Saturday, December 19, 2015

"I'm an Outie!": Squittering My Way to ADS-B Compliance

I recently had my airplane upgraded to comply with the FAA's ADS-B mandate. For anyone in the dark about what that means and interested in learning more, read on! For those already familiar with ADS-B and what it's all about, the discussion of my chosen path starts under "Choices".

A Brief History of ATC Tech - Abridged

Air Traffic Control (ATC), that benevolent overseer of the National Airspace System, exists for one principal purpose: preventing airplanes from bumping into each other. ATC's implementation of radar after World War II provided the ability to "see" aircraft at a distance rather than relying on estimates of ground speed and track periodically called in by en route pilots (though those practices still persist decades later in remote areas with poor radar coverage).

By itself, this "eye" is imperfect. It can be confounded by terrain, weather, or other obstacles. It is best reflected by metal, meaning that small fabric-covered or contemporary composite airplanes return weak echoes. When air traffic density increases, radar returns tend to become large undifferentiated blobs, making it difficult for controllers to keep track of which aircraft is which. Most importantly, although radar enables controllers to see aircraft range and bearing, all ships are depicted as though stuck to a flat plane; there is no insight given into altitude. Considering that aircraft operate in a three dimensional realm, this most basic form of radar (now called "primary radar") is a relatively crude tool for separating traffic.

These limitations led to the development of secondary surveillance radar (often simply referred to as "secondary radar"). Secondary radar uses a stronger signal for greater range and relies upon an active response from an interrogated aircraft rather than a passive reflection of the radio beam. Secondary radar was developed from another World War II innovation, the appropriately named "Identify Friend or Foe" (IFF) system in which a transponder on board the interrogated ship responds with identifying information. Modern transponders are pilot-programmable to respond with 1 of 4,096 possible identifying codes. These "squawk" codes are assigned by ATC on initial contact and correlated to, aircraft type, registration number, and flight plan. This information can then be displayed directly on the controller's screen. Thus, the "undifferentiated blob" problem from primary radar is greatly reduced and individual targets can be positively identified provided that they are participating in the system (squawking a discrete code).

Coupling the transponder to an on-board altitude encoder means that the transponder will not only identify itself upon interrogation (Mode A), but also broadcast the ship's pressure altitude (typically to the precision of 100 feet). This altitude reporting capability, known as Mode C, provides ATC with a three dimensional picture of air traffic.

Mode C Mandate

In 1986, a Piper Archer climbed without clearance into overlying controlled airspace above suburban Los Angeles and collided with a DC-9, resulting in the deaths of everyone on board both aircraft and several on the ground. Part of the issue was that the Archer was not equipped (nor was it required to be equipped) with a Mode C transponder and ATC did not know its altitude. This incident led to, among other things, a mandate for all aircraft operating in controlled airspace, within 30 miles of a Class B airport (Newark, Chicago, Detroit, Atlanta, etc.), and/or above 10,000 feet (MSL) to equip with (and use) Mode C transponders.

I don't know how common Mode C transponders were at the time. Warrior 481's documentation indicates that she was originally delivered in late 1978 with a King KT-76A Mode C transponder. Warrior 481 notwithstanding, these expensive boxes were not ubiquitous and aircraft owners howled about the cost to equip. Everyone was faced with a choice: equip or stay out of controlled airspace. Simple aircraft like Piper Cubs and Aeronca Champs, lacking electrical systems and possessed with limited range, were generally not upgraded. Many of these aircraft are still called-out by ATC as primary targets only; they are invisible to secondary radar and ATC has no inkling of their altitude.

Another transponder type, Mode S, was not originally broadly embraced by civil aviation in the United States. In addition to squawk code and pressure altitude, Mode S boxes broadcast specific identifying information about the aircraft and provide capability for an uplinked Traffic Information Service (TIS, now called "TIS-A" to differentiate it from what's coming next) that provides traffic data displayable on a moving map in the aircraft. TIS is somewhat limited in that it only functions within line of sight from certain radar installations.

