[mmaunder]

Wow that’s badass. It’s tempting to argue that a GPS based RNAV approach could have similar performance but this has several advantages:

The ground based equipment dictates approach path with no updates to onboard approach databases on aircraft needed.

RNAV can’t do curved approaches on a glide slope as far as I know.

ILS minimums are generally slightly lower than LPV which is the RNAV approach type with lowest minimums.

This supports older aircraft with ILS (localizer with glide slope) that don’t have a WAAS capable GPS.

Edit: Also, using the Mode S transponder with precision approach radar instead of ADSB-out for the aircraft position means older planes without ADSB (because they don’t fly under a mode C veil) can be supported and it also means the base is trusting their equipment for the aircraft position rather than trusting an aircraft that potentially doesn’t have an SBAS GPS on board.

[turrican]

RNAV RNP approaches can contain curved descents. They’re usually used in areas with high terrain but I don’t see why they couldn’t be used down here. LPV minimums seem to usually be very close to ILS minimums these days.

That said, it’s always nice to have a ground-based alternative approach. I wonder if they have sustained issues with GPS this far south.

[jcalvinowens]

> RNAV can’t do curved approaches on a glide slope as far as I know.

RNP approaches can, Monterey has one: https://aeronav.faa.gov/d-tpp/2408/00271RRZ28L.PDF

[p_l]

Main issue with RNAV is that GPS is just not that precise. WAAS helps a bit but it's not as good as having advanced local augmentation or things like RTK, and those are also non-trivial to deploy (not to mention there's no common standard for ground augmentation for aircaft to use).

So ILS or MLS had the benefit of being able to provide better precision without things like realtime ionosphere monitoring and correction. TLS let's you use all sorts of augmentation mechanisms without modifying the aircraft too.

[lxgr]

I would honestly not be surprised if pure GPS approaches were discontinued in the near future, giving all the spoofing incidents recently, and the shocking vulnerability of avionics systems to them. (I would have expected the GPS/INS to reject highly unplausible GPS signals in favor of the IRS, and huge clock deviations in favor of a simple internal quartz oscillator, but apparently neither is the the case.)

TLS seems much less vulnerable to spoofing, given the signal strengths involved.

The idea of an onboard TLS spoofer seems both feasible and very scary, though, now that I think about it...

[throwaway48476]

It would help if RTK was widely deployed, especially on cell phone towers.

[lxgr]

That already exists: GBAS is the aviation equivalent to RTK. It provides integrity monitoring and differential signals via VHF. It's better than WAAS, since it's ground-based, and accordingly significantly harder to drown out in a large area.

But neither WAAS nor GBAS are authenticated, so are ultimately both are spoofable.

[MobiusHorizons]

Would RNAV be able to handle the runway moving nontrivial distances? That seems to be one of the problems this solves.

[lxgr]

You'd have to update the approach plates and their digital version in the FMS's database occasionally, but that has to happen anyway, given that I believe almost all of them still use magnetic north as a reference. (Not sure if modern FMSes can store that data in true north format internally, but at least once the runway gets officially renamed I bet that requires an update.)

[jeffcoat]

Even switching from magnetic to true is unhelpful close enough to the South pole. All directions are north.

We have to switch to "grid" north to make sense of directions there.

https://en.wikipedia.org/wiki/Universal_polar_stereographic_...

[lxgr]

The problem isn't exactly that "all directions are North", right? That is true only when you're smack on the pole; even a meter away from it, things are well-defined again in terms of directions.

Problems start when you're moving around or doing trigonometry, though; following bearings starts getting weird even at otherwise negligible distances, for example, and runway designators on the opposite thresholds might not be exactly 180 degrees "apart".

But it's not like these effects don't exist everywhere on earth at some scale; they get gradually more pronounced the closer you get to the poles, so I'd be surprised if avionics software would get really confused by them.