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Automated direction finders are only reliable in relative close proximity to NDBs, due to various adverse effects. In particular, the night effect limits their useful range for navigation independently of the NDB transmission power.

However, there was at least one historical navigation system, Omega, that has offered global coverage through high-powered VLF beacons.

In contrast to NDBs and ADFs, Omega was based on hyperbolic navigation, and in particular compared the phase difference between signals received from different beacon stations. These stations were transmitting in the VLF band, with their transmissions reaching over ten thousand kilometers due to the unique propagation characteristics of that band.

What I dont understand: If there is a capability to transmit a radio signal that far, why can't the same principle be used to determine the direction to the transmitter using an ADF, using radio direction finding principles, and then determining one's position by intersecting the determined bearings to the various transmitting stations as is possible using NDBs?

Is there something unique to the VLF band that makes it less prone to the distortions affecting NDBs, or is it subject to them as well, but the phase comparison mechanism is somehow less prone to errors and measurement ambiguities than radio direction finding would be over the distances involved?

Has Omega just solved for these problems by modeling VLF propagations sufficiently?

Or is it something else entirely?

Note: This question was originally part of Was radio navigation used for oceanic crossings before GPS? If not, why not?, but I've spun it off of that for clarity.

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ADF radio sets tuned to NDB transmitters use frequencies just below the AM radio band, which do not constitute VLF signals. Their useful propagation distance is therefore short, typically a hundred miles or so, for a transmitter radiating a couple hundred watts. 500 mile range would require 50,000 watts or so and a huge antenna array. Reliable 1000 mile range needs 100,000 watts or more and an enormous antenna farm at those frequencies.

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  • $\begingroup$ What I'm wondering is: Could they be extended to use VLF beacons (existing ones used for other purposes, like Omega, or new and dedicated ones, for NDB-like usage)? Or would the precision of such hypothetical "VLF ADFs" be equally bad or worse at oceanic distances, making a more complicated system (based on hyperbolic navigation or ranging, potentially via complicated propagation correction tables such as those used for Omega) the only option? $\endgroup$
    – lxgr
    Commented Jun 5 at 16:43
  • $\begingroup$ sorry, no idea. -NN $\endgroup$
    – niels nielsen
    Commented Jun 5 at 21:01
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    $\begingroup$ @lxgr with the introduction of INS in the 1930s (or early 1940s) the need for the very limited scenario of oceanic traffic for the long range NDB disappeared, so the systems were abandoned in no small part to save cost. Similarly, most short range NDBs have been removed to make place for VORs, and now many of those are being removed because GPS is commonly available (and no, I'm not going into the wisdom of those decisions, apart from the obvious financial savings not having to maintain those beacons) $\endgroup$
    – jwenting
    Commented Jun 6 at 11:32
  • $\begingroup$ @jwenting That makes sense to me! To make sure I get the point – the idea is that INSes became accurate enough quickly enough to make radio beacons uneconomical (and maybe even less accurate given typical travel times and NDB angular error rates, I suppose)? If so, I think that would make for a good standalone answer! $\endgroup$
    – lxgr
    Commented Jun 6 at 13:16

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