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GPS Awareness Enabling Algorithms for Theater and Space Environment


OBJECTIVE: Algorithms that can enable operational GPS receivers devoted to position, navigation or timing, (PNT) to also usefully monitor the space environment. DESCRIPTION: GPS measurements of ionospheric total electron content (TEC) are the primary source of input data supporting the specification and forecasting of ionospheric effects and impacts on DoD radio-based systems, including communication, navigation, surveillance and geolocation. All two-frequency GPS receivers measure TEC in order to remove the position error effect it causes. DoD Space Environment missions most lack TEC where it is most needed: in operational warfighting theaters, and over oceans. Yet DoD vehicles, equipped with two-frequency GPS, are often common in these very areas. If operational GPS receivers in DoD ships, aircraft, and ground vehicles were enabled to measure and report useful TEC observations such capability would dramatically increase space environment awareness in these regions. (Note that"reporting", that is, output and communication of these TEC measurements, is outside of this topic.) Such enhancement in capability to model and forecast space environment effects would have high payoff for the classes of DoD radio-based systems mentioned above. New real-time algorithms are required that can operate within GPS PNT receivers and derive reasonably accurate measurements of ionospheric TEC, along with a valid metric of the uncertainty of this TEC data. This new uncertainty metric is vital since this enables modern space environment models to use the TEC data appropriately. Developing a robust and adaptable uncertainty determination process is a key objective of this effort, as this will enable high payoff by means of exploitation of large quantities of low-accuracy data. To be useful, candidate algorithms must be able to operate with only parameters available within DoD GPS user equipment (UE), and must have low impact on receiver-internal processing loading. In both the measurement and the uncertainty process, algorithms will also need to mitigate recognized GPS measurement issues such as receive antenna multipath and biases from both the receiver system and satellites. For the desired application, however, there are new challenges which include dynamic signal continuity, that is, bridging signal dropouts due to motion, obstructions, etc., and the requirement for occasional complete restart of processes. Algorithm bootstrap startup speed, and robustness against increasing levels of motion, dropouts, etc. will also be challenges. In addition, signal quality monitoring for such issues as noise, interference, and ionosphere scintillation will be important in setting uncertainty levels and determining measurement validity. PHASE I: Design and develop algorithms that can enable operational GPS receivers to also usefully monitor ionospheric TEC, and deliver results of a feasibility study for algorithm application to GPS receivers in general and GPS User Equipment (UE) in particular. PHASE II: Demonstrate the capability of candidate TEC and uncertainty - monitoring algorithms in various classes of GPS receivers, in particular GPS UE, by simulation studies and validated field measurements with varying platform dynamics and dropout characteristics. Deliver code and performance metrics for candidate algorithms. PHASE III: Enable DoD GPS receivers to monitor the ionospheric TEC will forecast theater space environment impact on DoD radio-based systems. New capability to measure TEC and its uncertainty for DoD GPS UE will help the civilian sector by providing improved GPS performance by reducing errors due to TEC. REFERENCES: 1. Anghel, Adela; Carrano, Charles; Komjathy, Attila; Astilean, Adina; and Letia, Tiberiu,"Kalman filter-based algorithms for monitoring the ionosphere and plasmasphere with GPS in near-real time,"Journal of Atmospheric and Solar-Terrestrial Physics, Volume 71, Issue 1, January 2009, Pages 158-174, ISSN 1364-6826, DOI: 10.1016/j.jastp.2008.10.006. 2. Kintner, Paul M., Jr., Professor,"A Beginner"s Guide to Space Weather and GPS," 3. Jee, G., H.-B. Lee, Y. H. Kim, J.-K. Chung, and J. Cho (2010), Assessment of GPS global ionosphere maps (GIM) by comparison between CODE GIM and TOPEX/Jason TEC data: Ionospheric perspective, J. Geophys. Res., 115, A10319, doi:10.1029/2010JA015432.
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