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Feature Article 1

FEATURE ARTICLE

Matt Debski, Aircraft Owner WVFC tdebber@alum.mit.edu

Stratux ADS-B In Receiver Build Report

Whenever I start to write about ADS-B, it's hard to know if my audience is fairly knowledgeable on the topic or if the acronym is meaningless.  This article won't be another "what is ADS-B and why is it important to me" article, but I'll do a brief introduction then move into my experience putting together a relatively inexpensive receiver.

ADS-B (automated dependent surveillance-broadcast) is a key component of FAA's NextGen upgrade to air traffic control.  ADS-B Out transponders must be installed in aircraft by January 2020 in order to operate in places where a Mode C transponder is required today.  Current transponders respond to ground-based radar interrogation signals with the squawk code.  ADS-B transponders transmit much more information autonomously.  The information transmitted includes GPS-based position and velocity and a unique aircraft identifier.  Other aircraft can receive this information as well as ground-based air traffic control facilities.

To encourage the more willing adoption of this technology, ground facilities transmit traffic and weather information that can be received by ADS-B In receivers.  This information can be displayed on in-panel GPS equipment and many popular tablets.  ADS-B In capability is not required by the 2020 mandate, but most ADS-B Out equipment includes the In receiver as well.  Note that there is no cost for a subscription, as opposed to XM weather services.

While the ADS-B Out transmitter must be installed in the panel by an A&P, there are several portable ADS-B In receivers available today.  These cost between $550 and $1200, with the more expensive units also including attitude information.  An open-source project called Stratux makes it relatively easy to build your own ADS-B In receiver that is compatible with many tablet applications.  Over the holidays, I ordered the parts for a Stratux and had it up and running with pleasantly little trouble in about an hour of work.

There are several different routes to go if you'd like to build your own receiver.  The first stop is the Stratux web site, http://stratux.me  After an introductory paragraph, the site lists several different options for how much do-it-yourself you care to do.  Each list includes links to Amazon so that you can prepare a shopping list and have the parts on their way to your door in a few minutes.  I chose the full-featured (dual-band), most DIY option, with a price of around $130.  As an Amazon Prime member, I had the parts two days later.

The most difficult part of the build process was determining which set of directions to follow.  The web site has one set of directions, several of the parts come with links to directions, and there are directions in the articles linked at the bottom of the web site.  In the end, I went with a combination of the directions that were linked from the case I ordered and the EAA article.  The assembly comprises mounting the small computer in the case, mounting the cooling fan on the case, attaching two heat sinks to components on the computer, attaching the antennas to the computer and mounting them on the case, inserting the flash card containing the software into the computer, and connecting the battery and optional GPS to the computer.  The only tools required are screwdrivers and a small wrench.  There is no soldering and no software installation nor configuration.

After completing these steps, the newly-constructed ADS-B receiver powers on and a few LEDs on the computer begin to blink.  To confirm operation of the device, you connect to the WiFi access point presented by the computer and open a web page.  Doing this successfully indicates the device is operational.  The diagnostic web page includes information about the peripherals connected to the computer (the radios and GPS).  You can also view the raw data received by the ADS-B receiver if there are aircraft nearby.

I was pleasantly surprised to get to this step with so little trouble and no debugger needed.  In reality, given the nature of the project, any debugging and repairs would probably have been limited to sending a defective part back to the manufacturer and ordering a new one.  There are not many "user-serviceable" components.

After verifying correct operation of the receiver, it's time to connect it to a compatible tablet app.  The stratux.me website lists the many apps with which it's compatible.  I use ForeFlight.  To connect the receiver to the tablet, you connect the tablet to the device's WiFi access point.  This does mean that you cannot both have the tablet connected to an Internet WiFi access point and the ADS-B receiver.  For most applications, this should not be a problem, as you're not usually flying a small GA aircraft and connected to an Internet WiFi access point.  After connecting the tablet to the receiver, the application should indicate that it's receiving ADS-B information.  If you choose to get the GPS receiver, the tablet will also use GPS data from the receiver.  This can be both more accurate than the tablet's GPS and help save tablet battery life.

I now use my Stratux on most flights out of the traffic pattern.  It is incredibly useful in finding traffic even when in contact with ATC and receiving Flight Following.  I have not used the weather functions much yet, other than to verify that I do receive up-to-date METARs.  The battery lasts for several hours, longer than a full tank of avgas.  Due to the limitations of ADS-B In, there are occasionally false negatives (where traffic does not appear on the display even though it is there), but there are no false positives.  Given that the cost of the project was less than an hour of flying time and was enjoyable to put together, I recommend this to anyone who does not already own one of the manufactured ADS-B receivers, is at least a little bit handy, and would prefer to save a few hundred bucks over having a supported product in a more polished case and form factor.

I'll be hosting a seminar on ADS-B followed by a Stratux Build Session on Wednesday, April 12 at 7:30pm at Palo Alto.  If you'd like to join the build session, go to the Stratux web site (http://stratux.me) and purchase either the Dual-band or Single-band shopping list.  Bring your parts to the seminar and we'll go through the build steps together.  I'll be able to answer any questions about the build.  We can all help each other complete the project so that people walk away with working ADS-B receivers (or know which component was DOA and can order a replacement).  E-mail me with any questions about the Build Session.

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