The United States military intends to launch satellites to learn more about how radio transmissions react in the upper atmosphere.
The Defense Advanced Research Projects Agency (DARPA) has launched Ouija, a new program that will track high-frequency radio waves in the ionosphere using sensors on "low-orbiting satellites." DARPA has issued a request for proposals for one element of the project and will issue a second solicitation at a later date.
The ionosphere is well known for being the region of the atmosphere where auroras can be seen depending on solar activity and the Earth's magnetic field. The Ouija mission will focus on an area of the ionosphere between 125 and 185 miles (300 and 400 kilometers) in altitude, much beyond the International Space Station's orbit, which circles our planet at an average height of 250 miles (400 km).
DARPA officials said in an April 22 statement that understanding how radio waves operate in this environment will be critical in assisting future warfighters. Because of the high density of charged particles (mostly electrons) that can modify the route of radio transmissions, signal propagation in the ionosphere is notoriously unpredictable.
According to Britannica, the term "ionosphere" was first used in the 1920s, and by the 1950s, organizations like the Institute of Radio Engineers had recognized the atmospheric zone's effect on radio waves. Satellite studies of this region began soon after the first satellite launch in 1957 and continue to this day.
"Much of the early research on the ionosphere was carried out by radio engineers and was stimulated by the need to define the factors influencing long-range radio communication," Britannica says.
Solar activity and the 11-year solar cycle can change the density of the ionosphere, making it even more difficult to predict how radio communications are affected in this region. The sun, which is scheduled to reach the end of its cycle in 2025, has already emitted a number of X-class or massive flares in recent weeks.
DARPA intends to contribute to ionospheric research in two areas. The first method involves launching multiple small satellites into orbit and using the Ouija scientific payload to "measure electron density by both direct sampling, and indirectly via radio occultation using navigation satellites." This research field is currently accepting applications.
The necessity for a high-frequency (HF) mission payload with a wide frequency range and minimal noise was recognized by DARPA. However, because HF antennas optimized for such operations tend to be lengthy, designing its antenna will be problematic. Longer antennas are more difficult to install in space and can cause drag in the atmosphere.
The second technical area is an electron density model that will be validated using on-orbit data in the future. DARPA stated that it would share more information about their plans in this area at a later time.