NRL Sensor to Measure Natural Airglow in the Upper Atmosphere

11/20/2003 - 75-03r
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The first of five Special Sensor Ultraviolet Limb Imager (SSULI) remote sensing instruments, developed by the Naval Research Laboratory (NRL), was launched on October 18, 2003 on board the DMSP F-16 satellite. SSULI is the first operational instrument of its kind and provides a new technique for remote sensing of the ionosphere and thermosphere from space. SSULI's measurements will provide scientific data supporting military and civil systems and will assist in predicting atmospheric drag effects on satellites and reentry vehicles.

The last of the Lockheed Martin Titan II vehicles to ever fly launched the Air Force's Defense Meteorological Satellite Program (DMSP) F-16 satellite and the SSULI sensor into low earth orbit from Vandenberg Air Force Base, California. SSULI has been powered on and is receiving data.

"Global observation and characterization of the thermosphere and ionosphere is an important goal for Department of Defense (DoD) and civilian users," said Stefan Thonnard, the SSULI Principal Investigator at NRL. He discussed the significance of the planned SSULI observations, saying, "Many critical systems are affected by signal propagation through the ionosphere, including radio communication, over-the-horizon radar, radio direction finding, and the Global Positioning System (GPS). In particular, during large geomagnetic disturbances, ionospheric irregularities can greatly affect the position accuracy of GPS systems. Furthermore, real-time observations of the global ionosphere and thermosphere are a significant need to help mitigate the impacts on these systems. The ionosphere is strongly controlled by solar and geomagnetic forces on short time scales, and on longer time scales by neutral density variations, gravity waves, and photochemistry. The SSULI data will be used to investigate the Sun-Earth connection while providing new capabilities to the DoD."

SSULI measures vertical profiles of the natural airglow radiation from atoms, molecules, and ions in the upper atmosphere and ionosphere from low earth orbit aboard the DMSP satellite. It builds on the successes of the NRL High Resolution Airglow/Aurora Spectroscopy (HIRAAS) experiment flown aboard the Space Test Program (STP) Advanced Research and Global Observations Satellite (ARGOS). SSULI makes measurements from the extreme ultraviolet (EUV) to the far ultraviolet (FUV) over the wavelength range of 80 nanometers to 170 nm with 1.5 nm resolution. SSULI determines the electron density and neutral density profiles from the naturally occurring airglow produced by the atmospheric constituents. SSULI uses a spectrograph fed by a mirror capable of scanning below the satellite horizon from 10 degrees to 27 degrees every 90 seconds. These observations constitute a vertical slice of the Earth's atmosphere from 750 km to 50 km in altitude. In addition to providing near real-time atmospheric and ionospheric specification, use of these data will permit the development of new techniques for global ionospheric remote sensing and new models of global electron density variation.

Commenting on the practical application of the instrument, Bill Raynor, the program manager at NRL said, "As our civil, commercial, and national security use of space continues to increase, natural atmospheric phenomena that can disrupt day-to-day operations are a growing concern. The scientific dataset collected by SSULI promises to provide a unique new tool to understand and predict impacts to ongoing operations caused by these upper atmospheric effects."

An extensive data processing suite was developed to support on-orbit observations and flight operations. It includes data reduction software developed at NRL that uses unique scientific algorithms and graphical interfaces for the user community. After launch, the SSULI sensor, software, and derived atmospheric specification will undergo an extensive one-year validation. After validation, SSULI products will be distributed by the Air Force Weather Agency to support operational DoD systems. The four remaining SSULI instruments will fly on subsequent DMSP Block 5D-3 spacecraft missions approximately every two years.

The launch of DMSP F-16 and SSULI brings an end to the long, successful history of the venerable Titan II. The October 18 flight was the last time a Titan II will fly. You can find more information on the launch and the Titan II by accessing the following web site:

Additional information about the SSULI instrument and its data processing software is available at

The NRL SSULI instrument, an operational DoD sensor, measures the natural airglow radiation from atoms, molecules, and ions in the upper atmosphere and ionosphere.

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