Build a Passive Radar With Software-Defined Radio

Usually, regarding radio-related initiatives, my home in New York City is an awful location. If we could see and hear the radio signals, it could make an EDM party seem like a sensory deprivation tank. Radio interference is a problem that plagues the city. However, for the first time, I saw that I could use this electromagnetism kaleidoscope to my advantage, using the help of a passive radar station.

Contrary to traditional radars, passive radar does not send pulses and search for reflections. Instead, it utilizes signals from the surrounding environment. An antenna that is a reference picks up signals from, for instance, a cell tower, and a surveillance antenna is set to a similar frequency. The signal from the reference antenna and that of the surveillance are contrasted. If a reflection of an object is observed, then the time taken to reach the surveillance antenna will give an approximate distance. Frequency shifts reveal how fast the thing moves due to the Doppler effect. Doppler effect.

I was intrigued by passive radar as I was looking to test a brand new radio that is software-defined (SDR) on its tests. I’ve been following amateur SDR advancements to IEEE Spectrum since 2006, the year that SDR became something within a maker’s budget. The most significant advancement came in 2012 when we discovered that USB stick TV tuners that used an RTL2832U modulator chip could create extraordinarily cheap but highly effective SDR receivers. A flurry of curiosity about SDRs ensued. Based on the interest generated by this trend, several manufacturers are now producing premium but comparatively affordable SDRs. This includes USB sticks that are RTLx-based and equipped with higher-quality support components and designs than TV tuners from the beginning and new receivers, such as that of the RSPDx. These SDRs can transmit and receive, like those that use the HackRF One or Lime Mini.

I was thinking about getting back into SDR using the help of one such device when I came across this CrowdSupply offer for US 399 KrakenSDR. It’s a receiver-only device. However, it’s more than two or three tuners, not even five! The tuners are built in the RTL R820T2/R860 chipset and are integrated with hardware to automate coherence synchronization between them.

The KrakenRF SDR and The Raspberry Pi 4 [middle bottom require a decent amount of power through USB C cables, so an energy pack (top middle] is necessary to operate on a mobile basis. This Pi can be connected to SDR via a data connection, and it is connected to the SDR by coaxial cable to 2 TV antennas, left and right and left. JAMES PROVOSTThat means that, for instance, you can put five omnidirectional antennas within the form of a circle and perform the direction-finding radio function by watching the time a signal is received by each one. In normal circumstances, an amateur looking to find direction must swivel around a directional antenna, an effort that takes work to perform while driving, for example, the vehicle.

The KrakenSDR’s ability to perform passive radar attracted my attention as a brand-new feature in low-cost radio technology, which is why I paid the cash. Next was to purchase appropriate antennas. The radio manufacturer, KrakenRF, recommends directional Yagi TV antennas for two reasons. The first is that while KrakenSDR works with a variety of signals, including FM radio or cell tower transmissions, Digital TV signals are ideal to work with since they’re equally distributed across the broadcast band of the channel as opposed to the more narrow and more erratic signals from the FM stations. (KrakenRF advises that if one has to utilize the FM channel, choose the one that is heavy-metal “since heavy metal is closer to white noise.”) The other reason is that having an antenna in a direction away from the source used to determine it means it’s less likely to get overwhelmed with reference signals.

I bought two compact and lightweight television antennas for $19. It was essential to have portability as I had to carry the entire setup to the apartment’s roof, and my location in the outside borough of the city offered additional advantages. The first is that the sky above is an ongoing supply of aircraft departing from NYC’s airports. Significant metal components moving in a background of no color are excellent radar test subjects. The second reason is that my roof has an incline to the Empire State Building, allowing me to choose as a reference signal among more than a dozen TV channels broadcasting from the spire.

I set up my rig with a powerful battery pack, the KrakenSDR cables and antennas, and a Raspberry Pi 4 to process data from the SDR. KrakenRF provides images of SD cards to the Pi, including an operating system configured to work with the already-installed open-source software. It also allows you to set up the Pi as an access point for Wi-Fi using a Web-based interface. I wish that more companies would take this method since installing open-source software can be difficult when you try to recreate the exact system environment that it was created. Suppose you decide to run the KrakenSDR software elsewhere than the Pi. In that case, having a reliable setup can be helpful as a reference and lets you evaluate the performance of your hardware.


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