radio interferometry

2 Articles

A drone is shown, carrying underneath it a white plastic box. On the side of the box are two patch antennas. A camera extends from one end of the box, and a large GPS antenna from the other end.

Synthetic Aperture Radar Drone Gets Interferometric Imaging

September 28, 2026 by 10 Comments

It’s been more than a year since [Henrik Forstén] built the first iteration of his synthetic-aperture radar (SAR) imaging drone, and he’s certainly been productive in the meantime. Not only did he develop a much more powerful autofocus algorithm to clean up the radar images, but he also extended the software to create high-resolution interferometric images.

The main limitation of the original radar system was the GPS, which only had a resolution of about one meter; the autofocus algorithm owed much of its improved clarity to an improved estimation of the drone’s position. A simpler, though more expensive, solution was to add an RTK-capable GPS receiver. RTK (Real-Time Kinematic) receivers use a fixed ground station to constantly transmit a correction signal, letting them reach a couple centimeters of accuracy. Since the drone doesn’t actually need to know its position in real time, it can also use PPK (Post-Processing Kinematic) positioning, which compares recorded GPS signals after the flight to obtain similarly accurate positions.

[Henrik] also implemented a few other hardware improvements, including stabilizing the phase-locked loop used to generate the radar’s frequency sweep. The controller FPGA’s SD card interface had too low a bandwidth to record data in real time, so [Henrik] also implemented a simple, fast compression algorithm to speed that up. Most significantly, he also developed a program for interferometric imaging. The drone flies the same path twice at different altitudes; by comparing phase information from different passes, it’s possible to detect a target’s elevation. Normally, the radar program assumes constant elevation, making tall objects seem to lean toward the radar source; an interferogram, on the other hand, allowed [Henrik] to generate a detailed elevation map.

[Henrik] is no stranger to synthetic aperture radar; we’ve previously covered a bike-mounted iteration and a budget SAR system. If the concepts behind this are still a bit fuzzy, we’ve also covered a guide to making your own SAR setup.

Hackaday Prize Entry: Radio Telescope Interferometer

October 31, 2017 by 7 Comments

Radio telescopes are one of the dark arts of science. Not only do you have to deal with RF wizardry, the photons you’re detecting are so far out of the normal human experience that you really don’t know what you’re looking at. It’s hard, but that’s the point — there’s a lot to learn with a radio telescope.

[alfazoOm]’s entry in the 2017 Hackaday Prize seeks to counteract a two-part problem: first, there is a dearth of educational radio interferometers in Latin America. Secondly, in Colombia, there’s only so much clear sky so radio astronomy is the preferred technique. Even though they’re so close to the equator, a lot of the northern stars can be seen as well. His interferometer, IMFR11GHz, answers both of those challenges.

IMFR11GHz is a Michelson interferometer, in which a light source is split into two beams, which are reflected by mirrors back to the detector. [alfazoOm] is basing his telescope off of the Stony Brook radio interferometer, though he is designing custom hardware that can position the dish in whatever direction the operator desires with an Alt-Az mount. The control system consists of an ESP32 microcontroller with an IMU and two stepper motors controlling azimuth and elevation. This is awesome citizen science, and a great entry in the Hackaday Prize.

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