  This paper presents field measurement-based channel characterization for
air--to--ground (A2G) and air--to--air (A2A) wireless communication systems
using two drones equipped with lightweight software-defined radios. A
correlation-based channel sounder is employed such that the transmitting drone
broadcasts the sounding waveform with a pseudo-noise sequence and the receiving
drone captures the sounding waveform together with the location information for
the post-processing analysis. The path loss results demonstrate that the
measurement and flat-earth two-ray results have similar trends for A2G while
the measurement and free space path loss are similar to each other for A2A. The
time delays between the direct path and multipath components are widely spread
for A2A while the multipath components are mostly concentrated around the
direct path for A2G generating a more challenging communication environment. We
observe that the reflections from several buildings having metal roofs and
claddings on the measurement site cause sudden peaks in the root-mean-square
delay spread. The results indicate that the A2A channel has better
characteristics than the A2G under similar mobility conditions.
