Glaciers and Ice Sheets Mapping Orbiter concept

This is the published version. Copyright 2006 American Geophysical UnionWe describe a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. Our primary objective is to develop this new technology for obtaining spaceborne estimates of the thickness of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate. Our new technology concept employs VHF and P-band interferometric radars using a novel clutter rejection technique for measuring surface and bottom topographies of polar ice sheets from aircraft and spacecraft. Our approach will enable us to reduce signal contamination from surface clutter, measure the topography of the glacier bed at better than 1 km intervals with an accuracy of 20 m, and paint a picture of variations in bed characteristics. The technology will also have applications for planetary exploration including studies of the Martian ice caps and the icy moons of the outer solar system. Through the concept developed here we believe that we can image the base and map the three-dimensional basal topography beneath an ice sheet at up to 5 km depth

Dual-Band Multi-Channel Airborne Radar for Mapping the Internal and Basal Layers of Polar Ice Sheets

Rapid thinning of the Jakobshavn and a few other outlet glaciers in Greenland and the Antarctic has been observed in the past few years. The key to understanding these dramatic changes is basal conditions. None of the spaceborne radars, that have been providing a wealth of information about the ice surface, is capable of measuring ice thickness or mapping bed conditions. At the Center for Remote Sensing of Ice Sheets (CReSIS), we have developed an airborne radar system to map the internal and basal layers to obtain a 3-dimensional representation of the ice sheets in Polar Regions. We have also devised advanced signal processing techniques to overcome the effects of surface clutter. We have developed a radar for measuring ice thickness up to a 5000 m depth from low-altitude (500 m) and high-altitude (7000 m) aircraft. This airborne radar system can operate at two bands: very high frequency band (VHF-band) (140 MHz to 160 MHz) with a peak power of 800 W and P-band (435 MHz to 465 MHz) with a peak power of 1.6 kW for collecting data to develop effective ice sheet models. The pulse signal has a duration of 3 us or 10 us. The radar has 1 transmitter and 6 receivers inside the aircraft and an 8 element dipole antenna array mounted beneath the wings of the aircraft. This system is designed to have 32 coherent integrations and pulse compression due to which a high loop sensitivity of at least 208 dB was obtained. This system was tested and data were collected in the recent September 2007 field experiment over various parts of Greenland. From the initial observations of the collected data it can be deduced that the signal losses at 450 MHz are more than predicted by existing models and clutter masked the weak bed echoes when the data were collected at higher altitudes both at 150 MHz and 450 MHz

Signal Generation for FMCW Ultra-Wideband Radar

One of the greatest concerns facing the planet earth today is global warming. Globally the temperatures have risen and this has caused rise in sea level. Since a large percentage of the population lives near the coast sea level rise could have potentially catastrophic consequences. One of the largest uncertainties in projections of sea level rise is the changes of mass-balance of the ice sheets of Greenland and Antarctica. To predict the rise in sea level we need accurate measurements of mass-balance. One of the methods of determining mass-balance is through surface ice elevation measurements. In order to measure surface ice elevation, map near surface internal layers and measure the thickness of snow over sea ice Ultra-Wideband (UWB) Frequency-Modulated Continuous-Wave Radars are being developed at CReSIS. FMCW radars are low-cost low-power solution to obtain very fine range resolution. However, nonlinearities present in the transmit frequency sweep of the FMCW radar can deteriorate the range resolution. The main objective of the thesis was to produce an ultra linear transmit chirp signal for UWB Radars. This was done by using the Voltage-Controlled-Oscillator (VCO) in a Phase-Locked Loop configuration. To check the linearity of the chirp beat frequency was generated using delay line as a synthetic target and captured on the oscilloscope. This beat signal data were further analyzed for linearity and we found that the frequency response of the beat signal was a focused Sinc wave as opposed to a smeared signal in case of nonlinear chirp. Also the phase of the beat signal data was linear with respect to time

Investigations into Optically Controlled Phase Contrast, Polarisation Switchable Narrow Band RF Detection Techniques.

This thesis describes an investigation into S-band microwave frequency phase-contrast imaging. Resolution is a critical issue so system enhancements such as optical remote connection and polarisation-dependant sensing have been implemented within an end-to-end sensing system. Initially, the feasibility of phase-contrast measurements was considered and the limits of phase and amplitude measurements established. A switching matrix was then designed and incorporated into a tri-antenna array to demonstrate triangulation-based location. Commercial, linearly-polarised antennas were then used to demonstrate basic object location. A comprehensive experimental investigation into optical transmission of phase sensitive data using Radio over Fibre (RoF) techniques is then described. Reflective technology and directly modulated Vertical Cavity Surface Emitting Lasers (VCSELs) are assessed for suitability as are Coarse Wavelength Division Multiplexed (CWDM) architectures. These are believed to be a novel contribution in the imaging context as are the techniques employed to enhance and extend the matching and performance of the optical devices. A directly modulated VCSEL based CWDM method was then used over the extended range of 1 km of standard single mode optical fibre. Subsequently, dual polarisation plane techniques were used to generate sequential, orthogonally-separated measurements, which required the development of a suitable antenna. The design, modelling, construction and deployment of a high cross-polar isolation, patch antenna is then described. An antenna with single symmetrical forward lobes (on both polarisation planes) and low back radiation pattern was devised so enabling sensing from a single coincident point. With the device integrated into the final measurement system the resulting “Polarisation Switched, Narrowband, RF Probe System Using a VCSEL Optical Feed” was used to demonstrate improved resolution of a phase contrast RF measurement system at an optically-remoted distance of 1km

Multiband Multistatic Synthetic Aperture Radar for Measuring Ice Sheet Basal Conditions

Abstract — Ice sheet models are necessary to understand ice sheet dynamics and to predict their behavior. Of the primary inputs to these models, basal conditions are the least understood. By observing the forward and backscatter across a wide frequency range (over two octaves) the basal conditions can be established with a high level of confidence. For this purpose, we developed a multistatic synthetic aperture radar system that operates on three frequency bands (75-85 MHz, 140-160 MHz, and 330-370 MHz). The radar system is designed to use pulse compression techniques and coherent integration to obtain high loop sensitivity (203 dB) necessary to overcome radio frequency losses in ice. The system will be tested at Summit, Greenland (72°34 ’ N, 38°29 ’ W) during July 2004. Keywords- ice, scattering, multistatic, synthetic aperture radar I