ADS-B is part of the FAA's NextGen initiative to upgrade the entire air traffic control system, which until now has revolved largely around ground-based radar tracking. By 2020, almost all aircraft operating in controlled airspace (aircraft originally built without an electrical system, including gliders, are exempt) must carry TSO'd equipment that can broadcast position reports (based on an approved position source---for normal category aircraft, that's a TSO'd GPS navigator) every few seconds. That has two big benefits: It provides ATC with much more accurate information on where aircraft are, which it can use to provide more efficient handling, and it offers the opportunity for pilots to know the position of other aircraft near them if they carry equipment to receive the signals being transmitted from those airplanes. Incidentally, while about half of the existing surveillance radars are scheduled for decommissioning, the rest will be kept as a cross-check and backup.

One major complication in the FAA's approach is that two separate (and incompatible) radio links are being used. Aircraft operating above 18,000 feet and/or flying internationally are required to use an upgraded "extended-squitter" Mode S transponder, operating on 1090 MHz. Those transponders are already used by most airliners and business jets---that's why those of us who buy portable ADS-B in receivers that support the Mode S link find ourselves looking at lots of high-altitude traffic. Unfortunately, for technical reasons, there's potentially a serious problem if every airplane in congested airspace is equipped with Mode S, and it has limited bandwidth that can't accommodate additional information like in-flight weather. So, for airplanes operating only in U.S. airspace below 18,000 feet, a second radio link is available, operating at 978 MHz. The equipment for using that link is called a Universal Access Transceiver (UAT). The FAA first tested UAT technology in the 1996-2006 Capstone program, which demonstrated that providing accurate aircraft position along with traffic and weather in the cockpit could significantly enhance safety in the Alaskan backcountry, where radar-based air traffic control service wasn't available.

To deal with the dual data links, the FAA contracted with Exelis Inc. to install over 600 ground stations covering most of the continental U.S. and Alaska. Those stations receive aircraft position reports and can also rebroadcast those reports.

For example, suppose two airplanes are approaching an airport with an ADS-B ground station. One aircraft is equipped with a UAT operating on 978 MHz, the other with a Mode S transponder on 1090 MHz. The ground station receives both positions and rebroadcasts the first aircraft position on 1090 MHz and the second aircraft position on 978 MHz. Assuming both aircraft have appropriate ADS-B in receivers and displays, they'll each see the position of the other aircraft. Now, suppose a transponder-equipped aircraft that's not ADS-B equipped joins the party. If there's a ground-based surveillance radar in the area, the position of that aircraft will be reported to nearby ADS-B ground stations, which will send the radar-based position to ADS-B in equipped aircraft.

In the last couple of years, a wide range of portable ADS-B in devices have appeared. If the transponder-equipped aircraft has one of those, it may see the signal from the ADS-B equipped aircraft, but the ground station won't automatically relay signals to it. The only way to reliably get position reports on all aircraft in the area is to be equipped with ADS-B out.

Now for the good news: Based on early experience with Capstone and related tests, most people have assumed that a full ADS-B in/out system would cost tens of thousands of dollars---and it can indeed cost that much, if you install TSO'd panel-mount ADS-B in, as well as ADS-B out. However, there's no requirement to do so. Both ADS-B traffic and weather are considered advisory-only, as a supplement to conventional traffic and weather sources. It's much cheaper to buy a portable ADS-B in receiver and handheld display than to pay an avionics shop to install panel-mounted equipment. For that matter, many piston singles lack the panel space for a TSO'd display.

That leaves one technical question: Is it legal for an avionics shop to configure TSO'd ADS-B out equipment to report that you're equipped to receive ADS-B in signals, if your ADS-B in equipment is portable? I asked about that, and an FAA representative confirmed that it's indeed legal to do so.

