Engineering topics
Publications and source records attributed to Nicholson, F. T..
This paper summarizes Galileo's orbit determination activities leading up to its encounter with asteroid 243-Ida on August 28, 1993. In addition to the nominal 2-way S-band range and Doppler radio metric data obtained from the Deep Space Network (DSN), several navigational aids were brought together to make this encounter successful.
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As the time of the Ida encounter by Galileo approached, some unfortunate circumstances occurred, causing a very worrisome but exciting encounter, especially with regards to navigation. This paper reports on Galileo's orbit determination stratagy during the period after the last Earth encounter through the Ida flyby. Details in the modeling of Galileo's orbit, and in the use of various navigation tools, will be explained and the results of several key orbit solutions will be given.
The Galileo mission is an ambitious attempt to explore the Jovian system by spacecraft. This voyage of exploration is a logical successor to the reconnaissance voyages of Pioneers 10 and 11, Voyagers 1 and 2, and Ulysses. These spacecraft merely flew past Jupiter, spending relatively little time in its system. Galileo differs from these earlier spacecraft in that it will remain within the Jovian system, studying the planet and its four major satellites for a period of two years. Insertion into orbit around Jupiter will occur on December 7, 1995. The portion of the mission encompassing observations of Jupiter and the major and minor satellites, and magnetospheric mapping, has become known as the Jovian tour. During this period Galileo will encounter each of Europa, Ganymede, and Callisto at least three times on trajectories that will bring it to altitudes from 200 to 3100 kilometers.
This paper discusses orbit determination results for the Galileo satellite tour. Lacking a high gain antenna, the mission will use a low gain antenna for communication and tracking. This change implies far less navigation data will be available than previously expected. A baseline orbit analysis was completed assuming this decreased data schedule. Variations on this baseline were studied to determine sensitivity to data loss. Results indicate that the probability of completing the tour is less than 90 percent, although future improvements in orbit determination promise to raise the probability of completion above 90 percent.
This paper presents the orbit determination strategy and results in navigating the Galileo spacecraft from launch through its Venus and first earth flybys. Many nongravitational effects were estimated, including solar radiation pressure, small velocity impulses from attitude changes and eight trajectory correction maneuvers. Tracking data consisted of S-Band Doppler and range. The fitting of Doppler was difficult since one of the cpacecraft's two antennas was offset from the spin axis, thus producing the sinusoidal velocity fluctuation seen in the data. Finally, Delta Differential One-way Range data was used during the last three months of the earth approach to help deliver the spacecraft to within desired accuracy.
This paper presents an overview of the orbit determination for Galileo's epochal encounter with the asteroid Gaspra on October 29, 1991. Topics discussed are a ground-based observation campaign to improve Gaspra's ephemeris before encounter, and the use of optical navigation together with Doppler and range data. The paper concludes by noting the steady improvement in the B-plane dispersions during the two months prior to encounter.
Results are given which demonstrate the ability of the orbit determination system to satisfy accuracy requirements in support of the Galileo Project's planned tour of Jupiter's satellites. The results are derived through the application of mission operation strategies and assumptions. It is shown that the achievement of the requisite orbit determination accuracies is predicated on the availability of optical navigation data. It is further demonstrated that the unavailability of tour optical navigation data yields not only degraded orbit determination accuracies which fail to meet propellant budget and science instrument pointing requirements, but also produces, for the planned 200 km flyby of Europa, an approximate 0.02 risk of having the spacecraft collide with the satellite.
The configuration of the Galileo mission, in which the Probe and Orbiter are joined as a single spacecraft (until five months before Jupiter encounter, when the Probe will be released into the atmosphere for the Io flyby) is discussed, together with the major mission objectives, and the aspects of the trajectory. Special attention is given to the descriptions of the orbit determination process, error source assumptions (based on the Voyager experience at Jupiter), and data assumptions. The orbit determination results for the interplanetary and Jupiter approach phases of the mission for the previously planned launch in 1986 are presented, together with the preliminary results of navigation studies of the current mission scheduled for a launch for late 1989.
Orbit determination characteristics of the Jupiter approach phase of the Galileo mission are described. Predicted orbit determination performance is given for the various mission events that occur during Jupiter approach. These mission events include delivery of an atmospheric entry probe, acquisition of probe science data by the Galileo orbiter for relay to earth, delivery of an orbiter to a close encounter of the Galilean satellite Io, and insertion of the orbiter into orbit about Jupiter. The orbit determination strategy and resulting accuracies are discussed for the data types which include Doppler, range, optical imaging of Io, and a new Very Long Baseline Interferometry (VLBI) data type called Differential One-Way Range (DOR).
A summary and evaluation of the Galileo-Mars flyby navigation techniques is presented. The navigational requirements of Galileo as it swings by Mars are going to be met with interferometric angular measurements (VLBI) and range and range-rate measurements. Like VLBI, dual spacecraft differenced range is less sensitive to Mars ephemeris errors and tracking station location errors than conventional range and Doppler. Similarly, differenced range provides angular information about the separation between the Mars Viking Lander I and the Galileo spacecraft. In covariance studies, dual spacecraft range coupled with conventional range and Doppler is shown to estimate the Galileo-Mars flyby distance to better than 10 km which is comparable to the VLBI performance. For the Galileo-Mars flyby, dual spacecraft differenced range promise to be an excellent backup to VLBI if the Mars Viking Lander remains operational.
The utilization of the Viking lander as a beacon for navigating the Galileo spacecraft for its Mars flyby is investigated. Covariance analyses show that the improvement in the flyby navigation accuracy is significant compared with the conventional radiometric navigation. A validation experiment using Viking lander and orbiter indicates that the expected accuracy can be obtained.