Analysis of ice-sheet temperature profiles from low-frequency airborne remote sensing

Abstract Ice internal temperature and basal geothermal heat flux (GHF) are analyzed along a study line in northwestern Greenland. The temperatures were obtained from a previously reported inversion of airborne microwave brightness-temperature spectra. The temperatures vary slowly through the upper ice sheet and more rapidly near the base increasing from ~259 K near Camp Century to values near the melting point near NorthGRIP. The flow-law rate factor is computed from temperature data and analytic expressions. The rate factor increases from ~1 × 10−8 to 8 × 10−8 kPa−3 a−1 along the line. A laminar flow model combined with the depth-dependent rate factor is used to estimate horizontal velocity. The modeled surface velocities are about a factor of 10 less than interferometric synthetic aperture radar (InSAR) surface velocities. The laminar velocities are fitted to the InSAR velocities through a factor of 8 enhancement of the rate factor for the lower 25% of the column. GHF values retrieved from the brightness temperature spectra increase from ~55 to 84 mW m−2 from Camp Century to NorthGRIP. A strain heating correction improves agreement with other geophysical datasets near Camp Century and NEEM but differ by ~15 mW m−2 in the central portion of the profile

Microwave Radiometry at Frequencies From 500 to 1400 MHz: An Emerging Technology for Earth Observations

icrowave radiometry has provided valuable spaceborne observations of Earth’s geophysical properties for decades. The recent SMOS, Aquarius, and SMAP satellites have demonstrated the value of measurements at 1400 MHz for observ- ing surface soil moisture, sea surface salinity, sea ice thickness, soil freeze/thaw state, and other geophysical variables. However, the information obtained is limited by penetration through the subsur- face at 1400 MHz and by a reduced sensitivity to surface salinity in cold or wind-roughened waters. Recent airborne experiments have shown the potential of brightness temperature measurements from 500–1400 MHz to address these limitations by enabling sensing of soil moisture and sea ice thickness to greater depths, sensing of temperature deep within ice sheets, improved sensing of sea salinity in cold waters, and enhanced sensitivity to soil moisture under veg- etation canopies. However, the absence of significant spectrum re- served for passive microwave measurements in the 500–1400 MHz band requires both an opportunistic sensing strategy and systems for reducing the impact of radio-frequency interference. Here, we summarize the potential advantages and applications of 500–1400 MHz microwave radiometry for Earth observation and review recent experiments and demonstrations of these concepts. We also describe the remaining questions and challenges to be addressed in advancing to future spaceborne operation of this technology along with recommendations for future research activities

Statistical estimation and tracking of refractivity from radar clutter

In many maritime regions of the world, such as the Mediterranean, Persian Gulf, East China Sea, and the Californian Coast, atmospheric ducts are common occurrences. They result in various anomalies such as significant variations in the maximum operational radar range, creation of regions where the radar is practically blind (radar holes) and increased sea clutter. Therefore, it is important to predict the real-time 3-D environment in which the radar is operating so that the radar operator will at least know the true system limitations and in some cases even compensate for them. This dissertation addresses the estimation and tracking of the lower atmospheric radio refractivity under non-standard propagation conditions frequently encountered in low altitude maritime radar applications. This is done by statistically estimating the duct strength (range and height-dependent atmospheric index of refraction) from the sea-surface reflected radar clutter. Therefore, such methods are called Refractivity From Clutter (RFC) techniques. These environmental statistics can then be used to predict the radar performance. The electromagnetic propagation in these complex environments is simulated using a split-step fast Fourier transform (FFT) based parabolic equation (PE) approximation to the wave equation. The first part of this thesis discusses various algorithms such as genetic algorithms (GA), Markov chain Monte Carlo samplers (MCMC) and the hybrid GA-MCMC samplers that are used to estimate atmospheric radio refractivity for a given azimuth direction and time. The results show that radar clutter can be a rich source of information about the environment and the techniques mentioned above are used successfully as near real-time estimators for the data collected during the Wallops'98 experiment conducted by the Naval Surface Warfare Center. The second part of this dissertation focuses on both spatial and temporal tracking of the 3-D environment. Techniques such as the extended (EKF) and unscented (UKF) Kalman filters, and particle filters (PF) are used for tracking the spatial and temporal evolution of the lower atmosphere. Even though the tracking performance of the Kalman filters was limited for certain duct types such as the surface-based ducts due to the high non-linearity of the split-step FFT PE, they performed well for other environments such as evaporation ducts. On the other hand, particle filters proved to be very promising in tracking a wide variety of scenarios including even abruptly changing environment