Suneel I. Sheikh

SPACECRAFT NAVIGATION USING X-RAY PULSARS

The feasibility of determining spacecraft time and position using x-ray pulsars is explored. Pulsars are rapidly rotating neutron stars that generate pulsed electromagnetic radiation. A detailed analysis of eight x-ray pulsars is presented to quantify expected spacecraft position accuracy based on described pulsar properties, detector parameters, and pulsar observation times. In addition, a time transformation equation is developed to provide comparisons of measured and predicted pulse time of arrival for accurate time and position determination. This model is used in a new pulsar navigation approach that provides corrections to estimated spacecraft position. This approach is evaluated using recorded flight data obtained from the unconventional stellar aspect x-ray timing experiment. Results from these data provide first demonstration of position determination using the Crab pulsar.

Noise analysis for X-ray navigation systems

Much as the Global Positioning System has ushered in an era of autonomous navigation on a global scale, X-ray navigation (XNAV) offers the possibility of autonomous navigation anywhere in the solar system. X-ray astronomers have identified a number of X-ray pulsars whose pulsed emissions have stabilities comparable to atomic clocks. X-ray navigation uses phase measurements from these sources to establish autonomously the position of the detector, and thus the spacecraft, relative to the solar system barycenter. This paper describes the development of a general noise model for X-ray navigation instruments. Key noise terms are identified and simple analytic expressions provided for each. This noise model is used to predict the performance of a typical XNAV system that could be used as the primary navigation resource on missions, including those beyond the orbit of Jupiter.

Spacecraft Navigation Using Celestial Gamma-Ray Sources

As the future progresses for space exploration endeavors, spacecraft that are capable of autonomously determining their position and velocity will provide clear navigation advances to mission operations. Thus, new techniques for determining spacecraft navigation solutions using celestial gamma-ray sources have been developed. Most of these sources offer detectable, bright, high-energy events that provide well-defined characteristics conducive to accurate time alignment among spatially separated spacecraft. Using assemblages of photons from distant gamma-ray bursts, relative range between two spacecraft can be accurately computed along the direction to each burst’s source based upon the difference in arrival time of the burst emission at each spacecraft’s location. Correlation methods used to time-align the high-energy burst profiles are provided. A simulation of the newly devised navigation algorithms has been developed to assess the system’s potential performance. Using predicted observation capabilities...

Validation of Pulsar Phase Tracking for Spacecraft Navigation

Previous theoretical work has shown that pulsars have fascinating potential as aids for autonomous, deep space navigation. In this paper, phase tracking is investigated as a technique to measure the distance traveled by a spacecraft over an observation interval. Phase tracking was experimentally validated using photons emitted by a modulated x-ray source and detected by a silicon-drift detector using the NASA Goddard X-Ray Navigation Laboratory Testbed. Models of the Crab pulsar and PSR B1821-24 were used. Three simulated, one-dimensional trajectories were considered: one stationary, one with constant velocity, and one with constant acceleration. For constant signal frequency, a maximum-likelihood phase estimator was used. Otherwise, the observation window was broken into smaller blocks over which maximum-likelihood phase estimates were looped through a digital phase-locked loop to reduce errors and estimate the Doppler frequency. The maximum-likelihood phase estimator tracked the initial phase within 0.1...

The use of small x-ray detectors for deep space relative navigation

Currently, there is considerable interest in developing technologies that will allow the use of high-energy photon measurements from celestial X-ray sources for deep space relative navigation. The impetus for this is to reduce operational costs in the number of envisioned space missions that will require spacecraft to have autonomous, or semiautonomous, navigation capabilities. For missions close to Earth, Global Navigation Satellite Systems (GNSS), such as the U.S. Global Positioning System (GPS), are readily available for use and provide high accuracy navigation solutions that can be used for autonomous vehicle operation. However, for missions far from Earth, currently only a few navigation options exist and most do not allow autonomous operation. While the radio telemetry based solutions with proven high performance such as NASA’s Deep Space Network (DSN) can be used for these class of missions, latencies associated with servicing a fleet of vehicles, such as a constellation of communication or science observation spacecraft, may not be compatible with autonomous operations which require timely updates of navigation solutions. Thus, new alternative solutions are sought with DSN-like accuracy. Because of their highly predictable pulsations, pulsars emitting X-radiation are ideal candidates for this task. These stars are ubiquitous celestial sources that can be used to provide time, attitude, range, and range-rate measurements — key parameters for navigation. Laboratory modeling of pulsar signals and operational aspects such as identifying pulsar-spacecraft geometry and performing cooperative observations with data communication are addressed in this paper. Algorithms and simulation tools that will enable designing and analyzing X-ray navigation concepts for a cis-lunar operational scenario are presented. In this situation, a space vehicle with a large-sized X-ray detector will work cooperatively with a number of smaller vehicles with smaller-sized detectors to generate a relative navigation solution between a reference and partner vehicle. The development of a compact X-ray detector system is pivotal to the eventual deployment of such navigation systems. Therefore, efforts to design a smallpackaged X-ray detector system along with the hardware, software and algorithm infrastructure required for testing and validating the system’s performance are described in this paper.