Henrik Forstén's Radar Drone Captures Detailed Imagery on Autonomous Flights — for Under $900

Finnish electrical engineer Henrik Forstén has built a drone equipped with a homebrew polarmetric synthetic aperture radar imaging system, flying autonomously and capturing surprisingly detailed radar imagery for under $900.

"I have made several homebuilt radars and done some synthetic aperture imaging testing with them on the ground," Forstén explains by way of background to the project. "I have wanted for a long time to put a radar on a drone and capture synthetic aperture images from air. When I last looked at this few years ago, medium sized drones with payload capability were around 1,000 EUR and up. I bought the cheapest Chinese no-name 7-inch FPV [First-Person View] kit and a small GPS + compass module to support autonomous flying with the goal of making a light weight synthetic aperture radar system that it can carry."

A traditional single-channel radar can only measure the distance from the emitter to the target; to calculate the angle of the target requires multiple channels, typically with the support of a large-scale antenna. "Instead of making a single large antenna," Forstén notes, "it's possible to move a single radar and take multiple measurements at different positions. If the scene remains static, this approach yields the same results as having one many channel radar system with big antenna."

Forstén's homebrew radar, built on a budget of just €500 (around $520), uses this approach to scale down an imaging radar system to a scale where it can be feasibly installed on a low-cost FPV quadcopter body. The radar is implemented on an AMD Zynq 7020 FPGA, built atop a custom PCB that includes 512MB of DDR3 memory, 32GB of eMMC memory, SD Card storage expansion, an inertial measurement unit (IMU), and USB 3.0 connectivity — "not needed for drone usage," Forstén explains, but present to allow the board to be connected to a host computer for other projects in the future.

The radar board is connected to a pair of PCB-based custom-built dual-polarized slot-fed stacked patch horn antennas, looking like square "eyes" for the drone. "This structure can achieve much wider bandwidth than a single patch," Forstén explains, "making it tolerant to frequency shift caused by inaccuracy of FR4 permittivity. The second patch also slightly increases the gain."

The drone is capable of autonomous flight, thanks to the GPS receiver included in the stock flight controller and IMU added to the radar PCB, and captures distance and phase information as it flies. This is then processed using a backprojection algorithm and separate backpropagation autofocus algorithm to convert the readings into visual images — with incredible detail, considering the low cost and homebrew nature of the device.

"The synthetic aperture radar drone can image at least up to 1.5km [around a mile] and likely even farther if flown higher," Forstén says. "It weighs under 1kg [2.2lbs] including the radar, drone, and battery. The system can capture HH, HV, VH, and VV polarizations. A gradient-based minimum entropy autofocus algorithm is capable of producing good good-quality images with a wide antenna beam using only non-RTK GPS and IMU sensor information. The total cost of the drone was about €200 [around $210], €600 [around $625] for two radar PCBs, and about 10 months of my free-time after work. I'm very happy with the performance of the system considering its low cost."

The project is written up in detail on Forstén's website, including links to the source code under the permissive MIT license and a copy of the radar board's schematics.