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Science Highlight: Navigating Low Earth Orbit Via RAIDS
Unlatched HREP RAIDS configuration fully rotated Unlatched HREP-RAIDS configuration fully rotated -- on-orbit the instrument is rotated 180 degrees. (Image courtesy of the Naval Research Laboratory)
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When Christopher Columbus set off to circle the globe, he did so without knowing exactly what he would find. He did know, however, what he was looking for: improved human exploration. Likewise, the International Space Station navigates the thermospheric layer of Earth's atmosphere searching for answers to advance long-duration space flight. Flying within low Earth orbit, the station uses instruments like the Remote Atmospheric and Ionospheric Detection System, or RAIDS, to collect data on the atmosphere. In particular, this device measures the thermosphere, which creates atmospheric drag on space vehicles and satellites, and is effected by solar activity. RAIDS also studies the ionosphere, which has a strong influence on radio, radar, and satellite navigation signals.

Dr. Scott Budzien, of the Naval Research Laboratory, asserts that understanding thermosphere and ionosphere via RAIDS is important to human space flight. "…analogous to the concept that understanding ocean currents allowed the first oceanic voyages, and understanding tropospheric weather was crucial for development of modern aviation. So too must we understand the near-Earth space environment for the future of routine space travel."

RAIDS is an ultraviolet and visible remote sensing instrument observes the Earth's bright dayside atmosphere faint airglow, measuring global electron density profiles at altitudes of 100 to 350 km. It launched on September 11, 2009 and resides on the Japanese Experiment Module -- Exposed Facility, or JEM-EF, of the Kibo module.

Due to light sensitivity, there is a small margin of error during measurements. If RAIDS scans too close to the Earth, overexposure degrades sensitivity; too far from the Earth and there is not enough light. Likewise, a long viewing path (~1,800 km behind the space station) means small fluctuations in pitch create large alterations in altitude readings. Over the long term, the station momentum management capability allows for pitch oscillation control within three different ranges (± 1.25, ± 0.88, or ± 0.25 degrees), depending on the mission parameters. The advertised pointing accuracy of the station is ± 3.5 degrees, but recent updates by space station Guidance, Navigation and Control improved attitude oscillation control to an impressive ± 0.2 degrees. This increased stability amplified RAIDS science data return by a factor of two.

The information from RAIDS will provide the most comprehensive survey of the ionosphere and thermosphere in over 20 years, improving knowledge of the Earth's atmosphere and guiding human spaceflight towards advanced exploration.
by Jessica Nimon
NASA's Johnson Space Center
ISS Program Science Office