Improved Ionosphere D-region Specification for High Frequency Propagation

SBIR topic DON27BZ02-NV016 (U.S. Department of Defense – Navy), Phase I and II. Open to U.S. small businesses. Opens: December 2, 2026. Closes: January 6, 2027. HF and VLF skywave propagation depend on knowledge of the altitudinal structure of the ionosphere for reliable system operation. The D-region ionosphere, a region typically between 60-90 km altitude above the Earth’s surface, is particularly challenging to predict with sufficient fidelity for skywave mission planning because the region is difficult to measure with in situ measurements. Remote sensing methods provide a limited dataset from which to develop detailed physical and assimilative models that can reliably produce the regional behavior on a day-to-day basis. Operational models like the D-Region Absorption Product (D-RAP) produce maps of outage areas as a function of radio frequency (RF) to inform HF system users of anticipated degradation or blackouts due to radiation increases. These predictions primarily address HF absorption due to solar X-ray flares at low and mid-latitudes and do not address the VLF propagation use case. While solar X-ray flares are important effects with Navy relevance, the effects of flares typically only last for a few hours and occur 10-20 times a year. For sufficient use in HF absorption and VLF propagation applications, a D-region ionosphere is required to capture seasonal, solar cycle, and day-to-day variability at all latitudes in addition to solar flare phenomenology. The intended capability is improved D-region specification and forecast in the 60-70 km and 80-90 km altitude regions for VLF propagation and HF absorption, particularly day-to-day variability and in the high latitude regions at night. Climatological models like the International Reference Ionosphere (IRI) produce an altitude profile at D-region altitudes. The IRI model is based on about a hundred historical rocket profiles, so it captures some of the basic day/night, latitudinal, and seasonal…