Highly Accurate Wide-Range FM-CW Radar Altimeter Adam Sarhage

Goal Improve accuracy of a standard FM-CW radar altimeter at both low and high altitudes. Aircraft use radar altimeters to assist with landing, so accuracy at low altitudes is important. Accuracy at high altitudes is important for terrain-aided navigation application. Think military… Notes: Radar will be operated in the Ku band. (12 – 18 GHz) Authors are employed with S. Korea’s Agency for Defense Development

Recap of FM-CW Radar Simplified block diagram from class notes

Recap of FM-CW Radar time Tx B Rx LO near far frequency LO bandwidth is driven by TX bandwidth and time between closest and farthest returns.

Challenges with Traditional Radar Altimeters Wide-range altitude operation results in wide variations in the observed beat frequency. A wide IF bandwidth is therefore necessary to capture the wide range of altitudes. Antenna-to-antenna mutual coupling degrades performance at all altitudes.

Novelties with The Presented Design Optical delay in transmitter for reduced dynamic range Adjustable TX and RX gain with search algorithm for determining appropriate gain settings Direct digital synthesis using the reference clock as the offset frequency of the phase-locked loop for high linearity and low phase noise

Block Diagram of Presented Design

Block Diagram of Presented Design

Adjustable TX/RX Gain Transmitter power and receiver gain are adjusted through implementation of a search algorithm. Sweep bandwidth is also adjusted. Three consecutive valid altitudes must be correlated to exit the search mode. Search mode is re-entered when the altitude is determined to no longer be valid. Note: Each “Unit” is 4 ms

Adjustable TX/RX Gain

Demonstrated Performance An optical delay simulator was used for measuring accuracy at an altitude of 200 m. It was calibrated with a network analyzer, and the measured error was 5 cm. A crane setup was used for measuring accuracy at an altitude of 20 m. It was calibrated with a laser rangefinder and IMU, and the measured error was 1 cm.

Comparison to Garmin GRA 5500 Altitude accuracy: +/- 1.5 ft (3 – 100 ft AGL); +/- 2% (> 100 – 2500 ft AGL) Altitude Range: -20 – 2550 ft AGL Altitude Output Time Constant: 0.1 s max Transmitter Output: 4.25 – 4.35 GHz “Gated” FMCW Power 1.0 W nominal

References Choi et al.: Design of an FMCW Radar Altimeter for Wide-Range and Low Measurement Error Allen, EECS 725 Notes: Radar Measurements II Garmin Website: https://buy.garmin.com/en-US/US/p/135561#specs