Angle Ambiguity Analysis of Antenna Baseline for Interferometric Radar Altimeter

Abstract

In this study, the angle ambiguity of a single baseline is analyzed to design an antenna baseline for an interferometric radar altimeter (IRA). To overcome the shortcoming of a single baseline in which angle ambiguity occurs during every phase period, the IRA adopts multiple baselines for its antenna and estimates the target angle by employing the phase comparison monopulse algorithm. However, the problem with multiple baselines is that an angle ambiguity that is difficult to solve by signal processing occurs when the phase error is large. To minimize this problem at the antenna design stage, the angle ambiguity is analyzed according to the phase error and the baseline spacing. The analysis is performed by conducting a parameter study based on the Monte Carlo simulation, and the validity of the analysis is verified from the test data.

Altimetry Method for an Interferometric Radar Altimeter Based on a Phase Quality Evaluation

“…The target angle estimation is based on the PCM technique of monopulse radar. However, considering that it is difficult to resolve the angular ambiguity with a typical single-baseline antenna, in this paper, we employ a method based on a multiple-baseline antenna to estimate the azimuth of the target by comparing the phase differences of the target signals received by each antenna, as in several earlier studies [ 14 , 27 ].…”

“…This may be an angular ambiguity inherent in the multiple-baseline antenna-based PCM technique [ 14 ], an inherent error in the DEM due to the accuracy of DEM production, or a phase error due to the superposition of the phases of multiple targets in one range cell ( ).…”

“…In Equation ( 13), 𝐿𝐿 𝑘𝑘 is the interantenna spacing, and 𝜃𝜃 𝑘𝑘 is the angle of the target obtained from Ant L and Ant R. 𝑚𝑚 𝑘𝑘 is a variable representing the periodicity of the measured phase difference ∆∅ 𝑘𝑘 (= ∅ 𝑘𝑘 − ∅ 0 ), which has angular ambiguity that occurs at every period (2π) [28]. To resolve this angular ambiguity, the assumption that 𝜃𝜃 0 is most reliable when the difference (∆𝜃𝜃 𝑚𝑚𝑖𝑖𝑖𝑖 ) between sin(𝜃𝜃 1 ) and sin(𝜃𝜃 2 ) is minimized is used to obtain 𝑚𝑚 𝑘𝑘 , as shown in Equation (14). The obtained values of sin(𝜃𝜃 1 ) and sin(𝜃𝜃 2 ) are arithmetically averaged, as shown in Equation (15), to obtain the estimated angle (𝜃𝜃 � ) of 𝜃𝜃 0 [18,27].…”

“…To resolve this angular ambiguity, the assumption that θ 0 is most reliable when the difference (∆θ min ) between sin(θ 1 ) and sin(θ 2 ) is minimized is used to obtain m k , as shown in Equation (14). The obtained values of sin(θ 1 ) and sin(θ 2 ) are arithmetically averaged, as shown in Equation (15), to obtain the estimated angle ( θ) of θ 0 [18,27].…”

“…To minimize these errors, researchers have investigated radar altimeters for aircraft to measure not only the range of the target but also the angle of the target [10][11][12][13][14]. Among these altimeters, an interferometric radar altimeter (IRA) [10][11][12] estimates the target angle using the phase comparison monopulse (PCM) technique [15,16] with monopulse radar [17,18].…”