A overview of ATSC spectrum sensing based on detection of the pilot signal is provided. Then the theoretical performance for such a spectrum sensor is given. The formula for the probability of mis-detection (one minus the probability of detection) is derived for several different channels. The channel models that are consider are: additive white Gaussian noise, a Rayleigh fading channel with long coherence time and a multiple receive antenna Rayleigh fading channel with long coherence time and finally a Rayleigh fading channel with short coherence time. The formula for the probability of false alarm is also derived. Based on the formulas for probability of mis-detection and probability of false alarm observations are made about how changes in various design parameters impact the sensing performance. Finally, and example is given using typical parameter values.