Here's a quick breakdown (Note: This is the 'quick' answer, reference FAA guidance for specifics):
If you are flying above 18,000 Ft., operating under Part 135, or plan to fly in a country requiring it, you will need a Mode S Transporter with Extended Squitter (ES).
If flying within Class 'B' Airspace after ADS-B mandate, you will need a Certified ADS-B Out w/Certified WAAS GPS for a TC'd aircraft.
Unfortunately, given the diversity of the airframes, condition of these airframes, avionics configurations, and the constant changes within the avionics industry any attempt on my part to capture a 'one-size fits all cost associated with these requirements would be irresponsible on my part. Again, the intent of this article is to provide a better understanding of what your options are so that you, as the aircraft owner, are in a better position to know what you need and/or want. This knowledge then empowers you to determine your costs through a trusted Avionics Shop.
However, 2016 and 2017 have seen some changes that have simplified many choices. Two things have occurred:
January 2017 has seen several manufacturers end their promotional pricing which in the end, increases the cost for these options.
The year 2016 has seen the introduction of Transponder solutions that INCLUDE a built in GPS. The significance of these products is important to note. Why? Because when one compares the typical costs to install a remote mounted UAT for compliance to the cost of replacing a Transponder for compliance, you will find that though the option of Transponder replacement most likely will be more money, the amount more is generally less than $1,000. So there is an argument to me made from a long term cost benefit perspective to go with the Transponder replacement rather than the less expensive UAT option. If you go with the UAT option for compliance and at some time in the future you end up having to replace your transponder, you will end up spending far more than a $1,000 for this replacement. Not only that, but you end up with an extra antenna on the aircraft, multiple boxes, and more weight (though not really significant). Bottom line is that going the Transponder route, in most cases, provides a cleaner end game. And should the day come when another country such a Canada or Mexico decides that a Mode S Transponder is required for entry, going with the Transponder option for compliance today prevents a need for change in the future.
It is hard to predicts the future. There are two questions that I am often asked concerning the future.
Will the 2020 date get pushed back?
Will the prices drop?
The only way I know to answer the first question is to say that it wouldn't be the first time that the FAA has pushed back a proposed date. It has also been speculated that if all aircraft, which will need ADS-B Out, were to schedule this work with their Avionics shop as of 1/1/2014, there would not be enough shops and manpower to perform all the upgrades by 2020. If this is true, what will happen? Considering that this rule will have been in effect for 13 years, it is hard to determine which side of the fence this will fall. To answer the price question, there are multiple manufacturers but the products that have been promised have pretty much already reached the market and I have not heard any rumors regarding anything new since so this leads me to believe that there is no major impactful solution on the horizon. History tells us that technology and competition typically causes a drop in equipment price but I am not so sure that will be the case here. Why? Supply and demand could be a factor and the fact that to date, certification can only be accomplished via an STC with a Flight Manual Supplement (FMS) or a Field Approval. There is an exception for non-TC'd aircraft. This raises another question - If manufacturer 'A' is the WAAS GPS source and manufacturer 'B' is the ADS-B Out source, which of the two manufacturers will provide the STC and FMS? In this case, if manufacturer 'A' decides to change their software to accommodate their own ADS-B solution what will happen? This is further compounded when discussing display of ADS-B In.
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These may all seem like reasons to wait to upgrade so let me close with this. If you can make a commitment to a manufacturer's approach to ADS-B and upgrade sooner rather than later, you are done thus can avoid the speculations and the anticipated lead times. It may also better assure easier sell of the aircraft in the future. There is also talk of financing assistance for those who may be less than capable of upgrading freely. As we move forward, all other things being equal, it makes sense to upgrade sooner rather than later. The labor costs have the potential to rise - especially if avionics shops get backed up trying to meet the demand. So it comes down to this...If you spend the money now, you could reap the benefits of ADS-B immediately. If you chose to not spend it at all (maybe you are planning on selling the aircraft in 5-6 years), then expect it to cost you something on the 'back-end'. Given the costs of ADS-B, buyers of your aircraft will most likely not buy without a sale price reduction of the aircraft to offset the costs of compliance. In this case, you in effect, will have spent the money and have never had the opportunity to experience what ADS-B has to offer.