ON THE ACHIEVABLE UPPER LIMIT OF THE ELEVATION ANGLES OF EFFECTIVE DIRECTION FINDING OF TARGETS OVER THE SEA BY THE ROOT-MUSIC METHOD PROVIDED WITH ADEQUATE A PRIORI PARAMETERS

Abstract

Subject and Purpose. The paper investigates the accuracy of measurements of the elevation angles of targets located near the surface of a disturbed sea in the area from the upper limit of their extremely small values to the free space zone. The main part of the study concerns measurements using the root-MUSIC method based on the analysis of the eigenvalues of the autocorrelation matrix of signals received from the target. Under the same conditions, the accuracy of measurements using the amplitude sum-difference (monopulse) method was obtained. The purpose of the work was to determine the upper limit of the location angles up to which the accuracy of the root-MUSIC method exceeds the accuracy of the monopulse method measurements, and thereby determine the area of its eff ective use. Methods and Methodology. The study was conducted using computer modeling of the measurement of elevation angles by two types of radio direction finders. The first one used the root-MUSIC measurement method, and the second one used the monopulse method. Both operated at a wave of 3.2 cm and had the same receiving antenna aperture of 2.5 m. The modeling was performed for sea waves from calm to heavy. In addition to interference from the sea, interference caused by internal noise of the direction finders’ receiving channels was taken into account. The modeling calculated the distance dependencies of the measurement errors of the target’s position angles moving at a constant height of 20 m at a distance from 2 to 0.5 km. This ensured the overlap of the position angles in the range of 0.57...2.28 of the width of the sum directional pattern of the monopulse direction finder. Results. Under the same sea waves and using equal apertures of the receiving antennas, the values of the errors in measuring the target elevation angles by both methods were obtained. Recommendations for choice the a priori parameters of the root-MUSIC method have been developed, which ensure high measurement accuracy in diff erent interference conditions. Conclusions. It was found that the root-MUSIC method has a higher accuracy of measuring the elevation angles of low-altitude targets over the sea compared to the widely used monopulse method up to position angles equal to twice the width of the sum directional pattern of the monopulse direction finder antenna.

Keywords

• root-music
• monopulse method
• a priori parameters
• elevation angle
• low-altitude target
• measurement errors
• sea waves
• multipath propagation
• thermal noise
• computer modeling