TIS, FIS, and Squitters

The times, they are a-changin'. Again.

The FAA is transitioning air traffic control away from a dependence on radar to a "satellite-based" system (i.e., GPS). The technology is called Automatic Dependent Surveillance - Broadcast, shortened to the unwieldy initialism "ADS-B". Equipping to ADS-B standards is mandated by January 1, 2020 for all aircraft flying in places that require Mode C transponders today. This translates to new, expensive hardware for a lot of aircraft. Owners are howling again, just as they did over the Mode C mandate back in the Eighties.

The basic principle of ADS-B is to use an accurate GPS position source in all aircraft to broadcast each ship's altitude, airspeed, and position along with identifying information. WAAS ("Wide Area Augmentation System") GPS systems are recommended as position sources because they meet ADS-B performance requirements, but official FAA documents do not explicitly require WAAS. This broadcasting of identity and position is known as "ADS-B Out" and is received by ATC and other airborne aircraft equipped to receive "ADS-B In". In other words, everyone should be able to see everyone else; an intriguing idea in concept. The ADS-B mandate, however, only requires ADS-B Out. ADS-B In is optional.

Benefits touted for ADS-B from the perspective of ATC include more precise location data on aircraft to enable reduced separation in congested airspace, better coverage than the current radar system, and more frequent aircraft position updates (once per second, rather than the once per every five to twelve seconds provided by radar). For end users, ADS-B carries the promise of subscription-free datalinked weather (FIS-B) and traffic (TIS-B) available in the cockpit.

The international frequency standard for ADS-B is 1090 MHz, the same frequency used by Mode S transponders. That's convenient; however, ADS-B Out requires a richer data burst (or "squitter") from aircraft than legacy Mode S transponders can deliver. This led to the development of Mode S transponders with ES ("Extended Squitter") capability. The difference between a "squawk" and a "squitter" is that squawking is done in response to interrogation (by, for example, secondary radar) whereas squittering is a data broadcast without an external stimulus (it's also an archaic term for diarrhea, which seems vaguely appropriate).

Still with me? Great! Here's where it all gets complicated.


Within the United States, the FAA uses an additional second frequency for ADS-B: 978 MHz (known as the UAT or "Universal Access Transceiver" frequency). FAA uses the UAT frequency to broadcast that wonderful free weather (FIS-B, "Flight Information System-Broadcast") that I can receive with  my Stratus ADS-B In equipment. The primary reason for adding a second ADS-B frequency was to avoid overburdening the bandwidth on 1090 MHz with the additional data.

Thus, US-registered aircraft may comply with ADS-B Out by either equipping with a 1090ES Mode S transponder or a UAT device broadcasting in concert with an existing Mode C transponder. The Mode C is still required because radar will remain in service as a backup in the event of satellite outages.

Aircraft owners are thus confronted with a choice. Typical mission profiles may be used as a guide to determine which ADS-B solution is the best choice. Those airplanes flying above 18,000 feet or operating internationally are required to use a 1090ES solution. Therefore, large airliners can be expected to adopt this path to compliance. Aircraft strictly operating in the United States below 18,000' can utilize either frequency. So, amusingly, 1090ES is actually a more "universal" frequency than the so-called "Universal Access Transceiver" frequency.

By creating a situation in which aircraft are broadcasting position data on two different ADS-B frequencies in the United States, the FAA also created a conundrum. An airliner broadcasting its data on 1090 MHz might be invisible to another ship only equipped to broadcast and listen on 978 MHz. Demonstrating the truism that complexification begets complexification, the FAA slapped a Band-Aid on this issue by installing ground based repeaters around the country that collect aircraft ADS-B Out information on both frequencies, then re-broadcast it on both frequencies to ensure that all participating aircraft can see each other (a rather elaborate and expensive Band-Aid, I think). Some ship-based hardware, like the inexpensive Stratus 2 that I use for ADS-B In, conveniently receives on both frequencies. ADS-B Out solutions, on the other hand, will utilize one frequency or the other to avoid wasting bandwidth.

