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Comment by LeChuck | Hacker News Reader

> Even as the train moves toward you, and you move toward it, the train’s image maintains a relatively constant position on your retina[...]

That's funny. The very first thing you learn on a ship is to watch out for exactly this. Any ship that maintains it's position relative to yours is on a collision course!

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This is how old air-to-air missiles worked. They would steer until the image of the target stopped moving. This was something simple enough that you could make it work electronically in the 1960s.

> This is how old air-to-air missiles worked. They would steer until the image of the target stopped moving.

That seems to be a weird way of describing "steer so the target is always in center of the image".

Not necessarily on center. Constant bearing but decreasing range will result in a collision.

Exactly! If the target was in the center, that would mean the missile was pointed to the target's current location. The target would quickly drift out of view!

Instead you want to intercept the target. You want to reach the point where the target will be, and at the same time as you. So keep the target to the side, at a constant angle.

Imaging missiles typically have the sensor mounted on a gimbal so the target can be centered in the image even when the missile body is not pointed at the target. But yes, the line-of-sight to the target will generally be off of the missile boresight.

I implemented this once in a video game. If the missile steers so it bears directly on the target, but has limits on its ability to turn, then it will often miss the target or spin around the target.

If you instead steer so the target’s bearing doesn’t change, and ignore whether that bearing is in the center or somewhere else, the missiles are much more likely to hit.

Yes this is called constant bearing decreasing range (CBDR) and it's a core lesson when you're training for your pilot's license as well.

Same reason why all road vehicles have brake lights. Too difficult to detect the (decreasing) speed of a vehicle in front of you.

Its because of this that road authorities usually place "mandatory head lights on" road signs on long straight single lane pieces of road. (because if there is high traffic there will be people taking advantage of the long straight to overtake slower cars, and during the day there is the possibility that a driver will not interpret a head on collision correctly)

It’s also how baseball players catch balls, how many animals catch their prey, etc.

There’s no need to estimate position and speed of objects and then extrapolate them to predict whether course collide and how long that will take. It’s easier to directly derive that from optical information (think of it this way: if that train is 10% as far away, and also goes 10% faster, it will still hit you at the same time. That’s a degree of freedom brains don’t have to solve for)

(Couldn’t find a really good link. You’ll have to do with https://en.m.wikipedia.org/wiki/Ecological_psychology, http://www.scholarpedia.org/article/Optic_flow#Active_and_ec..., https://www.latimes.com/archives/la-xpm-1995-04-28-mn-59922-...)

Proportional navigation: https://en.wikipedia.org/wiki/Proportional_navigation

Or it's moving away from the same point.

OK, fair enough: that's why they actually talk about CBDR - constant bearing, decreasing range.

Same for pilots

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