Showing what VOR and ILS Aviation Signals Look like in SDR#

Over on YouTube user RedWhiteandPew has uploaded two videos showing what VOR and ILS signals look like in SDR# with an RTL-SDR dongle. VOR and ILS are both radio signals used for navigation in aviation. 

VOR stands for VHF Omnidirectional Range and is a way to help aircraft navigate by using fixed ground based beacons. The beacons are specially designed in such a way that the aircraft can use the beacon to determine a bearing towards the VOR transmitter. VOR beacons are found between 108 MHz and 117.95 MHz.

RedWhiteandPew writes:

Here I am picking up the VOR beacon from KSJC. The coolest part is at the end of the video. I believe the signal moving back and forth is caused by the Doppler effect, because VORs transmit their signals in a circular pattern. The VOR wiki article has a GIF that shows how it works here…. If you play and pause the video at different points before I zoom in, you can see that the two signals on the side are the opposite phase.

ILS stands for Instrument Landing System and is a radio system that enables aircraft to land on a runway safely even without visual contact. It works by using highly directional antennas to create four directional lobes (two in the horizontal plane, two in the vertical) that are used to try and ensure the aircraft is centered and leveled on the approach correctly. The ILS frequencies are at 108.1 – 111.95 MHz for the horizontal ‘localizer’, and at 329.15-335.0 MHz for the vertical ‘glide slope’.

RedWhiteandPew writes:

Here I have tuned into one of KSJC’s ILS frequencies. You are able to hear the faint identifier beeping transmitting its ISL ID code which is ISJC. For comparison, I used to morse code translator website.

The reason I am hearing ISJC and not ISLV even though they are on the same frequency is because the localizers transmitting the signal are directional along the length of the runway. Since I am located to the south east of the airport, and I am within its transmitting beam, I am able to listen to it on a scanner.

If you’re interested in these signals then this previous post about actually decoding them might be of interest to you.

Visualizing the Traveled Path of a Weather Balloon, Tanker Boat and Gliders with an RTL-SDR and CesiumJS

Over on YouTube user pascal poulain has uploaded a short video that shows a timelapse of the flight path of a weather balloon in Cesiumjs as it rises and falls, as well as a time lapse of a marine tanker docking, with the signals received with an RTL-SDR. In a third video pascal also shows a visualization of glider flights tracked via FLARM and the Open Glider Network which also obtains most of it’s data through RTL-SDR contributors.

Cesiumjs is a tool similar to Google Earth. The main difference is that it works on a wider array of devices through a web browser without the need for any plugins. It is often used for visualizing data on the globe. An example of some of its many demos can be found here.

We’re not sure what tools pascal used, but over on GitHub there is a tool called airtrack which can be used together with dump1090 to display flights in real time on Cesiumjs.

Localizing Transmitters to within a few meters with TDOA and RTL-SDR Dongles

Back in August we posted a number of videos from the Software Defined Radio Academy talks held this year in Friedrichshafen, Germany. One of those talks was by Stefan Scholl, DC9ST and titled Introduction and Experiments on Transmitter Localization with TDOA. This was a very interesting talk that showed how Stefan has been using three RTL-SDR + Raspberry Pi setups to locate the almost exact position of various transmitters with time difference of arrival (TDOA) techniques. TDOA works by setting up at least three receivers spread apart by some distance. Due to the speed of radio propagation, the transmitted signal will arrive at each receiver at a different time allowing the physical origin point of the signal to be calculated.

Now over on his blog Stefan has created a very nice writeup of his work with RTL-SDRs and TDOA that is definitely worth a good read. He first explains the basics of how TDOA actually works, and then goes on to explain how his RTL-SDR based system works. He discusses the important challenges such as transferring the raw data, synchronizing the receivers in time and the signal processing required. 

Stefans TDOA System
Stefans TDOA System

He tested the system on various transmitters including a DMR signal at 439 MHz, a mobile phone signal at 922 MHz, an FM signal at 96.9 MHz and an unknown signal at 391 MHz. The results were all extremely accurate, locating transmitters with an accuracy of up to a few meters.

Stefan has also uploaded all his MATLAB code onto GitHub.

