  This paper details the design of an autonomous alignment and tracking
platform to mechanically steer directional horn antennas in a sliding
correlator channel sounder setup for 28 GHz V2X propagation modeling. A
pan-and-tilt subsystem facilitates uninhibited rotational mobility along the
yaw and pitch axes, driven by open-loop servo units and orchestrated via
inertial motion controllers. A geo-positioning subsystem augmented in accuracy
by real-time kinematics enables navigation events to be shared between a
transmitter and receiver over an Apache Kafka messaging middleware framework
with fault tolerance. Herein, our system demonstrates a 3D geo-positioning
accuracy of 17 cm, an average principal axes positioning accuracy of 1.1
degrees, and an average tracking response time of 27.8 ms. Crucially, fully
autonomous antenna alignment and tracking facilitates continuous series of
measurements, a unique yet critical necessity for millimeter wave channel
modeling in vehicular networks. The power-delay profiles, collected along
routes spanning urban and suburban neighborhoods on the NSF POWDER testbed, are
used in pathloss evaluations involving the 3GPP TR38.901 and ITU-R M.2135
standards. Empirically, we demonstrate that these models fail to accurately
capture the 28 GHz pathloss behavior in urban foliage and suburban radio
environments. In addition to RMS direction-spread analyses for
angles-of-arrival via the SAGE algorithm, we perform signal decoherence studies
wherein we derive exponential models for the spatial/angular autocorrelation
coefficient under distance and alignment effects.