When it comes to ADS-B Out and the 2020 mandate, aircraft owners have decisions to make.


One choice is to not equip at all. Equipping a basic Cessna 150 that does not already have a suitable GPS position source may easily exceed 50% or more of the value of the entire airplane! Operators of small, inexpensive aircraft that do not frequent controlled airspace may choose not to equip and join the segregated community started in the Eighties and already populated with a lot of Piper Cubs. 

Given the type of flying I do, this is not an option without a significant loss in capability and freedom.

My Warrior, as a normally aspirated aircraft with a service ceiling in the low teens, will never fly above 18,000 feet (unless something goes very, very wrong). But we do often pass through Canada, which uses the 1090ES standard for ADS-B. Though Canada does not have a mandate to equip at the present time, who knows what the future holds? Most owners of aircraft like mine flying within the borders of the United States can pursue a 978 MHz solution. As a result, I investigated both paths. Although there are many products from various suppliers, I found myself narrowing down the options to Garmin's product line on the basis of cost, performance, compatibility with equipment already on board my aircraft, and the experience of the avionics shop I use.

If I pursued a 1090ES solution, I could leverage my existing WAAS GPS (the Garmin GNS 430W) and simply replace my venerable King KT-76A transponder with a 1090ES Mode S transponder. I looked specifically at the Garmin GTX 330ES transponder. Installation would be relatively straightforward and there would be no need for invasive work like the installation of new antennae. This would be an ADS-B Out solution only, but I could continue using my Stratus 2 for ADS-B In weather (FIS-B) and traffic (TIS-B).

For a UAT solution, I considered the Garmin GDL 88, which serves as an ADS-B In and Out solution. For this option, I would keep my existing Mode C transponder (the GDL 88 detects the Mode C squawk code and synchronizes its output to match) and add a FlightStream 210 that would send the ADS-B In data via Bluetooth to my iPad for display. Honestly, I am thrilled by the idea of the FlightStream 210, which also allows passing flight plans back and forth between the iPad and the Garmin 430W in the instrument panel. It is MUCH easier to enter flight plans on the iPad than the Garmin.

However, the GDL 88 / FlightStream 210 option was a more costly solution (by a few thousand dollars), particularly on the labor side owing to the need for a more invasive installation, including new antennae. Though excited about the potential for FlightStream, particularly now that it is compatible with ForeFlight, I struggled with the added expense. Furthermore, 1090ES remains the best bet for Canadian airspace and the way I use my airplane currently. As a result, I chose the less expensive and internationally-compliant Mode S transponder route.

Installation and First Impressions

I delivered Warrior 481 to the Genesee County Airport on November 28, 2015 for Brian at Boshart Enterprises to do his thing. I had the privilege of  hitching a ride back to Sodus in my hangar-neighbor's very capable Bonanza. Via FlightStream, he had his Garmin 530W talking to ForeFlight on his iPad and the passage of data back and forth was seamless. I was envious, but not envious enough to shell out the extra cash.

Installation on my aircraft went without a problem and I picked up the airplane on the following Friday.

My ship is now compliant with the ADS-B mandate. In concert with my Garmin 430W as a position source, the GTX 330ES (bottom of the radio stack, pictured squawking 1200) broadcasts the airplane's identity and flight information as required. I was able to use ForeFlight/Stratus to verify that Brian programmed the transponder to broadcast the correct tail number (not that I was genuinely concerned).

Now that my ship broadcasts ADS-B Out, I receive a more complete traffic (TIS-B) picture on ForeFlight/Stratus than before because the ADS-B ground transmitters only work when activated by a nearby aircraft equipped for ADS-B Out. This was the FAA's sneaky way of saying, "if you want the traffic information, you need to equip." I also see TIS-A traffic information on my Garmin 430W courtesy of the Mode S transponder when I am within range of a suitable radar facility (e.g., Buffalo, Rochester, Syracuse). It aligns well with what I see by ADS-B In on the iPad, at least until I fly out of range of the TIS-A service and the nice British lady that lives in my audio panel proclaims, "traffic unavailable".

