Transponder Landing System enables precision approaches at McMurdo in Antarctica | Hacker News Reader

Transponder Landing System enables precision approaches at McMurdo in Antarctica

flightradar24.com

148 pointsrmnwski41 comments

Unsurprisingly, the challenges of getting to, living in, and managing infrastructure in Antarctica is a frequent theme of posts shared here. Some previous discussions of note:

* How to Operate an Airport in Antarctica (also via flightradar24.com): https://news.ycombinator.com/item?id=26340384 (68 comments)

* Nuclear Power at McMurdo Station, Antarctica: https://news.ycombinator.com/item?id=27549484 (192 comments)

And perhaps my favorite blog, brr.fyi makes frequent appearances here. An example:

* South Pole Water Infrastructure: https://news.ycombinator.com/item?id=40615530 (63 comments)

For all Antarctica posts: https://brr.fyi/tags#south-pole

Sadly, brr.fyi is done with the Antarctica stint and currently looking for another job.

I’m sure he won’t find it difficult to land another interesting job. I once emailed him and got a reply from the Antartica! Super cool.

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.

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.

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

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.)

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_...

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.

> 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

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.

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...

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

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.

Year round aviation access to Antartica. Well Done! Every step towards McMurdo becoming a normal city despite its location seems like such an incredible accomplishment.

Can this work at pole station too? I realize there's a lot of other considerations landing there in the winter (fuel freeze temp?) but the less isolated it becomes, the more science we can get.

I'm guessing there are a whole bunch of assumptions and simplifications in standard aviation practice around lat/long, GPS, and magnetic compasses - which all fail at the pole(s).

I bet it's "quite exciting" to be a pilot trying to fly at the pole doing anything apart from flying straight and level right past.

Your longitude readout becomes useless as the lines of longitude converge. Your GPS altitude becomes wildly inaccurate because the orbital inclinations of the constellation means they never get above 45 degrees or so from the horizon. Your compass is pretty much trying to point straight down (and at the magnetic pole which is some way apart from the rotational axis of the earth pole).

And it's cold, likely very bad weather, the landscape make orienting yourself and even seeing upcoming mountains challenging, and you are a long long way from a safe landing spot and even further from any realistic help.

I think a lot of aviation navigation is by named waypoints on charts. I am definitely not an expert, but I believe some of these would be visually identifiable in good weather, or even identified by radar transponders. Compas deviations would certainly be an issue, but I believe are important to account for even in much more populated areas. Aviation does use gps now, but many of the previous systems still exist and are quite clever. The ILS system this is based off of is such a system.

Flying over north pole became quite popular due to great circle routes going that way. use of GPS in civilian aviation actually came from compass getting confused during polar flight resulting in heading reversal. GPS satellites also supported polar operations from the start as the critical user needed flights over north pole. (this is one of those cases where typical map projections confuse people).

That said, magnetic compass deviations are common all over the world due to things like iron ore veins, so maps have corrections available.

GPS isn't used for altitude in aircraft in practice, it's too inprecise - barometric altimeter plus radar altimeter are the precise instruments for that.

> use of GPS in civilian aviation actually came from compass getting confused during polar flight resulting in heading reversal

Do you have a source for that? I was under the impression that INS-based navigation was much more precise than for that to have been an issue by the time GPS became available to civil aviation.

While GPS was apparently always planned to be provided for civilian navigation, Korean Air Flight 007 being shot down due to error in navigating over north pole led to explicit mention by White House:

https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007#Af....

KAL007 was an usage error of the INS mode of the aircraft's autopilot, though, which was itself working as designed (although that design was a bit of a footgun in retrospect).

The exact same error could have happened for a Pacific crossing near the equator. Magnetic aberration being larger near the poles had nothing to do with it.

Landing at McMurdo Station seems a lot like landing on a giant, slow-moving aircraft carrier.

I wonder how similar their ILS is to the ILS used on navy aircraft carriers?

Aircraft carriers have two systems.

The Instrument Carrier Landing System (ICLS) is basically just an upgraded version of the civilian ILS which they squashed down onto a ship. It broadcasts a beam of radio waves into the air and any aircraft can pick it up and follow the glide slope.

There is also the Automatic Carrier Landing System (ACLS), which is roughly equivalent to this system. Radar receivers on the carrier are fed into a computer, which calculates the aircraft's position and transmits back commands back to the aircraft's autopilot.

The cool part about this Transponder Landing System is that it doesn't require any equipment upgrades to the aircraft. An aircraft equiped with original ILS equipment from the 60s can use it.

Adding for flavor and context: The ACLS is colloquially referred to as the "magic carpet", and we are now decades in to the classic disagreement about pilot skills versus automation. It's just in this case the consequence is splattering into the stern of a ship.

The glacier moves orders of magnitude slower than an aircraft carrier does, so I doubt that the TLS used does any comparable automatic adjustments. It's probably more than enough to periodically survey the runway and input that data manually.

Reminded me of a blog I stumbled upon by a woman who, just for kick, decided to work and live in Antarctica:

https://wandereatwrite.com/how-i-got-paid-to-live-in-antarct...

This was a great read

How they do aircraft positioning is the most interesting aspect here. The article says its multilateration, but I don't think it's quite the tradional simple multilat. Instead they are doing some cleverness with two angle of arrival detectors arranged perpendicularly, which feels novel?

Found this random paper that has more details

https://www.icasc.co/wp-content/uploads/2023/02/Application_...

There a couple companies that make a DILS where the localizer and glideslope can be temporarily deployed. The intent is for emergency or short term use.

How does the TLS work with multiple aircraft landing at once? With ILS, the signal broadcast is static but it seems like it will now be per aircraft.

I would expect that these runways are not high volume. Temporary/shifting/special needs.

This is about McMurdo. I'd be surprised if they ever had multiple flights landing at once.

Exactly. I could see a 'convoy' of aircraft for a resupply, but I also expect even that might be just a few a day (because of weather and such, to avoid excessive aborts or returns due to weather or unloading delays).

The entire fleet qualified for McMurdo seems to be six planes and three helicopters, if I'm reading this right.

https://www.usap.gov/sciencesupport/scienceplanningsummaries...

I would guess it to be pretty rare that multiple aircraft would be on approach at once, and if so, I'd imagine one could hold at a distance to allow approaches to be serialized.

I've flown to/from McMurdo on a total of 4 types of planes... C-17, C130 (kiwi AF), L-100 (Safari, contracted by Italian antarctic program) and LC-130 (to/from pole).

What an experience! Can I ask your role in visiting?

Basically I work on experiments (balloons and in-ice) that are attempting to detect ultra high energy neutrinos interacting in ice sheets via radio.

This continues to be one of the cooler ongoing projects that produces significant, physical results. Good luck finding the special neutrinos!

There are a few other airports that have this, and my understanding is that only one aircraft can do the approach at a time. If another plane tunes in the ILS they will see the localizer and glideslope indications for the other plane.

I’m not sure how the TLS figures out what transponder to look at, I guess either the controller enters in the code of the plane on approach or there’s some reserved transponder code for the approach.

In theory, you can broadcast different signals on different frequencies, one per aircraft.

ICE31 in the air right now https://www.flightradar24.com/ICE31/36b2c5d5

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