Measurements of transionospheric radio propagation parameters using the FORTE satellite

We report initial measurements of ionospheric propagation parameters, parti cularly the total electron content (TEC), using the recently launched FORTE satellite. FORTE, which orbits the Earth at an altitude of 800 km and an i nclination of 70 degrees, contains a set of wideband radio receivers whose output is digitally recorded. A specialized triggering circuit identifies t ransient, broadband radio events, which include radiation from lightning, t ransionospheric pulse pairs, and man-made sources. Event data are transmitt ed to the ground station for analysis. In this paper we examine signals tra nsmitted from an electromagnetic pulse generator operated at Los Alamos. Th e transmitter produces nearly impulsive signals in the VHF range. The recei ved signal is dispersed by the ionosphere, and the received signal can be a nalyzed to deduce the total electron content along the path. By comparing t he slant TEC thus measured with results from a ray-tracing code, we can ded uce the vertical TEC to 800 km. Data from eight passes are presented. These types of data (in larger quantities) are of interest to operators of radar altimeters, who need data to corroborate their corrections for the ionosph eric TEC. The combination of FORTE TEC data to 800 km and TEC measurements to 20,000 km (the Global Positioning System orbital altitude) can provide u seful information for assessing the validity of models of plasmaspheric ele ctron density. Initial estimates of the plasmaspheric density, on two dayti me passes, are about 6 TECU. The signal received by FORTE, which is linearl y polarized at the transmitter, is split into two magnetoionic modes by the ionosphere. The receiving antenna is also linearly polarized and therefore receives both modes. By measuring the beat frequency between the two modes , we can deduce the product of the geomagnetic field and the cosine of the angle between the field and the propagation vector. The possibility of usin g the measured slant TEC and the beat frequency to geolocate impulsive sign als is discussed.