NRL SSULI: Satellite to measure ionosphere electron density

Says Andrew Nicholas, Principal NRL Investigator, "What we're looking at is naturally occurring air glow emissions from the upper atmosphere." 

Shown is the daily averaged electron density over two years; as a function of altitude and day of the year in the post-sunset, equatorial ionosphere. 

F18 SSULI measured the nightside oxygen 135.6 nm radiative recombination emission intensity, capturing the ionosphere's variations daily and over the long-term as associated with seasons and solar cycles. 

Credit: U.S. Naval Research Laboratory

On April 3rd, 2014, a satellite carrying a U.S. Naval Research Laboratory (NRL) space weather instrument will launch from Vandenberg Air Force Base.

Special Sensor Ultraviolet Limb Imager (SSULI)

Called the Special Sensor Ultraviolet Limb Imager (SSULI), "SSULI makes accurate measurements of the upper atmosphere and ionosphere that are ultimately useful to the warfighter," says Dr. Scott Budzien, the NRL Program Manager.

"Down in the lower atmosphere, the characteristics of weather that are important are temperature, precipitation, wind, and so on," he says.

"But in the ionosphere, the aspects that are important for our systems are electron density, the morphology and gradients of electron density, and the height where the ionosphere lies."

SSULI measures the density of the ionosphere (as ions or electrons per cubic centimeter). Says Andrew Nicholas, the Principal Investigator, "We are measuring naturally occurring airglow emissions from the upper atmosphere."

The density profiles go into the Department of Defense's weather system, which the U.S. Air Force has run since the 1940s to collect observations about terrestrial and space weather.

"SSULI helps provide a very good specification of the state of the atmosphere, a nowcast," says Budzien.

With a more accurate nowcast, forecasting models better predict space weather into the future.

Space weather is important for military operations, because how signals are transmitted or reflected influences the reliability of radar and of communication and navigation systems.

Configuration of Operational Polar Satellites

The April 3rd Defense Meteorological Satellite Program (DMSP) mission, Flight-19 (F19), is the fourth to carry a SSULI from NRL.

The DMSP satellites are low-earth orbiting satellites, flying at about 830-840 kilometers (km) above the ground.

"They fly in a sun-synchronous orbit, which means they are always at the same local time," says Budzien.

"The one we are launching in April is going into a terminator orbit, right at the day-night boundary."

NRL built five SSULIs in the early 1990s; the last launch is currently planned for 2016. But, says Budzien, "Based on the lessons learned from SSULI, we've developed the design for a smaller, more sensitive instrument."

Still just a concept, Budzien hopes to identify a sponsor who will help "to continue to provide improved products for the warfighter."