Design & implementation of algorithm for linear sweep generation and signal processing for an FMCW radar altimeter

Radar Altimeters (RA) are widely used as a landing aid for aerospace vehicles because of its better measurement accuracy particularly at low altitude ranges. This paper describes the linear frequency modulated signal generation and the signal processing algorithm developed for an FM-CW altimeter. The altimeter uses two modulation rates to satisfy the measurement accuracy criteria for different applications. For generating the linear frequency modulated signal for both modulation rates, a Direct Digital Synthesis (DDS) based methodology is adopted. The altitude of the aerospace vehicle is measured based on the instantaneous frequency difference (beat signal frequency) between the transmitted and received signals. The beat signal is processed by Fast Fourier Transform (FFT) techniques which provide an additional gain so that the frequency of the required signal can be measured even if the signal to noise ratio is near zero dB. The proposed Radar Altimeter signal processing algorithm decides the modulation rate based on the measured altitude and the strength of the received signal. Higher resolution is ensured at lower altitudes by using a higher modulation rate.