Mike Hall, a former Air Force fighter pilot, has exactly that kind of setup in his Mooney: A TSO'd Garmin GTX 330 Mode S transponder for ADS-B out and a Garmin GDL 39 portable ADS-B receiver. The GDL 39 is a "dual-band" device that receives traffic on both 978 and 1090 MHz. Hall displays traffic on an iPad running the Garmin Pilot app. He also has active traffic and says the ADS-B traffic not only provides most of what the active system does, but has several advantages. "Active traffic from my Garmin GTS 800 is superb, but only works out to about 12 miles. In the terminal area, TAS gives me real-time airplane-to-airplane info, but sometimes the antenna is blanked, and I don't see another target, or a target may disappear. ADS-B is less likely to do that, and it gives vectors, as well as target and altitude separation."

The panel-mounted active traffic system does have some benefits. "Head-down traffic display on a tablet isn't ideal, especially in busy airspace. The one thing you don't get is warnings, which TAS/TCAS will do, though that's not always helpful---it yells at me every time I depart from White Plains," Hall says.

But, Hall is adamant that portable ADS-B traffic is much better than depending on the Mark I eyeball when flying in busy airspace. "There's so much traffic out there. I used to fly fighters, but really never appreciated how many airplanes are there until I got traffic. Light airplanes are actually harder to see than fighters!"

The GDL 39 gives Hall weather, as well as traffic. "ADS-B NEXRAD is not as nice as satellite, but usable for strategic weather avoidance. You also get METARs and TAFs, which works really well in Garmin Pilot. Really good in-flight weather---that's a big step forward in safety. And ADS-B provides good traffic, useful for both strategic and tactical situations. I fly into the New York City area regularly---with this, I have the ability to look ahead and have a heads up if I'm liable to get a hold."

And, according to Hall, ADS-B offers a lot of capability for a low price. He's spent over $70,000 on avionics upgrades including a full glass panel. Of that, ADS-B required "a couple thousand" (dollars) for an upgrade to his existing transponder, a portable ADS-B in receiver, iPad and display app. He did run into one "gotcha:" Garmin's ADS-B upgrade for the GTX 330 didn't include a cable to connect it with the GNS 430 GPS he uses as a position source. Fixing that required an additional trip to the avionics shop.

Incidentally, while the flight information service broadcast (FIS-B) weather provided by ADS-B isn't as nice as satellite-based weather products, it's slated to improve: FAA Advisory Circular 000-63A discusses plans for enhancing FIS to add lightning, turbulence, icing, cloud tops, more frequent AWOS update, and delivery of tailored FIS data to aircraft at different altitudes and vicinity to selected airports.

Fully functional traffic and weather for a few thousand dollars is one very positive reason to consider an upgrade before the January 1, 2020 deadline. And, of course, if you're fortunate enough to have plenty of panel space and an unlimited budget, you can install a fully TSO'd system with a panel-mounted multifunction display (MFD). Avidyne and Garmin will also be happy to sell you hybrid traffic systems that combine ADS-B and transponder-based traffic. And, if all that comes to over $10,000 (as it easily can), there's low-cost financing available from a public-private partnership called the NextGen GA Fund (see the More ADS-B Information links sidebar).

Whatever kind of system you choose, another reason to upgrade sooner than later is that there are a limited number of avionics shops and a lot of airplanes that need to be upgraded. Garmin's Bill Stone told me, "Over the last 10 years, there has been a significant decrease in avionics shop capabilities---a lot of guys are retiring, and we aren't training enough new technicians. So we have a decrease in shops and a huge number of airplanes. There are 1,436 business days between now and January 1, 2020. To equip every airplane, over 100 airplanes per day need to be upgraded---30,000 to 40,000 per year!"

To sum up:
• To fly in controlled airspace after January 1, 2020, you must be equipped for ADS-B out.
• If you fly above 18,000 feet or outside the U.S., you must use 1090 MHz Mode S extended squitter for ADS-B out. Otherwise, you can use a 978 MHz UAT.
• If you use a 978 MHz UAT for ADS-B out, you must also keep your Mode C transponder for radar compatibility.
• Both 1090 MHz Mode S (ES) and 978 MHz UAT ADS-B out solutions require an approved position source (usually a TSO'd GPS).
• Equipping with both ADS-B out and ADS-B in will get you reliable traffic (and in most cases, weather).
• ADS-B out equipment must be TSO certified and (for normal category aircraft) professionally installed.
• ADS-B in equipment can be noncertified and portable.