Example localization of a DMR transmitter
Example localization of a DMR transmitter
Localizing the position of a mobile phone base station (Stars indicate known base stations)
Localizing the position of a mobile phone base station (Stars indicate known base stations)

Gospell GR-227: New SDR Based Digital Broadcast Radio Adapter for Cars

Over on the SWLing Post blog we’ve seen news of this new SDR based car radio called the Gospell GR-227. Gospell is a Chinese manufacturer of various broadcast consumer radio products including DRM receivers. It is intended to be an adapter for your car that lets you listen to digital broadcast stations such as DAB/DAB+ on VHF and DRM on UHF, but it can also be used for standard AM and FM reception. From the product sheet it looks like it will simply plug into you car USB port, and output audio through that port into your cars head unit. Control of the unit is through an Android app.

There doesn’t seem to be anything stopping someone from using this outside of a car though, so perhaps depending on the price and software hackability available it might make a good PC or Raspberry Pi based HF receiver for all modulation types too.

Over on the Gospell Facebook page are images showing the Gospell running at IBC 2017 and next to other upcoming SDR based digital broadcast receivers like the Titus II.

Gospell SDR Connected to a Car Radio Head Unit
Gospell SDR Connected to a Car Radio Head Unit

No word yet on a release date or pricing. The press release reads:

Chengdu, China, September 04, 2017 – A new adaptor specifically designed for in-car use that simplifies digital radio on the road will be introduced at IBC by Gospell.

GR-227 is a small, low-cost adaptor that acts as an aftermarket add-on to car stereos receiving high-quality digital broadcast programs and data application, and serving it to the car audio system over a USB cable. Based on software defined radio technology, GR-227 is compatible with DAB, DAB+, DRM and is DRM+ ready. It is also powerful enough to support digital audio decoding such as extended HE-AAC (xHE-AAC).

GR-227 literally works with any kind of car stereos with a USB port. Our patent pending technology allows the adaptor to behave like a thumb drive when plug into a USB port and makes it compatible with most of the music players not only in car but also for home use.

To make the most of GR-227, the Gospell Smart Tune App for Android has been included to add more features. When partnered with an Android powered car stereo, the App not only allows for playback of the broadcast audio program but data application which brings much fun to car entertainment.

By connecting the supplied triple band active antenna which can be attached to the windscreen through the SMA antenna connector, the reception in DRM, FM and DAB bands can be significantly improved, offering maximum flexibility between different broadcasting standards.

Installing the plug-and-play GR-227 adaptor to your car is easy and doesn’t require changing your car stereo. It is one of the easiest ways to upgrade your car radio to digital without replacing anything.

The Gospell’s aftermarket car adaptor range starts with USB model but more will follow to support more car stereo types.

Haochun Liu, DRM director, Gospell, said: “By leveraging SDR, we can now combine multiple broadcasting standards together to offer flexibility and cost advantages, coupled with easy installation without the necessity of buying a new car stereo as in traditional solutions.”

For additional information, please visit or contact Gospell sales at [email protected]

About Gospell

Founded in 1993, Gospell Digital Technology Co Ltd (GOSPELL). is a private hi-tech enterprise with R&D, manufacturing, business consultancy and planning, trade, delivery, project implementation and after sales service, acting as a complete DTV and triple-play solution provider for Digital TV/OTT related projects. Headquartered in GOSPELL INDUSTRIAL PARK at Chenzhou, Hunan Province for CPE related production manufacturing, GOSPELL also has its office in Shenzhen for business/marketing management and administration, in Chengdu for R&D and headend/transmitter system production/debugging and Customer Service Center, and in 12 cities in China as well as international offices in India, Africa and Mexico.

[First seen on]

Comparing SSB, NFM, Codec2 and Opus with QRadioLink and an RTL-SDR

Earlier in the month we posted about Adrian M’s video that showed his QRadioLink software running on Android with an RTL-SDR. QRadioLink is a digital amateur radio voice decoder and encoder, that currently supports modern digital voice codecs like Codec2 and Opus. It’s compatible with a wide range of SDRs including the RTL-SDR, as well as TX capable SDRs for transmitting.

Over on YouTube Adrian M has recently uploaded a new video showing a comparison of QRadioLink receiving SSB, NFM, Codec2 and Opus voice signals at the same initial power levels. The results show that the digital modes are generally much clearer and static free even at low TX levels. He writes:

The Linux SDR transceiver application QRadioLink uses here an RTL-SDR dongle for reception. The QRadioLink transmit chain is using an USRP B200 with output power set at about half the maximum. The Codec2 digital mode works down to a low CNR (6 dB) where even SSB is hard to copy. The Opus mode provides good voice quality at a level where analog narrow FM is noisy.
The code for QRadioLink is fully open-source, licensed under GPLv3, and can be found on Github, where it’s undergoing development. Bug reports, patches and suggestions are welcome.

