ADS-B system diagram
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In an industry famous for its ridiculous acronyms, ADS-B stands out for being uniquely confusing. Everybody uses the term, but few really know what it means. And who can blame them–it’s incredibly complicated. Unlike WAAS or LORAN, you can’t even pronounce it!

So what is ADS-B? Why should you care about it? Can you just ignore it?

No. While ADS-B may be confusing, it’s probably the most important technological change you will have to deal with as a pilot over the next two decades. So suck it up and spend some time learning the language.

ADS-B system diagram

ADS-B involves ground stations, GPS satellites and panel-installed avionics.

What is it?

At heart, ADS-B is really just a new way to manage air traffic. As such, it will eventually replace radar as Air Traffic Control’s (ATC) primary tool for separating aircraft. It’s different from radar in that it does not depend on controllers in a central location watching radar scopes. Instead, aircraft self-report their GPS position in a networked environment, so pilots can see the entire air traffic picture around them. There is also the added benefit of datalink weather available through ADS-B.

ADS-B stands for Automatic Dependent Surveillance-Broadcast. It’s a dreadful name that only an engineer could love, but it happens to be fairly descriptive. Let’s look at each part of it:

  • Automatic–properly-equipped aircraft automatically report their position, without need for a radar interrogation
  • Dependent–ADS-B depends on aircraft having an approved WAAS GPS on board and an ADS-B Out transmitter
  • Surveillance–it is a surveillance technology that allows ATC to watch airplanes move around
  • Broadcast–aircraft broadcast their position information to airplanes and ATC

This system doesn’t need radar to work properly, but it will depend on a network of ground stations to receive aircraft reports and send them back to ATC. These stations also transmit weather and traffic information back up to properly-equipped aircraft. This network currently consists of over 400 stations, and the complete network is supposed to be finished by early 2014. So when you hear that ADS-B moves from a “ground-based” radar system to a “satellite-based” system, it’s only partially true.

By the way, you’ll often hear the phrase NextGen used interchangeably with ADS-B. Technically, NextGen (or the Next Generation Air Transportation System) is the FAA’s omnibus plan for modernizing air traffic control. ADS-B is a critical part of NextGen, but it’s only one part of it.

Now that we know what ADS-B is, how does it work?

Out vs. In

ADS-B is made up of two main parts: ADS-B Out and ADS-B In. Out is of interest to controllers, while In is mostly of interest to pilots.

ADS-B ground station map

When completed in early 2014, the network of ADS-B ground stations will provide nearly nationwide coverage.

ADS-B Out is a surveillance technology for tracking aircraft–it’s what ATC needs to manage traffic. It reports your aircraft’s position, velocity and altitude once per second. This transmission is received by ATC and nearby aircraft and this data makes up the equivalent of a radar display. Most aircraft will be required to have ADS-B Out by 2020 (see below).

ADS-B In allows an aircraft to receive transmissions from ADS-B ground stations and other aircraft. This is how pilots can get subscription-free weather and traffic in the cockpit. Adding ADS-B In is strictly optional. While it offers some great benefits, the FAA is only concerned about you equipping with ADS-B Out–the free weather and traffic is simply the carrot to get you to write a check.

Note that there are various combinations of these two: Out-only equipment that simply meets the FAA requirement, In-only portable devices that receive weather, and ADS-B In/Out products that do it all. One thing to keep in mind–there is no such thing as a portable ADS-B Out device. All Out equipment must be panel-installed.

1090 vs. 978

You would think that would be the end of the confusion with ADS-B, but unfortunately you would be wrong. Due to concerns about frequency congestion (and other issues too boring to detail here), there are two different datalink technologies that meet the ADS-B requirement: 1090 MHz ES and 978 MHz UAT. As the names imply, these are simply different frequencies used by the equipment to transmit and receive data.

Garmin GTX 330

Mode S transponders like Garmin’s GTX 330 can sometimes be upgraded to ADS-B Out.

1090 Extended Squitter (ES) is based on 1090 MHz, just like our Mode A/C/S transponders. In fact, some Mode S transponders (like Garmin’s GTX 330) can be upgraded to an ES transponder by upgrading the software and adding a WAAS GPS. This is the only technology accepted outside the US and above 18,000 feet, so it will be popular with turbine airplanes. ES receivers can detect other aircraft with ES transmitters air-to-air, and they can receive other traffic information uplinked from ADS-B ground stations. But there is no weather datalink on 1090.

