I'm trying to come up with the D&D 3.5e world's functional equivalent to a real life Non-Directional Beacon (NDB).
Are there any existing spells in D&D 3.5e that can be cast on durable objects (say, stone monoliths) to make them into some sort of beacon that can be homed upon (or better yet, used for direction-finding purposes) from a long distance (miles, not feet) by some other means than sight (say, with the aid of a spell or magic item used or carried by whoever desires to home in on it)? Magic items are welcome instead, provided they are durable enough to survive being placed outdoors!
Source material is unrestricted save for epic level spells (a straight Wiz20, Clr20, or Drd20 should be able to make a beacon). It should cost less than constructing an inn or other permanent structure, and should not require special jewels or other exotic materials in the monolith itself. However, if the homing/direction-finding function can work over the many miles required, then the range of imbuing the object does not matter — it's not a major problem if a touch-range spell requiring several hours of work is needed to set one of these up, as the beacon-monoliths are set-and-forget. The homing function itself, though, must be available cheaply (as a first or second level spell if not a cantrip or orison for divine casters; bonus points if it's available as an arcane spell as well); a permanent magic item is acceptable instead provided it is not bound to a specific instance of one of these beacons, small enough for a user or their mount to wear, and costs less than oh, say, 10,000gp.
Custom magic item research should not be a problem, nor is custom spell research. Houserules can be proposed here — I'm workshopping this here because I don't know how to translate what I want from the RL terms I understand it in to something a DM who is utterly unfamiliar with the real-life IFR environs can understand.
Also, teleportation itself isn't workable (teleport isn't accessible enough, and probably has too short of a distance, besides, this needs to work on a day-in-day-out basis), and find the path may not work so well in an aerial environment (can you set up a holding fix with it?). The same accessibility argument holds for Plane Shift…
Best Answer
The spell "Locate City" has a range of 10 miles per caster level, and allows the sense of distance and direction to the "nearest community of a minimum size designated by you at time of casting." With nearest being counted without movement through solid objects.
Thus, with a sufficiently large caster level, a sufficiently accurate census, and a sufficiently large population density of villages and cities, it should be quite possible to get a very good idea of where a person is based on gated counts of censuses.
The trick to building a navigation network out of this is by creating artificial communities in "known positions" which count for purposes of this spell. (How to do that is functionally a DM call, but a metropolis built in great miniature with undead squirrels haunt-shifted into it to count as "occupants" suspended in the sky isn't a... horrible option.) For each of these metropoli, anchor to them to the sky via 3 immovable rods within a spacing dedicated by the modal range of the spell being used (which itself becomes a question of assets and magic item economies.)
Much like with dGPS, since you can set the community size of each of these, it turns into a 4-colour map problem equivalent. By knowing north, having a chart of these undead beacons, and casting the spell N times (where N is the set of granularities allowed for by "minimum population" in the spell), you can get a direction and distance to each of the nearest "population-unique" beacons. Then it's a matter of triangulating against the map to identify where the individual is.
Circle dance would be preferable (as it has no maximum range), save for the fact that it requires firsthand knowledge of a creature, which rather reduces the utility, (though being taken on a tour of the "sky thrones" and meeting the undead squirrels sitting on each would itself, be hilarious). If you're OK with houserules, you can probably assert that each squirrel's truename can count as firsthand knowledge of them for purposes of circle dance. Then it's merely a matter of referencing the appropriate truenames of the beacons, and finding where the vectors intersect. But that's much less cool.
...
Or they can just cast find the path (or Knowledge(Geography) perhaps boosted to absurd levels with divine insight or improvisation) to get there and if visibility prevents landing due to some obscure house-rule, use dispel fog to clear a lane to your landing area and blindsight and/or true seeing and/or short range teleportation and/or feather fall to deal with any traps or concerns at the landing pad.