Predicted Performance of an X-Ray Navigation System for Future Deep Space and Lunar Missions

In November 2017, the NASA Goddard Space Flight Center (GSFC) Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) experiment successfully demonstrated the feasibility of X-ray Pulsar Navigation (XNAV) as part of the Neutron Star Interior Composition Explorer (NICER) mission, which is an X-ray Astrophysics Mission of Opportunity currently operating onboard the International Space Station (ISS). XNAV provides a GPS-like absolute autonomous navigation and timing capability available anywhere in the Solar System and beyond. While the most significant benefits of XNAV are expected to come in support of very deep-space missions, the absolute autonomous navigation and timing capability also has utility for inner Solar System missions where increased autonomy or backup navigation and timing services are required, e.g., address loss of communication scenarios.The NASA commitment to develop a Gateway to support exploration of the Moon and eventually Mars, as well as current and future robotic missions such as James Webb Space Telescope (JWST), New Horizons, and much more, certainly will tax the existing ground based infrastructure in terms of availability. There- fore, an extended look at the feasibility and potential performance of XNAV for comparable missions is warranted. In this paper, we briefly review the XNAV concept and present case studies of its utility and performance for a Gateway orbit, Sun-Earth libration orbit, and a deep space transit trajectory

Effects of Uncertainties in the Atmospheric Density on the Probability of Collision Between Space Objects

The rapid increase of the number of objects in orbit around the Earth poses a serious threat to operational spacecraft and astronauts. In order to effectively avoid collisions, mission operators need to assess the risk of collision between the satellite and any other object whose orbit is likely to approach its trajectory. Several algorithms predict the probability of collision but have limitations that impair the accuracy of the prediction. An important limitation is that uncertainties in the atmospheric density are usually not taken into account in the propagation of the covariance matrix from current epoch to closest approach time. The atmosphere between 100 km and 700 km is strongly driven by solar and magnetospheric activity. Therefore, uncertainties in the drivers directly relate to uncertainties in the neutral density, hence in the drag acceleration. This results in important considerations for the prediction of Low Earth Orbits, especially for the determination of the probability of collision. This study shows how uncertainties in the atmospheric density can cause significant differences in the probability of collision and presents an algorithm that takes these uncertainties into account to more accurately assess the risk of collision. As an example, the effects of a geomagnetic storm on the probability of collision are illustrated.Plain Language SummarySpacecraft collision avoidance is particularly challenging at low altitudes (below  700 km). One of the main reasons is that, at these altitudes, satellite trajectories are strongly perturbed by atmospheric drag, a force particularly hard to model. The sources of errors mostly come from the complex coupling between the Sun and the Earth’s environment. This system drives the density of the Earth’s atmosphere on which the atmospheric drag directly depends. In other words, uncertainties in the atmospheric density result in large uncertainties in the satellite trajectories. The probability of collision, which is computed from the prediction of the satellite trajectories, thus cannot be predicted perfectly accurately. However, mission operators decide whether or not a collision avoidance maneuver has to be carried out based on the value of the probability of collision. Therefore, it is essential to characterize the level of uncertainty associated with the prediction of the probability of collision. The research presented here offers an approach to determine the uncertainty on the prediction of the probability of collision as a result of uncertainties in the atmospheric density. The ultimate goal is to assist mission operators in making the correct decision with regard to potential collision avoidance maneuvers.Key PointsUncertainties in the atmospheric density result in uncertainties in the probability of collisionProbability distribution functions of the probability of collision resulting from uncertainties in the atmospheric density are derivedMonte Carlo procedures are used to compute the probability of collisionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144643/1/swe20687_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144643/2/swe20687.pd

Predicted Performance of an X-Ray Navigation System for Future Deep Space and Lunar Missions

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Lunar Navigation Beacon Network Using Global Navigation Satellite System Receivers

With the increasing traffic in the lunar regime as part of NASA efforts to return humans to the moon. In order to support these missions, new capabilities are needed to support autonomous navigation and inter-asset communication. Additionally, with maturation and flight demonstration of increasingly capable small satellites, there is an opportunity to embed technology into a small spacecraft as part of companion missions. This paper addresses one such architecture, taking advantage of a lunar lander vehicle to host a companion spacecraft to build out lunar navigation and communication capability. The backbone of this spacecraft is the Navigator GPS receiver. This hardware has continually broken records on high altitude GPS coverage and has the potential to support autonomous navigation at lunar distances. This research proposes a large cubesat built around this technology and catching a ride to the moon via a lander mission. The concept of operations includes the spacecraft deploying prior to the lunar sphere of influence and maneuvering to enter into a lunar orbit. With the Navigator receiver, this spacecraft is capable of a large amount of autonomy, with a limited need for ground-based orbit determination. This spacecraft will fly alongside the lander, acting as a navigation reference during cruise, descent, and post-landing for mission validation. To assess this mission scenario, three aspects are covered in detail herein: the feasibility and mission requirements for entering into a lunar orbit given deployment along a lander surface-bound trajectory, the performance capability of the receiver along this transfer trajectory and in lunar orbit, and the ability to support navigation of the lander itself. These three areas are discussed in detail, providing results that support feasibility of the mission and determination of initial requirements