| dc.description.abstract | To improve ice sheet models used to estimate future sea level rise, major contributions must be made to ice thickness datasets. To date, ice thickness measurements have primarily been made by using depth sounding radars on airborne platforms, such as those fielded by the Center for Remote Sensing of Ice Sheets (CReSIS) group at the University of Kansas (KU). Measurements collected over the last three decades by CReSIS and other groups are used to produce bedrock elevation and ice thickness maps of the Antarctic and Greenland ice sheets. However, there are still large geographic areas that have never been sounded. In addition, the datasets used to generate the full bedrock Digital Elevation Model (DEM) are collected using disparate systems over large temporal baselines, which can result in interpolation errors. Proposed in this document is an uncontrolled orbital radar sounding constellation of nanosatellites designed to fill coverage gaps of the major ice sheets. Similar concepts have been proposed in the literature, but this work contributes a unique vehicle analysis for this uncontrolled orbital constellation mission. Nanosatellites are of interest due to the benefits of limited cost and development time. Given the small size of the vehicle, in order to achieve the necessary cross-track aperture size, a constellation is required. Using multiple antenna channels from individual satellites limits design complexity and implementation time for a single vehicle, while maintaining the benefits of multi-channel radar data collection. A primary design driver for this report is the use of current technologies to allow for rapid implementation after publication; thus only an uncontrolled constellation (the nanosatellites include no propulsion or station-keeping technology) is considered. The unique challenge of this design is determining the likelihood of success for an uncontrolled constellation, especially as it relates to the accuracy of launch vehicle jettison. Only the uncontrolled constellation is investigated but all of the concepts discussed in this document are relevant to a controlled mission concept, but the complexity of propulsion systems can greatly increase development time and cost; therefore the simplest solution must be analyzed first. This document presents the mission design in the context of other fielded airborne systems and uses the available literature to define mission requirements. These and other mission requirements are set to define an initial system architecture. This architecture includes radar operating parameters and analysis as well as defining satellite operations on-orbit and the accompanying satellite hardware. Satellites designed for this work utilized only Commercial Off The Shelf (COTS) hardware as the standard practices of commercial products will limit complexity and development time. Analysis of the power and data cycles of a single satellite showed that reasonable margin is present such that the battery does not fully deplete, the data does not accumulate too quickly, and nominal mission operations can be conducted for more than 40 orbits uninterrupted. Once defined, the architecture is then investigated from an orbital mechanics perspective. To do so required the development and validation of an orbit propagator. In the validation process, it was found that it is difficult to find propagation datasets to verify against; thus this document also provides a validation dataset for future assessments of this mission. By propagating the orbits of eight uncontrolled nanosatellites, considering perturbations from J2 and drag, the viability of this mission is fully evaluated. This evaluation is the major contribution of this work. Evaluating the constellation requires first defining the ideal orbital elements for each constellation constituent such that ideal spacing, as defined by radar array performance, is maintained over the targets and chance of collision is minimized during close approaches. The ideal configuration is evaluated as a baseline to determine the approximate data coverage as well as the Useful Mission Lifetime (UML). However, errors in orbital insertion play a major role in the ability to conduct this mission for a significant lifetime. Using a Monte Carlo analysis, the effects of orbital insertion errors is assesed to determine mission lifespan. The culmination of this evaluation is the first in-depth look at the orbital mechanics for uncontrolled multi-satellite proximity constellation missions. It is shown in this report that an uncontrolled constellation is not well suited for this mission type. An ideal configuration of nanosatellites is found that utilizes offsets in Inclination, Right Ascension of the Ascending Node, and True Anomaly. This ideal configuration could collect over 100,000 line-km of data within 20 orbits. However, after 20 orbits it is expected that satellites in the constellation would collide leading to mission end and concerns of subsequent space debris. Similarly, it is found from the Monte Carlo jettison analysis that sub-millimeter accuracy is required in position and velocity for the launch vehicle to place satellites into the ideal configuration without excessive risk of collision. Considering the time to confirm with each satellite after jettison and the risk of collision, it is suggested that satellites should not be jettisoned directly into the ideal configuration, even for a controlled mission concept, should it be pursued. Future work beyond this report should focus on confirming that the controlled concept is capable of entering and maintaining the ideal configuration after jettison for a significant period of time. The Monte Carlo error analysis indicates that future concepts for this mission will not only require a propulsion systems, but will also require a system with the ability to accurately measure relative satellite spacing as small deviations from the ideal configuration will greatly increase collision risk. | |
