Publications

The Terminal Doppler Weather Radars (TDWRs) installed at major airports around the country are intended to enhance the safety of air travel by the detection and timely warning of hazardous wind shear conditions in the airport terminal area. To meet these objectives, scan strategies to efficiently cover the protected airspace were developed after extensive testing at several sites with different meteorological environments. Since the topology and geometry differ at each TDWR location, special considerations were necessary to define the specific scan sequences for each site. This report describes the criteria used to establish these scan sequences, including the determination of the lowest practicable elevation angle for each site - the "surface scan," which is used to detect microburst surface outflows, and other special scans such as the "MTS scan," which is used to illuminate the remote Moving Target Simulator (MTS).

Techniques for the suppression of ground and storm clutter to permit the detection of low altitude windshear by pulse Doppler radars are described. Novel features of the system include the use of clutter residue and range aliased weather echo editing maps which edit out the range-azimuth cells on a "data adaptive" basis.

The FAA is developing the Terminal Doppler Weather Radar system to automatically detect low altitude wind shear due to microbursts and gust fronts. Detection of this phenomenon presents a significant radar engineering challenge due to the need to observe low reflectivity events in the presence of strong clutter from ground objects and range aliased weather returns. This paper describes a number of unique approaches to clutter recognition which have been validated with the TDWR test bed radar.

The Lincoln Laboratory Terminal Doppler Weather Radar (TDWR) testbed operated in Denver, CO in 1987-88. This radar is a prototype of the wind shear detection radars scheduled to be installed by the FAA to provide warnings of possibly hazardous wind shear conditions in airport terminal areas. To obtain the required coverage at low altitudes (down to 100-200 meters above ground level), the antenna beam is required to scan at or very near the earth's surface. Strong ground clutter returns at these low elevation angles present a major problem in the detection of low reflectivity wind shear signals and pose a significant challenge to the mission of these radars. To address this problem, steps along several fronts are taken to mitigate the effects of clutter contamination. These include the use of narrow pencil-beam antennas to minimize ground illumination, suppression by high-pass clutter filters, and the use of clutter residue map editing. This report deals with the latter step, and focuses on the clutter environment experienced at the testbed site during April-October 1988 and its effect on clutter residue map usage. Since the clutter environment is subject to change over time -- due either to man-made or natural causes -- the residue maps require periodic updates to reflect the changing nature of the clutter. This is particularly important for radar systems such as these which rely on automated algorithms to detect subtle patterns and features in the radar returns. To study the frequency with which residue maps required replacement in Denver, clutter measurements recorded during this period were analyzed and are presented in this report as a series of clutter residue maps. The maps are compared and the short and long term changes analyzed. It is concluded that the overall changes during this time were relatively small and gradual, and that map updates at one to two month intervals were sufficient. The generation of the residue maps is described and the importance of collecting clutter data on clear, weather-free days, without the presence of anomalous propagation conditions is addressed. This report also describes the use of median estimation in the construction of the maps as an effective method of eliminating the occasional strong returns from moving reflectors, such as aircraft and vehicles, which would otherwise distort the maps.