Collaborative Research: NSWP--Scintillation-Scale Ionospheric Imaging using GPS and other RF Data in Inverse Diffraction Algorithms

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This project will infer fine-scale electron density variations of the equatorial ionosphere by developing and applying estimation algorithms to data recorded from scintillating GPS radio signals. The resulting electron density profiles will provide images of the bubble structures that are thought to cause the phenomena of radio frequency scintillations and equatorial spread-F. The project will include algorithm development, data collection campaigns, and data analysis. The algorithms to be developed will perform diffraction tomography calculations; the input data are received Radio Frequency (RF) signal amplitudes and phases from an array of receivers that are experiencing scintillations. The algorithms will estimate electron density profiles by developing and applying model estimation/inversion techniques to models such as a phase screen with Fresnel forward scattering. The estimation techniques will exploit the availability of dual-frequency GPS scintillation data that can be monitored because of new civilian GPS L2 signals now becoming available. They will also exploit the ability of GPS to make group-delay measurements; use of these data will follow development of a model predicting how group delay is affected by a disturbed ionosphere. The algorithms will be applied to data from two types of equatorial scintillation collection campaigns. One type of campaign will collect data using small arrays of wide-band digital storage receivers and MATLAB post-processing to acquire and track scintillating signals. The second type will use a medium sized array of real-time GPS software radio receivers. Some of the effort will go toward completing development of the latter type of receiver. Two distinctive features of this project are its use of very robust receiver technology, which will enable collection of very accurate RF amplitude and phase data for very strong scintillations and its development of generalized estimation/inversion algorithms. These algorithms that go beyond the capabilities of currently existing or proposed algorithms in their use of multi-frequency data, in their problem formulations, and in their algorithmic implementation based on optimization or fixed-point methods. The project focuses on very strong scintillations since these are the most important from an operational standpoint since they have the biggest impact on communications and navigation signals. The project's broader impacts include the development of instruments that can be used in education as well as in research and the improved communication and navigation system reliability that will result from an improved understanding of scintillations.