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Whether in the field of battle, search-and-rescue or humanitarian aid efforts, the ability to share real-time, networked information between ground, sea and airborne forces is rapidly becoming the defining factor in a mission’s success. However modern satellite and other communications systems in higher frequecies are under constant threat from adversaries like Russia and china have developed sophiticated Electronic warfare (EW) syatems through which they can jam them. NATO members and partner forces are vulnerable to disruption of satellite communications, particularly along the alliance’s eastern flank where Russian armed forces continue to conduct electronic warfare.
Electronic warfare elements deployed within theaters of operation threaten to degrade, disrupt or deny VHF, UHF and SATCOM communication. In this scenario, HF radio is a viable backup mode of communication. Communication satellites are also easy targets for Russian and Chinese antisatellite systems both ground Ascent missiles and coorbital killer satellites .
Warfighters depend on high-frequency (HF) radio transmissions to operate military systems across the space, air, ground, and maritime domains. Current understanding of how HF waves propagate through the electromagnetically noisy ionosphere typically depends on ground-based methods. To more accurately understand HF propagation in space requires scientific measurements taken from within the ionosphere itself.
HF operates BLOS in the 2 to 30 MHz frequency band by either reflecting off the ionosphere (called Skywave) or refracting off the surface of the earth (called Surface Wave). Different frequencies will reflect off the ionized layers depending upon their height and ionization density which varies depending upon time of day and solar activity.
Solar activity as well as the 11-year solar cycle can change the density of the ionosphere, making it much more difficult to predict how radio communications in this region would be affected. The sun, which is scheduled to reach the end of its cycle in 2025, has already produced a number of X-class and big flares in recent times.
Thus, multiple HF frequency assignments are required to ensure 24/7/365 reliable communications. It is possible to predict which frequencies will be propagating based upon time of day and solar conditions as well as physical measurement of the ionosphere layers via sounding stations.
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 earth at a typical height of 400 km (250 miles).
DARPA’s new Ouija program aims to use sensors on low-orbiting satellites to provide new insights into HF radio wave propagation in the ionosphere, which spans the upper edges of the Earth’s atmosphere to the lower regions of space. The program seeks to quantify the space HF noise environment and improve characterization of the ionosphere to support warfighter capabilities.
“Ouija will augment ground-based measurements with in-situ measurements from space, in very low- Earth orbit (VLEO), to develop and validate accurate, near real-time HF propagation predictions,” said Jeff Rogers, Ouija program manager in DARPA’s Strategic Technology Office. “The VLEO altitude regime, approximately 200 km – 300 km above Earth, is of particular interest due to its information-rich environment where ionospheric electron density is at a maximum. Fine-grained knowledge of the spatial-temporal characteristics of electron density at these altitudes is required for accurate HF propagation prediction.”
In an April 22 statement, DARPA officials claimed that understanding how radio waves operate in this environment will be critical in assisting future warfighters. Because of the high density of the charged particles (mostly electrons) that can modify the route of radio transmissions, the propagation of the signal in the ionosphere is highly unpredictable.
The program includes two technical areas. The first technical area, announced in a solicitation issued April 21, 2022, seeks to develop, qualify, launch, and operate multiple small satellites carrying scientific and mission instrumentation. The Ouija scientific payload will measure electron density by both direct sampling and indirectly via radio occultation using navigation satellites. It is anticipated that the scientific payload will use or adapt commercial-off-the-shelf (COTS) components, but innovative instrumentation proposals that enhance the functionality of the scientific payload over a COTS baseline are welcome.
“The HF mission payload will require a high sensitivity, high dynamic range, low noise HF measurement subsystem,” Rogers said. “The antenna for this subsystem is a particular challenge, as efficient HF antennas that operate at the lower end of the frequency band are long, presenting deployment and space vehicle drag challenges.”
The second technical area, which will be fully detailed in a separate solicitation at a later date, aims to develop assimilative models that ingest direct, in-situ, measurements of electron density from a satellite in VLEO. The derived electron density models will be fed into HF propagation code then validated with data measured on-orbit. The goal is to improve fidelity over current state-of-the-art assimilative models by incorporating high resolution (in time and space) local measures with low latency updates.
Ouija employs a simplified Other Transactions (OT) process aimed at lowering the bureaucratic barrier for companies to make proposals, especially those seeking to work with DoD or DARPA for the first time.
DARPA Awards Spire Global Contract to Design VLEO Satellite
The Defense Advanced Research Projects Agency (DARPA) has awarded Spire Global a contract to deliver a preliminary design for a satellite that would carry an array of sensors to very low Earth orbit (VLEO) for in-situ ionosphere measurements. The contract will leverage Spire’s pioneering Space Services model to design a satellite that could carry the agency’s sensor to very low Earth orbit to improve characterization of the ionosphere
The design will be a modified version of Spire’s Low Earth Multi-Use Receiver (LEMUR) satellite. Spire operates a constellation of more than 100 LEMUR cubesats equipped with sensors to gather weather data, track ships and airplanes, and provide customers with other space-based services.
DARPA made the award under its Ouija program, which will use satellites to quantify and characterize high-frequency (HF) radio wave propagation in the ionosphere to support novel HF capabilities. The ionosphere reaches from the upper edges of Earth’s atmosphere to the lower regions of space.
“Spire is proud to be supporting DARPA’s efforts to advance our understanding of the ionosphere. Spire has built and launched over 150 satellites in the decade since the company was founded, and we’re excited to bring that heritage and experience in ionospheric data collection to this project,” said Kamal Arafeh, Senior Vice President of Sales, Spire. “For innovative programs like Ouija, the Space Services model provides a fast and cost-effective platform to build and scale new technology in space.”
Spire will use its Space Services model, which offers customers “fast and scalable access to space through a subscription model that eliminates the high upfront cost of building and maintaining infrastructure in space,” the company said. “Organizations can leverage Spire’s established space, ground, and web infrastructure to deploy and operate a constellation of satellites, a hosted payload, or a software application in space. Spire handles the end-to-end management, from manufacturing to launch to satellite operations, and the customer operates the system through a web API.”
