Below are a few of the radar applications being pursued in the Radar Science & Engineering section, including:
Unlike photogrammetry (where stereo image pairs are used to produce highly accurate topographic maps, but which relies on having sufficient scene contrast consistently across the field of view) or lidar altimetry (which provides very high-resolution measurements of the surface height at the sub-spacecraft point, yet whose sampling is very sparse), radar interferometry augments a conventional radar with a spatially separated receiving antenna, allowing the estimation of height over a wide swath, and, if the synthetic aperture technique is used, at a high spatial resolution. By comparing the interferograms obtained over a given scene before and after a geophysical event such as a volcano eruption, an earthquake or a subsidence event, one can infer the intervening surface deformation map. One set of applications is to identify, monitor and quantify anthropogenic causes for observed deformations, including processes such as water pumping and fossil fuel extraction.
Another application of interferometric topography mapping is to infer the geophysical processes that help shape the surface, such as understanding the putative subsurface ocean on Jupiter's moon, Europa, from an interferometric-topography-derived estimation of the parameters governing the tidal motion.
A third application of interferometry that is actively pursued in Section 334 is the detection of very fine-scale coherent changes of the surface, whether due to natural or anthropogenic processes, by estimating the radar-wavelength-scale correlation rather than trying to estimate the interferometric phase itself.
Radar interferometry is also being applied to the field of ocean and surface water altimetry, with the planned Surface Water Ocean Topography (SWOT) mission. SWOT will feature an interferometric altimeter capable of monitoring water levels in rivers, lakes and, indeed, in the oceans, including coastal areas, at a very high horizontal resolution (between 50 and 300 m2). The use of interferometry gives the instrument a wide swath, allowing the kind of frequent global coverage necessary for hydrologists to refine our understanding of the hydraulics of wetlands, to monitor the changes in surface water discharge and storage, and to improve our ability to model and forecast flood hazards.
The very low topography of southern Florida is evident in this color-coded shaded relief map generated with interferometric radar data from JPL's Shuttle Radar Topography Mission
. The image on the left is a standard view, with the green colors indicating low elevations, rising through yellow and tan, to white at the highest elevations (about 60 meters above sea level). In the view on the right, elevations below 5 meters above sea level have been colored dark blue, and lighter blue indicates elevations below 10 meters. These data identify the areas that are vulnerable to flooding associated with storm surges, so that planners can develop appropriate mitigation measures.
For more information, please contact Scott Hensley or Paul Rosen.
Ocean Vector Winds Applications
By observing every spot within its swath from two different viewing angles and interpreting simultaneously the two measurements of the wind-driven normalized surface cross-section, the QuikSCAT satellite's SeaWinds scatterometer radar (built by Section 334) produces estimates of the near-surface horizontal wind speed and direction over the ocean – except where the surface is obscured by significant amounts of precipitation, of the kind that a hurricane produces. Yet it is precisely within the rain bands (and eyewall) of hurricanes that accurate estimates of the wind field are most useful to forecasters. Section 334's hurricane subgroup is developing a data-assimilation approach to combine passive microwave radiometry and mesoscale numerical modeling with the scatterometry measurements to produce reliable, accurate estimates of the wind field in the hurricane vortex.
The left panel shows the wind estimated by the SeaWinds standard algorithm for Hurricane Isabel on 16 September 2003. The middle panel shows the corrected wind estimates accounting for the precipitation attenuation. The right panel shows the radiometer-measured precipitation, illustrating that the correction is greatest in the most active parts of the storm.
For more information, please contact Svetla Hristova-Veleva.
Radar Ecology Applications
Understanding the behavior of forest fires and planning for fire management require maps showing the distribution of wildfire fuel loads at medium-to-fine spatial resolution across large landscapes. Radar sensors from airborne or spaceborne platforms have the potential of providing quantitative information about the forest structure and biomass components that can be readily translated to meaningful fuel load estimates for fire management. Dr. Sassan Saatchi has used multi-frequency polarimetric synthetic aperture radar (SAR) imagery acquired over a large area of Yellowstone National Park by Section 334's airborne SAR to estimate the distribution of forest biomass and canopy fuel loads. From the data, Dr. Saatchi derived semi-empirical algorithms to estimate crown and stem biomass and three major fuel load parameters, namely canopy fuel weight, canopy bulk density, and foliage moisture content. These algorithms should prove very useful for supplying timely inputs to existing fire forecasting and management models.
For more information, please contact Sassan Saatchi.
Though the primary goal of QuikSCAT's SeaWinds scatterometer is to map the horizontal wind vector field over Earth's oceans, the surface cross-sections that are measured by this instrument have many other applications, including the identification and mapping of different classes of sea ice, ranging from older, thicker perennial ice to younger, thinner seasonal ice. Dr. Son V. Nghiem developed an approach to combine the SeaWinds measurements with a computer model based on sea ice drift observed by the International Arctic Buoy Program and apply it to monitor Arctic perennial ice cover evolution. The results showed that between 2005 and 2007, the extent of the Arctic's thick, year-round sea ice cover was reduced by about 23%. This drastic reduction of perennial winter sea ice is the primary cause of the fastest-ever sea ice retreat on record observed in the summer of 2007, and of the subsequent smallest-ever extent of total Arctic sea ice cover.
For more information, please contact Dr. Son V. Nghiem.
Cloud Radar Applications
The CloudSat satellite was launched in 2006 and is the first platform to carry a radar dedicated to the measurement and vertical "profiling" of clouds. In addition to clouds, CloudSat's radar can also detect and help estimate snowfall and light rain. While this radar does not scan across the track of the satellite and therefore does not provide great coverage, over polar regions it is the only instrument that is directly sensitive to snowfall and light rain. Section 334's precipitation subgroup is therefore using the CloudSat radar's measurements to train the passive microwave radiometers on a NASA platform that flies in formation with CloudSat and on a NOAA platform whose orbit is serendipitously close (in space and time) to CloudSat's. Three other NOAA orbiters carry a replica of the NOAA microwave radiometer, AMSU-B, and we plan to use the CloudSat-trained algorithm to reprocess the AMSU-B observations from 1998 forward and thus produce daily precipitation amounts over the Arctic and Antarctica.
The Arctic as seen by AMSR-E's highest frequency channel (89-GHz) on September 1, 2006 (left panel), and by AMSU-B's 183-GHz channel 5 on the same day (right panel). Although the AMSR-E radiometer flies in tandem with the CloudSat radar, its channels are too sensitive to the ice that covers polar regions and therefore cannot be used to "see" any snowfall, in contrast with AMSU-B's shorter wavelengths. The black curves trace the spots observed by CloudSat on that day.
For more information, please contact Simone Tanelli.
Ground Penetrating Radar
Ground-penetrating radar (GPR) can probe the subsurface from a few tenths of centimeters to hundreds meters in the frequency band between 1 and 100 MHz depending on ground geo-electric properties (complex permittivity and conductivity) and subsurface heterogeneity. GPR data on the subsurface of volcanic, icy and arid terrains are key elements in understanding the geological evolution of Earth’s subsurface. Section 334’s GPR subgroup also supports performance studies and prototyping for future Lunar and Martian GPRs mounted on Rover platforms. While icy materials are known to be favorable environments for deep radar sounding, subsurface exploration in volcanic contexts (that are also representative of several telluric planetary surfaces) are quite challenging, due to the potential presence of ferric and ferrous oxides of different types that reduce the penetration depth and increase the ambiguities in the interpretation of GPR returns and the subsequent description of the subsurface structure (including the detection of water and hydrates).
For more information, please contact Essam Heggy and visit http://dielectric.jpl.nasa.gov .