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Characterizing the Impact of Ionospheric Wave Structures on Coordinate Registration


OBJECTIVE: To enable characterization of the impact of ionospheric wave structures on coordinate-registration for over the horizon radar and other applications. DESCRIPTION: Due to its long range ability, wide area of coverage per installation, and low cost per square km of coverage, over the horizon radar (OTHR) has the potential to fill surveillance gaps in North American defense and security. In particular the vast air/oceanic expanse represented by the Atlantic/Pacific/Caribbean basins require affordable surveillance options. At the operational heart of an OTHR lies the transmission of a beam of radio waves into the sky where it reflects off the ionosphere and out to locations beyond the line-of-site horizon (2000 - 5000 km). Once there it can scatter off targets of interest and then takes another sky route to one or more detection sites. Knowledge of the ionosphere is critical to successful operation of any OTHR device. The optimal frequency for transmission is set by ionospheric conditions, and such knowledge is also critical to coordinate registration (CR). CR is the process by which the information received is converted into an estimate of the location of a target. In general, knowledge of the ionosphere, along with a measure of the time of flight of the radar beam, allows determination of the direction and distance to the target. Current approaches to CR range from a"mirror in the sky"paradigm, where the reflection height is assumed constant and the ionosphere acts like a horizontal mirror at that altitude, to explicit ray tracing through a model ionosphere which may be adjusted with observational data. The more advanced approach allows for inclusion of such features as horizontal gradients and changing reflection heights as a function of frequency, azimuth, and zenith. Obviously such an improved approach requires improved knowledge of the ionosphere. One relatively new feature of our understanding of the ionosphere is the ubiquitous presence of migrating wave structures, sometimes referred to as traveling ionospheric disturbances (TIDs). These are typically understood as gravity waves (similar to ocean waves) and have been observed at a variety of wavelengths and amplitudes. Such structures are not resolved by the ionospheric modeling described above and it appears that the error ellipse associated with CR is often set by such waves in the ionosphere. Initially the potential impact of TIDs in various OTHR relevant settings must be understood. This should be based on a survey of state of the art TID observations and generation mechanisms and will require state of the art modeling to understand how and where TIDs are impacting CR. The simulations must be detailed enough to distinguish between the impacts of different types of TIDs on different types of targets. Vertical as well as horizontal propagation should be investigated. Once the magnitude of the impact in various situations is understood, measurements capable of resolving local TID structures will be required to be incorporated into the simulations and compared to actual operational systems. The fielded system should represent an innovative solution to performing routine measurements of wave structures in the bottom side ionosphere. It would be very useful for the system to be scalable in terms of area of coverage, thus low cost per area is also desirable. The final integrated hardware/software system should be able to inform the cost and benefit of fielding TID resolving hardware as part of arbitrary systems requiring CR. PHASE I: Produce a tool capable of simulating 3D HF propagation through a flexible ionospheric environment and providing output as used/produced by an OTHR system, e.g. vertical and oblique incidence ionograms, Doppler space, group path and bearing to a target. Simulate the full system, e.g. include the ability to invert specified measurements to retrieve TID characteristics in the simulated ionosphere. PHASE II: Field and operate a system capable of resolving TID structures at the range of scales relevant to CR. Couple the measurements from that system to the simulator produced in Phase I through an assimilative ionospheric model also capable of ingesting data from other sources. Validate the model with ground truth, e.g. predict sensitivity of retrieved fields to sampling rate or frequency stability. Characterize the effects on simulated OTHR retrievals. Compare to data from an operational OTHR system. PHASE III: Provide a tool to OTHR and CR users capable of including the effects of TIDs in their operations. Improved specification of HF radio circuit paths provides insight into HF communications systems design/performance, and signal fade/loss margins. Move R & D level understanding of TIDs forward. REFERENCES: 1. Cannon, P. S.,"Mitigation and exploitation of the ionosphere: A military perspective,"Radio Science, 44, RS0A20, doi:10.1029/2008RS004021, 2009. 2. Headrick, J. M., and Anderson, S. J., RADAR HANDBOOK, Third Edition, Chapter 20, McGraw-Hill Companies, United States of America, 2008. 3. Nickisch, L. J., Hausman, M. A., and Fridman, S. V.,"Range rate Doppler correlation for HF propagation in traveling ionospheric disturbance environments,"Radio Sci., 41, RS5S39, doi:10.1029/2005RS003358, 2006. 4. Vadas, S. L., and Crowley, G.,"Sources of the traveling ionospheric disturbances observed by the ionospheric TIDDBIT sounder near Wallops Island on 30 October 2007,"J. Geophys. Res., 115, A07324, doi:10.1029/2009JA015053, 2010. 5. Zhou, C., Zhao, Z., Yang G., Chen, G., Hu, Y., and Zhang, Y.,"Evidence of low-latitude daytime large-scale traveling ionospheric disturbances observed by high-frequency multistatic backscatter sounding system during a geomagnetically quiet period", J. Geophys. Res., 117, A06302, doi:10.1029/2012JA017605, 2012. 6. Chum, J., Athieno, R., Bae, J., Bureov, D., Hruka, F., Latovicka, J., McKinnell, L. A., and indelrov, T.,"Statistical investigation of horizontal propagation of gravity waves in the ionosphere over Europe and South Africa", J. Geophys. Res., 117, A03312, doi:10.1029/2011JA017161, 2012.
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