Ionospheric Sounders In Aircraft

Investigation of ionospheric phenomena over large areas of the world has been pursued by Air Force Cambridge Research Laboratories since 1953 by means of ionospheric sounders carried in large aircraft. Technical details of the first airborne ionospheric sounding system utilizing a modified C-3A Automatic Ionosphere Recorder in a JKC-97 aircraft, are described briefly. Emphasis is given to the limitations and special considerations connected with an airborne ionospheric sounder such as adequate antenna, types of interference, power, effects of temperature and vibration, and peculiarities of airborne data. Mutual operation with other projects on the aircraft conducting various photometric measurements, is discussed in regard to the severe rf interference produced by the ionospheric sounder. (Author).

Experiments were conducted to determine whether acoustic waves could propagate from the lower atmosphere into the ionosphere and be detected by hf radio waves. A supersonic aircraft was flown on a trajectory designed to focus the shock wave energy, generated over a long flight path, into a relatively small region of the daytime E-layer which was being probed by a phase path vertical sounder. Three series of tests, consisting of four flights each, were made. In each series, perturbations were found in the frequency spectrum of reflected radio waves that could be well correlated with the expected disturbance from aircraft-generated shock waves. (Author).

In response to Air Force needs for specifying and forecasting conditions of the Arctic ionosphere, AFCRL's Airborne Ionospheric Laboratory, a highly instrumented NKC 135 jet aircraft, was used in an extensive Arctic research program that started in 1967. To date, 69 flights were made and ionospheric and optical data covering major parts of the auroral oval, mainly the midday sector and the night sector, were collected. During the winter months of 1967, 1968, 1969, and 1970 special emphasis was put on the investigation of the day sector. These flights covered a rather unique and poorly investigated area of the auroral oval, and a detailed report on the observations is given here. A thorough analysis of the data collected in the day sector reveals the existence of a band of continuous particle precipitation south of the location of the discrete aurora. This precipitation hardens towards the south and results in E- and D-region ionization. These observations compare very well with measurements of precipitating particles done by satellite. The investigation of the evening and night sectors showed similar but less organized trends than on the day side. During several flights the continuity of the auroral oval was verified. The investigation indicates that, with additional measurements, it may be possible to develop a model of the Arctic ionosphere. With control measurements from a few ideally located ground stations, the dynamics of the Arctic ionosphere might then be utilized for forecasting propagation conditions in the Arctic. (Author).

Data collection in the field of environmental science using analog methods has become highly problematic due to the immense information density of most measurements. One possible solution to this problem is the selection of parameters prior to the recording of the data. Certain aspects of the proposed solutions, as well as modern data analysis methods, require preprocessing and digital recording of the information. A sounding system comprising an airborne Granger 3905-1 Ionospheric Sounder and a Pulse Amplitude Digitizer uses the approach of parameter selection. Up to six arbitrarily selected frequencies can be monitored simultaneously; one ionospheric echo per frequency can be gated out and recorded in digital form on tape. This system, together with automatic calibration is the ideal basis for multi-frequency absorption and drift measurements. As a part of an airborne ionospheric laboratory, this equipment has been successfully used for absorption measurements during two eclipse expeditions, for investigations of the South Atlantic Magnetic Anomaly, and for the study of the influence of the albedo on the D-region ionization. Further experiments such as the study of ionospheric irregularities and the ground reflectivity are considered. (Author).