David A. Quinn

SATELLITE FORMATION FLYING USING AN INNOVATIVE AUTONOMOUS CONTROL SYSTEM (AUTOCON) ENVIRONMENT

This paper describes the formation of a partnership between two competing technologies with very different approaches to the problem of enhanced formation flying (EFF) on the New Millennium Program (NMP) Earth Orbiter (EO)-l mission. This includes a brief description of the two approaches that were independently proposed by the Goddard Space Flight Center (GSFC)/Stanford University and JPL teams. The actual mission combines these two approaches within a single autonomous control architecture called AutoCon™. The software is designed so that a control mode switch can be set by the ground flight operations team to invoke either EFF algorithm. The advantage of this approach is that both EFF technologies can be incorporated onboard EO-1 within the AutoCon™ framework. In addition, the details of each proposed algorithm need not be divulged provided that the algorithms conform to the specifications of AutoCon™. Forming a partnership between two competing technologies represents a significant programmatic challenge. This paper discusses the programmatic issues and several of the technologies that have been developed to perform the EFF mission. In the process, several recommendations are provided that should streamline similar partnerships on future NMP missions.

Tethered Formation Configurations: Meeting the Scientific Objectives of Large Aperture and Interferometric Science

With the success of the Hubble Space Telescope, it has become apparent that new frontiers of science and discovery are made every time an improvement in imaging resolution is made. For the HST working primarily in the visible and near-visible spectrum, this meant designing, building and launching a primary mirror approximately three meters in diameter. Conventional thinking tells us that accomplishing a comparable improvement in resolution at longer wavelengths for Earth and Space Science applications requires a corresponding increase in the size of the primary mirror. For wavelengths in the sub-millimeter range, a very large telescope with an effective aperture in excess of one kilometer in diameter would be needed to obtain high quality angular resolution. Realistically a single aperture this large is practically impossible. Fortunately such large apertures can be constructed synthetically. Possibly as few as three 3 - 4 meter diameter mirrors flying in precision formation could be used to collect light at these longer wavelengths permitting not only very large virtual aperture science to be carried out, but high-resolution interferometry as well. To ensure the longest possible mission duration, a system of tethered spacecraft will be needed to mitigate the need for a great deal of propellant. A spin-stabilized, tethered formation will likely meet these requirements. Several configurations have been proposed which possibly meet the needs of the Space Science community. This paper discusses two of them, weighing the relative pros and cons of each concept. The ultimate goal being to settle on a configuration which combines the best features of structure, tethers and formation flying to meet the ambitious requirements necessary to make future large synthetic aperture and interferometric science missions successful.

Libration Point Navigation Concepts Supporting the Vision for Space Exploration

This work examines the autonomous navigation accuracy achievable for a lunar exploration trajectory from a translunar libration point lunar navigation relay satellite, augmented by signals from the Global Positioning System (GPS). We also provide a brief analysis comparing the libration point relay to lunar orbit relay architectures, and discuss some issues of GPS usage for cis-lunar trajectories.

A Lunar Communications and Navigation Satellite Concept for the Robotic Lunar Exploration Program

The Robotic Lunar Exploration Programs (RLEP) second mission (RLEP-2) has studied entering a candidate crater near the South Pole of the Moon to search for water ice. The geometry is such that there is no line-of-sight communications to either planned mission elements near the rim of the crater or to Earth. This has motivated the conceptual design of a relay satellite architecture to support the RLEP-2 mission, and potentially follow-on RLEP missions requiring communications and navigation support. The conceptual design presented provides up to 73% coverage of mission elements in the candidate crater, while also providing simultaneous forward and return services to a Lander near the rim. Further, the conceptual design uses flight proven hardware to lower technical and schedule implementation risk.

An algorithm for enhanced formation flying of satellites in low earth orbit

With scientific objectives for Earth observation programs becoming more ambitious and spacecraft becoming more autonomous, the need for innovative technical approaches on the feasibility of achieving and maintaining formations of spacecraft has come to the forefront. The trend to develop small low-cost spacecraft has led many scientists to recognize the advantage of flying several spacecraft in formation to achieve the correlated instrument measurements formerly possible only by flying many instruments on a single large platform. Yet, formation flying imposes additional complications on orbit maintenance, especially when each spacecraft has its own orbit requirements. However, advances in automation and technology proposed by the Goddard Space Flight Center (GSFC) allow more of the burden in maneuver planning and execution to be placed onboard the spacecraft, mitigating some of the associated operational concerns. The purpose of this paper is to present GSFC’s Guidance, Navigation, and Control Center’s (G...

STEREO superior solar conjunction mission phase

With its long duration and high gain antenna (HGA) feed thermal constraint; the NASA Solar-TErestrial RElations Observatory (STEREO) solar conjunction mission phase is quite unique to deep space operations. Originally designed for a two year heliocentric orbit mission to primarily study coronal mass ejection propagation, after 8 years of continuous science data collection, the twin STEREO observatories entered the solar conjunction mission phase, for which they were not designed. Nine months before entering conjunction, an unforeseen thermal constraint threatened to stop daily communications and science data collection for 15 months. With a 3.5 month long communication blackout from the superior solar conjunction, without ground commands, each observatory will reset every 3 days, resulting in 35 system resets at an Earth range of 2 AU. As the observatories will be conjoined for the first time in 8 years, a unique opportunity for calibrating the same instruments on identical spacecraft will occur. As each observatory has lost redundancy, and with only a limited fidelity hardware simulator, how can the new observatory configuration be adequately and safely tested on each spacecraft? Without ground commands, how would a 3-axis stabilized spacecraft safely manage the ever accumulating system momentum without using propellant for thrusters? Could science data still be collected for the duration of the solar conjunction mission phase? Would the observatories survive? In its second extended mission, operational resources were limited at best. This paper discusses the solutions to the STEREO superior solar conjunction operational challenges, science data impact, testing, mission operations, results, and lessons learned while implementing.