Publications

Field measurements of terrain reflection data at L band and C band have been taken during 1980 at several test sites at Hanscom airport, Fort Devens, and Camp Edwards, Massachusetts. These field data are used to (1) characterization of the L band and C band multipath environments, (2) validation of terrain mutlipath propagation models, and (3) assessment of TRSB elevation angle estimation performance with various aperture sizes. Conventional beamsum, maximum likelihood, and maximum entropy angular power spectral estimates have been employed as a means to characterize the multipath environment. The maximum entropy angular power spectral estimate offered highest resolution of the various multipath signals. The L-band field measurement results indicated that the terrain multipath were specular reflections with a high multipath levels. L-band M/D ratios ranging from -5 dB to 1 dB were observed in a variety of terrain conditions. At C band, diffuse ground reflections were evident at some measurement sites, especially at Camp Edwards J2 range site, where small scale terrain roughness was fairly visible. However, these C-band disffuse reflections appeared to be at fairly low levels, e.g., -15 dB to -20dB relative to the direct signal. The C-band peak specular multipath levels of -10dB to -2 dB were slightly lower than those of the L-band for the same terrain geometry. The phenomenon of hte "focusing" ground reflections, i.e., more than one specular ground reflection present at the same time, was observed at both L-band and C-band. For the L-band, this generally occurred in the rolling type of terrain. However, for the C-band, this also happened in the fairly flat terrain at Hanscom airport site.

The angular resolution and tracking of closely spaced targets is a classical radar problem which is receiving increased attention, and terrain multipath (e.g., reflections) has long been recognized to be a principal limitation on the achievable accuracy of radar elevation trackers at low elevation angles. A variety of techniques have been proposed for improved elevation angle estimation: however, comparative analysis based on field comparable data has not been available to date. In this paper, distributions of multipath scattered power, described in a companion paper, are used to compare several elevation angle estimation techniques: (1) conventional monopulse; (2) off-boresight monopulse; (3) double null monopulse; (4) single edge processing as is used for flare processing in the Microwave Landing System; and (5) a maximum entropy technique based estimator.

This paper presents the results of an experimental program to obtain a better quantitative understanding of low angle microwave propagation phenomena needed to assess the potential for improved elevation tracking performance. It has long been recognized that terrain multipath (e.g., reflections and/or shadowing) are a principal limitation on the achievable accuracy of radar elevation trackers and/or landing navigation aids at low angles; however, there has been a paucity of relevant experimental data over irregular terrain. The experimental data presented were obtained with a 26 lambda L-band array and a 57 lambda C-band array at a variety of sites in eastern Massachusetts with vegetated and/or rolling terrain. It is shown that specular reflections appear to be the predominant multipath source and these are predictable from a model based on scattering from tilted plates.

This report presents work done during phase 3 of the US national Microwave Landing System (MLS) program toward developing a computer simulation model of the MLS multipath effects, the experimental validation ot the model, and the application of the model to investigate multipath performance of ICAO proposals for the new approach and landing guidance system. This first volume of the report presents an overview of the overall simulation results as well as a description of the refined mathematical models and validation of the propagation portion of the simulation. Specific propagation topics include: 1) preliminary results for validation of models for reflections from rough and/or rising terrain based on L-band field data. 2) validation of the models for building reflections based on field measurements at a number of operational airports. 3) validation and refinement of the models for angle guidance system shadowing by obstacles such as buildings and other objects.