We consider the performance of a fast frequency hopping M-ary frequency shift keying spread spectrum rank sum diversity combiner. The spread signals are received in partial band interference and the parameters of this intentional interference are unknown. For the BFSK (M=2) case and a Rayleigh fading channel, the analytical performance of the rank sum receiver is compared to that of the linear receiver. Simulations are carried out for the rank sum receiver in a non-fading channel and compared to simulated performance of the clipper receiver and product combiner receiver (PCR). The performance of the rank sum combiner, in the non-fading channel, is comparable to the product combiner receiver and almost always is worse than the clipper receiver. In the Rayleigh fading channel, the rank sum receiver performs considerably better than the linear receiver when the jamming fraction is relatively small.