Bike-Mounted Synthetic-Aperture Radar Makes Detailed Images

Synthetic-aperture radar, in which a moving radar is used to simulate a very large antenna and obtain high-resolution images, is typically not the stuff of hobbyists. Nobody told that to [Henrik Forstén], though, and so we’ve got this bicycle-mounted synthetic-aperture radar project to marvel over as a result.

Neither the electronics nor the math involved in making SAR work is trivial, so [Henrik]’s comprehensive write-up is invaluable to understanding what’s going on. First step: build a 6-GHz frequency modulated-continuous wave (FMCW) radar, a project that [Henrik] undertook some time back that really knocked our socks off. His FMCW set is good enough to resolve human-scale objects at about 100 meters.

Moving the radar and capturing data along a path are the next steps and are pretty simple, but figuring out what to do with the data is anything but. [Henrik] goes into great detail about the SAR algorithm he used, called Omega-K, a routine that makes use of the Fast Fourier Transform which he implemented for a GPU using Tensor Flow. We usually see that for neural net applications, but the code turned out remarkably detailed 2D scans of a parking lot he rode through with the bike-mounted radar. [Henrik] added an auto-focus routine as well, and you can clearly see each parked car, light pole, and distant building within range of the radar.

We find it pretty amazing what [Henrik] was able to accomplish with relatively low-budget equipment. Synthetic-aperture radar has a lot of applications, and we’d love to see this refined and developed further.

[via r/electronics]

The Death Of A Weather Satellite As Seen By SDR

What is this world coming to when a weather satellite that was designed for a two-year mission starts to fail 21 years after launch? I mean, really — where’s the pride these days?

All kidding aside, it seems like NOAA-15, a satellite launched in 1998 to monitor surface temperatures and other meteorologic and climatologic parameters, has recently started showing its age. This is the way of things, and generally the decommissioning of a satellite is of little note to the general public, except possibly when it deorbits in a spectacular but brief display across the sky.

But NOAA-15 and her sister satellites have a keen following among a community of enthusiasts who spend their time teasing signals from them as they whiz overhead, using homemade antennas and cheap SDR receivers. It was these hobbyists who were among the first to notice NOAA-15’s woes, and over the past weeks they’ve been busy alternately lamenting and celebrating as the satellite’s signals come and go. Their on-again, off-again romance with the satellite is worth a look, as is the what exactly is going wrong with this bird in the first place.

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L Band Satellite Antennas Revealed

[SignalsEverywhere] has a lot of satellite antennas and he’s willing to show them off — inside and out — in his latest video that you can see below. Using software-defined radio techniques, you can use these antennas to pull off weather satellite images and other space signals.

A lot of these antennas are actually made for some commercial purpose like keeping ships connected to Inmarsat. In fact, the shipborne antenna has a nice motorized system for pointing the antenna that [SignalsEverywhere] is hoping to modify for his own purposes.

With what appears to be standard NEMA 17 steppers onboard, it should be relatively easy to supplant the original controller with an Arduino and CNC shield. Though considering the resale value these particular units seem to have on eBay, we might be inclined to just roll our own positioner.

The QHF QFH antenna is another interesting teardown. The antenna makes a helix shape and looks like it would be interesting to build from scratch. There isn’t a lot of details about the antenna designs, but it is interesting to see the variety and range of antennas and how they appear internally.

L band is from 1 GHz to 2 GHz, so signals and antennas get very strange at these frequencies. The wavelength of a 2GHz signal is only 15cm, so small antennas can work quite well and are often as much mechanical designs as electrical. The L band contains everything from GPS to phone calls to ADS-B.

We’ve seen radiosonde antennas reborn before. Dish antenna repurposing is also popular.

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Antenna Tuning For GHz Frequencies

Antenna tuning at HF frequencies is something that radio amateurs learn as part of their licence exam, and then hone over their time operating. A few basic instruments and an LC network antenna tuner in a box are all that is required, and everything from a bit of wet string to ten thousand dollars worth of commercial antenna can be loaded up and used to work the world. When a move is made into the gigahertz range though it becomes a little more difficult. The same principles apply, but the variables of antenna design are much harder to get right and a par of wire snippers and an antenna tuner is no longer enough. With a plethora of GHz-range electronic devices surrounding us there has been more than one engineer sucked into a well of doom by imagining that their antenna design would be an easy task.

An article from Baseapp then makes for very interesting reading. Titled “Antenna tuning for beginners“, it approaches the subject from the perspective of miniature GHz antennas for IoT devices and the like. We’re taken through the basics and have a look at different types of antennas and connectors, before being introduced to a Vector Network Analyser, or VNA. Here is where some of the Black Art of high frequency RF design is laid bare, with everything explained through a series of use cases.

