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ADS-B Out: Why Won’t TSO C129 Systems Work?

Apr 26, 2017
By: Paul Damschen, Manager of Product Management and Flight Test Operations

As the FAA's Automatic Dependent Surveillance-Broadcast (ADS-B) Out 2020 mandate draws closer, it's more important than ever to educate yourself on the difference between the varying GPS Technical Standard Orders (TSO) that may apply to current Flight Management Systems (FMS). In hand with that, is to also understand compliance to Advisory Circular (AC) 90-165B and the required position sources.

The FAA has taken the position that their regulatory requirements should be "technologically agnostic" or simply, they determine what the equipment has to do and the manufacturers get to determine what technology is best to solve the requirements they set. Therefore, you'll no longer see regulations that speak to specific types of technology. Over time, the technology involved in the solution may change as new technologies emerge. This idea is actually captured in the ADS-B Out AC stating; "The FAA does not know of any currently available non-GNSS position sources that can meet the performance requirements of § 91.227."

Bottom line, the FAA may revise the ADS-B Out AC as time progresses, but for the moment, the only technology that is known to satisfy the ADS-B Out requirements in FAR 91.227 are GPS-based, although AC 20-165B allows for any technology or solution that is rule compliant.

That leads us back to the TSO 129/146 differences. Why won't TSO C129 systems work? The short answer is that the ADS-B Out requirements weren't set back when TSO C129 came into existence. Believe it or not, it's been 25 years since TSO C129 first came into existence! Since then, several pieces of FAA documentation have been developed to support ADS-B Out and the FAA's NextGen air traffic systems, and the sum total of those requirements weren't established until 2009. Some early TSO C146 GPS systems aren't compliant to the requirements of ADS-B Out, but to our knowledge, those have been updated to be compliant by this point if they are still in production.

In addition, the FAA no longer supports TSO C129 applications. As a manufacturer, we couldn't even apply for a new TSO C129 approval if we wanted to. If you are still producing TSO C129 systems, any major changes to your GPS engine to make it compliant for ADS-B Out would require a new TSO application; and guess what, it won't be accepted or granted.

Long story summarized, TSO C129 systems were all designed long before ADS-B Out requirements were completed. Only current or updated production TSO C146 GPS engines will meet those mandatory ADS-B Out requirements (and you should check!). GPS position is only a small part of those requirements in FAR 91.227, so don't fall for the belief that a TSO C129 GPS system will be compliant. In all likelihood, it won't be.

Operators should be wary of shortcuts or "bandaid" fixes that add-on a lower quality sensor to feed the responder for ADS-B Out. Remember, RNP requirements are coming along and many of the same requirements for ADS-B Out will also be necessary to operate in RNP airspace, meaning your TSO C146/145 sensor will ultimately have to drive your navigation systems as well.

To learn more about ADS-B Out, download a copy of our "Understanding Compliance with Automatic Dependent Surveillance-Broadcast (ADS-B) Out" White Paper.

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