QRadioLink is available over on GitHub.

Using our new Dipole Antenna Kit

Over on our store we’ve recently released our new receive only dipole antenna kit which now replaces the older magnetic whip style antennas from the previous kit. This was done for a few reasons including 1) We believe that the dipole kit is much more versatile and will enable beginners to get better reception right of the bat, 2) magnets of any type are no longer welcome on most airmail parcel carriers (though they still get through for now). While the magnetic whip still works perfectly fine, the dipole kit should make it easier to get the antenna outside, and it also allows for a simple v-dipole configuration for satellite reception.

The units are currently in stock at our Chinese warehouse either bundled with an RTL-SDR or as an individual antenna set. The individual antenna set only has a small quantity at the moment, so get in quick if you want them as they’re only $9.95 USD shipped. We expect more stock to be ready in 1-2 months. Amazon USA should also be stocked with these units in about ~2 weeks from this post date. ALSO, please note that there is a large Chinese holiday between 1 – 7 October, and during this time parcels will not ship out.

This post is a guide on how to use the dipole antenna set in various configurations. First we’ll show and explain about what’s included in the set:

  • 1x dipole antenna base with 60cm RG174 cable and SMA Male connector. This is the dipole base where the telescopic antennas connect to. The short run of RG174 is decoupled from the base elements with a ferrite choke. This helps to prevent the feed line from interfering with the dipole radiation pattern. On the inside of the base the two dipole element sides are connected with a 100 kOhm resistor to help bleed off any static. The dipole has a 1/4 inch female screw on the bottom, which allows you to use standard camera mount products for mounting.
  • 1x 3 meter RG174 coax cable extension. This coax cable extension allows you to place the antennas in a place that gets better reception. E.g. outside on a window, or higher up.
  • 2x 23cm to 1 m telescopic antennas. The telescopic dipoles are detachable from the dipole base via a M5 thread which allows for greater portability and the ability to swap them out. These long telescopic antennas cover VHF and to UHF.
  • 2x 5cm to 13cm telescopic antennas. These smaller antennas cover UHF to 1090 MHz ADS-B, and even still work decently up to L-band 1.5 GHz frequencies.
  • 1x flexible Tripod mount with 1/4″ male screw. This mount allows you to mount the dipole on a variety of different locations. E.g. a pole, tree branch, desk, door, window sill. The legs of the tripod are bendy and rubberized so can wrap securely around many objects.
  • 1x suction cup mount with 1/4″ male screw. With this mount you can mount the dipole on the outside of a window, on a wall, car roof/window, or on any other smooth surface. To use first clean the surface with window cleaner or isopropyl alochol. Then place the suction cup on the cleaned surface and close the lever to activate the suction.
What's included in the new Dipole kit
What’s included in the new Dipole kit

Dipole Orientation

Signals are normally transmitted with either horizontal, vertical or right hand/left hand circular polarization (RHCP/LHCP). This is essentially the ‘orientation’ of a signal, and an antenna with the same polarization should be used too for best performance. A dipole can be used in either vertical or horizontal polarization, just by orienting it either vertically or horizontally.

If you mismatch vertical and horizontal polarization or RHCP and LHCP you’ll get an instant 20dB loss. If you mismatch vertical/RHCP, vertical/LHCP, horizontal/RHCP, horizontal/LHCP you’ll only get a 3dB loss.

There are also ways to optimize the radiation pattern with dipoles. For example for LEO VHF satellites you can use a V-dipole configuration. You can also make a somewhat directional antenna by using a bent dipole configuration. Some more examples of dipole configurations can be found on KK4OBI’s page on bent dipoles.

Terrestrial Signal Reception

Most signals broadcast terrestrially (on Earth) are vertically polarized.

To use the dipole for vertically polarized signals, all that you need to do is orient the elements vertically (up and down).

Satellite Reception

The dipole can be used in a V-Dipole configuration for polar orbiting satellite reception. See Adam 9A4QV’s post where he wrote about how he discovered that it was possible to use dipoles in this configuration for excellent satellite reception. The idea is to use the dipole in horizontal polarization. This gives 3dB loss on the RHCP satellite signals, but also nicely gives 20dB loss on terrestrial signals which could be overloading your RTL-SDR.