978 products are sometimes called UAT, for Universal Access Transceiver. This is only available in the US, and only below 18,000 feet, so it is aimed mostly at piston aircraft. Like a 1090 ES receiver, UATs can detect other airplanes with transmitters on the same frequency (978 MHz) air-to-air and also receive the rest of the traffic picture from ADS-B ground stations. But weather is also transmitted over 978 MHz, an added bonus.

This ends up being a real mess. You can have all kinds of different equipment: 978 Out only, 978 Out/In, 1090ES Out only and even a combined 1090ES Out/978 In. At the end of the day, you should choose the Out frequency that matches your flying. If you fly above 18,000 feet or outside the US, 1090ES is your only option. If you don’t, a 978 UAT could work. After you’ve chosen your Out frequency, the only other decision is whether you want ADS-B weather; since that’s only available on 978, that’s a simpler decision.

Stratus 2S

Portable ADS-B receivers like Appareo’s Stratus allow pilots to receive free datalink weather via 978 MHz.

Weather and Traffic

Since weather and traffic come into play so much during any discussion of ADS-B, let’s define some terms: FIS-B and TIS-B. These are the two products that we can receive via ADS-B In.

Flight Information Services-Broadcast (FIS-B) is just a fancy name for datalink weather. Only available with a 978 MHz receiver, the end product is very similar to what we’re used to seeing with XM Weather. NEXRAD radar, METARs, TAFs, TFRs, AIRMETs and other information is continuously updated in flight, and all this can be displayed on either a panel-mount MFD or a portable device like an iPad. There is no monthly subscription fee with FIS-B (your tax dollars paid for it), which is a nice feature. But unlike XM Weather, ADS-B weather uses the network of ground stations, not satellites. That means coverage, while pretty good now and getting a lot better, is not as universal as XM.

Traffic Information Services-Broadcast (TIS-B) is what the name suggests–datalink traffic. But leave it to the FAA to make this complicated. Unlike ADS-B weather, which is broadcast to anyone in range of the ground stations, ADS-B traffic is a custom report that is only sent to aircraft with ADS-B Out. If you’re flying with an ADS-B Out transmitter in your airplane, you’ll get an excellent picture of all traffic within roughly 30 miles of you. But if you’re not flying with an ADS-B Out transmitter (say, with a portable ADS-B In receiver), TIS-B is fairly unreliable. Read this article for complete details on this confusing subject.

Changes for ATC

Remember that, while datalink weather and traffic are nice, the whole point of ADS-B is for ATC. And the FAA has some grand plans for how ADS-B will transform the way it does business, claiming it will reduce aviation’s environmental impact, improve safety and increase capacity at airports. A lot of this seems awfully optimistic, and will not be a reality for many years (if ever).

But there are some more realistic improvements that will probably come to pass sooner. Since ADS-B is so much more accurate than radar, separation minimums can be reduced. This should lead to at least a little more direct routing and some increased capacity. Because ADS-B does not require radar, air traffic control will be available in many remote areas that cannot be served by radar. ADS-B will also impact ground operations, giving controllers the ability to prevent runway incursions and ground traffic conflicts.

But of course this won’t come free.


Final ADS-B Out rules were finalized only in 2011. Those rules say that by 2020, all aircraft will be required to have ADS-B Out equipment to fly in Class A, B and C airspace, plus Class E airspace above 10,000 feet but not below 2,500 feet. So in general you’ll need ADS-B Out most of the places you need a Mode C transponder today–and you’ll need to keep that Mode C transponder, because radar will be the backup for ADS-B.

ADS-B airspace

ADS-B Out will be required by 2020 for flight in most controlled airspace.

That does mean some pilots will not have to upgrade to ADS-B Out. If you fly a Cub on sunny Saturdays away from major airports, you’ll be exempt. But if you use your airplane for any type of transportation flying, plan on equipping with ADS-B Out by 2020.

This ADS-B Out transmitter must be a panel-installed, certified solution (again, no portable ADS-B Out option). An approved WAAS GPS source is also required, to make sure your reported position is accurate. Remember, though, there is no mandate for ADS-B In equipment.

There are a number of products available now to satisfy this requirement, from major avionics manufacturers like Garmin, as well as new products on the drawing board from Avidyne and others. Prices vary significantly, but average about $5000 (plus installation).

What should I do?

This may all sound overwhelming, and the FAA certainly has made things complicated. But the end result is pretty simple: by 2020, you will most likely need to install an ADS-B Out transmitter in your panel (or upgrade your Mode S transponder if you have one). The only questions are what solution to install and when to do it.

