  This paper deals with adaptive radar detection of a subspace signal competing
with two sources of interference. The former is Gaussian with unknown
covariance matrix and accounts for the joint presence of clutter plus thermal
noise. The latter is structured as a subspace signal and models coherent pulsed
jammers impinging on the radar antenna. The problem is solved via the Principle
of Invariance which is based on the identification of a suitable group of
transformations leaving the considered hypothesis testing problem invariant. A
maximal invariant statistic, which completely characterizes the class of
invariant decision rules and significantly compresses the original data domain,
as well as its statistical characterization are determined. Thus, the existence
of the optimum invariant detector is addressed together with the design of
practically implementable invariant decision rules. At the analysis stage, the
performance of some receivers belonging to the new invariant class is
established through the use of analytic expressions.
