ASEN 6519:

Graduate Seminar for Astrodynamics and Satellite Navigation Focus Area

Fall 2007 Seminar Schedule

Date	Speaker	Affiliation	Title/Topic
9/4/07	Chris Lane	CU	Relative Navigation in High Earth Orbit
9/11/07	Jeff Parker	CU	Low-Energy Ballistic Lunar Transfers Conventional lunar missions require 3-5 days and a great deal of energy to transfer cargo to the Moon. The research being presented discusses an alternative, low-energy method to transfer material to the Moon. A Ballistic Lunar Transfer (BLT) may be used to send 30 - 40% more cargo to the Moon given the same launch vehicle as a conventional transfer. The main downside is that BLTs require at least 90 days to reach the Moon; nevertheless, they are particularly useful to transfer satellites, cargo, and even space stations to the Moon.
9/18/07	Kathryn Hamera	CU	An Evolvable Lunar Communication and Navigation Concept The Global Exploration Strategy and proposed lunar architecture revealed by NASA present a roadmap for future U.S. missions. The exploration plan begins with robotic precursor missions, followed by human sorties, eventually progressing to a permanent base on the lunar surface. NASA?s Lunar Architecture team concluded that this permanent base would be located near the lunar South Pole. The location of an outpost at the Lunar South prevents direct communication links with the Earth. A Lunar relay element will be necessary to provide critical communication and navigation support for the upcoming missions. This paper presents a highly evolvable low-cost lunar relay constellation concept using small satellites in Earth-Moon Halo orbits. The initial constellation is designed to provide coverage of the Lunar South Pole in support of the inaugural robotic missions and is easily expanded to provide global coverage. Two satellites in an L2 Halo orbit provide continuous South Pole coverage for the initial constellation. A final constellation providing nearly continuous global coverage can be achieved by the addition of two spacecraft in a Halo orbit about L1. A basic set of requirements and desired capabilities for the constellation is developed, based on NASA reports and Constellation C3I compatibility. A preliminary spacecraft bus and subsystems design is presented along with expected performance.
9/25/07	Scott Mitchell	Ball Aerospace	Multiple asteroid rendezvous missions An approach to selecting a relatively low delta-v mission that will rendezvous with three asteroids is described. This method is used to select two multi-asteroid rendezvous missions. The first visits one asteroid from a list of high priority asteroids (only one is possible with reasonable delta-v capability), then visits two other nearby asteroids. This design considers trajectories which result in a very long mission duration (~15 years). The second mission considers only objects which are very near to Earth and uses trajectories which result in a relatively short mission duration (~3 years).
10/2/07	Andria Bilich	NOS/NOAA	GPS carrier phase multipath and signal-to-noise ratio (SNR) measurements Multipath, where a signal arrives at the receiving antenna by more than one path, is a significant and largely unmodeled source of GPS range error and therefore positioning error. The signal-to-noise ratio (SNR) is a quantity routinely reported by GPS receivers that shares a direct link with multipath errors in carrier phase measurements. This seminar reviews the theory behind the SNR-phase multipath relationship, and through a few examples demonstrates how reliable and accurate SNR data contains a wealth of temporal and spatial information about a station?s multipath environment and multipath phase errors. Examples of SNR-multipath applications include maps of SNR frequency and amplitude used to characterize the multipath environment, modeling SNR oscillations to yield carrier phase multipath corrections, and using SNR amplitude information from ground-reflected signals to monitor changes in soil moisture.
10/9/07	Harvey Mamich	Lockheed Martin Space Systems	Overview of On-board Navigation for NASA's Orion Crew Exploration Vehicle The Orion Crew Exploration Vehicle (CEV) will replace the Space Shuttle and serve as the Apollo-style, next-generation spaceship to carry humans to the International Space Station, the Earth?s Moon and beyond. Pad Abort 1 (PA-1) will initiate a series of test flights scheduled to begin in September 2008. The first orbital flights of the Orion Crew Vehicle are set for 2011 with the first crewed Orion flights planned for 2013. This presentation will discuss the collaborative effort between LMSSC and NASA ? Johnson Space Center to assess the Rendezvous, Proximity Operations and Docking (RPOD) approach for the CEV mission to the International Space Station: the design philosophy, challenges, reference trajectories, sensors and performance analyses. Finally, an introduction to the cis-Lunar navigation approach will be presented as time permits.
10/16/07	Deepti Sharma	Ball Aerospace	Use of Ballutes for Performing Controlled Reentry The use of ballutes in place of propulsion to perform deorbit and controlled deorbit appears to be feasible. The ballute sizes required to deorbit various spacecraft sizes in various periods were calculated. Controlled deorbit using only a ballute was investigated using the Hubble Space Telescope as a case study. A 25 m diameter ballute will cause HST reentry within 3.5 years, without using propulsion. The landing accuracy was estimated to be approximately +/- 7 degrees along the orbit track, using realistic short-term atmospheric density variations. Although the orbit during which the spacecraft impacts may not pass over a remote part of the southern Pacific Ocean where controlled reentry impacts have traditionally occurred, it is likely that a suitably remote portion of some ocean, with this landing location uncertainty, can be found under any orbit groundtrack.
10/23/07	no seminar
