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Preliminary space flight results of attitude determination using GPS are presented from a spacecraft in low Earth orbit. Relative position measurements accurate to the sub‐centimetre level are made among multiple GPS antennas mounted on the space vehicle. A Trimble Navigation TANS Quadrex (a GPS receiver specially adapted for attitude determination by Stanford University) is used as a differential carrier phase sensor for the flight. Four GPS antennas are mounted on the zenith face of RADCAL, a polar orbiting, gravity‐gradient‐stabilized Air Force Space Test Program Satellite, built by Defense Systems, Inc. The four antennas are equally spaced about the perimeter of the 30 inch diameter cylindrical spacecraft bus. The Quadrex receiver measures the phase of the L‐band GPS carrier (1575 MHz) at each of up to four antennas for up to six GPS satellites simultaneously. From these measurements, an initial assessment of attitude determination in space is performed in post‐processing. For RADCAL, the attitude solution is greatly overdetermined. In a preliminary evaluation of system performance, the system accuracy is determined through measurement self‐consistency. Analysis of the attitude motion in the context of a gravity gradient dynamic model yields further insight into the system performance.

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In early 1985, an improved ground Operational Control System (OCS) will maintain the navigation service. Primary among the OCS improvements over past GPS navigation systems is a global network of ground antennas (to upload satellite navigation data) and tracking/monitor stations. In addition, a refined Kalman filter will continuously estimate the Global Positioning System (GPS) satellite ephemerides and clock phases and frequencies. Although the user-GPS interface remains unaltered, the accuracy of the GPS navigation service is expected to improve by a factor of three.Using real and simulated GPS pseudo range radiometric tracking data, The Aerospace Corporation has completed a detailed error analysis which shows that the satellite clock noise contributes more than 90 percent of the total satellite-touser pseudo range error. If the number of OCS uploads is increased to three per day (as planned), then the accuracy of the navigation service is also expected to improve by nearly a factor of three because the clock-noise contribution to the range error increases linearly with time. Also, this analysis shows the consequence of satellite ephermeris uncertainties in the GPS navigation application.

Specs Physics Laboratory; Atmoapherlc and Ionospheric physics, radiation from the atmosphere, density and composition of the atmosphere, auroraa and alrglow; magnetoapherlc phyalca, coamlc raya, generation and propagation of plaama waves In the magnetoaphere; aolar physics, studlee of solar magnetic fields; space astronomy, x-ray astronomy; the effects of nuclear explosion«, magnetic storms, and solar activity on the earth's atmosphere. Ionosphere, and magnetosphsre; the effects of optical, electromagnetic, and particulate radiation« In «pace on apace systems. THE AEROSPACE CORPORATION ÄTSf '"'"'•-'--^ --iEl Segundo, California THE LOGACS (Low-G Accelerometer Calibration System) experiment, which contained a miniature electrostatic accelerometer (MESA), was placed in a low-altitude polar orbit on 22 May 1967. The experiment provided approximately 100 hours of acceleration data from which the accelerometer scale factor, accelerometer null bias, and atmospheric drag on the satellite were calculated.Extensive comparisons of the data to various model atmospheres are made. These analyses are the more interesting because of the intense solar and geomagnetic activity during the latter part of the LOGACS flight. From the data, both midlatitude and polar models of atmospheric density have been developed over altitudes for which LOGACS data were available.An analysis was also performed to determine wind characteristics normal to the LOGACS' orbit plane. The results confirm that the atmosphere (up to 100 nmi) has high-velocity winds in the high-latitude regions as a result of the great magnetic storm. Theoretical analysis shows these winds can be described as convective motion due to excessive heating of the polar thermosphere. Models of atmospheric density have been developed over the altitudes for which LOGACS data were available. Both midlatitude models and polar and auroral models are formulated and presented.An analysis was also performed on the LOGACS data to determine the wind magnitude and characteristics normal to the orbit plane of the in experiment. The results confirm that the earth's atmosphere (up to the altitude of 100 nmi) rotates with the earth and find that hiph-velority winds are present in high-latitude regions as a result of the Rreat magnetic storm,