How do I design an RF altimeter for a drone or unmanned aircraft?

Designing an RF altimeter for a drone or unmanned aircraft uses a downward-looking FMCW (Frequency Modulated Continuous Wave) radar to measure the distance from the aircraft to the ground surface below. The RF altimeter transmits a frequency-swept signal toward the ground and receives the reflected echo. The time delay between transmission and reception, measured as a beat frequency in the FMCW dechirp process, is directly proportional to altitude: altitude = (f_beat x c x T_sweep) / (2 x BW). The key design parameters are: operating frequency (4.3 GHz for traditional aviation radar altimeters; 24 GHz or 77 GHz for compact drone altimeters where size/weight are critical), transmit power (typically 1-100 mW for altitudes up to 500 m; the ground provides a strong radar return due to its large area), antenna (a small patch or horn antenna pointed downward with moderate gain of 6-15 dBi; beamwidth of 30-60 degrees provides resilience to aircraft attitude changes), sweep bandwidth (determines altitude resolution: 100 MHz bandwidth gives 1.5 m resolution; 500 MHz gives 30 cm; 4 GHz gives 3.75 cm), sweep time (1-10 ms; faster sweeps allow higher update rates for use in landing control loops), and processing (FFT of the beat signal yields the range profile; the dominant peak corresponds to the ground altitude).