James M. Stiles

Coherent radar ice thickness measurements over the Greenland ice sheet

We developed two 150-MHz coherent radar depth sounders for ice thickness measurements over the Greenland ice sheet. We developed one of these using connectorized components and the other using radio frequency integrated circuits (RFICs). Both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice. We used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers. We operated both radar systems on the NASA P-3B aircraft equipped with GPS receivers. Radar data are tagged with GPS-derived location information and are collected in conjunction with laser altimeter measurements. We have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of Program for Regional Climate Assessment (PARCA) investigations and the North Greenland Ice Core Project. Radar echograms and derived ice thickness data are placed on a server at the University of Kansas (http://tornado.rsl.ukans.edu/Greenlanddata.htm) for easy access by the scientific community. We obtained good ice thickness information with an accuracy of ±10 m over 90% of the flight lines flown as a part of the PARCA initiative. In this paper we provide a brief description of the system along with samples of data over the interior, along the 2000-m contour line in the south and from a few selected outlet glaciers.

Optimal Landsat TM band combinations and vegetation indices for discrimination of six grassland types in eastern Kansas

Hyperspectral sensors can make narrow-band measurements for several hundred regions of the electromagnetic spectrum, and with increasing frequency, multi-dates of remotely sensed data are being used for Earth observation purposes. The use of more spectral bands is creating greater demand for larger computer storage capacity and faster data processors. This study evaluates the use of raw Thematic Mapper (TM) band combinations and several derived vegetation indices to determine optimal vegetation indices and band combinations for discriminating among six grassland management practices in eastern Kansas. Results showed that among the transformed dataset, the Greenness Vegetation Index was found to be the best for discriminating among grassland management types. When evaluating the raw TM bands, TM4 (NIR) was always selected in Discriminate Analysis as the best discriminating variable. There is no significant improvement in grassland discrimination by using a combination of the raw TM bands and the vegetation indices. Increasing the number of TM bands by using multiple dates of imagery will improve discrimination accuracy up to a point, but the use of too many bands (greater than 10 or 12) can actually decrease discrimination accuracy.

Processing of multiple-receiver spaceborne arrays for wide-area SAR

The instantaneous area illuminated by a single-aperture synthetic aperture radar (SAR) is fundamentally limited by the minimum SAR antenna area constraint. This limitation is due to the fact that the number of illuminated resolution cells cannot exceed the number of collected data samples. However, if spatial sampling is added through the use of multiple-receiver arrays, then the maximum unambiguous illumination area is increased because multiple beams can be formed to reject range-Doppler ambiguities. Furthermore, the maximum unambiguous illumination area increases with the number of receivers in the array. One spaceborne implementation of multiple-aperture SAR that has been proposed is a constellation of formation-flying satellites. In this implementation, several satellites fly in a cluster and work together as a single coherent system. There are many advantages to the constellation implementation including cost benefits, graceful performance degradation, and the possibility of performing in multiple modes. The disadvantage is that the spatial samples provided by such a constellation will be sparse and irregularly spaced; consequently, traditional matched filtering produces unsatisfactory results. We investigate SAR performance and processing of sparse, multiple-aperture arrays. Three filters are evaluated: the matched filter, maximum-likelihood filter, and minimum mean-square error filter.

Intrapulse Radar-Embedded Communications

The embedding of a covert communication signal amongst the ambient scattering from an incident radar pulse has previously been achieved by modulating a Doppler-like phase shift sequence over numerous pulses (i.e., on an inter-pulse basis). In contrast, this paper considers radar-embedded communications on an intrapulse basis whereby an incident radar waveform is converted into one of K communication waveforms, each of which acts as a communication symbol representing some predetermined information (e.g., a bit sequence). To preserve a low intercept probability, this manner of radar-embedded communications necessitates prudent selection of the set of communication waveforms as well as interference cancellation on receive. A general mathematical model and subsequent optimization problem is established for the design of the communication waveforms, from which three design strategies are developed. Also, receiver design issues are discussed, and an interference-canceling maximum likelihood receiver is presented. Performance results are presented in terms of the communication symbol error rate as well as a correlation-based metric from which intercept probability can be inferred. It is demonstrated that, given persistent radar illumination with a pulse repetition frequency (PRF) of 1-2 kHz, intrapulse radar-embedded communications can theoretically achieve data-rates commensurate with speech coding (for the interval of the radar dwell time) with the potential for even higher data-rates if additional diversity is appropriately incorporated.

Polyphase-coded FM (PCFM) radar waveforms, part II: optimization

This paper addresses polyphase code optimization with respect to the nonlinear frequency modulation waveform generated by the continuous phase modulation implementation. A greedy search leveraging the complementary metrics of peak sidelobe level, integrated sidelobe level, and spectral content yield extremely low range sidelobes relative to waveform time-bandwidth product. Transmitter distortion is also incorporated into the optimization via modeling and actual hardware. Thus the physical radar emission can be designed to address spectrum management and enable the physical realization of advanced waveform-diverse schemes.