The market for ADS-B products is hot right now, with features going up and prices coming down. Garmin’s GDL 82 and GTX 335 show that the avionics giant is serious about owning the ADS-B market, but there are a number of smaller companies either shipping ADS-B boxes of their own (like the wingtip SkyBeacon). Some of these are Out-only, in an attempt to check the box for 2020 compliance as cheaply as possible. Others are full-featured Out/In products that can connect to a variety of MFDs.

Of course there is already a red hot market in portable ADS-B receivers, like the Stratus, GDL 52 and ForeFlight Sentry. These are easy and inexpensive ways to get subscription-free weather on your iPad or portable GPS, and have become quite popular over the past year. But they do not address the 2020 mandate for ADS-B Out.

One option that could become more appealing is to combine these two products: install an ADS-B Out transponder in the panel, but use a portable receiver for ADS-B In. That would comply with the 2020 rule at a fairly low cost, but give you complete weather and traffic datalink. The only major drawback here is that your weather and traffic would not be displayed on the panel, but rather on an iPad or other portable device. And remember that you’ll need an approved GPS source for your ADS-B Out box–either a WAAS GPS or a GPS receiver built-in to your transponder.

When to purchase an ADS-B Out solution is a tougher decision. If your transponder breaks, it’s probably sensible to replace it with a full ADS-B Out unit instead of spending the money on a soon-to-be-outdated Mode C transponder. And if you want free in-flight weather and traffic, it may also be worth it. Just remember to factor in shop time – many are packed, so while you might be able to buy ADS-B Out avionics, you may not be able to get them installed.

No matter what decision you make, it seems clear that ADS-B is coming to US airspace. In time, it might even be a good thing.

John Zimmerman
32 replies
  1. Steve Phoenix
    Steve Phoenix says:

    You mention that a good time to buy ADS-B OUT is if the mode C quits in the next few years rather than spend more money on the obsolete transponder. But not quite true, because up above you mention that the mode C will still need to be onboard, even with ADS-B OUT, as a backup. So that broken transponder still will need to be fixed.

    Also, I have not seen any mention of maintenance requirements for the ADS-B equipment. I wonder if there will be a need for a biennial check like on the transponders. Otherwise, how would one know if the silly thing is even transmitting the correct position, or at all, to the world?

    I’m sure the $5000 entry price is going to be a tough nut for many. It will be interesting to see how that shakes out.

    • Tod R. Lanham
      Tod R. Lanham says:

      Just as a point of information, altitude information for ADS-B will not be based off of GPS altitude. It will be based off of your mode C encoder, pressure altitude. This is why the TSO states the equipment must be based off a single source mode C device. ATC must have a standardization to base separation from, put to GPS device next to each other on the ground and they will not necessarily read the same altitude.

  2. John Fasching
    John Fasching says:

    I am equipped for ADSB-IN. There had been only two ground stations in Colorado, both very near the New Mexico border. The FAA web site that shows where ADS-B ground stations are located was almost a year old. I asked AOPA if they could nudge the FAA to update that website. They did. Then nothing happened on that web site for so very long I again asked AOPA to prod the FAA. I suspect that in order to shut me up they showed eight (8) ADS-B stations in as very small cluster in extreme western Colorado. Those stations don’t actually seem to exist. but someone at the FAA that manages that site thought that would quiet me. ADS-B does still not even exist in some states, and has only a very few in many more states. Even the imaginary ones in western Colorado don’t seem to work. That ADS-B installation program is a real slow program with little feedback or timely reporting to would-be users. Its time to kick butt and take down names !

  3. John Townsley
    John Townsley says:

    Like Steve Phoenix, I’m curious about the recertification and maintenance cycle for ADS-B equipment. I’ve found that the required checks for my 406 mhz ELT are about 10 times more expensive than the previous 121.5 ELT because specialized (and very expensive) equipment is required, plus the batteries are expensive – a lot more expensive. Bottom line: TANSTAAFL. Or, in English: There Ain’t No Such Thing As A Free Lunch. It’ll be “nice” to have all that great information to prevent Mid Air collisions. Unfortunately, that’s not the accident that occurs most often. CFIT and continued flight into IMC are the big killers. So do we mandate a techno fix for those problems next?

    I also agree with Steve’s assessment that “…the $5000 entry price is going to be a tough nut for many.” That’s 25% of the value of some older piston aircraft like C150’s and C172’s. Unless something changes in a big way, those aircraft will continue to be used for flight training and personal transportation just shy of seven years hence. I don’t own a crystal ball, but none-the-less expect (no predictions!) the piston GA fleet to contract quickly about ADS-B time…

  4. Paul Reed
    Paul Reed says:

    AS an older pilot, and one who is not really much involved in computer stuff, this all tells me that I will no longer be an aircraft owner nor probably an active pilot in 2020. I have a hard time figuring out the features of my cell phone(it is not a smart phone), so how am I supposed to figure out all this other stuff. A WAAS certified GPS(you must be kidding) and the ADS-B Out machine: far out of my ability to pay. Anyone want a 65 Debonair?

