Clearing the Crypto-Fog Tips for Decoding and Deciding Among ADS-B Equipment Options

--by James Williams, FAA Safety Briefing

Tick. Tock. Tick. Tock. The clock is ticking and, by the time you read this article, you will have less than a year to get your aircraft ready to meet the FAA’s ADS-B Out mandate. I’ve been watching this process unfold since the beginning. It’s been an interesting ride. Since I joined the FAA in 2006, I have seen the ADS-B Out mandate go from concept to a proposed rulemaking to final rule (that in less than a year), and then into effect. I remember vividly in May 2010 thinking that 2020 was such a long way off. Not anymore! It’s here, so let’s get down to what you really care about: how do you comply?

When the Speed of Light Just Isn’t Fast Enough

Most people don’t know a whole lot about radar and, frankly, why should they? So you might have nodded off when you read some of the initial “radar is an aging technology” pitches for ADS-B. A more compelling point is the inherent limitations of radar, most of which can be explained with simple physics and geometry. Radar works by transmitting radio waves and then receiving the reflected return of those waves. Those radio waves travel at the speed of light.

While that sounds really fast in terms of our normal understanding of travel speeds, it’s actually a significant limitation. That’s because the receiver has to “wait” ever so slightly for the return, so the transit time of the radio wave from the antenna to the object and back limits the radar in terms of range and update times. In general terms, the faster a radar antenna turns, the faster it updates, but at the expense of a shorter effective range. Therefore, every radar system is inherently a compromise between range and update interval. No amount of technology can solve that problem. The speed of light appears to be a limitation across the universe. It limits space travel, radar, and communications. Radar also becomes less accurate over distance due to angular spread.

ADS-B doesn’t have these problems. Each user broadcasts a highly accurate position signal every second, one that is not dependent on a radar interrogation to update everyone involved, including the FAA. How does ADS-B do that? It’s that “B,” which stands for Broadcast. Having each aircraft determine and then broadcast its own location eliminates the directionality of radar.

Let’s Talk Tech

There are essentially three components necessary to make the ADS-B system work, and this is where your equipment needs come in. To keep it simple, ADS-B requires three things: a source of position information, a transmitter, and a receiver. Technically speaking, you really only need the first two components to satisfy the FAA ADS-B Out mandate, but the absence of a receiver deprives you of the system’s full benefits. Most pilots would probably agree that most of the benefits to users come from ADS-B In. But first let’s look at the required technology.

We’ll start with the easiest part, the position source. This is required no matter what, even if you opt only for the required ADS-B Out. FAA recommends a Wide Area Augmentation System (WAAS) GPS. Many popular products from companies like Garmin, Avidyne, Aspen, FreeFlight, and Bendix King meet (or can be upgraded to meet) WAAS standards. If you don’t have a WAAS GPS and don’t think that adding one is in the cards — they can be quite expensive — all hope is not lost. Some of the ADS-B solutions either include an internal WAAS position source or can be purchased with an optional internal WAAS position source.

The next piece of equipment is an ADS-B Out transmitter. This is a radio that functions like your transponder. Instead of waiting for a radar interrogation, though, it automatically broadcasts your location and other specified information to ADS-B ground stations and other properly equipped aircraft at one-second intervals.

The final piece of equipment is optional. An ADS-B In receiver allows you to take advantage of all the ADS-B Out information from other aircraft, in addition to that from the FAA. It also allows you to receive subscription-free weather information via Flight Information Service-Broadcast (FIS-B), with one important caveat that we’ll cover later. The additional situational awareness and free weather make the incremental investment in ADS-B In well worth considering.

The Sentry ADS-B In receiver is one option that allows you to add TIS-B and FIS-B information to an ADS-B Out only solution. Photo courtesy of ForeFlight

While I’ve described these items as three separate pieces of equipment, they are often sold and sometimes even packaged as a single unit. I’ve separated them to illustrate the flexibility you have in tailoring an ADS-B solution to your particular situation. Maybe you have an approved WAAS GPS already and don’t want ADS-B In. In that case, a simple ADS-B Out unit would probably be most effective. If you don’t have any modern avionics and want to go all in, you might elect for a full ADS-B Out and In unit along with a brand new WAAS navigator. Maybe you want both ADS-B Out and In, but don’t have WAAS and don’t want to write a check for a fancy new navigator. No problem. Simply select an approved Out/In unit with integrated WAAS. The point is, no matter how unique your situation, there is likely a solution out there that will meet your needs.

An example of a 978 MHz UAT installation that combines with an existing transponder. Diagram courtesy of Garmin

What’s the Frequency Kenneth?

One of the biggest questions when looking at ADS-B is 978 or 1090 — megahertz (MHz) that is. As part of the ADS-B rule, the FAA allowed two different ways to meet the ADS-B Out requirement if you always fly below 18,000 feet MSL. One was a 978 MHz Universal Access Transceiver (UAT) and the other is the 1090 MHz Extended Squitter (ES).