Electromagnetic scattering from grassland. I. A fully phase-coherent scattering model

A microwave scattering formulation is presented for grassland and other short vegetation canopies. The fact that the constituent elements of these targets can be as large as the vegetation layer make this formulation problematic. For example, a grass element may extend from the soil surface to the top of the canopy, and thus the upper portion of the element can be illuminated with far greater energy than the bottom. By modeling the long, thin elements of this type of vegetation as line dipole elements, this nonuniform illumination can be accounted for. Additionally, the stature and structure of grass plants can result in situations where the average inner-product of coherent terms are significant at lower frequencies. As a result, the backscattering coefficient cannot be modeled simply as the incoherent addition of the power from each element and scattering mechanism. To determine these coherent terms, a coherent model that considers scattered fields, and not power, is provided. This formulation is then used to provide a solution to the multiple coherent scattering terms, terms which include the correlation of the scattering between both dissimilar constituent elements and dissimilar scattering mechanisms. Finally, a major component of the grass family are cultural grasses, such as wheat and barley. This vegetation is often planted in row structures, a periodic organization that can likewise result in significant coherent scattering effects, depending on the frequency and illumination pattern. Therefore, a formulation is also provided that accounts for the unique scattering of these structures.

Embedding information into radar emissions via waveform implementation

In this paper an approach for the joint design of multiple receive filters is described that enables the use of different waveforms during a radar CPI while minimizing the attendant Doppler coherency degradation due to range sidelobe clutter modulation. This capability allows for a set of unique radar waveforms to serve the dual purpose of acting as communication symbols while acceptable radar performance is maintained. The receive filter design approach is based upon the well-known Least-Squares (LS) mismatch filter formulation. The novel modification is the iterative adaptation of the desired response such that all the waveform/filter pair responses are driven to be identical.

Electromagnetic scattering from grassland. II. Measurement and modeling results

For pt.I see ibid., vol.38, no.1, p.339-48 (2000). The validity of a coherent, grassland scattering model is determined by comparing the model predictions with direct measurements of a representative grass canopy. A wheat field was selected as the test target, and polarimetric, multifrequency backscattering data were collected over an entire growing season, along with a complete set of ground-truth data. The L-band measured data demonstrated a strong dependence on azimuthal look direction in relation to the row direction of the wheat. The C-band measurements likewise showed an interesting backscattering response, wherein /spl sigma//sub /spl nu//spl nu///sup 0/ actually increased with incidence angle for many cases. The coherent scattering model provides backscattering data that match and predict these measured data and most of the other measured data well. The model shows that at L-band, the incoherent scattering power alone is insufficient for predicting the measured results, as the coherent terms can dominate the total scattered energy. Additionally, the model, which accounts for this nonuniform illumination of the wheat elements, demonstrates the peculiar data observed for C-band. Likewise, it is demonstrated that the fidelity used to model grass constituents (e.g., curvature) is required to match the scattering measurements accurately.

Polarimetric calibration of SIR-C using point and distributed targets

In preparation for the Shuttle Imaging Radar-C/XSAR (SIR-C/XSAR) flights, the University of Michigan has been involved in the development of calibration procedures and precision calibration devices to quantify the complex radar images with an accuracy of 0.5 dB in magnitude and 5 degrees in phase. In this paper, the preliminary results of the SIR-C calibration and a summary of the University of Michigans activity in the Raco calibration super-site is presented. In this calibration campaign an array of point calibration targets including trihedral corner reflectors and polarimetric active radar calibrators (PARCs) in addition to a uniform distributed target were used for characterizing the radiometric calibration constant and the distortion parameters of the C-band SAR. Two different calibration methods, one based on the application of point targets and the other based on the application of the distributed target, are used to calibrate the SIR-C data and the results are compared with calibrated images provided by JPL. The distributed target used in this experiment was a field of grass, sometimes covered with snow, whose differential Mueller matrix was measured immediately after the SIR-C overpass using The University of Michigan polarimetric scatterometer systems. The scatterometers were calibrated against a precision metallic sphere and measured 100 independent spatial samples for characterizing the differential Mueller matrix of the distributed target to achieve the desired calibration accuracy. The L-band SAR has not yet been adequately calibrated for inclusion here. >

A scattering model for thin dielectric cylinders of arbitrary cross section and electrical length

A scattering solution for long, thin, dielectric cylinders of arbitrary cross section and electrical length is presented. The infinite-cylinder scattering formulation is shown to be an asymptotic solution for the finite-cylinder case, regardless of cylinder electrical length or cross section. The generalized Rayleigh-Gans (GRG) approximation for circular cylinders is shown to be a specific case of this general formulation, and therefore, the assertions of GRG are explicitly proven. A moment-method (MM) solution for thin circular cylinders is likewise presented and is used to examine and quantify the asymptotic errors associated with this solution.