011). To the best of our knowledge, this is the first
constant-factor approximation algorithm for this problem.
