The increasing availability of low cost, commercial off-the-shelf unmanned aerial systems (UAS) technology has made it more accessible for researchers to investigate air-to-ground wireless communications channel phenomenology. Furthermore, using software defined radio (SDR) transceivers as the UAS communications platform provides the flexibility for researchers to characterize the air-to-ground channel across multiple frequency bands. In this paper we investigate the time and frequency dispersion characteristics of UAS air-to-ground wireless channel at C band (5.8 GHz), with signal bandwidth of 20 MHz, using empirical measurements collected in outdoor residential and mountainous desert terrains. We characterize the time and frequency dispersion of the UAS air-to-ground channel by evaluating the root mean square (RMS) delay and Doppler spread. For the residential environment, the median RMS delay spread was approximately 0.03μs. For the mountainous desert environment, the median RMS delay spread was approximately 0.06μs.