Passive HF radar sounding: Work with DARPA
NASA image of total electron content in the ionosphere
As part of a pair of DARPA awards (DARPA-PS-26-03, DAIRS and FROSTY programs), I am currently working with a multi-institutional team of PIs on projects seeking to leverage passive target imaging in HF radar environments. In particular, these programs seek to extend passive interferometric methods and their array-based counterparts to imaging both environmental (e.g., ionospheric layering, ocean state) and distributed (e.g. over the horizon and local airborne and seaborne) targets.
Currently, we are in the second iteration of this program (FROSTY) and are developing a variety of neat algorithms and approaches to solve this problem.
Unlike ambient seismic environments, HF environments lack the typical persistently (somewhat) isotropic source distributions that have revolutionized the use of noise and passive methods in general and instead feature multifold natural and anthropogenic sources with power profiles that vary by orders of magnitude. As such, simply bulk interferometric methods cannot directly be interpreted in the context of Green's functions, and this is further frustrated by the inclusion of moving targets.
In the gray zone between isotropic source distributions and single strong illuminators, interferometric methods often fail, but arrays can be used to systematically integrate coherently returned scattering through double beamforming approaches if care is taken to account for the different properties of the source environment. The current version of this program (FROSTY) is starting up as of Summer 2026, and I am always looking for interested math-oriented students to get their hands dirty.
Range-Doppler video created via passive interferometric methods for a linear array of HF receivers on the East coast of the US. Various moving targets of different character can be identified.
Example beam contributions for two distinct scatterers embedded in a synthetic medium surrounded by a combined noise and CODAR waveform excitation. In practice, the highly anisotropic nature of the source environment necessitates careful; segmentation of the contributions of each source to the scattering problem.