    • Daniel
      Daniel says:

      Giving your airplane away? You could always donate it. My school is always looking for new planes for our A&P certification course.

  5. Liad
    Liad says:

    I’m a student pilot and a computer geek. My CFi is old school like Paul here. I introduced my CFi to the iPad, wingX, a zaon system and now Sythetic vision and ads-b. as a 20,000 hours airline pilot , he is amazed at what these “computers” can do in a small GA plane. My advice to all of u old timers is… barter! We can teach you the tech stuff, you teach us how to stay alive and become and old aged pilot. :-)

    • John Zimmerman
      John Zimmerman says:

      Great perspective, Liad. The good news about ADS-B is that, once it’s installed and working, there isn’t all that much for the pilot to do. It’s fairly automatic.

      It’s not as complicated as something like… VORs. I bet nobody was born knowing how to fly those.

  6. Steven Stokoe
    Steven Stokoe says:

    While some of the planes I fly have ADS-B the balloons I fly do not. We do not fly in class A, B, C airspace normally but flights beyond 10,000 feet do happen frequently and there is no panel to install the instruments into because all of our instruments are portable though certificated to the aircraft. Would the FAA grant waivers for the use of panel equipment in portable cases? It is a long way out but just a thought.

  7. Dan Jenkins
    Dan Jenkins says:

    I guess at 71 I could possibly be considered an “old timer”, but I welcome all the new changes because they keep the contact points firing (in the brain) and the study habits alive and well. Don’t jump off the train until you are thrown. It’ll keep you young.

  8. Tom Corcoran
    Tom Corcoran says:

    What isn’t quite clear to me yet is… with the ADS-B system, will we still be getting traffic advisories from a ground controller or will this be a another distraction which keeps us looking inside rather than out the window?

    • John Zimmerman
      John Zimmerman says:

      Tom, we’ll have to wait and see on some of this. But ATC will still be there. The traffic picture you’ll see is like TIS now–advisory.

  9. Rafael Sierra
    Rafael Sierra says:

    I am a full time CFI, CFII, 70 and have worked with computers since the Commodore 64. I have a good background and understanding of new technologies in avionics, their systems and equipment and have adapted to Glass Decks and portables well after several thousand hours of satellite nav system flying. However, I do not discount the existing ground based VLOC navigational guidance or paper backups. ADS-B is in its infancy, the OEMs and users are still on STDBY waiting for better and cheaper and enforcement. GPS/WAAS is a done deal, if one flies without them then one is somewhat handicapped as the GPS/WAAS environments are more exacting and efficent. As pilots, we must undersand that this new technology requires extensive study and good hands-on practice as inflight processes can get to be a handful. I started using GPS receivers and WAAS add-on as soon as they were avaiable and still continue to be amazed by all, but still ready for a battery failure.

  10. Jeremy Clark
    Jeremy Clark says:

    Whilst all this wonderful stuff is confusing or enlightening those pilots that are privileged to fly in US airspace, please spare a moment for those of us that fly in some of the not=quite-so-advanced airspace’s. A lot of my flying these days is in places such as Uganda, Kenya, Tanzania, even Guyana and Chile, in GA light aircraft. Not a one of the planes I have flown can spell ‘glass cockpit”. Steam gauges and I are close friends. My question is, should a miracle occur and I buy a brand spanking new Cessna something, or a Baron maybe, all kitted out with everything mentioned in the above article, which of that Xmas tree full of goodies is likely to be of benefit as I approach Mwanza for a full stop landing?

  11. Ed Pedro
    Ed Pedro says:

    Many good points and questions here. I can see the upside in that improved separation will lead to greater safety…(potentially). I can see the downside…additional costs to pilots in an already flailing market!
    Remember though, FAA=Government. 2020 may be the (target date) for implementation, but these folks haven’t had a budget for nearly 4 years. I don’t think I’m going to rush out and buy equipment for a good while! Don’t want to be the guy that bought the first anythings on the market only to find out the following year (anything) cost half as much and was twice the product.

  12. Tod R. Lanham
    Tod R. Lanham says:

    John, very nice article but I believe your first picture my be inaccurate and mislead many. The only communication from satellites to the airplane are the GPS devices receiving the GPS signal. The communication satellite is used for the GBT sitting on top of the mountain or oil platform to relay information back to ATC where landlines are not available. The airplane does not talk to the satellite.

