Comment by albertzeyer | Hacker News Reader

Comment by albertzeyer | Hacker News Reader

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Wow, that is a lengthy tutorial.

I only skimmed over it now, so maybe I missed it: How do all these developed methods compare to existing real autopilot systems? Are there even existing autopilot systems which work similarly? Did the author come up with these methods just by themselves, or were they adapted from existing systems? Would those developed methods make sense in a real autopilot system?

Also, it seems that this involved a lot of trial-and-error, and the solutions come with a number of heuristics and hyper parameters to tune. I wonder if you can do better and avoid that as much as possible.

First of all, you would need to have an actual metric which covers all the things you would care about. Obviously no fatal crash, but then also maybe somewhat smooth flight (you can measure that) and how close you want to get to each waypoint, while maybe taking not too much fuel. And given those, let it find the best possible path. I guess this is optimal control theory then. When you have an actual metric which you can measure, then you can quantitatively compare all the methods, also automatically optimize any parameters.

I haven't read the tutorial (yet), but I know a bit about autopilots.

Autopilots depend a lot on which kind of plane it is in, but for the most part, autopilots aren't really "auto pilots". They consist of various fairly basic primitives rather than more complex decision making.

As a rule of thumb, the complexity of the autopilot is proportional to the size of the plane.

Basic autopilots can maintain headings, maintain (barometric) altitudes (i.e. above sea level) and similar very basic operations. For these autopilots it is the pilot's job to ensure the plane is operating within the envelope and does not depart from controlled flight and the throttle is controlled by the pilot.

As an example, the autopilot in an F-16 fighter jet has four things it can do: maintain attitude, maintain altitude, fly toward nav point and fly a compass heading. That's it.

Autopilots in airliner jets are more complex and can handle a set of nav point, ascent/descent, arrival and, depending on the airfield, approach and landing. They also feature autothrottles. Their implementation is still fairly simple, since it's a matter of matching throttle, attitude and heading with the position and altitude of each nav point. The autopilot doesn't have to do any pathfinding or more complex decision making, in order to keep it simple, predictable and bug free.

Are there autopilot systems that do any sort of drone, bird, or other aerial object avoidance?

Then, aren't regional cultural ethics necessary to solve a trolley problem; when there are multiple simultaneous predictable accidents and you can only prevent one.

Does "autopilot" solve the trolley problem for the liable pilot?

There's collision avoidance systems, but those are really for "not flying into another plane". You're going so fast that by the time you can pick up that you're about to hit a drone or bird, you've destroyed it -and possibly yourself- already.

Military drones are a different matter: you will never find yourself in the same air space, because you will never be told they're there. They are ghosts, much like airplanes that run without a transponder "because it's legal". You don't see another aircraft unless you know exactly where to look, or you're right on top of them.

Flying things could sqwak with the inverse of the Doppler transform when subsonic

https://news.ycombinator.com/item?id=37886514 :

> FAA UAS RemoteID: https://www.faa.gov/uas/getting_started/remote_id

Collision avoidance in transportation: https://en.wikipedia.org/wiki/Collision_avoidance_in_transpo...

How does autopilots using barometric pressure maintain altitude, when these can vary wildly in non-calm situations? I would expect at least gps added to the mix (although with russia blocking skies randomly that may not be the smartest option these days). Or somehow keeping inclination (dont know the proper name of instrument here).

It uses the pressure altitude relative to the sea level pressure you configure in the plane (the altimeter in inHg / QNH). Above transition altitude (in the US usually 18000) you switch to standard pressure (29.92 inHg), so for those higher flight levels we ignore pressure differences. But for altitudes below this, yes you constantly have to change the altimeter in the plane or else your altitude / flight level chance / VNAV descent / climb will be incorrect. Of course you can always get the pressure by tuning into a ATIS / AWOS etc radio station for a nearby airport if you aren't getting the latest pressure from the ATC enroute.

GPS is usually not used for altitude.

Just to add for completeness, for ICAO/the rest of the world:

Transition altitude (ascent) and transition level (descent) is typically defined per FIR (flight information region), but can be airport-dependent. It is listed in the relevant airport charts. 7000 is fairly common, but it varies. Altimeter setting is measured in hPa/millibars (with the standard being 1013.25, which corresponds to 29.92 inHg).

To expand on verst's answer, the altimeter setting is defined based on your location, to roughly match meteorological conditions. When flying IFR or otherwise directed by ATC, you will be given altimeter settings. They will also be repeated during arrival and landing. In addition, they are given using an automated weather service called ATIS. ATIS broadcasts are given an incremental letter, that way you can verify you have the latest update. When checking in with a controller you will give the letter, and they will tell you if you're outdated.

The most important thing is that everyone in an area reference the same altitude/altimeter setting for traffic avoidance. Whether or not that is off from the actual altitude by a few hundred feet or not is secondary, since ground avoidance typically uses a lot bigger safety margins. Airliners and military planes have radar altimeters to measure height above ground when flying in IMC (low vis), while smaller planes typically fly VFR (based on visual rather than instruments).

> Airliners and military planes have radar altimeters to measure height above ground when flying in IMC (low vis),

But the RA are used primarily in the later phases of the approach. The barometric altimeter setting remains vitally important throughout most IFR ops in all categories of aircraft. (Radar altimeters are a key component of the ground proximity warning system GPWS in transport aircraft.)

> while smaller planes typically fly VFR (based on visual rather than instruments).

General aviation aircraft are quite frequently well-equipped with instrumentation for IFR ops. I’ve flown approaches down to published minima in single engine aircraft and have (legally) departed in low IMC conditions that grounded Part 121 flights. (Not necessarily best judgement…but here to tell the tale that I’ve become wiser in older age…)

> But the RA are used primarily in the later phases of the approach.

Indeed, the context of my sentence was using it for ground avoidance.

> General aviation aircraft are quite frequently well-equipped with instrumentation for IFR ops.

Yes, but most GA flights are VFR flights. Again, the context was ground avoidance and my point was that since they are usually used for VFR, more advanced means of ground avoidance are not generally required.

What about drones? They would need to have a complete solution to the problem, right?

> I wonder if you can do better and avoid that as much as possible.

You can! However, that's basically another 100 pages of "and now let's throw all this code away again, because it's time to switch to self-tuning PD controllers with a drastically decreased polling interval".

Really an autopilot is a combination of as "super dumb, but effective" control processes (the dumber the better, because less code equal less bug equal more alive) that are ideal for PD control, but that does require running the control loop as close to "real time" as you can, whereas we started with "let's not bog down the system, and run twice a second" (because MSFS is programmed in a, let's call it "special" way, and it's basically always CPU bottlenecked)

So... exercise for the reader right now =D

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