I'll be honest, I'm disproportionately thrilled to have a "VFR" button on my transponder for one-push squawking of 1200. The new transponder also has some nice features like display of pressure altitude (through which I have learned that my ancient altitude encoder is slow to warm up) and a collection of timers that includes a programmable count-down timer that ends with an audio call out of "timer expired". It seems like the latter might be nice for timed instrument approaches during those high workload moments nearing the missed approach fix.


Over the last twelve years of owning Warrior 481, a lot of new instrumentation has gone into the panel. Those replaced components without appreciable core or resale value have begun to accumulate on my desk.

I think I'm getting closer to having my own realistic flight simulator. Thankfully, my desk is exempt from the ADS-B mandate and I can just keep squawking VFR as seen here.


  1. Can you post the source for your note that WAAS-equipped GPS is required? I cannot find that requirement in any official source.

    1. raise an excellent question. I confess that I did not look into it very carefully because I already had that base covered.

      The FAA has an FAQ that indicates WAAS is NOT a requirement:

      In fact, it appears that, while GPS is the only position source that meets the 14 CFR 91.227 performance requirements for ADS-B, WAAS is not mandated:

      However, other FAA sources indicate that WAAS assures that the position source will meet ADS-B performance requirements and that the FAA recommends WAAS:

      Reading between the lines, it appears that non-WAAS GPS sources can be submitted for approval as position sources for ADS-B (and may or may need meet performance requirements), but that WAAS GPS systems will meet the requirements.

      You raised an excellent point and I will revise the text in the article to reflect that. Thanks!

  2. Chris,

    Glad you're ADS-B out compliant! :) What are your thoughts on the weather through the ADS-B in?

    1. Hi Gary - I am glad to have post soon to come for the reason why.

      I've been using the ADS-B weather for 1.5 years now and it has worked well for me. I know it gets a bad rap from some because of the reduced resolution versus XM, but with the delay that the system operates on, I don't think I really need that resolution - I'm not trying to thread a needle with it (nor should I be), I want to know trends and general regions to avoid. It has worked well for that.

  3. This is honestly one of the best explanations of the complexification I've ever read. So well done on that front. As a renter, besides considering building my own little ADS-B box, I've remained acutely - and somewhat intentionally - ignorant about the specific implementation of all this new gadgetry.

    Oh, and do I spy another venerable piece of hardware on your desk? That would be a WRT54G, of course.

    1. Thanks, Steve.

      "Venerable"? It seems like only yesterday that was the new router... sigh I almost scrapped it a few years ago because it needed to be reset almost daily, but another friend suggested I install DD-WRT on it and it has not required an intentional reboot since.

    2. Yep, I've been running DD-WRT on mine (a rebadged TP Link) for a few years now. Not totally bulletproof but that's more a function of the hardware than the firmware. But I'm actually now in the process of upgrading to enterprise-grade hardware (Ubiquiti UniFi line).

      Anyway, yeah, all this ADS-B talk pushed me over the edge last night; I already ordered a R820T2 SDR + antenna to hook up to Avare on my Nexus tablet. Traffic and weather in the cockpit for $8 in hardware - hard to beat that!

    3. I have to say, on my router, DD-WRT has been so much more stable than the native firmware!

      It is hard to beat that! I'm beginning to think I overpaid for my Stratus 2...

    4. I'll certainly write up a post once I've tested everything out. Hard to believe one can have in-cockpit everything for under $10 but that may be where we're at in 2016. Pretty crazy, even as a tech nerd, considering how much time I spend NORDO in Cubs!

    5. So yeah... the parts arrived. Plugged everything in, clicked a couple buttons, and voila - traffic in Avare on my tablet! I need to test everything out in the air but it's pretty impressive for less than 1 Hamilton.

    6. Very cool! It's nice having the additional eye,nthough you'll find its effectiveness for traffic to be spotty (increasing, but spotty) unless you yourself are broadcasting ADS-B Out. It's been amazing to me how much traffic I "see" that I would have never noticed before.