    • John Zimmerman
      John Zimmerman says:

      Good point–you are 100% correct. The satellite piece is purely GPS, which is one reason why I don’t like it when FAA people talk about NextGen moving from “ground-based radar to satellite-based ADS-B.” It’s not really the case.

  13. Galen Hanselman
    Galen Hanselman says:

    John, very well written and informative article.

    It’s my understanding that the ADS-B is reliant on line-of-sight with ground based stations. H-m-m-m. Most of Idaho is “up on end” and line of sight in canyons is not possible. The chosen ADS-B frequencies are very directional and don’t follow terrain well.

    I would venture to guess that once the ground-based stations are installed to serve Idaho, the reliable altitude for ADS-B services will be around 14,000′ for about 3/4 of the state. (I fly above tree tops most of the time.) However, the major cities are located along the open Snake River Plain and ADS-B should work there. Sounds like mountain/canyon pilots will have to rely on XM satellite for weather.

    • Tod R. Lanham
      Tod R. Lanham says:


      Just about all frequencies (UHF and VHF) in aviation are line sight, except those in the HF band. The benefit of the Ground Based Transceiver that is being installed for ADS-B is you place one on top of the mountain and one in the valley and this will cover all aircraft when properly spaced.

  14. Steve Ells
    Steve Ells says:

    I just returned from the Aviation Electronics Association convention where I heard a ADS-B expert fron Freeflight say that for every one of the 220,000 airplanes in the US to get an ADS-B out system installed at one of the existing AEA member shops by the 2020 due date something like 179 systems would have to be installed every working day between now and 2020.
    In my estimation that’s impossible.
    FWIW,the $5000 number only includes the ADS-B out equipment; a WAAS equipped GPS is also required. John–is a MFD also required?
    Ric Peri, head of regulatory affairs at AEA seemed to be hinting that in his estimation the 2020 date would be extended

    • John Zimmerman
      John Zimmerman says:

      I think you’re 100% right Steve–there’s no way all the airplanes that need ADS-B Out can get it in 6 1/2 years.

      MFD is not required. The most basic option would be to take an existing GTX330 and modify it to be ES. That assumes you have a WAAS GPS on-board. If not, that’s additional. But all the In features are optional.


    This blog should have annual updates. Very interesting – I am still hanging on to my money for the GTX-330 out (ES) modification as I sense an extension of the 2020 deadline.

  16. Tod R. Lanham
    Tod R. Lanham says:

    Gentleman, I respectfully disagree. The FAA can’t afford to extend the deadline, this is the bases for NextGen programs to move forward. The community had 8 years to prepare, most of the GBT’s are already in place. There may be 220,00 airplanes on the aircraft registry but I don’t believe that there are that many in airworthy condition nor that many that people will pay to have their aircraft equipped. Prices and competition are pretty well set, if you don’t have a product to compete at this point it would take almost 3 years to develop one and get it certified. That time would leave you out of the market to recover the cost of development. Rafeael there is nothing wrong with your theory however I predict that Garmin will eventually raise the price from $1200.00 for there portion of the upgrade. The additional cost of wiring, testing, and certification will continue to climb each year, as it always does.

  17. Tom Ingle
    Tom Ingle says:

    My question is, as I don’t have a transponder in my Cessna 150, will I be able to get “Flight Following” with ADS-B from ATC?
    If I can get “Flight Following” with ADS-B will I need ADS-B “IN” as well as “OUT”?

    Sorry to be asking these dumb questions but I’m confused !!

    Finally: Can I use the above in Southern Ontario, Canada where I AM able use my iPad with USA coverage?

  18. Tod
    Tod says:

    ADS-B also requires information from a transponder. In todays world where there is good RADAR coverage ATC can provide flight following on a primary target aircraft work load permitting, that should continue. There is currently no requirement that mandates ADS-B IN in the United States. If you are in Canada currently there is no plans to implement ADS-B requirements in areas that they have current RADAR coverage.

    There are no dumb questions in aviation you just need some education in this area to understand what ADS-B really is all about. Try reading the Garmin site the have an very good program that will help you understand ADS-B.

    I don’t understand your Canadian question.

  19. Larry Channel
    Larry Channel says:

    I have a Garmin GTX 330 transponder and a Garmin 430W GPS. What do I need to get ADS-B Out compliance? Upgrade the GTX software? Any idea of cost?

    PS: Just ordered a Stratus 2 for ADS-B In info.

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