The ILS is an invaluable aid to pilots, especially those operating to/from the many airports where poor weather is common (low-visibility operations would be completely impossible without an ILS); however, ILSs are expensive and maintenance-intensive, with the result that many smaller airports lack ILS capability. Also, the requirement for a considerable quantity of pre-emplaced infrastructure obviously precludes one from performing an off-airport instrument landing.
On the other hand, the technology is available to guide an aircraft down to a precision landing at an arbitrary point on the ground, and much of this technology is already mandatory equipment on aircraft; GPS (which most if not all aircraft already have anyway) could be used for horizontal positioning and guidance and for plotting a landing route about the local terrain, the radar altimeter (also standard aircraft equipment) would provide vertical guidance, and a forward-looking radar system (also already standard, in the form of the aircraft's weather radar, which should only require an additional driver or two to add a terrain-sensing mode) would allow fine control of the aircraft's flightpath for the landing itself (where GPS, with its ~10m CEP, is insufficiently precise and accurate for a safe touchdown) and allow the aircraft to orient itself in space (as GPS tells the aircraft where it is, but not which direction it's pointing). Additional equipment that would be useful and could easily be added would be a second weather radar (to allow the pilots to continue to monitor the weather even with one radar in terrain-mapping mode), a Doppler radar system (to warn the pilots of dangerous windshear, microbursts, etc.), and a forward-looking lidar system (to provide advance warning of clear-air turbulence along the approach path).
With the appropriate software, these instruments could be used to plot and fly a safe instrument landing at a noninstrumented airport, or even at a non-airport; this latter capability would be extremely useful for medevac and SAR pilots (whose duties, by their very nature, involve operations to and from off-airport locations, frequently in poor weather) and for military helicopter pilots (same reason).
Are there any aircraft already equipped with such a "self-contained ILS"?