This paper characterizes the performance of direct sequence spread spectrum (DS-SS) signals from the detection probability point of view. The detection probability of DSSS signals is estimated using wideband radiometer receiver over flat fading channel. Simulations are performed to evaluate detection probability of DS-SS signals over flat fading channel for various time bandwidth product values. The results are compared with the detection probability of DS-SS signals over AWGN channel. The results show that the fading parameter degrades the detection probability of DS-SS signals. The performance of DS will be discussed later in the presence of imperfect channel estimation errors.