For 137 MHz satellites like NOAA and Meteor M2 extend the larger antenna elements out to about 53.4 cm each (about 2.5 sections). Angle the dipole so it is horizontal and in a ‘Vee’ shape, at about 120 degrees. Place the dipole in the North-Source direction.

With an appropriate L-band LNA like the Outernet LNA the dipole can also somewhat work to receive L-band satellites. Using the smallest antenna collapsed, use a V-dipole configuration and point it towards the L-band satellite. Ideally use a reflector too. In the image below we used a simple cookie tin as a reflector. A hole was drilled into the center and the mount used to clamp in the antenna. This together with the Outernet LNA was enough to receive AERO and STD-C.

L-band v-dipole with reflector tin

L-band v-dipole with reflector tin

Receiving Inmarsat signals with the Outernet LNA

Receiving Inmarsat signals with the Outernet LNA

Choosing the Antenna Element Length

Like with the whip you can use an online calculator to calculate the optimal length for your frequency of interest. We recommend this dipole calculator. The exact length does not matter too much, but try to get the lengths as close to what the calculator says as you can. With the dipole you want both elements to be the same length.

In reality extending the antenna to almost any random length will work just fine for most strong signals. But if you’re really trying to optimize those weak signals you’ll want to fine tune the lengths.

Basically the longer the antenna, the lower it’s resonant frequency. The shorter the antenna, the higher the resonant frequency. You want to be close to the resonant frequency. Remember that there is about 2cm of metal inside the antenna itself which needs to be added on. Below is a cheat sheet for various lengths and frequencies. Note that the length refers to the length of one side only.

  • Large Antenna, 5 Sections, 100cm + 2cm is resonant @ ~70 MHz
  • Large Antenna, 4 Sections, 80cm + 2cm is resonant @ ~87MHz
  • Large Antenna, 3 Sections, 60cm + 2cm is resonant @ ~115 MHz
  • Large Antenna, 2 Sections, 42cm + 2cm is resonant @ ~162 MHz
  • Large Antenna, 1 Section, 23cm + 2cm is resonant @ ~ 285 MHz
  • Small Antenna, 4 Sections, 14cm + 2cm is resonant @ ~445 MHz
  • Small Antenna, 3 Sections, 11cm + 2cm is resonant @ ~550 MHz
  • Small Antenna, 2 Sections, 8cm + 2cm is resonant @ ~720MHz
  • Small Antenna, 1 Section, 5cm + 2cm is resonant @ ~1030 MHz.

See the SWR plots at the end for a more accurate reading of the resonance points. But in most cases no matter what you extend the length to the SWR should be below 5 at most frequencies which results in 2.5 dB loss or less. More accurate info on VSWR loss graphs can be found in this document from the ARRL “Understanding SWR by Example” (pdf).

Using the Mounts

The mounts and RG174 extension allow you to more easily use the dipole antennas outside. But please note that like our older magnetic whip we do not recommend permanently mounting this antenna outdoors. This antenna is designed to be a portable antenna that you put up and take down at the end of the day – not for permanent outdoor mounting. It is not protected against water, not grounded so cannot handle a lightning strike and could be damaged with dirt and grime build up. For permanent mounting you could conceivably fill the inside and hinges of the dipole with silicon putty or maybe even hot glue and ground the antenna yourself, but we have not tested this. The stainless steel antennas won’t rust, but dirt and grime could gum up the collapsing mechanism.

The suction cup mount allows you to easily place the antenna on a window, or any smooth surface. To use it first clean the surface thoroughly with isopropyl alcohol or glass cleaner. Then apply the suction cup and close the lever to lock it in place. The lever requires some force to push down, and this ensures a strong grip. You can then angle the antenna in the orientation that you need using the ball socket. Once in place close the ring to lock the ball socket in place.

The flexible tripod mount is useful to mounting the dipole to almost everything else. Including tables, doors, poles, trees etc. The legs of the tripod have a flexible wire inside and rubber sheath so they can be bent into a position to grip almost anything.

Some examples of how to use the mounts.
Some examples of how to use the mounts.

Tightening the hinge

Once you’ve got the orientation of the dipoles the way you want, you might want to tighten the hinge so the elements don’t move so easily anymore. To do this simply take a small screwdriver and tighten the screw on the hinge.

