Ionosphere Investigations – DARPA’s Ouija Programme Gets Underway

The Ionosphere
The ionosphere is the essential ingredient for skywave high frequency communications. DARPA’s Ouija programme will use satellites to help understand the behaviour of this atmospheric layer.

The US’ Defence Advanced Research Projects Agency’s Ouija programme will delve into the mysterious world of ionospheric high frequency communications.

Skywave HF (three to 30 megahertz) transmissions use the ionosphere to avoid the curvature of the planet. This layer of the atmosphere is between 26 nautical miles/nm (48 kilometres/km) and 521nm (965km) above Earth. HF waves cannot penetrate the ionosphere meaning they are reflected to Earth. If HF transmissions are projected at an angle towards the ionosphere, they will bounce off at an angle. This mimics how a snooker ball bounces off the cushion of the table. This attribute let HF traffic traverse intercontinental distances. However, the ionosphere is a capricious place. Space weather like the solar wind, the stream of charged particles released by the sun’s corona, greatly affects levels of ionisation. This impacts how efficiently HF communications are performed.

The Defence Advanced Research Projects Agency (DARPA) has launched a programme called Ouija to better understand the ionosphere. The programme was revealed in a press release published by DARPA in April. The document said that Ouija will use Low Earth Orbit (LEO) satellites to measure the ionosphere’s behaviour. LEO satellites orbit at below 1,079nm (2,000km) above Earth. These satellites will specifically take measurements from altitudes of between 108nm (200km) and 162nm (300km). Ionospheric electron density reaches its maximum at these altitudes, making them particularly suitable for HF communications. DARPA released its first solicitation for these satellites on 21st April.

Pathfinder Satellites

The Ouija satellites will “provide high fidelity in-situ ionospheric measurements that enable accurate predictions of regional ionosphere conditions,” according to the programme solicitation. This will allow the development of propagation models with “unprecedented resolution in space and time.” These models should help further refine HF use by improving the prediction of how the ionosphere may behave. Ouija is interesting as measuring ionospheric activity has traditionally depended on ground-based infrastructure. Taking ionospheric measurements from space is a new approach. Ouija’s measurements will be blended with those taken using ground-based equipment helping improve accuracy still further.

The key metric for ionospheric measurement is electron density in the ionosphere at any given time. The Ouija satellites will help calculate this through direct sampling and radio occultation. The latter detects changes in radio signals as they pass through the Earth’s atmosphere. “It is anticipated that the scientific payload will use or adapt commercial-off-the-shelf components” the solicitation said.

An initial Pathfinder satellite will be launched containing the mission payload as part of the programme’s Phase-1B. Phase-2 will see the launch of a further six satellites based on this design and lessons learned from the first spacecraft. The additional six spacecraft will help increase ionosphere measurement density. The solicitation expects the Pathfinder satellite to be launched in April/May 2024. The additional six satellites are expected to follow in November 2025. As Ouija is a research project, the satellites will have a relatively short lifespan of less than one year.

Military high frequency radio is experiencing a renaissance thanks to its practicality as an alternative to satellite communications. The Ouija programme will be an important step forward in helping improve the understanding of the ionosphere’s capricious nature.

by Dr. Thomas Withington