STR to develop new modeling and simulation for HF radio waves to help improve communications and sensing

July 6, 2023
HF radio is notorious for RF interference; the ionosphere is changes constantly, and can influence HF radio and radar signals from minute-to-minute.

ARLINGTON, Va. – U.S. military researchers needed a company to develop new ways to model the ionosphere in real time to help predict the propagation of high-frequency (HF) radio waves for improved communications and sensing. They found their solution from Systems & Technology Research LLC (STR) in Woburn, Mass.

Officials of the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., announced a $6.1 million contract to STR last week for the Ouija TA-2 project to develop real-time modeling and simulation for assimilative ionospheric and HF radio propagation.

The ionosphere is the ionized part of the upper atmosphere of Earth, from about 30 miles to 600 miles above sea level, which is ionized by solar radiation. It influences radio propagation to distant places on Earth by reflecting HF signals.

HF radio uses signals in relatively long wavelengths of between 10 and 100 meters. HF radio bands lie between the commercial AM and FM broadcast bands, and operate from 3 to 30 MHz. HF radio waves are notable for their ability to propagate signals for long distances by bouncing signals off the ionosphere.

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HF radio also is notorious for static from thunderstorms and other radio frequency interference. The ionosphere is changing constantly, and can influence HF radio signals from minute-to-minute, and from season-to-season.

One goal for STR in the Ouija TA-2 project is to develop near-real-time assimilative ionospheric computer models that can mimic ionospheric disturbances at scales of 100 kilometers and below.

These models must assimilate ionospheric measurements taken with the scientific instrumentation packages to be flown on the Ouija TA-1 CubeSats in very low-Earth orbit (VLEO), in addition to standard vertical and oblique sounder data. The scientific instrumentation on the Ouija spacecraft will include Langmuir probes and similar devices to measure electron density and other quantities of interest.

The objective is to predict the characteristics of the ionosphere at unprecedented resolution and fidelity in near-real-time, DARPA researchers say.

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The second goal for STR is to develop high-fidelity HF radio propagation models to help predict ground-to-VLEO radio wave propagation, which will be validated using on-orbit measurements taken from the Ouija spacecraft HF payload, which will receive test signals from cooperative terrestrial transmitters.

Researchers expect industry to develop HF radio propagation models by linking ionospheric models using on-orbit measurements to an HF propagation prediction model that will provide high-fidelity predictions of ground-to-space HF radio propagation.

The scientific payload will measure ionospheric characteristics in near-real-time using Langmuir probes, magnetometers, and global navigation satellites system (GNSS) devices to estimate electron density profiles using radio occultation. The HF payload will consist of an HF antenna and receiver to receive test signals from terrestrial transmitters.

One objective is the ability to predict the ionosphere at using high-fidelity models that update at a rate of 10 seconds per update, rather than minutes per update, which should be sufficient to predict HF radio propagation for ground-to-space HF links.

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The project's nine-month first phase will begin modeling before data from the Ouija VLEO satellites is available. Instead, it will use sounder measurements from a terrestrial HF radio transmitter to low-Earth orbit (LEO) satellites equipped with an HF receive payload.

The one-year second phase will assimilate data from one VLEO satellite and produce electron density distributions. The 16-month third phase will assimilate on-orbit data from six Ouija satellites. DARPA researchers say they expect to award several contracts.

On this contract, STR should be finished with the first phase by early next year. For more information contact STR online at, or DARPA at

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