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January 4, 2007

Second SSUSI Instrument Flies on Defense Meteorological Satellite

SSUSI Instrument

Since 1990, APL has built five Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instruments for the Air Force Defense Meteorological Satellite Program (DMSP), which provides current weather data to service members worldwide.

The goal was to launch the SSUSI-carrying satellites beginning in 1996 to replace aging DMSP spacecraft. The instruments and their operating software were ready on schedule, but a combination of factors — including the old satellites holding up better than expected — delayed the first launch until 2003.

On Nov. 4, the second SSUSI instrument finally launched into orbit onboard the DMSP F17 satellite, boosted by a Delta IV rocket from Vandenberg Air Force Base in California, into a near-twilight, sun-synchronous orbit.

"Fortunately for the Air Force, APL provides program continuity — we act as their corporate memory," says the Space Department's Dr. Paxton, principal investigator on the SSUSI project. "People here understand the instrument, even though it's going on about 17 years since the project began."

The delay had an unforeseen benefit: allowing APL staff to improve the SSUSI instruments before the first one launched. The Global Ultraviolet Imager (GUVI) onboard NASA's APL-built TIMED spacecraft was nearly identical to SSUSI, but launched first, in 2001. Paxton and his colleagues were therefore able to apply lessons learned from GUVI's performance to improve SSUSI.

The instruments include spectrographic imagers and photometric systems that focus on the ionosphere and thermosphere. The spectrographs measure ultraviolet emissions in five different wavelength bands, while the photometers measure relative brightness.

The photometers provide data on the height of the ionosphere, which varies as the atmosphere warms and cools as a result of changing solar emissions. Knowing the height of the ionosphere, which acts as a reflector for radio or radar waves, can help operators better pinpoint over-the-horizon targets or people with whom they're trying to communicate.

The spectrographs were designed to monitor the aurora and the density waves it creates, which move through the upper atmosphere from the poles toward the equator. These waves can increase drag on satellites and alter the propagation of communications signals. But as GUVI demonstrated, the spectrographs can also capture images of turbulent density "bubbles" in the ionosphere, a phenomenon that had previously been detected only through disruptions in radio signals.

Until GUVI flew, says Paxton, "we couldn't prove that we could image these ionospheric bubbles." And the bubbles can have a profound impact on military communications and operations by distorting radio and radar waves. With the launch of DMSP F17, there are three APL-designed and -built instruments in low-Earth orbit providing this unique capability.

The UV images of the aurora and the derived environmental data from the SSUSI instruments provide the Air Force Weather Agency with near real-time information on the Earth's space weather environment. This information can be used in a number of applications including maintenance of high-frequency communication links and related systems and calculation of the atmospheric drag experienced by satellites in Earth orbit.

"What we're doing is providing space weather situational awareness, to give users a global view of space weather impacts," says Paxton. "We're helping the customer provide new services and helping DoD realize that space weather is a phenomenon that affects them every day."

He adds, "We've had lots of very good people working on this project, from design to software, and it has been fun." All three branches of the Space Department were involved in SSUSI, with input from the Atmospheric and Ionospheric Remote Sensing, Space Science Instrumentation, Science Applications, Space Systems Applications and Mechanical Systems groups. Approximately 35 APL staff members have worked on the project, plus an additional 50 to 60 people with the Air Force, Navy, universities and companies.