RTL-SDR As a Spectrum Analyzer

Hackaday has brought to attention a blog post by Kerry Wong which shows how the RTL-SDR can be used as a simple and inexpensive spectrum analyzer. In the past we’ve already posted numerous examples of the RTL-SDR being used as a spectrum analyzer but Kerry’s post discusses some of the do’s and don’ts that you need to think about when using a SDR as a spectrum analyzer and also provides some measurements.

During his tests he discovered that popular software like RTLSDR Scanner and SDR# either distort the spectrum or don’t display the signal amplitude correctly. Only GQRX and osmocom_fft seemed to give an accurate depiction of the spectrum.

Kerry also discusses how to calibrate the spectrum display to show proper power levels, how to set the gain for spectrum analysis and discusses some thoughts on LO leakage.

Using an RTL-SDR as a spectrum analyzer with osmocom_fft
Using an RTL-SDR as a spectrum analyzer with osmocom_fft

Improving HF Reception By Disconnecting the Switching Power Supply on the RTL-SDR

By using an upconverter, direct sampling mod, or experimental software driver the RTL-SDR can be used for HF reception. However, a problem with HF reception and the RTL2832U chip is that it uses a switching power supply in its design. This switching power supply causes significant amounts of noise spurs to appear in the HF spectrum.

Japanese RTL-SDR experimented ttrftech has recently posted about a modification to the RTL-SDR which he performed (note in Japanese, use Google translate to read). The purpose of the switching power supply in the RTL2832U is to step 3.3v down to 1.2v. In his modification, ttrftech disconnected the switching power supply and instead stepped 3.3v down to 1.2v by using three diodes. This works as each diode has a voltage drop across it of 0.7v.

His results show that there is a significant reduction in noise spurs at HF frequencies.

Switching power supply replaced by three diodes.
Switching power supply replaced by three diodes.
Unmodded reception at 7 MHz
Unmodded reception at 7 MHz
Modded reception at 7 MHz
Modded reception at 7 MHz

Watching a VHS Tape using a RTL-SDR

Over on YouTube user DogsRNiceMineCraft has uploaded a video showing a VHS tape being played using an RTL-SDR. To do this he connected the RF out port on his VHS tape player by wrapping the RTL-SDR stock antenna cord around the RF out cable from the VCR. He then used the TV Sharp software to view the VHS tape.

The playback quality is very poor, but the concept works!

viewing a vhs tape on a computer using sdr

SatNOGS – Hackaday Prize Winner uses RTL-SDR in Design

The popular Hackaday blog recently announced the winner of their grand competition to win a trip to space or $200k. The goal of the competition was to design and build the best example of “an open, connected device”. The winner of the competition is SatNOGS, a system that hopes to enable a low cost network of satellite ground stations thus enabling greater access to satellite data. The radio receiver used in the SatNOGS hardware is a standard RTL2832U R820T RTL-SDR dongle.

The SatNOGS hardware is a system that uses high gain antennas, tracking motors, a RTL-SDR and a PC running GNU Radio and other software to automatically track, receive and record satellites as they pass over head. The open source software works to automatically schedule observations and record them to an online database.

More information about SatNOGS can also be found on their website https://satnogs.org/.

The third prize winner of the Hackaday prize was the ‘PortableSDR’, which we posted about previously.

SatNOGS Hardware Tracking a Satellite
SatNOGS Hardware Tracking a Satellite
SatNOGS Hardware with RTL-SDR Dongle Visible
SatNOGS Hardware with RTL-SDR Dongle Visible
SatNOGS Project – THP Finals

ADS-B Onboard a 737 with Realtime Primary Flight and Navigation Display

Recently we found this video from 2013 on YouTube by user carcharias04 showing an RTL-SDR being used for ADS-B on board a 737-800 commercial jet. In the video he uses a custom program that interfaces with RTL1090 and XHSI, which is a navigation display program for the popular flight simulator known as X-Plane.

With his RTL-SDR, RTL1090, his custom software and XHSI running he is able to see a real time display of the primary flight and navigation displays which are the same or similar to the instruments used by the pilots in the cockpit.

Unfortunately, it seems like the uploaders custom interface program is not available anywhere that we know of.

Update 1: The software is this interface available on GitHub. Schumann-resonance from the comments section has uploaded a precompiled binary file here http://www57.zippyshare.com/v/49667810/file.html.

