Summary
The DAIRS program will explore novel sensing and signal processing methods for potential application to maritime vehicle tracking and environmental characterization. While the techniques are general, the primary emphasis will be in the high frequency (HF) band, nominally 4 to 15 MHz.
Key applications in this frequency band are surface-wave over-the-horizon radar (SWOTHR) for air and maritime vehicle tracking, oceanographic SWOTHR, and sounding for ionospheric characterization. Developments in this program will be in three focus areas:
Passive noise radar
The geophysics, oceanographic, and astronomical sciences communities have developed sensing methods that use indigenous environmental noise sources for remote sensing. The geophysics community has successfully used acoustic noise from wind and microtremors, along with geophone arrays to characterize ice structure in Antarctica. Similar examples can be found in oceanography and radio astronomy. While current passive radar methods are primarily based on estimating or knowing the temporal properties of the waveforms, this program will build on the body of work covering passive remote sensing to investigate and develop techniques for passive noise radar, without the need to know or estimate the temporal properties of the received waveforms.
Clutter mitigation
With current surface wave radar technology, there is a sensitivity versus spread-Doppler clutter trade that effectively limits the reliable maximum vehicle tracking range to about 100km. Long-range operation necessitates the use of operating frequencies below about 6 MHz to reduce losses in radio wave propagation along the sea surface. At these lower frequencies, unmitigated clutter from ionospheric backscatter and turbulence can seriously impair radar performance. The DAIRS program will aim to develop methods that are based on the physics of surface wave and spread clutter propagation to mitigate this unwanted spread-Doppler clutter.
Classification
Narrow bandwidth radars, particularly those operating in the HF band, are typically not used to classify the maritime ship traffic that is observed. The long wavelengths used for HF radar, particularly at the lower frequencies that support long range propagation, can be of vehicle scale, which has historically prohibited the identification of platform features that are typically used for classification purposes in microwave radars.
The HF surface wave radars band frequency allocations provided by the International Telecommunication Union (ITU) and the United States (U.S.) National Telecommunications and Information Administration (NTIA) are discontinuous and do not have what would be considered a single “wide-band” allocation.
This program will explore the use of longer observation periods along with multi-frequency and multi-angle observation processing to classify maritime vehicles. The intent is to perform a “first principles” look at classification for low frequency and relatively narrow band radar sensors.