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Comment by Someone1234 | Hacker News Reader

> You know how modern ones skim along terrain? Terrain maps.

Terrain maps require active radar. Active radar increases your detectability.

> If you are using terrain maps to navigate, then you don't need GPS - so this system has no hopes of working against low altitude cruise missiles.

Some currently deployed US Navy Cruise Missiles already utilise GPS. Including the Block III TLAM[0].

They support INS (inaccurate), TERCOM (which has a radar signature), and DSMAC (which relies on TERCOM or GPS, and only increases final target accuracy).

You'd definitely use GPS if you could use GPS, it is the most reliable and quietest system. This system proposes to neuter your ability to use GPS in Russian territory.

> Also, the cruise missiles would need to be flying lower than the cell towers

That isn't how radio signals work.

[0] http://www.navy.mil/navydata/fact_display.asp?cid=2200&tid=1...

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You're underestimating the mind-blowing awesomeness of high-end inertial navigation in general, and the awesomeness of inertial navigation inputs mixed in with other sensor systems in particular. It should be possible to build a system that can handle some sophisticated jamming and still keep flying with a high degree of precision...

...but the spectacle of Iran downing one of our drones a few years back using GPS spoofing makes me wonder if maybe defense contractors are hiring engineers who couldn't get jobs working at the phone company, or something. So yeah. Bleh.

(the maker of the laser ring gyros used in the F-15 used to brag that the plane could go up, patrol, and land and the indicated coordinates were in the same hangar. I'm sure the tech has only gotten better.)

I think it's just that they weren't designed with spoofing in the threat model.

That's the point of filtering inputs from multiple navigation sources, though. One of them should be able to go wrong, it should be apparent, and the rest of the system should just be able to continue to function. There's ample experience integrating GPS, inertial, stellar (!), and so on.

Mostly I was responding the parent's comment about inertial, which is way off base. Inertial navigation is amazing.

> Active radar increases your detectability.

An active radar pointed at the ground, so you'd need a pretty extensive detection network. Also, I don't think stealth is much of a concern once the cruise missiles are over the target's territory. BTW, TERCOM doesn't need to be continuously radiating when in use, it only needs to get a fix frequently enough to keep INS within acceptable accuracy bounds.

> That isn't how radio signals work.

It is when you're talking about trying to interfere with a weapons grade radio, by transmitting to the sidelobe instead of mainlobe.

> An active radar pointed at the ground, so you'd need a pretty extensive detection network.

So they can't navigate a sudden cliff in the way?

If the cliff materialized between the time the path was plotted and uploaded to the missile prior to launch, then no. The missile has an internal map, the radar measurements are like mile markers on the highway - used to find your position on the map, not used to tell you what is ahead (that is what the map is for).

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