ABSTRACT Spaceborne GPS atmospheric science is now blossoming, with small missions in preparation in several countries. These missions require high performance GPS flight receivers with capabilities well beyond most space needs. The full promise of spaceborne GPS science can be realizedonly with dcdicatcd constellations of orbiting GPS receivers specially designed for science use. Initial proposals are for a pilot constellation of a dozen or so microsats launched at once into a single orbit plane. In the future we may see a large constellation of hundreds of tiny satellites, each with a mass of less than 1 kg, enveloping the earth in multiple orbit planes. This will requireadvances in spaceborne GPS receiver architecture and microsatellite design. Key tasks include reducing a high performance receiver to a 1.5watt, credit-card size instrument; devising efficient 3-axis stabi Iization for microsats; and providing low-power cell phone communication from space, There will be a sizable commercial payoff to this work. ‘~he miniature receiver will have enormous appeal as both aflight instrument and a high end terrestrial receiver for surveying, geodesy, and aircraft app]icat ions. Moreover, the commercial value of atmospheric data for use in weather prediction COUIC1 one day be substantial. lNTROIJUC~’ION With the maturing of the Global Positioning System and the appearance of increasingly affordable spaceborne receivers, GPS usage is expanding rapidly into the worldof space flight projects. Indeed, owing to the great utility and convenience of autonomous onboard positioning, timing, and attitude determination, basic navigation receivers are coming to be seen as almost indispensable to future low earth missions. This development has been expected and awaited since the earliest days of GPS. Perhaps more surprising, howe.vcr, has been the emergence of directspaceborne GPS science and the blc)ssoming of new science. applications for high performance geodetic space receivers. Science applications of spaceborne G]% include ccntimctcr-]evel precise orbit determi nation to support ocean ah imctry; F.arth gravity model improvement and other enhancements to GPS global geodesy; high resolution 3D ionospheric imaging; and atmospheric limb sounding (radiooccultation) to produce precise profiles of atmosl)heric density, pressure, temperature, and
water vapor ciistribution. l’igurc 1 offers a sinlpli(”iect summary of the 13arth science now emerging from spaceborne GPS.
}1’ig. 1. Some kcy science applications for a sp;lceborne array of GPS receivers. At present, Iiar(h science from spaceborne GPS is cterivcd front jus[ two missions. The firstis the lJ.S.-l~rcnch Topcx/Poseidon mission, launched aboard an Ariane rocket in August 1992, which carries a six-channc], dual-frequency, P-code receiver developed by Motorola. The (3PS antenna on ‘1’opcx/Poseidon provides a nearly hcmisphcrica] ficlct of view directed towards the zenith, affording no view of the F,arth’s limb and tlms no view of the atmosphere and little view of the ionosphere.Over the past three. years, C;}YS data from ‘1’opex/Poseidon
support will be found to make this idea a reality, And while the initial step is likely to be modest--a pilot constellation of perhaps 12 satellites- –we foresee that within a decade up to several hundred tiny 1-kg microsats will be in place providing a vast flow of data that will transform the study of the earth’s climate,...