Appareo Systems manufactures the popular $899 Stratus device, sold by Sporty's Pilot Shop. The current Stratus Second Generation combines dual-band ADS-B in with WAAS-enabled GPS, and a solid-state attitude and heading reference, in a portable unit that mounts to a window. Stratus communicates with an iPad or other display device via wireless Internet and is supported by ForeFlight Mobile software. Visit www.appareo.com/stratus.

Avidyne offers the TSO'd AXP340 Mode S transponder, which supports ADS-B out on 1090 MHz. The company is working to certify combined ADS-B in and transponder-based active traffic display on its TAS600-series traffic devices, and will support display of traffic from those devices on its IFD540/440-series GPS navigators later this year. Visit www.avidyne.com.

Bendix/King's KT 74 is a Mode S transponder capable of meeting ADS-B out requirements when used with an approved position source. A trade-up program is available for some older King transponders. Visit www.bendixking.com.

The $549.99 XGPS170 from Dual Electronics is a portable 978 MHz ADS-B in receiver and GPS with built-in bluetooth wireless connectivity to a wide range of display devices, including iPads and Android-based tablets. It's supported by at least 11 apps. Visit www.gps.dualav.com/explore-by-product/xgps170/.

For a limited time, Freeflight Systems is offering a complete ADS-B in/out solution built around the TSO'd RANGR 978, a 978 MHz UAT. The company also offers an ADS-B compatible Mode S transponder for high-altitude operators. Visit www.freeflightsystems.com/adsbnow.

Garmin Ltd. offers a wide range of ADS-B equipment, including the GDL 88 TSO'd UAT, GTX 330/33/23 transponders, GDL 39 portable dual-band ADS-B receiver and GTS-series ADS-B enhanced active traffic systems. Pricing is $599 (GDL-39) and $849 (GDL-39 3D). Visit www.garmin.com/us/intheair/ads-b/.

Levil Aviation's $1,195 iLevil SW combines a 978 MHz ADS-B in receiver, AHRS- and WAAS-enabled GPS in a portable package with integrated solar cells that provide continuous recharging during daylight hours. Visit www.aviation.levil.com/.

NavWorx offers its ADS600 line of ADS-B in and out UAT devices, available with or without a built-in GPS position source. They're compatible with a range of certified displays, and a WiFi adapter is available to display traffic and weather on portable devices. The company is also in the process of certificating a patent-pending device that will monitor output from a Mode C transponder and match squawk codes to meet FAA single point of entry requirements. Pricing starts at $2,399. Visit www.navworx.com.

The Clarity line of dual-band ADS-B receivers from Sagetech also offer WAAS-enabled GPS, built-in wireless Internet for connection to portable displays and patent-pending phased array antenna and "data burst" technologies that receive signals from all directions and record ADS-B messages while displays are turned off. Pricing starts at $1,150. Visit www.sagetechcorp.com/general-aviation-solutions/clarity-ads-b.cfm.

SkyGuard TWX offers portable UAT devices, including transmit-only, receive-only and a two-way transmit/receive unit. Note that the latter is approved by the FCC, but not by the FAA. According to SkyGuard, it's "currently under FAA authorization testing," however, the FAA has said repeatedly that portable devices won't be approved to meet the 2020 ADS-B out mandate. Visit http://adsb.skyguardtwx.com.

Radenna LLC sells SkyRadar dual-band UAT receive-only ADS-B in devices with built in GPS, with and without AHRS capability. Pricing starts at $689. Visit www.skyradar.net.

SkyVision Xtreme offers 978 MHz ADS-B in/out UATs with wireless Internet capability for external display on iPad or similar devices. Pricing starts at $3,295, not including a certified GPS or antenna. The company offers a suitcase-style portable system for the same price, advertised as upgradable "free of charge" for installation in compliance with the FAA mandate. Visit www.skyvisionxtreme.com.

Trig Avionics offers a range of ADS-B compatible 1090 MHz Mode S transponders. Their TT31 is a "plug and play" replacement for Bendix/King KT 76A and 78A transponders (a replacement tray is required for full ADS-B functionality). The TT31 typically sells for less than $3,000. Visit www.trig-avionics.com.