Precise relative motions of formation flying space vehicles

Abstract

This paper presents an analysis of the relative motions between formation flying space vehicles in orbit about the Earth and Mars. Formation flying is proposed for: cross calibration of science instruments on follow-on missions and new technology demonstrations, moving from large platforms to several smaller vehicles with distributed instrumentation (virtual platforms) and spacebome interferometry. In Mars orbit, a small constellation for in-situ navigation is being considered to enable automated operations and to reduce Earth based tracking requirements. To establish achievable control accuracies for these types of missions, dynamics in the orbit design space are examined thoroughly. Aspherical and third body gravity, atmospheric drag, and solar radiation pressure are the primary dynamic forces responsible for producing relative motions. High fidelity models of these forces are used to examine the relative motion characteristics of circular LEO and GEO orbits and circular low and synchronous Mars orbits. Introduction Simplest are formations of two space vehicles, separated only in true anomaly (i.e., co-planar with nearly the same altitude and eccentricity). This type of formation is proposed for: follow-on missions requiring sensor cross calibration, new technology validation, and for complementary science via distributed instrumentation on several vehicles. * Navigation and Flight Mechanics Section (818) 354-0425, AIAA Member Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Titlel?, U.S. Code. The Government has a royaltyfree license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved are reserved by the copyright owner. Navigation operations consist of maintaining a prescribed separation without dedicated satellite-tosatellite links. This is referred to as non-cooperative formation flying. Navigation functions can be automated or implemented as an autonomous capability by taking advantage of common dynamic force modelling cancellation. An example of a cross calibration mission is the TOPEX/Poseidon follow-on mission called JASON-1. Formation flying in this case is required to cross calibrate the altimeters that provide the primary science measurements. A technology validation example is the New Millennium Program's Earth Orbiter-1 (EO-1) mission. EO-1 will fly one minute (-450 km) behind the Landsat-7 land imaging mission and carry a lighter weight, lower cost version of the Landsat multispectral imaging system. Image co-registration requires control of the mean along track orbit separation to six seconds (-45 km). Virtual platforms comprised of complementary sensors are under consideration as an alternative to the large EOS platforms. Possible benefits are reduced implementation risk with comparable or improved science return and reduced mission operations costs. For each of these simple formations, orbit control consists primarily of maintaining the mean semimajor axis, accounting for secular changes caused by atmospheric drag. Occasional inclination control due to lunar perturbations is also required. More complex is a formation with true anomaly and inclination differences. This type of formation has been proposed for the New Millennium Program's Deep Space-3 (DS-3) mission. At geosynchronous altitude, DS-3 consists of