Update 2: To get it to work you need to first set the Table 2 name in RTL1090 to “tableb”, then run RTL1090 first before opening RTL1090-XHSI. Then enter the ICAO of the flight you’d like to use in the text box at the top of the interface window. Now data should begin to appear in the RTL1090-XHSI Window. Now you can open XHSI and it should automatically begin using the ADS-B data.


RTL1090 ADSB live on board Boeing 737-800

Decoding the Russian Parus (Cosmos) Navigation Satellites with the RTL-SDR

Once again Happysat, who previously wrote in to RTL-SDR.com to let us know how to receive dead satellites with the RTL-SDR has again written in to let us know about his latest achievements.

Happysat has recently been using a RTL-SDR to decode the discontinued Russian Parus (Cosmos) Navigation Satellites. These are low earth orbit satellites operated by the Russian Space Forces that are used for military communication and navigation. Since 1974 there have been 99 Parus satellites launched, but there are only three currently active.

With an RTL-SDR, SDR# and decoder software, Happysat was able to decode data from the satellite which includes the current Moscow time and various location and telemetry data.

Russian Parus (Cosmos) Satellite Decoded Data
Russian Parus (Cosmos) Satellite Decoded Data

Happysat writes:

The Russian Military Parus satellites are/where used for low-earth orbiting navigation information and store-dump radio communications relay service for Red Navy surface vessels and submarines.

Each satellite is in a near-circular orbit of about 1000 km (620 miles).The orbits are polar (pass over the poles of the earth) and stay stationary in space so that as the earth rotates, the satellite covers different parts of the planet.

There are three currently operational, Cosmos 2407, 2414, and the last one launched in this series (April 2012) Cosmos 2463 with a lifespan of 4 years.

They are discontinued now as the GLONASS Navigation systems did take over the service which are providing a better accurate GPS position.

Why the Parus-Satellites are currently still broadcasting data is unknown. Most probably due the older vessels and submarines are still using Shkhuna Radio systems.

The satellites transmit two radio carriers, one on VHF which is FM modulated with the navigational data around 10 watts, and one on UHF which is unmodulated (tracking Beacon).

Already in 1980 the British Kettering Group was able to decode the encryption. The first 18 Bit of data contain the Moscow time, the other Bits contain the positions and orbits from the other active Parus-Satellites. Very similar data like our GPS output and the Orbcomm satellites (on 137 Mhz) with OrbcommPlotter (explained also on rtl-sdr).

With RTL-SDR and SDRSharp its possible also to decode the Russian Military Parus satellites.

Alan Cordwell did write a Java decoder that will decode the navigation data from the VHF transmission in non-real time. cosmos_export.rar [Mirror] It is experimental software and very basic.

Unfortunately his website is offline but still on web-archive (link to web archive)

You will need to record the audio from the satellite and save it as a 44100, 8-bit, mono .wav file. Which means SDRSharp included wave recording is not suitable to do this you need a external program like Audacity or any favourite which can handle the requirements above.

Then you will need to apply narrow bandpass filtering to it at 3, 5 and 7 kHz. Without this filtering step only a small portion of the data will be available, i’m still struggling with this step ;)

He did use Cool Edit Pro to do this, there are no doubt other apps available like Audacity. Included is a sample filtered audio file for you to try it with, it’s in the rar archive as well.

Unpack the archive to a convenient location a folder called cosmos-export will result. Execute the file cosdec.jar to run the application. Open a file with file/open and in the file chooser dialog select the wav file. To decode it go to Actions/Process File.

To write all output to a log file: check the Dump to log option in File menu (log file will be created with same name as audio file but with .log extension appended). Datascope does as it suggests; it launches a little frame that shows (using graphics) the waveform of the data recovered from the audio. The frequency axis is upside down! you’ll see the 3, 5 and 7kHz bits with 7 at the bottom.

There is another program (Sorcerer) which can decode in real-time only, the current actual atom Moscow time from the satellites.

[sorcer download]

Start sorcerer and go to the menu add decoder on the left FSK and choose COSMOS NAVDATA. Move the first bar slider in the spectrum to 3 kHz so the second and third are on 5 and 7 KHz as seen in the screenshot.

Its possible to run this application and record audio at the same time which is later on needed for the cosmos_export Java program.

The frequencies are as follows:

Cosmos 2407 and 2414
VHF Frequency NavData 149.970 MHz, Tracking Beacon 399.920MHz

Cosmos 2463
VHF Frequency NavData 149.940 MHz Tracking Beacon 399.840MHz

Orbital parameters and predictions:

You need to run Orbitron in SDRSharp to take care of the Doppler!

