THE NASA BARREL (BALLOON ARRAY FOR RADIATION RELATIVISTIC ELECTRON LOSSES) MISSION USED INSTRUMENTED HIGH-ALTITUDE BALLOONS TO REMOTE-SENSE ELECTRON PRECIPITATION FROM EARTH S RADIATION BELTS OVER BROAD GEOGRAPHIC REGIONS. EACH PAYLOAD S PRIMARY INSTRUMENT WAS A NAI SPECTROMETER WHICH MEASURED BREMSSTRAHLUNG X-RAYS FROM PRECIPITATING ELECTRONS. IN MEASURING ENERGETIC ELECTRON LOSSES BARREL WAS A MISSION OF OPPORTUNITY IN SUPPORT OF NASA S VAN ALLEN PROBES MISSION. FORTY PAYLOADS WERE DEPLOYED IN TWO ANTARCTIC SUMMERS FROM THE BRITISH ANTARCTIC STATION HALLEY BAY AND THE SOUTH AFRICAN ANTARCTIC STATION SANAE CREATING AN ARRAY OF SLOW-MOVING MEASUREMENT STATIONS.