Overview of the LASSII experiment on the Combined Release and Radiation Effects Satellite

Introduction T HE Combined Release and Radiation Effects Satellite (CRRES) was launched into Earth orbit on July 25, 1990. Onboard CRRES is the Low Altitude Satellite Study of Ionospheric Irregularities (LASSII) experiment. The objective of LASSII is to study plasma irregularities in the ionosphere caused by naturally occurring and artificially created plasma instabilities. Specific investigations include equatorial spreadF, chemical releases, ionospheric modification by intense radio waves, and solar flare disturbances of the ionosphere. These studies will extend our understanding of the effects of ionospheric irregularities on radio-wave propagation in ionospheric ducts, through scintillating regions, and from spacebased radars. We present an overview of the LASSII instruments, their measurement characteristics, and the planned orbit operations. The LASSII experiment was originally proposed to the Department of Defense space test program in 1977 as a Shuttlelaunched free flyer experiment that would be placed in orbit for a 1-2 week period, then recaptured for return to Earth and refurbishment, and reflight on subsequent Shuttle missions. In that concept, LASSII was to provide up to 10 different diagnostic measurements of the ionospheric plasma environment at lowand medium-magnetic latitudes, with the goal of providing a detailed understanding of spread-F irregularities, ionospheric chemical reactions, and plasma turbulence associated with these processes. In its original version, the LASSII experiment was similar to what is now referred to as a "lightsat" experiment. Although LASSII had a high priorty ranking, there was difficulty in getting enough funding to support the experiment; it was therefore decided to modify the experiment and combine it with other DoD and NASA missions, which resulted in LASSII becoming part of CRRES in 1981. The CRRES mission represented the first joint DoD/NASA space experiment of its kind, and would use the Shuttle to place CRRES into a circular orbit of 350 km altitude and 27 deg inclination. LASSII became the prime experiment to provide in-situ diagnostics of the NASA chemical releases to be performed in low Earth orbit (LEO). This low earth orbit mission would last 90 days, during which a total of about 48 chemical release experiments would take place. After those initial 90 lys, CRRES would be boosted into a geosynchronous trans:r orbit (GTO), where the rest of the radiation effects part of ie mission would continue for at least one year. In this plan, ASSII would have performed most of its original mission .most unchanged. Major changes to the CRRES mission were required after ie Shuttle Challenger was lost. As a result of the restructured ad constrained Shuttle schedule, it was decided that CRRES ould be launched aboard an expendable launch vehicle (the lias-Centaur). This new launch mode required a substantial reduction in weight for CRRES, in which about half of the chemical canisters for the NASA experiment were off-loaded, to be launched into the ionosphere from sounding rockets. LASSII experiments were reduced in weight by removal of a swept frequency plasma wave analyzer. In addition, the telemetry resources of the spacecraft were reduced so that the high speed data rates (256 and 64 kbps) originally planned for LASSII were eliminated, leaving only the low rate of 16 kbps. The planned sequence of operations involving separate LEO and GTO operations were also combined so that formerly LEO operations now took place during the perigee portion of the GTO orbit. These various changes in overall operations resulted in general descoping of the LASSII experiment and of the original plans; however, the basic mission to study the low-latitude ionosphere and to diagnose low-altitude chemical releases remained intact. One phase of the LASSII experiment plans was actually enhanced by the restructured mission, and that was to perform in-situ measurements in coordination with powerful ground-based high-frequency radio wave transmitters. This enhancement came about because the orbit inclination had to be reduced to 18 deg, thereby allowing CRRES to fly over the ionospheric diagnostics facility at Arecibo, Puerto Rico. Current plans call for the use of the hf ionospheric heater at Arecibo to conduct experiments in artificial stimulation of plasma instabilities in the ionosphere.