Though many of you will at some time or other work with these frequencies it’s very likely that few of you will do this kind of design exercise. It’s hard work, and there are so many ready-made RF modules upon which an engineer has already done the difficult part for you. But it does no harm to know something about it, so it’s very much worth taking a look at this piece.

It’s an area we’ve ventured into before, at a Superconference a few years ago [Michael Ossmann] gave us a fundamental introduction to RF design.

Retrotechtacular: The OSCAR 7 Satellite Died And Was Reborn 20 Years Later

If I were to ask you what is the oldest man-made orbiting satellite still in use, I’d expect to hear a variety of answers. Space geeks might mention the passive radar calibration spheres, or possibly one of the early weather satellites. But what about the oldest communication satellite still in use?

The answer is a complicated one. Oscar 7 is an amateur radio satellite launched on November 5th 1974, carrying two transponders and four beacons, all of which operate on bands available to amateur radio operators. Nearly 45 years later it still provides radio amateurs with contacts just as it did in the 1970s. But this bird’s history is anything but ordinary. It’s the satellite that came back from the dead after being thought lost forever. And just as it was fading from view it played an unexpected role in the resistance to the communist government in Poland.

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RTL-SDR: Seven Years Later

Before swearing my fealty to the Jolly Wrencher, I wrote for several other sites, creating more or less the same sort of content I do now. In fact, the topical overlap was enough that occasionally those articles would get picked up here on Hackaday. One of those articles, which graced the pages of this site a little more than seven years ago, was Getting Started with RTL-SDR. The original linked article has long since disappeared, and the site it was hosted on is now apparently dedicated to Nintendo games, but you can probably get the gist of what it was about from the title alone.

An “Old School” RTL-SDR Receiver

When I wrote that article in 2012, the RTL-SDR project and its community were still in their infancy. It took some real digging to find out which TV tuners based on the Realtek RTL2832U were supported, what adapters you needed to connect more capable antennas, and how to compile all the software necessary to get them listening outside of their advertised frequency range. It wasn’t exactly the most user-friendly experience, and when it was all said and done, you were left largely to your own devices. If you didn’t know how to create your own receivers in GNU Radio, there wasn’t a whole lot you could do other than eavesdrop on hams or tune into local FM broadcasts.

Nearly a decade later, things have changed dramatically. The RTL-SDR hardware and software has itself improved enormously, but perhaps more importantly, the success of the project has kicked off something of a revolution in the software defined radio (SDR) world. Prior to 2012, SDRs were certainly not unobtainable, but they were considerably more expensive. Back then, the most comparable device on the market would have been the FUNcube dongle, a nearly $200 USD receiver that was actually designed for receiving data from CubeSats. Anything cheaper than that was likely to be a kit, and often operated within a narrower range of frequencies.

Today, we would argue that an RTL-SDR receiver is a must-have tool. For the cost of a cheap set of screwdrivers, you can gain access to a world that not so long ago would have been all but hidden to the amateur hacker. Let’s take a closer look at a few obvious ways that everyone’s favorite low-cost SDR has helped free the RF hacking genie from its bottle in the last few years.

Continue reading “RTL-SDR: Seven Years Later”

Drone On Drone Warfare, With Jammers

After the alleged drone attacks on London Gatwick airport in 2018 we’ve been on the look out for effective countermeasures against these rogue drone operators. An interesting solution has been created by [Ogün Levent] in Turkey and is briefly documented on in his Dronesense page on Crowdsupply. There’s a few gaps in the write up due to non-disclosure agreements, but we might well be able to make some good guesses as to the missing content.

Not one, but two LimeSDRs are sent off into the air onboard a custom made drone to track down other drones and knock them out by jamming their signals, which is generally much safer than trying to fire air to air guided missiles at them!

The drone hardware used by [Ogün Levent] and his team is a custom-made S600 frame with T-Motor U3 motors and a 40 A speed controller, with a takeoff weight of 5 kg. An Adventech single board computer is the master controller with a Pixhawk secondary and, most importantly, a honking great big 4 W, 2.4 GHz frequency jammer with a range of 1200 meters.

The big advantage of sending out a hunter drone with countermeasures rather than trying to do it on the ground is that, being closer to the drone, the power of the jammer can be reduced, thus creating less disturbance to other RF devices in the area – the rogue drone is specifically targeted.

One of the LimeSDRs runs a GNU radio flowgraph with a specially designed block for detecting the rogue drone’s frequency modulation signature with what seems to be a machine learning classification script. The other LimeSDR runs another *secret* flowgraph and a custom script running on the SBC combines the two flowgraphs together.

So now it’s the fun part, what does the second LimeSDR do? Some of the more obvious problems with the overall concept is that the drone will jam itself and the rogue drone might already have anti-jamming capabilities installed, in which case it will just return to home. Maybe the second SDR is there to track the drone as it returns home and thereby catch the human operator? Answers/suggestions in the comments below! Video after the break. Continue reading “Drone On Drone Warfare, With Jammers”