On the NASA/GSFC/Geophysics, Geodynamics and Space Geodesy Program, SGT is the prime contractor. Activities within this program include:
ICESat and ICESat-2: An important element of the climate system with long memory is that of the cryosphere. The Earth’s ice sheets both exert control over changes in the Earth’s climate, but also are important to monitor, for they will exhibit secular changes due to its changing state. ICESat carried a laser altimeter and was launched in early 2003 to uniquely monitor long wavelength cm-level changes that reflect trends in mass exchange between the oceans and ice sheets, and the deformation of the ice sheets themselves. Added to the 20-year legacy of mapping the polar regions with radar altimeters, ICESat has revolutionized the accuracy by which ice sheet topographic changes can be discerned. With surface spot sizes of 65 meters in diameter, and dense transects of data, this mission has also significantly advanced our synoptic mapping capabilities over all landforms. ICESat-2, scheduled to launch in 2017 will further enhance our knowledge of the cryosphere. The instrument on ICESat-2 utilizes an advanced photon-counting laser that has multiple beams and fires 10000 times per second. With the volume and distribution of these 10 meter spots on the earth’s surface, ICESat-2 will further the science started by ICESat over a decade ago.
ICESat was retired in February 2010 due to a technical malfunction with its laser systems, leaving NASA without an operating satellite dedicated to ice measurements. NASA developed the IceBridge program to provide aircraft measurements using both lidars and radars to fill the gap between the ICESat and ICESat-2 data records. IceBridge flights started in October 2009, providing measurements of major ice sheets in both the northern and southern hemispheres.
ICESat and ICESat-2 measure the topography of the Earth’s surface to unprecedented accuracy approaching ±2 cm for non-sloped smooth surfaces (like those seem over much of the ice sheets). With highly accurate orbits, these missions provide highly significant measures of ice sheet mass flux, including the first ever measures of the thinning of the Arctic Ice canopy. SGT is providing a complete end-to-end level of support for this mission. We are engaged in commanding the laser altimeter instrument, monitoring engineering housekeeping data, performing the data ingest into our SGT developed Science Information Processing System, and producing all of the Level 1, 2, and 3 science data products. In addition we are heavily engaged in science product algorithm development, science product verification and calibration, and the reprocessing of the more than 2 billion laser returns to increasingly add value to these unique data. SGT is also heavily engaged in the cryospheric research made possible from these data with a team of PhD researchers supporting the Project Scientist. We have several Co-Investigators on the ICESat Science Team. From 2006 onward, SGT has authored or co- authored over 15 refereed journal articles on these investigations.
Planetary Laser Altimetry: SGT supported the processing and analysis of laser altimetry acquired by the Mars Observer Laser Altimeter (MOLA) on the Mars Global Surveyor (MGS), and the Near Earth Asteroid Rendezvous – Shoemaker (NEAR Shoemaker) Mission to the asteroid 433-Eros. Currently, SGT supports science operations and analysis for the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO) and the Mercury Laser Altimeter (MLA) on the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) satellite.
GRACE: SGT scientists are active within the GRACE Science Team using GRACE data to measure mass flux within the Earth system. The GRACE satellites fly in constellation and track one another with exquisite precision. From the measured relative motion of these satellites, we are able to measure changes in the gravity field arising from mass flux within the Earth’s system, as a form of remote sensing. Our participation within this team has led to a host of publications in major journals like SCIENCE and Geophysical Research Letters reporting mass changes over Greenland and Antarctica’s ice sheets, the deterioration of the Alaska mountain glacial system, and the monitoring of ground water changes within hydrological basins.
TOPEX/Jason/OSTM: The oceans play a dominant role in determining climate. Through the transport of heat from the equators to the pole, and by providing a sink for excess carbon dioxide, the oceans also play a key role in climate change. Access to the vast expanse of the oceans has always presented a problem for better understanding ocean circulation, and the limitations of ship based surveys have relegated study to certain geographic regions leaving much of the ocean under-observed. Space-based radar altimeter systems were developed to provide a synoptic and continuous monitoring of the oceans by providing direct measures of a boundary condition reflective of this circulation, that being the ocean’s topographic height and surface slopes. Altimetry is one of the most important tools for ocean change studies, and altimeters are the only systems available to make observations on the spatial and temporal scales upon which these changes occur. Consequently, a major focus during the last decade was to advance altimeter sensors and improve the large host of enabling models (e.g. orbits, tides, atmospheric refractive corrections) needed to extract ocean circulation signals from altimeter measurements.
From its launch in the fall of 1992, the TOPEX/Poseidon Mission created a revolution in oceanography through the application of radar altimetry. Through its ability to deliver global sea surface height measurements, which are of an accuracy comparable to that acquired by tide gauges (±2 cm), TOPEX data are now being assimilated directly into some of the most accurate ocean circulation models. Basin scale ocean processes are routinely revealed, and events like the El Nino are unambiguously seen to form, advance, and dissipate through altimeter studies. Jason, the TOPEX follow on, launched in 2002, has continued the TOPEX legacy and by overflying the same ground track, has provided a continuous record of sea surface height to the present. The Ocean Surface Topography (OSTM/Jason 2) mission launched in June of 2008 currently furthers this critical measurement enabling the generation of a sea surface height Climate Data Record that now spans more than two decades. In the spring of 2015 Jason-3 is scheduled for launch to extend these high fidelity observations into the next decade.
SGT scientists have supported the TOPEX, Jason, and OSTM Missions since the early 1980s. The support we have provided has helped to produce the gravity field needed for high precision (± cm radial accuracy) orbits, the geoid needed to isolate the dynamic ocean topographic signals, the precision orbits employed in the distribution of these data to the worldwide community, and science investigations including those demanding the most stringent accuracy requirements directed at measuring the rise in global sea level from these data. SGT scientists are on dozens of publications related to this work.