Missile Navigation: GPS v Map

choi

The vision thing l Bombs That Can Recognise Their Targets Are Back in Fashion; A new generation of smart weapons is in development. Economist, Dec 3, 2016.
http://www.economist.com/news/sc ... can-recognise-their

Note:
(a) "JDAM and similar systems work only when they can receive signals from GPS satellites. And such signals are weak—approximately as powerful as a standard television transmission would be if the transmitter were five times as far away as the Moon is. They are thus easily jammed."

Joint Direct Attack Munition
https://en.wikipedia.org/wiki/Joint_Direct_Attack_Munition
(The JDAM's guidance system was jointly developed by the United States Air Force and United States Navy, hence the "joint" in JDAM)
(b) Talking about guided missiles, which used to be guided by a map in the warhead: "it is GPS, cheaper and simpler than map-based guidance, that has subsequently dominated the business of automatic navigation.  America's original map-based cruise-missile guidance system came in two parts. The first, Terrain Contour Matching or TERCOM, took a missile to the general area of its target using a radar altimeter and a series of digital maps that showed the elevation of the ground under sections of the planned route. * * * Once it [missile]  was close to the target, a second system, the Digital Scene Matching Area Correlator (DSMAC), compared the view from a video camera with a set of stored images, in order to locate the bullseye.  Such a combined system was awkward and expensive, but at least it was the best available before GPS. Now, though, huge improvements in electronics have turned the tables. Israel is in the forefront, with a system which it calls Spice."
(c) Armed with a video camera both in the visible and the infra-red parts of the spectrum[,] Spice can operate in darkness, and can penetrate smoke and fog. Moreover, unlike DSMAC, Spice stores enough data to cover the entire route to a target. It has no need of an accompanying system similar to TERCOM."
(d) Besides GPS and map, there was a third system for missiles: inertial guidance.