Wideband air-to-ground radio channel measurements using an antenna array at 2 GHz for low-altitude operations

Wideband measurements were performed using a direct-sequence spread-spectrum measurement system in the air-to-ground radio environment to characterize propagation between an airborne transmitter and a ground-based receiving antenna array at a center frequency of 2.05 GHz. The transmitter was flown along constant-radius arcs at low altitudes around the receiver location to obtain measurement results for 7.5, 15, 22.5, and 30 degree elevation angles. An 80 megachip per second (Mcps) modulating PN sequence was transmitted by the airborne station. The receiver was located in a campus environment of four- to six-story buildings and rolling terrain. The receiver used a four-element antenna array and sampled received signals at 1 gigasample per second (Gsps) per channel. Power-delay profiles that approximated channel impulse responses were used to measure magnitude, phase, and delay of multipath signal components received at each element. Characterization parameters produced from the measurements include RMS delay spread, excess delay spread, multipath fading CDFs, antenna diversity gain, and gain achieved through spatial-temporal combining. The measurements presented here support analysis of wireless systems for intentional transmissions, such as data communications between ground nodes and low-altitude aircraft. In addition, these measurements support investigations into interference from ground sources to low-altitude aircraft (e.g., on instrument approaches) or interception of signals originating from ground sources.