EXPERIMENTAL EVALUATION OF THE BDD-I DOSIMETER FOR THE GLOBAL POSITIONING SYSTEM

The Burst Detection Dosimeter I (BDD-I) is a multipurpose silicon detector system that has flown on two Block I satellites of the Global Positioning System (GPS) series. These spacecraft occupy circular orbits about 20000 km above the surface of the Earth. BDD-I measures the radiation dose received by the GPS spacecraft, primarily from energetic electrons trapped in the Earth's radiation belt, but also from solar energetic particles and galactic cosmic rays outside the trapped radiation zone (when the spacecraft is near the Earth's magnetic poles). Absorbers, located in front of four separate silicon sensors, determine energy thresholds for measuring incident particle fluxes, and the magnitude of energy loss in each sensor provides an imperfect but useful separation between protons and electrons over a wide range of energies. This article describes the important mechanical and electronic features of this detector. Relations are given for converting the observed counting rate in the detector to parameters representing the incident flux of charged particles. The overall uncertainty in this determination of the integral electron flux is about 30%. The detector response was well characterized by the use of electrons with energies below 3 MeV. Monte Carlo simulations agree with measured responses to within the experimental uncertainty. The simulation results also enable assessments of the sensitivity of the experimental measurements to systematic effects such as an angular divergence in the incident electron beam. The article presents some sample data measured with the detector.