Experiments Launched To Study Earth's Atmosphere

NRL's Spatial Heterodyne Imager for Mesospheric Radicals (SHIMMER) instrument is the primary payload of STPSat-1. It is a compact, rugged, high-resolution ultraviolet spectrometer that will image the Earth's atmosphere. SHIMMER is the first satellite-based instrument to use the spatial heterodyne spectroscopy (SHS) technique, which significantly reduces the instrument's size and weight while retaining the spectral resolution and exceeding the sensitivity of comparable conventional instrumentation.

The two main goals of the SHIMMER mission are to demonstrate SHS for long-duration (greater than one year) spaceflight, and to measure altitude profiles of the hydroxyl radical (OH) between 40 and 100 km altitude. OH participates in the photochemical destruction of ozone and is also a proxy for water vapor, which is a tracer for large-scale circulation in the Earth's upper atmosphere.

The heart of SHIMMER is a monolithic interferometer, which allows SHIMMER to simultaneously observe the OH solar resonance fluorescence from 32 altitudes at a superior resolving power of 25,000. NRL's Space Science Division developed SHIMMER in cooperation with St. Cloud State University and the University of Wisconsin. NASA's Planetary Instrument Definition and Development Program supported the development of the monolithic interferometer.

"In addition to improving our understanding of the atmosphere, a successful flight of SHIMMER will be a tremendous step towards future SHS space instruments for terrestrial and planetary applications," says Dr. Christoph R. Englert, principal investigator of SHIMMER.

OH remains one of the least measured trace gases in the middle atmosphere. The first global measurements of OH were made by NRL's Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) in 1994 and 1997. MAHRSI produced the first maps of OH on a satellite deployed and retrieved from the space shuttle during two one-week missions. Compared to MAHRSI, SHIMMER is not only smaller by a factor of three in mass and volume, but it samples the atmosphere seven times faster due to its higher sensitivity.

"SHIMMER is undeniably a bold technological advance, but the mission concept is also scientifically very exciting. MAHRSI answered many questions about the global distribution of OH but raised many more that we could not answer with only a couple of weeks of data," notes Dr. Michael H. Stevens, project scientist for SHIMMER.

One additional goal of SHIMMER is to observe the equatorward edge of the polar mesospheric cloud (PMC) region, around 55° latitude. Originally, PMCs were thought to be caused solely by water vapor lofted from the lower atmosphere over the summer polar region. However, MAHRSI demonstrated that water vapor exhaust injected into the upper atmosphere from the space shuttle can also form PMCs. By observing both the OH (water vapor) and the PMCs, SHIMMER results will help quantify this contribution to PMCs.

The second NRL payload on STPSat-1 is the Scintillation and Tomography Receiver in Space (CITRIS). CITRIS will detect when and where radio wave propagation through the ionized atmosphere (called the ionosphere) is adversely affected by scintillation and refraction, and will provide a global map of ionospheric densities and irregularities. CITRIS is a four-frequency receiver that uses a multi-band antenna located on the ram or wake side of STPSat-1. CITRIS data will be used to improve current models of the ionosphere. Dr. Paul A. Bernhardt of NRL's Plasma Physics Division is the principal investigator and is supported by technical staff of the Plasma Physics Division and NRL's Naval Center for Space Technology.

The Earth's ionosphere is the primary source for errors in GPS and other navigation systems. Also, regions of turbulence in the ionosphere provide disruptions of GPS and communications signals that make the systems unusable. CITRIS will provide ionospheric data that will help the Navy locate harmful regions in the ionosphere. "In the future, we expect that the CITRIS measurements will provide a warning of impending outages for both civilian and military radio systems" says Dr. Carl Siefring the CITRIS Project Scientist.

CITRIS uses existing radio sources around the world to monitor ionospheric electron density structures. One source of radio signals is the NRL CERTO radio transmitters in low-earth-orbit on the DMSP/F15, COSMIC, CASSIOPE, C/NOFS and EQUARS satellites. With these space-based beacons and a global array of French ground beacons (DORIS), CITRIS will provide worldwide measurements of ionospheric refraction and radio scintillations.

The CITRIS team has been formulating new science algorithms that permit characterization of plasma structures along propagation paths between orbiting satellites. The primary advantage of the space-based receiver is to provide ionospheric specifications in regions where instrumentation is not available, such as remote regions at sea. The results of CITRIS on STPSat-1 will demonstrate the usefulness of these algorithms in the latitudes below 35 degrees where the ionosphere is often corrupted by natural plasma irregularities of equatorial origin. "We've provided digital signal processors in CITRIS which rapidly convert the received radio signals into useful data products" states Mr. Ivan Galysh, project engineer for CITRIS.

SHIMMER and CITRIS are joint efforts between the DoD Space Test Program (SMC/SDTW) and the Naval Research Laboratory.