LORAN, we hardly knew ye
<p>There&#8217;s been some upset in the blogosphere over the shutdown of the LORAN system of radionavigation beacons.  <a href="http://chicagoboyz.net/archives/11579.html">This post at Chicago Boyz</a> is representative (hat tip to Instapundit).  The author worries &#8220;Iâ€™m not totally sure that this was a good decision.&#8221; and various commenters are much more emphatic, bemoaning the lack of a backup for GPS.</p>
<p>In my capacity as lead of the <a href="http://gpsd.berlios.de/">GPSD project</a> I&#8217;ve been required to become a topic expert on the strengths and weaknesses of GPS and various competitors to it, including LORAN.  So here&#8217;s the  straight scoop: yes, a backup for GPS would be a really good idea.  No, LORAN was never plausible as that backup, so angsting about its passing on those grounds is just silly.</p>
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<p>GPS is locally vulnerable to ground-based jamming, and globally vulnerable to attack by satellite-killers.  Other than the U.S. and Russia, only the Communist Chinese are thought to have anywhere near the capacity for the latter, but that&#8217;s enough reason to worry; taking out GPS would be continuous with their strategic doctrine, which seems to puts high priority on unconventional warfare against enemy technological infrastructure.  See for confirmation the recent rash of crack attacks against the Pentagon apparently originating in China.</p>
<p>The biggest problem with justifying a life extension for LORAN on this basis is that its area coverage was totally inadequate.  The entire half of the world south of the Equator was a dead zone as far as LORAN-C was concerned; in fact coverage was never really good outside the land area and near coastal waters of U.S, Europe, and the Pacific Rim.  A secondary issue was that LORAN accuracy was very poor &#8211; 60 to 300 feet, with significant multipath problems in night operation.  The accuracy issue could have been addressed with a relatively cheap technology upgrade, but fixing the coverage issue would have required an expensive station-building program.</p>
<p>So, what <em>would</em> make sense as a backup for GPS?</p>
<p>For starters, multiple satellite constellations at different orbital altitudes; Galileo and GLONASS would be a good beginning, but only that.  If I were the Pentagon I&#8217;d actually be putting money on the table to get the Europeans off their lazy bureaucratic butts about Galileo.  (The first of those birds was supposed to go operational two years ago.)  GLONASS doesn&#8217;t look like a good bet; it&#8217;s kind of a shambles, and Russia&#8217;s accelerating descent into the status of demographically-collapsed third-world pesthole doesn&#8217;t inspire confidence that the problems will ever be fixed, especially since more than half the engineers who used to run GLONASS are probably now living in Israel.  </p>
<p>If I were designing a backup, it would be a fleet of solar-powered high-altitude aerostats.   They&#8217;d use laser ranging to fix their position with respect to ground stations at known locations and each other, and broadcast microsecond-timed here-I-am signals that could be used the way GPS signals are.  If a bad guy shot some down, launching more would be cheap. </p>
<p>The reason this wouldn&#8217;t have made sense even twenty years ago is that the local calculations to deduce a position from a network like this are a chrome-plated revolving bitch.  Your aerostats will be moving unpredictably, remember, so they have to be recomputing their own position from observations in real time.  It&#8217;s not like the satellite case where knowing your orbital elements and the time gives you your position to high accuracy; the aerostat net would need very powerful but dirt-cheap onboard computers, which was not a capability we could take for granted even as recently as the early 1990s.  </p>
<p>Then, of course, your navigation receiver would have to do trigonometry to deduce its position from the positions and times shipped by the aerostats in view. Humans couldn&#8217;t perform these fast enough to be useful, but receiver firmware can; this is well tested in GPS receivers.</p>
<p>Actually, an aerostat-based system would have one significant advantage over GPS &mdash; no accuracy loss due to variable signal delay in the ionosphere.  It might be worth building for that reason alone.</p>