TLE’s for the current satellites, (these are current as of 10th November 2014)

1 28380U 04028A   14313.17698750  .00000073  00000-0  59418-4 0  2927
2 28380  82.9601 332.2565 0038129 220.5020 139.3308 13.75978622517201

1 28521U 05002A   14314.08629907  .00000094  00000-0  65641-4 0  6502
2 28521  82.9510   6.9860 0040501 164.6577 195.5838 13.87513505496523

1 36519U 10017A   14313.57202739 -.00000009  00000-0 -27079-4 0  4041
2 36519  82.9553 122.2841 0036820   7.8477 352.3245 13.71357663227175

New ones can be downloaded at celestrak: musson.txt

Good luck and if anyone has good knowledge on how to apply narrow bandpass filtering at 3, 5 and 7 kHz, please write in the comments!

Note that Orbitron can be downloaded from http://www.stoff.pl/, and the SDR# plugin to interface with it can be downloaded from SatelliteTracker2.zip or here. A tutorial on using Orbitron with SDR# for Doppler correction can be found on our NOAA Weather satellite reception tutorial.

SDR# with Orbitron for Doppler Correction and Sorcerer for Decoding Cosmos
SDR# with Orbitron for Doppler Correction and Sorcerer for Decoding Cosmos
Image of the Russian Parus  Satellite
Image of the Russian Parus Satellite
Image of the Russian Parus  Satellite
Image of the Russian Parus Satellite
Which option to choose in Sorcerer
Which option to choose in Sorcerer

YouTube Airspy Review and Comparison

Over on YouTube user ranickel (aka W9RAN) has uploaded a video showing his review of the Airpsy software defined radio. Although the Airspy is not yet shipping, several independent reviewers received a unit early.

In the video W9RAN compares the Airspy against seven traditional analogue hardware radios and the RTL-SDR using a amateur radio beacon at 28 MHz. He also demonstrates the software decimation feature of the Airspy drivers and shows how it can increase the sensitivity and signal to noise ratio.

Airspy Review and Comparison by W9RAN

Android App RFAnalyzer Now on Google Play with Support for the RTL-SDR

Previously we posted about the new RFAnalyzer Android app for the HackRF which has a RF spectrum and waterfall display. Now RFAnalyzer is available on the Google Play store with experimental support for the RTL-SDR dongle. The app also now supports AM and FM audio demodulation.

The app is fully open source and the code and APK can be downloaded for free from its Git repository. Alternatively, the app can be downloaded from the Google Play store at a small cost of $0.99 USD.

To use the app you’ll need a USB OTG cable to connect your HackRF or RTL-SDR to your Android phone. More information on the app can be found on the authors blog.

An alternative Android app to RFAnalyzer is SDR Touch.

RF Analyzer Android App for the HackRF and RTL-SDR
RF Analyzer Android App for the HackRF and RTL-SDR

Military SATCOM Satellite Reception with the RTL-SDR

Recently on YouTube user Tom Mladenov has been uploading videos showing reception of Military SATCOM satellites using an RTL-SDR dongle and QFH antenna tuned to 137 MHz. One video shows how sometimes Russian civilian phone calls are routed to other continents via military satellite.

SATCOM – West Russian Phone Relay channel – RTL SDR

Another video shows what military SATCOM data channels look and sound like on the FleetSatcom 7 satellite.

UHF Military SATCOM Data channels – RTL SDR

Simple RTL-SDR Shielding with Copper Tape

Last month we posted about an RTL-SDR experimenter who shielded his RTL-SDR dongle using an aluminium case. Shielding the RTL-SDR helps to reduce out of band interference which can enter the device through the PCB itself.

Now Melih, another RTL-SDR experimenter has recently posted on his blog about his simpler shielding method that uses cheap copper tape. The copper tape was purchased from ebay, and is the type that is commonly used for creating a slug barrier in the garden.

Over on the Reddit thread discussing his work, there are some concerns about excessive crystal drift due to there being no ventilation holes. However, it seems that the general consensus is that lack of ventilation will not significantly affect crystal drift and may actually help to stabilise the crystal over time by keeping the internal temperature more constant.

You may also be interested in Melih’s previous post where he replaces the MCX connector on the dongle with an SMA female connector.

Copper Tape Shielding RTL-SDR
Copper Tape Shielding RTL-SDR