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Virtual Radar from a Digital TV Dongle

Track aircraft by reprogramming an inexpensive digital TV USB stick to receive Automatic Dependent Surveillance-Broadcast signals.

   It is now possible to track aircraft within a 160 km  radius and plot their positions on a real-time display — for about $20! There is a caveat, however — the tracked aircraft must employ a special ADS-B transmitter that continually transmits the aircraft’s flight parameters. The majority of all aircraft flying in US airspace will not be required to use this system until 2020. So, while you probably can’t track that Piper Cub buzzing your neighborhood on a Sunday afternoon, there is still plenty of fun to be had, and you won’t even need to heat up your soldering iron. All you need to do is obtain an inexpensive DVB-T stick that plugs into a computer’s USB connector (this type of device is often referred to as a dongle), download some free software, and build a simple antenna.


ADS-B

 
 Automatic Dependent Surveillance-Broad-cast, or ADS-B, is a replacement for (or supplement to) traditional aircraft position detection by ground-based radar that has been used for more than 50 years for air traffic control.1 This represents a major change in surveillance philosophy — instead of using radar to interrogate an aircraft and determine its position, each aircraft will find its own position using GPS and then automatically transmit this and other information to a network of ground stations. This change is a key part of the FAA’s NextGen — the Next Generation Air Transportation System, which is scheduled to be in full operation by 2020.

More than 27 countries are in the process of building ADS-B ground stations and equipping aircraft with the technology, so the number of aircraft that will show up on virtual radar screens will continue to grow in the coming years. ADS-B has the benefit of being less costly to build and operate than ground radar. It also provides better positional accuracy, which will improve safety, particularly at busy airports.


Virtual Radar System

  
My ADS-B receiving system consists of the three major elements shown in Fig-ure 1: a homebrew collinear antenna, a DVB-T TV tuner dongle, and a Windows-based PC running ADSB# and Virtual Radar Server software. While the FAA is putting millions of dollars into its ground station network, you can make yours for about $25.
 

Antenna

   ADS-B signals are transmitted at 1090 MHz. My antenna is a collinear vertical comprised of eight half-wave coaxial offset sections with a half-wave whip at the top. The antenna is Omni directional and has a gain of 6 dBi. It is designed to be assembled without the need for soldering.

Receiver

   In the US and most of North America, television broadcast stations have switched to digital using the ASTC standard, but in Europe, Australia, and parts of Africa, a different standard called DVB-T (Digital Video Broadcast-Terrestrial) is used. In these areas a small DVB-T stick or dongle is used to receive terrestrial TV broadcasts on laptops and computers.

   While the DVB-T stick was intended as a TV receiver, some clever software developers working on mobile communications in a group called Osmocom discovered that this inexpensive hardware could be repurposed for use as a VHF-UHF software defined radio (SDR) receiver. SDR applications soon added support for this new RF front end, which typically tunes as low as 15 MHz, to well over 1700 MHz. It is important to use a DVB-T TV tuner dongle that uses the sensitive Rafael Micro R820T tuner chip. Sensitivity is as good as most communications gear and continuously variable filter selectivity is made possible through DSP. Integrating these elements creates a low-cost ADS-B ground station that rivals the performance of units costing $500 and up.

Software

   Conversion from RF to digital is done with an open source application called ADSB# (read as "ADSB sharp") created by Youssef Touill and the SDR# development team. This Windows application automatically sets the receiver for optimal performance and processes data transmissions from aircraft within range and sends the raw ADS-B data to the real-time display program.

   Real-time display (see Figure 2) and data sharing is accomplished with the Virtual Radar Server (VRS) application. VRS decodes the ADS-B information and presents it along with other useful data on a real-time Google Maps display. Data from ADSB# is sent using Ethernet protocols so it is not necessary for VRS to be running on the same computer as ADSB#. Virtual Radar Server is a free, open-source development from Andrew Whewell in the UK and provides a great deal of flexibility to present information of interest. VRS and ADSB# include features for sharing data with other "hubs" that aggregate information in order to to provide a global view of aircraft traffic.

Construction
Parts List

A DVB-T dongle capable of 1090 MHz reception. Many online sellers exist, just search for "R820T" and "RTL2832" to be sure you get the right kind. Selling prices have dropped below $15, depending on whether you choose a US or China-based seller.

  • The DVB-T dongles most use MCX connectors, so you’ll need either an adapter or to    cut the supplied antenna cable and splice on a new antenna connector of your choice.
     

  • RG-6/U coaxial cable — enough to reach from your PC to the antenna mounting location, plus an extra 5 feet.
     

  • Three Type F male connectors for RG-6/U.
     

  • One Type F type 90° elbow adapter.
     

  • A 5 foot length of 3⁄4 inch PVC pipe.
     

  • One 3⁄4 inch PVC pipe cap.
     

  • One 3⁄4 PVC pipe T.
     

  • One 3⁄4 inch PVC pipe plug.
     

  • Tools — tape measure, electrical tape, utility knife, and a hacksaw.

Coaxial Collinear Antenna

 
 This antenna provides enough gain to hear plane transmissions from 160 km or more away, yet costs only a few dollars and takes less than an hour to make using the following procedure:
   Cut seven pieces of RG-6/U coax 7 inches long, and one piece 10 inches long. Then, expose 1 inch of the center conductor from one end of all eight coax pieces by rolling the coax against the blade of a sharp utility knife to cut through the vinyl outer jacket, foil shield, and foam insulation. Take care not to press hard enough to cut or nick the center conductor. Make sure the end is clear of stray shield wires or foil.

   Expose 11⁄2 inches of center conductor from the other end of each of the seven short pieces. You should end up with seven pieces of RG-6/U with 1 inch of the center conductor protruding from one end, and 112 inches from the other (see Figure 3). Inspect the cut ends and use the tip of the knife blade to clear any strands of wire or foil shield from the insulation around the center conductor. Expose 412 inches of the center conductor from the other end of the 10 inch piece — this will act as a vertical whip at the top of the antenna.

 
 

   It will be easier to assemble the antenna if the vinyl jackets are made more pliable by heating them. Place the eight pieces on a tray in your oven at its lowest setting (150° F maximum) for 10 to 15 minutes. [Alternatively, use a heat gun or hair dryer to soften the outer coax covering. — Ed.]
 

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Westlakes Amateur Radio Club Inc. York Street, Teralba NSW