In an LPV approach, the WAAS GPS (not the barometric altimeter) is used to generate the vertical guidance. But the decision altitude for going missed is determined using barometric altimeter. Why isn't the DA using WAAS GPS for consistency? This way the DA will be the exact same point on the glide slope no matter what the field temperature is.
2$\begingroup$ I'm not sure, but usually SBAS corrected altitude isn't shown to the pilot at all, right? All aircraft I'm familiar with only allow setting minimums based on barometric (DA) or radio (DH) altitude. $\endgroup$– BianfableMar 16 at 7:03
$\begingroup$ Just like an inner marker on an ILS approach, yes? $\endgroup$– wbeard52Mar 16 at 14:37
I'm pretty sure it's an artifact of the conservatism applied to the certification of WAAS LPV approaches. From a certification standpoint, the assumed vertical precision of GPS is not considered adequate compared to baro alt, even if technically it is.
With smartphones having GPS antennas that can resolve position down to inches, you could probably fly an LPV approach using an app on your phone (if someone made one) with more precision than a TSO'd aviation system, but the phone isn't certified.
It takes a long time for change to work its way through the system, and the technology tends to run quite a bit ahead of its application in aviation.
1$\begingroup$ I don’t think smartphones are WAAS equipped or have that accuracy. This makes me imagine a grisly scene of NTSB and local LE coming up on an aircraft accident with the smashed and burning wreckage in a ravine 300 yds before the approach end of a runway in blinding snow. Near the lifeless, bloody hand of what was once the captain, investigators find a smartphone with a Verizon ad playing, saying “Can you hear me now?” $\endgroup$ Mar 16 at 18:05
1$\begingroup$ Yes a smart phone can now resolve 3D position down to centimeters, whereas the aviation equipment is stuck with its legacy cert requirements. An new GPS antenna was developed a few years ago that made this possible. That's why delivery drones can deliver a package to your front door. Not saying you would ever try it. $\endgroup$– John KMar 17 at 3:37
$\begingroup$ @JohnK Do you have a source that GPS in smartphones is accurate to that level? I haven't seen anything better than a few meters claimed in any reputable source, and that is just for lat/long, not altitude. $\endgroup$ Mar 17 at 23:24
$\begingroup$ In any case, drone deliveries are operated under part 135, so presumably they would need to use similar certified GPS just like any other aircraft that requires an airworthiness certificate? $\endgroup$ Mar 17 at 23:28
$\begingroup$ It was an article about the development of a new antenna for cell phones developed IIRC by the University of Texas. I'd have to dig it up. $\endgroup$– John KMar 18 at 12:09
One thing never to forget in aviation is redundancy - if possible never trust a single system - better have two or even more independent sources to crosscheck their data. Such as verifying the correct GPS height with the barometer to be really sure you are not too close to the ground.