Category: Applications

Listening to an NFC Polling Signal from a Nexus 7 with an RTL-SDR

Over on YouTube user 2e26tenW has uploaded a video showing reception of the second harmonic of an NFC polling signal with his RTL-SDR. NFC stands for “Near Field Communication” and is a technology that enables smartphones and other devices to communicate with one another and some smart cards simply by bringing the two devices together.

In his experiment he uses a Nexus 7 tablet to poll an NFC enabled transportation card. As the RTL-SDR cannot receive the NFC frequency of 13.56 MHz directly without an upconverter or hardware or software direct sampling modified dongle, 2e26tenW instead tunes to the second harmonic at 27.12 MHz which allows him to receive the signal.

Video Showing Decoding of DGPS Beacons with SDR# and MultiPSK

Following on from our last post where dewdude showed how to decode DGPS signalsFrank K2NCC has uploaded a video on YouTube showing DGPS decoding in action. In his video Frank uses an Airspy plus ham-it-up upconverter, a Sirio discone antenna and for software he uses SDR# with audio piped into MultiPSK for decoding.

In the video you can clearly see the decoded DGPS messages showing the pseudorange corrections and station numbers. To decode DGPS with MultiPSK you will need to use the paid version which costs approximately $50 USD, however in the free version the DGPS will run for 5 minutes each time MultiPSK is opened before expiring.

Below is an example of a decoded message.

24/03/2015 02:06:09
Message type        : 9 (GPS partial correction set)
Station number      : 172 (Appleton WA USA 300.0 Khz TXID 871 100bps)
Z-count             : 4215 ( 42 mn 9.0 s )
Sequence count      : 2le factor=0.3)

Sat. ID|SF|UDRE|Pseudorange corr.  |Range rate corr.|IOD|CRC
25     |0 |1-4m|      -7.68 m      |   0.000 m/s    |62 |OK
31     |0 |1-4m|       1.54 m      |   0.000 m/s    |27 |OK
32     |0 |1-4m|       0.70 m      |   0.000 m/s    |99 |Error

Decoding Differential GPS Beacons with an RTL-SDR, Speclab and SDR#

Over on his blog “RTL-SDR DX” dewdude has been exploring the reception and decoding of Differential GPS (DGPS) signals. DGPS signals are transmitted by government authorities in the long wave band at around 300 kHz. These beacons are used to dramatically improve the accuracy of GPS (Global Positioning System) devices from their default accuracy of about 15 m down to about 10 cm. Unlike GPS signals which originate from satellites, the DGPS signal is terrestrial based and is broadcast from multiple known fixed positions. The signal itself contains information about the difference between the DGPS stations received GPS position and it’s known exact position. These differences can be used to correct other GPS receivers that receive DGPS signal.

By using his RTL-SDR (with upconverter or HF modification) dewdude was able to receive the DGPS beacon in SDR#. Then by piping the output audio into SpectrumLab’s DGPS decoder he was able to decode the data contained within the DGPS signal. His post contains a tutorial showing how to set up SpectrumLab to decode DGPS. If you’re interested in hearing what a DGPS signal sounds like, dewdude has uploaded a sound sample at the bottom of another post of his.

Decoding Differential GPS (DGPS) signals in SpectrumLab
Decoding Differential GPS (DGPS) signals in SpectrumLab

Using a USRP E310 for Digital Video Downlink and Scanning on a Drone

Balint, one of the researchers at Ettus Research (the company behind the USRP range of software defined radios) has recently uploaded a video to YouTube showing one of his projects where he is prototyping the use of a digital signal for transmitting digital FPV video on a drone. The drone carries a USRP E310 SDR and transmits a QPSK video down developed in GNU Radio to a receiver on the ground.

FPV strands for “first person view” and is a growing hobby where remote controlled aircraft such as quadcopter drones are flown in first person view using live video from an on board camera.

In another video balint also shows how the on board E310 can be used to transmit frequency scan FFT data via a WiFi link. This can be very useful for getting an antenna up high enough to get good reception for a scan.

Visualizing the electromagnetic spectrum with Frequensea and an RTL-SDR

Over on YouTube user Frederik De Bleser has uploaded a video showing his new open source toolkit called Frequensea which can be used for visualizing the electromagnetic spectrum with an RTL-SDR or HackRF. The software allows you to visualize the output as an FFT spectrum in various 3D display modes and is even compatible with the Oculus Rift, a virtual reality headset.

Frequensea can be downloaded from https://github.com/fdb/frequensea, and it currently has installation instructions available for OSX, Ubuntu and the Raspberry Pi.

New ADS-B Android App for Europe

Android app programmer Nikos recently wrote in to let us know about his new app called “Track your flight Europe”. His app can be used together with an RTL-SDR and USB OTG cable to track aircraft via ADS-B. The difference between Nikos’ app and other similar ADS-B apps is that his app is specifically designed for tracking the aircraft you are flying in by providing an offline map which does not require an internet connection to display.

As the app is currently in the alpha stages of development, Nikos is looking for people to help test it out. The alpha can be downloaded for free on the Android Google play store at https://play.google.com/store/apps/details?id=org.qtproject.example.Android_Application_ADSB_final.

Europe ADS-B App
Europe ADS-B App

RTL-SDR Tutorial: Measuring filter characteristics and antenna VSWR with an RTL-SDR and noise source

By using an RTL-SDR dongle together with a low cost noise source it is possible to measure the response of an RF filter. Also, with an additional piece of hardware called a directional coupler the standing wave ratio (SWR) of antennas can also be measured. Measuring the response of a filter can be very useful for those designing their own, or for those who just want to check the performance and characteristics of a filter they have purchased. The SWR of an antenna determines where the antenna is resonant and is important for tuning it for the frequency you are interested in listening to.

These tutorials are based heavily on information learned from Adam Alicajic’s (9A4QV), videos which can be found at [1], [2], [3], [4]. Adam is the creator of the LNA4ALL and several other RTL-SDR compatible products. Recently Tim Havens also posted some experiments with characterizing home made filters on his blog.

Characterizing Filters

Using just a noise source and RTL-SDR dongle it is possible to determine the properties of an RF filter.

In our experiments we used the following equipment:

Equipment

The BG7TBL noise source is a wideband noise source that can provide strong noise over the entire frequency range of the RTL-SDR. It requires power from a 12V source which can be obtained from a common plug in power supply. It also uses an SMA female connector, so you may need some adapters to connect it to your filter under test (adapters can be found cheaply on Ebay). Finally a quick warning: be careful when handling the circuit board after it has been powered for some time as some of the components can get very hot. Note that if the Ebay store runs out of these there is also a seller on Aliexpress with some available, just type “noise source” in the search bar.

The BG7TBL Noise Source
The BG7TBL Noise Source

If you have a ham-it-up upconverter and are good at soldering small surface mount components you might instead consider purchasing the noise source kit add on. Here is a video showing how to build and test the ham-it-up noise source.

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New GUI for rtl_power: QSpectrumAnalyzer

A new GUI for rtl_power has been released by programmer Mikos. Although there are already several rtl_power GUIs and spectrum analyser applications that exist, Mikos developed QSpectrumAnalyzer because he found that the alternatives were either slow, closed source or Windows only.

Rtl_power is a command line tool that can be used with an RTL-SDR to create a spectrum scan of a large swath of bandwidth that is greater than the RTL-SDRs maximum sample rate.

The project can be found at https://github.com/xmikos/qspectrumanalyzer and Mikos is open to pull requests on GitHub.

QSpectrumAnalyzer GUI for  rtl_power
QSpectrumAnalyzer GUI for rtl_power