10/30/07	Hanspeter Schaub	CU	Electrostatic Spacecraft Relative Motion Control Electrostatic (Coulomb) thrusting is a novel promising technology being considered to control the relative motion of spacecraft. The separation distances between craft considered range up to 100 meters. Such mission scenarios include wide field of view optical interferometery from GEO, flying small drone craft about a larger craft to perform inspection and surveillance missions, or forming large virtual structures. In 2000 a NASA NIAC study was performed Michigan Tech to investigate the natural charging that occurs with GEO satellites, as well as how well the SCATHA and ATS missions were able to actively control their charge. A surprising result of this work was that inter-spacecraft forces between two GEO satellites could achieve milli-Newtons levels with separation distances of dozens of meters. This presentation will provide an overview of the Coulomb thrusting research being conducted. After discussing the basic concept with its advantages and limitations, three specific Coulomb thrusting example applications are discussed. The Coulomb tether concept replaces the physical tether connecting two craft with the electrostatic force field. The virtual Coulomb structure research investigates developing a spacecraft concept which is composed of discretely distributed spacecraft components which are connected by the Coulomb force fields. All these solutions are static as seen by the rotating Hill frame. The final application discusses a spinning 3-craft formation which yields invariant shapes under the influence of electrostatic forces.
11/6/07	Eric Vinande	CU	A Software-Defined GPS and Galileo Receiver: Single-Frequency Approach This seminar presents a brief overview of an open-source, MATLAB software Global Positioning System (GPS) receiver. All functions from signal acquisition to position solution are described. The focus is then turned to developments at CU related to the software receiver. A Universal Serial Bus (USB) intermediate frequency (IF) data sampler module is described that researchers can use to collect data to support their research. Student projects that were the outcome of a graduate level global navigation satellite system (GNSS) receiver architecture course are also described. These projects included signal acquisition and tracking in an FPGA, multi-correlator signal tracking, L2C acquisition, and Galileo signal tracking. The validity of the existing software receiver as a base for future research was confirmed by the success of these projects. The results from Wide Area Augmentation System (WAAS) ranging, WAAS correction decoding, and navigation solution filtering are described all with the intent of improving positioning accuracy.
11/13/07	Bill Schreiner	UCAR	Precise Orbit Determination and Radio Occultation Retrieval Processing at the UCAR CDAAC: Overview and Results The Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) / Formosa Satellite 3 (FORMOSAT-3) is a new six-satellite radio occultation (RO) mission that was successfully launched in mid-April, 2006. The UCAR COSMIC Data Analysis and Archival Center (CDAAC) receives the COSMIC data in near real-time, and performs the functions of precise orbit determination, excess phase calibration, neutral atmospheric and ionospheric profile inversions, quality control checking, and dissemination of data products to weather centers within 3 hours of observation. The CDAAC also generates more precise post-processed data products for climate applications. This presentation will initially give an overview of the data processing details performed at the CDAAC related to Precise Orbit Determination (POD) of the COSMIC satellites, excess phase calibration, and radio occultation (RO) retrievals. Then, the presentation will give a summary assessment of COSMIC data quality.
11/20/08	Thanksgiving Break
11/27/07	David Wiese	CU	Alternative Mission Architectures for a Gravity Recovery Satellite Mission The Gravity Recovery and Climate Experiment (GRACE) mission has been providing unprecedented levels of accuracy in measuring the Earth's gravity field since its launch in 2002. GRACE employs a mission architecture consisting of a collinear satellite pair, with measurements of changes in distance between spacecraft used to infer variations in the Earth's gravity field. The measurements are constrained to the along-track direction due to the GRACE mission architecture, which contributes to longitudinal striping in the estimated gravity fields. This study examines four different mission architectures for a future gravity recovery satellite mission in hopes of improved mission performance: a two-satellite collinear pair, two two-satellite collinear pairs flying together, a two-satellite cartwheel formation, and a four-satellite cartwheel formation. Cartwheel orbits consist of a satellite formation performing relative elliptical motion about their center of mass as the formation orbits the Earth. Measurements of changes in distance between such spacecraft are, at times, directed towards the center of the Earth rather than along the orbital track. This geometric difference can be advantageous for estimation of the gravity field. The ability of each formation to recover the gravity field at both 250 km and 400 km nominal altitudes was compared using numerical simulations performed with JPL's GIPSY-OASIS software package. 60x60 gravity fields were estimated using 30 days of data. Data noise characteristics of an improved satellite-satellite ranging system were included, along with aliasing errors from Atmospheric and Ocean Dealiasing (AOD) models. Results showed the gravity field errors associated with the four-satellite cartwheel formation were approximately one order of magnitude lower than the collinear satellite pair when only data noise was considered. When aliasing was introduced, the gravity field errors for each formation were approximately the same. The cartwheel formations eliminated most of the longitudinal striping seen in the gravity field error. A covariance analysis showed the error spectrum of the cartwheel formations to be lower and more isotropic than that of the collinear formations.
12/4/07	Doug Engelhardt	DigitalGlobe	Astrodynamics of DigitalGlobe's Commercial Imaging Satellite Constellation DigitalGlobe's commercial remote sensing business is based on imagery from its QuickBird and WorldView satellites. The recent history of this nascent industry will be briefly reviewed. Aspects of the QuickBird and WorldView orbit design, determination, and prediction will be presented. Aspects include design of the orbits to optimize target revisit frequency, accuracy of GPS-based reduced dynamic orbit determination, and satellite lifetime prediction in light of the uncertainty in the upcoming solar cycle.