Enhanced GPS and comms the focus in AFRL/NASA CubeSat mission

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February 16, 2023

Sensor experiments recently deployed from the International Space Station (ISS) during late 2022 may lead to greater understanding of the Earth’s ionosphere that will in turn enable more accurate predictive modeling and lead to increased resilience in the navigation and communications realms.

The Air Force Research Laboratory (AFRL) reports that its new sensor experiment – riding along on NASA’s six-unit CubeSat – launched from the ISS on December 29, 2022. The mission of the CubeSat, named petitSat [Plasma Enhancements in the Ionosphere-Thermosphere Satellite], according to an AFRL update, is to study the Earth’s ionospheric layer to gain insight into weather conditions and disturbances in space and how they affect navigation and communication systems.

The AFRL sensor, developed at the agency’s Space Vehicles Directorate and operating out of NASA’s Goddard Space Flight Center, is called GRIDS [Gridded Retarding Ion Drift Sensor]. The AFRL team reports that GRIDS is a low size, weight, and power (SWaP) sensor built internally that will measure various ions in the ionosphere.

The ionosphere is the solar-radiation-ionized part of the upper atmosphere of Earth, from about 48 km to 965 km (29.83 miles to 599.62 miles) above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The National Oceanic and Atmospheric Administration (NOAA) explains that the ionosphere is important because it reflects and modifies radio waves used for communication and navigation. Other phenomena such as energetic charged particles and cosmic rays also have an ionizing effect and can contribute to conditions affecting the ionosphere.

The conditions in the ionosphere, and its constant changes, have practical repercussions, say teams at NASA, given the ever-increasing reliance on technology: This is the area through which radio communications and GPS signals travel. Variations here can result in distortions or even complete disruption of such signals.

Researchers at Penn State University’s College of Earth and Mineral Sciences report that large errors in GPS positioning can be attributable to shifts in the ionosphere and other portions of the Earth’s atmosphere. The long journey of the GPS signal through the virtual vacuum of space means that the signal can be affected as it passes through the earth’s atmosphere. Through both refraction and diffraction, the atmosphere alters the apparent speed and, to a lesser extent, the direction of the signal. These changes mean an apparent delay in the signal’s transit from the satellite to the receiver.

The Earth’s upper atmosphere is where most low-Earth-orbit satellites operate; their orbits can be affected by sudden density changes created by space weather, explains Ryan Davidson, AFRL senior research physicist and the experiment’s program manager.

The accumulation of so-called cold plasma in the form of bubbles and globs affects radio propagation of satellite-based navigation and communications passing in the vicinity of these plasma formations, which interfere with GPS and radar signals, Davidson asserts.

“This experiment will increase our fundamental understanding of how the ionosphere works and allow us to determine operational models and strategies to increase the resiliency of the Space Force’s space-based assets,” Davidson says. “The GRIDS sensor is designed to measure how much plasma is present in the atmosphere and in what direction it is moving. This should allow us to detect the presence of plasma bubbles and globs and give us information into how they are formed.

“In this experiment, AFRL hopes to determine the formation mechanism of plasma perturbations and use that information to feed predictive ionospheric models and make them more accurate to increase satellite navigation and communication, thereby increasing the protection of vital assets such as the Global Positioning System,” he explains.

The AFRL petitSat team expects, according to the AFRL report on the experiment’s deployment, that GRIDS will remain in orbit for approximately six months. Another NASA/AFRL GRIDS experiment is expected to launch in 2024.

[Figure 1 | A sensor experiment named GRID [Gridded Retarding Ion Drift Sensor] deploys from the International Space Station. The sensor, developed by the Air Force Research Laboratory’s Space Vehicles Directorate, is hosted on NASA’s six-unit cube satellite petitSat, or Plasma Enhancements in the Ionosphere-Thermosphere Satellite. NASA’s Heliophysics SPORT CubeSat, also studying the ionosphere, can be seen in the top left corner.]