Phase Estimation Method in a Radio Altimeter with a Continuous Frequency Modulated Signal

Introduction. The Cramér–Rao lower bound for the root-mean-square error of height estimation in radio altimeters with continuous frequency-modulated periodic signals is determined. Analysis of the bound showed that a periodic stepwise change in the emitted signal frequency between two levels, the difference between which is equal to the selected deviation value, is a type of frequency modulation that enables the potential altitude estimation accuracy of lowaltitude radio altimeters. However, such signals cannot be used in existing radio altimeters that are based on beat frequency estimation. An alternative solution is to implement a phase-locked altitude measurement mode.

Aim. To investigate the feasibility of implementing a phase altitude measurement mode in a low-altitude tracking radio altimeter using a phase-locked loop (PLL) for altitude estimation, and to verify its performance using mathematical simulation.

Materials and methods. The stated problem was solved by constructing a mathematical model of a tracking radio altimeter with the PLL to implement the phase method for altitude estimation.

Results. The conducted computer simulation demonstrated the operability of the PLL-based radio altimeter that utilizes the phase method of altitude estimation. When operating over a perfectly flat surface, the altimeter provides an unbiased and effective altitude estimate with signal-to-noise ratios greater than 5 dB. In the case of a rough surface, the quality of altitude estimation in the altimeter is approximately an order of magnitude higher than in tracking altimeters, which implement estimation of the beat signal frequency.

Conclusion. The data obtained via computer simulation demonstrate the potential for achieving the height estimation accuracy in a PLL-based radio altimeter that combines frequency and phase measurement modes. Future research will examine the influence of various factors on the performance of the radio altimeter and its circuit design.

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