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This paper presents evidence that systematic oblique incidence channel sampling transmissions are valuable for improving the reliability of air-ground HF communications when propagation conditions are marginal. This evidence results from air-ground radio communications trials supplemented by frequency sounding transmissions over flight paths of up to 1260 nautical miles into auroral zone latitudes from a ground station in Nova Scotia. The ground-air frequency sounding measurements established that propagation via clouds of auroral sporadic E was possible for a high percentage of time during geomagnetic storms. These modes provided a useful, and often the only, mode for communications over the air-ground path. In order to communicate effectively via auroral E modes, it was found necessary to use a wide selection of aeronautical frequencies. In addition, because of the rapid variation in HF propagation conditions, it was found desirable to renew channel sampling measurements once every five minutes on each frequency assignment. It was established that this procedure for the selection of operating frequencies during disturbances resulted in communications reliability that was appreciably higher than would have been obtained if other frequency selection techniques, alone, were followed.