Originally, UAT was envisioned to be the GA solution for ADS-B. UAT is still a perfectly acceptable solution for anyone who flies below 18,000 feet MSL and within the United States. UAT operates in addition to your existing Mode C transponder (which is still required in 2020), essentially adding ADS-B to your current set up. Some UAT systems have the advantage of being slightly less expensive for the hardware, although the Aircraft Owners and Pilots Association (AOPA) notes that they are sometimes more expensive to install. UAT might be the “easiest” solution to add to an existing setup.

UAT at 978 MHz is also the exclusive home of FIS-B information. But there are some drawbacks. The biggest issue is that UAT is currently supported only in the United States, meaning that any international flight requiring ADS-B is off the table. Another potential issue is that you have to maintain your existing Mode C transponder. If you need to add a new Mode C transponder for some reason, the UAT option may be less economical than the 1090 option.

So let’s talk about 1090 “extended squitter” (1090ES), which is an enhancement of the Mode S transponder. Mode S transponders already squit, but 1090ES sends more information. In case you are wondering about the somewhat funny word, “squit” is essentially the opposite of a squawk, meaning that it sends information automatically rather than waiting for a radar interrogation. The information from 1090ES is identical to that which UAT sends, but it goes at 1090 MHz rather than 978 MHz.

The Garmin GTX-330ES is an example of a 1090 MHz Extended Squitter. Photo courtesy of Garmin

In addition to being the accepted international standard and being required above 18,000 feet MSL in the United States, 1090ES also replaces your existing transponder. This means that one box will meet both the ADS-B and transponder requirement. The major drawback with 1090ES, aside from cost, is that a 1090ES ADS-B In system doesn’t receive FIS-B. We’ll talk about some fixes for that later. But other than cost and FIS-B compatibility, 1090ES makes a strong case for better operational flexibility, simplicity, and potentially even resale value.

I Challenge you to a Dual

With some customers torn between the pros and cons of each ADS-B Out option, manufacturers created an interesting solution. Enter the dual band receiver. From a technical perspective, this is the “ideal” solution. The dual band units generally have a 1090 ES transponder integrated with an ADS-B-In system that receives both 1090 MHz and 978 MHz. This choice gives the pilot the operational advantages of 1090ES Out, but with the benefit of 978 MHz ADS-B In, namely FIS-B. Another benefit is having ADS-B In on both 1090/978 MHz, which means that your aircraft can receive traffic information directly from both bands. A single band receiver, whether 1090 or 978, only receives traffic information from aircraft on the same band directly from the aircraft. Full traffic information would still be provided, but it would depend on having the information rebroadcast from FAA ground stations. This isn’t a huge benefit, but it’s something to keep in mind if you fly outside FAA ground station coverage on a regular basis. 1090ES Out transponders also meet Mode C/S requirements anywhere in the world.

Embracing a Split Strategy

While a dual band receiver might be the “technically ideal” solution, it might not be the most financially appealing one. In that case, you might opt to adopt what I’ll call a split strategy. The FAA is only requiring ADS-B Out. To meet the legal requirement, you can install an ADS-B Out system, 1090ES or UAT. These systems are generally cheaper than a dual band or single band Out/In system. Installing ADS-B Out meets the mandate but doesn’t get you FIS-B or any traffic information. To handle ADS-B In, there are a number of portable systems that allow you to add ADS-B In to your other cockpit devices like tablets or portable GPS systems. Many of these systems are even dual band receivers. While they don’t integrate into your avionics and are generally limited to displaying on portable devices, they do provide a low cost, no installation option to pilots who want to add ADS-B In to an ADS-B Out only installation. You can also add this option later on if you desire.

The Avidyne AXP-322 is a remote mounted 1090 MHz Extended Squitter that can be controlled through other cockpit systems like the pictured IFD 550. Photo courtesy of Avidyne

Finding the Best Path

The clock is running and, as we have stressed, the FAA is not going to stop the clock or add extra time. The market has responded, and ADS-B options are plentiful. They range widely in price and capability from units that meet the standard and nothing more, to those with a host of features beyond the basics.

Since price and capability are not always highly correlated, a bit of research is in order.

A good place to start is the FAA’s Equip ADS-B page: www.faa.gov/nextgen/equipadsb. You can search and see what options are available for your particular aircraft, and that may help narrow your choices. Next, look for an avionics shop with experience installing ADS-B equipment. They can provide options and estimated costs. They can also tell you which options are most popular given your aircraft and needs. This is a great way to benefit from the experience of thousands of pilots who have traveled down this path before you. You are best positioned to find your best path, but it doesn’t have to be a solitary struggle.

ADS-B Solutions

Content disclaimer: Products and services mentioned, and/or external, non-FAA links within, do not constitute official endorsement on behalf of the FAA.

James Williams is FAA Safety Briefing’s associate editor and photo editor. He is also a pilot and ground instructor.

This article was originally published in the January/February 2019 issue of FAA Safety Briefing magazine.
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FAA Safety Team

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