1. An adaptive signal detector to detect a signal in a spectrum space, including: a software defined receiver, the software defined receiver for calculating a cumulative decision statistic for determining a presence of the signal in the spectrum space; the cumulative decision statistic being determined by a sequential energy detector that interoperates: a block energy detector having a block termination rule, a block sampling instant and a block decision phase; and, a truncated sequential energy detector having a truncated sequential termination rule and a truncated sequential decision phase; the block energy detector and truncated sequential energy detector therein interoperated to provide the cumulative decision statistic in which the block energy sampling instants provide samples that are independent and identically distributed; and, the cumulative decision statistic determining the presence of the signal in the spectrum space based upon a non-Bayesian dynamic mathematical modeled relationship of the cumulative decision statistic.