Publications

An adaptive procedure for selecting radar pulse repetition frequency (PRF) has been developed as the primary means of minimizing the occurence of range aliased echoes within operationaly significant coverage aread (e.g., airport runways) of the Terminal Doppler Weather Radar (TDWR) system. This procedure underwent extensive testing at the S-Band TDWR testbed while located in Denver, CO, where it was judged to be highly successful at preserving the integrity of data collected within the vicinity of the Stapleton International Airport runways. The actual TDWR system will operate at a C-Band frequency, and an increase in potential range obscuration is expected over that experience by the S-band testbed. This report discusses the anticipated performance of the PRF selection procedure in the C-Band environment by extrapolating results obtained using S-Band testbed data. The results conclusively demonstrate the efficacy of adaptive PRF selection as a method by which to reduce potential range obscuration. A worst-case scenario, for example, indicates that over 20% of the TDWR radar collected about the airport runways has the potential for being contaminated with range aliased echoes at any given time during TDWR surveillance operations. With adaptive PRF selection, however, the expected obscuration is reduced to only 3%. (The corresponding figures for the S-Band testbed are shown to be 14& rather than 20% and 1% rather than 3%). While adaptive PRF selection can substantially reduce range obscuration, it cannot totally eliminate the problem. An enhancement to the PRF selection strategy, which further reduces the potential range obscuration, is introduced and recommended. Additionally, the complementary use of pulse-to-pulse phase modulation to exact valid velocity measurements in the presence of range contamination is discussed and preliminary experimental results presented.

The Federal Aviation Administration has recently awarded a contract for the procurement of 47 Terminal Doppler Weather Radar (TDWR) systems to be sited near high traffic airports. These systems will collect and process Doppler radar data that will be used by fully automated algorithms to identify hazardous meteorological wind shear events in real time (eg., microbursts and gust fronts.) This information will the be conveyed to aircraft pilots in order that potentially hazardous takeoffs or landings be averted. In a pulsed Doppler weather radar, one of the most serious causes of data quality degradation is due to range aliased echoes from distant storms [3]. This range contamination can occur in the immediate vicinity of a meteorological hazard, possibly obscure the event, and thus decrease the probability of detecting it. In other instances, range contaminated data can present a radar signature similar to that of a wind shear hazard, and perhaps cause an algorithm to issue a false alarm. In order for the TDWR system to achieve a high probability of detecting meteorological hazards, while maintaining a low probability of false alarms, an effective means of dealing with range contamination is required. An adaptive procedure by which to select the radar's pulse repetition frequency (Pm) has been developed as a primary means by which to minimize range contamination within the operationally significant coverage area of a TDWR system. This procedure will be developed within this paper and a quantitative assessment as to the anticipated effectiveness of this technique in the TDWR system will be provided.

The Terminal Doppler Weather Radar (TDWR) system shall provide high quality Doppler radar data on weather phenomena near high traffic airports. These data shall be used in real time by automated TDWR algorithms to detect weather situations which may be hazardous to the safe operation of aircraft within the vicinity of the airport. One of the major factors which could cause the degradation of the quality of these TDWR data is obscuration by 'distant' storm cells. This obscuration is caused by storms located beyond the range interval being sampled by the radar, yet whose radar echo ambiguously folds within the range interval of interest. These range aliased echoes could trigger false detections by the algorithms, and/or cause actual hazardous situations near the airport to remain undetected. By carefully selecting the pulse repetition frequency (PRF) of the radar, range obscuration from distant storms can be minimized over specified airport regions. This document describes techniques for predicting the obscuration as a function of PRF, and details the criteria which shall be used by the TDWR system to automatically and adaptively select an optimal PRF in order to minimize these obscuration effects. Weather radar, Radar range obscuration, TDWR, Radar range aliasing, Radar pulse, Repetition Frequency (PRF).