Of the hundreds of WAAS inquires I've seen, this one stood out. Someone asked, "Why?"
The question was, "You mentioned that some installations may require two WAAS receivers to be certified to fly LPV approaches. Why? I don't see that as a requirement in any of the FAA advisory material. What is it about WAAS and LPV that requires two Flight Management Systems and two GPS receivers?"
The answer, of course, is complicated. The following explanation was provided by Avionics expert and AEA Vice Chairman, Gary Harpster:
Prior to WAAS receivers, the Flight Management Systems were certified under TSO C129. TSO C129 allowed an operator to use a single FMS for a GPS approach as long as it was an overlay approach. This also required alternate navigational imputs, such as VOR, DME, ADF, etc.
The new WAAS receivers and Flight Management Systems are certified under TSO C145 and C146, allowing a very precise GPS approach without any additional inputs. These systems also have a unique feature: they are constantly doing an analysis of the GPS integrity. Part of this analysis is a cross check of the secondary system, which adds a high degree of reliability.
One of the challenges of Air Traffic Control is to put multiple aircraft into a confined corridor and keep them properly separated. The new WAAS receivers are designed to accomplish this- the secondary system is a necessary backup in case of the failure of the primary system.
Some manufacturers have designed WAAS monitors to act as a secondary FMS without a Cockpit Display Unit. This configuration provides the required redundancy without the expense of an entire second system.
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