NRL SHIMMER and CITRIS Experiments Launched on STPSat-1 to Study Earth's Atmosphere (Forwarded)

Public Affairs Office
Naval Research Laboratory
Washington, D.C.

3/9/2007

NRL Press Release 11-07r

NRL SHIMMER and CITRIS Experiments Launched on STPSat-1 to Study Earth's
Atmosphere

Two innovative experiments built at the Naval Research Laboratory launched
into low earth orbit on Thursday, March 8, onboard the Space Test Program
Satellite-1 (STPSat-1). Both payloads contain pioneering technology designed
to answer compelling scientific questions about the Earth's atmosphere. The
Atlas V launch from Cape Canaveral, Florida, included five additional
satellites, all part of the STP-1 mission.

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 deg 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.