Sample VSWR Plots

Small Antenna Collapsed

Small Antenna Collapsed

Small Antenna Extended

Small Antenna Extended

Large Antenna Collapsed

Large Antenna Collapsed

Large Antenna Extended

Large Antenna Extended

RG174 Cable Loss

RG174 Cable Loss

Other Notes

Note that this is NOT an antenna designed for TXing. It is an RX antenna only. So please do not TX with it unless you really know what you are doing as you could damage your TX radio. You’ll probably need to remove the 100kOhm static bleed resistor to TX anyway.

EOD Robots now packing USRP and HackRF Software Defined Radios

Thanks to the team of Robotics company Servosila for sharing the following press release with us which describes how their new EOD robot makes use of SDR technologies for electronic warfare.

We also wrote back to them and asked for a bit more information on the SDRs used. They wrote that there are two SDR options available for the EOD robot. Option one uses the Ettus Research USRP B205mini-i, and option two uses the HackRF One. This provides a good trade off between cost and functionality.

Servosila introduces Mobile Robots equipped with Software Defined Radio (SDR) payloads

Servosila introduces a new member of the family of Servosila “Engineer” robots, a UGV called “Radio Engineer”. This new variant of the well-known backpack-transportable robot features a Software Defined Radio (SDR) payload module integrated into the robotic vehicle.

“Several of our key customers had asked us to enable an Electronic Warfare (EW) or Cognitive Radio applications in our robots”, – says a spokesman for the company, “By integrating a Software Defined Radio (SDR) module into our robotic platforms we cater to both requirements. Radio spectrum analysis, radio signal detection, jamming, and radio relay are important features for EOD robots such as ours. Servosila continues to serve the customers by pushing the boundaries of what their Servosila robots can do. Our partners in the research world and academia shall also greatly benefit from the new functionality that gives them more means of achieving their research goals.”

Coupling a programmable mobile robot with a software-defined radio creates a powerful platform for developing innovative applications that mix mobility and artificial intelligence with modern radio technologies. The new robotic radio applications include localized frequency hopping pattern analysis, OFDM waveform recognition, outdoor signal triangulation, cognitive mesh networking, automatic area search for radio emitters, passive or active mobile robotic radars, mobile base stations, mobile radio scanners, and many others.

A rotating head of the robot with mounts for external antennae acts as a pan-and-tilt device thus enabling various scanning and tracking applications. The neck of the robotic head is equipped with a pair of highly accurate Servosila-made servos with a pointing precision of 3.0 angular minutes. This means that the robot can point its antennae with an unprecedented accuracy.

Researchers and academia can benefit from the platform’s support for GnuRadio, an open source software framework for developing SDR applications. An on-board Intel i7 computer capable of executing OpenCL code, is internally connected to the SDR payload module. This makes it possible to execute most existing GnuRadio applications directly on the robot’s on-board computer. Other sensors of the robot such as a GPS sensor, an IMU or a thermal vision camera contribute into sensor fusion algorithms.

Since Servosila “Engineer” mobile robots are primarily designed for outdoor use, the SDR module is fully enclosed into a hardened body of the robot which provides protection in case of dust, rain, snow or impacts with obstacles while the robot is on the move. The robot and its SDR payload module are both powered by an on-board battery thus making the entire robotic radio platform independent of external power supplies.

Servosila plans to start shipping the SDR-equipped robots to international customers in October, 2017.


About the Company
Servosila is a robotics technology company that designs, produces and markets a range of mobile robots, robotic arms, servo drives, harmonic reduction gears, robotic control systems as well as software packages that make the robots intelligent. Servosila provides consulting, training and operations support services to various customers around the world. The company markets its products and services directly or through a network of partners who provide tailored and localized services that meet specific procurement, support or operational needs.

Servosila EOD Robot
Servosila EOD Robot

Decrambling Split-Band Voice Inversion with the Deinvert Tool

In early September we posted about Oona Räisänen’s deinvert which is a tool that can be used to unscramble voice audio that has had voice inversion scrambling applied to it. Voice inversion works by scrambling the voice frequencies so that a simple eavesdropper will have trouble listening in. A special descrambling radio is required to listen in. This provides very little real security, but may be enough to stop people with cheap scanners from listening in. Oona’s deinvert tool allows us to take a scrambled audio sample recorded with an RTL-SDR or any other radio and decramble the inversion.

In her latest blog post Oona explains how her deinvert software works and how it can also be used to decode the more difficult split-band inversion technique. She also writes that at the default quality level, the deinvert software is fast enough to run in real time on a Raspberry Pi 1.

Deinverting an inverted audio signal
Deinverting an inverted audio signal