HASL AERIAL SURVEY SYSTEM le INTRODUCTION The measurement of fall-out over extensive areas has become necessary in view of the increased yield of radioactive debris from the larger nuclear devices, The use of aircraft for long range radiation survey makes feasible a rapid determination of the dise tribution of fall-out debris, The limit of detectability depends on the sensitivity of the measuring instrument, the energy and intensity of the surface radioactivity and the absorption of the intervening layer of air, When the altitude is known, it is then possible to relate an aerial radiation measurement to the intensity at the surface. The resolution, that is, the degree at which an area can be de linea ted is proportional to the aircraft altitude, The radius of the ground area in feet which is viewed by the detector is approxe imately equal to the aircraft altitude in feet, Since fall-out is distributed fairly wiformily over wide areas, the aircraft reading is directly related to the surface radiation field, This aerial survey system uses a scintillation cowmter which is sensitive over a range extending from 0,005 mr/hr to 200 r/nr, as measured in the aircraft. A signal from the airplane radar altimeter corrects the aircraft radiation reading to a radiation intensity at three feet from the surface. This level is recorded and simultaneously converted to a time=-modulated pulse-train by a “Telepulse” Coding Unit for radio transmission to a plot center. At this center, a "Telepulse* Receiving Unit decodes the information and presents the ground-level radiation intensity on a strip=chart =a] «=