Mark your calendars, technogeeks, because you’re overdue for
the unofficial sixth annual Geocachers’ New Year. For those who’ve never heard
of it, geocaching is a form of techno-empowered scavenger hunting in which the
players seek out targeted objects and locations based on Global Position System
(GPS) coordinates. May 1 is something of a Geocachers’ New Year because, on
that date six years ago, technological and political fiat indirectly legislated
the pastime into existence.

First, let’s offer some background on GPS. The U.S.
Department of Defense designed and deployed GPS for its own uses, and the 50th
Space Wing at Colorado’s Schriever Air Force Base centrally administers it.

The Department of Defense launched the first GPS satellite
in February 1978, but a complete “constellation” of 24 GPS satellites
wasn’t in orbit until Jan. 17, 1994. The constellation ensured that at least
four GPS satellites were always within line-of-sight of almost any place on

In simplest terms, any terrestrial GPS receiver can
calculate its exact position by measuring the Doppler shift of the signals
received from any four or more GPS satellites and matching a single
intersecting location for those ranges. To ensure that the GPS signal intervals
are reliable, each satellite boasts its own hyper-accurate atomic clock. At
least four GPS birds are necessary to compensate for ionospheric interference,
which could effectively slow or warp any single GPS signal by virtue of
weather, sunspot activity, and any number of electromagnetic factors.

However, the U.S. military originally had no intention of
allowing foreign organizations—or even American civilians—to employ the full
power of the GPS network. (Considering that GPS is accurate enough to guide
cruise missiles, this makes a certain amount of sense.)

In its early days of operation, the GPS civilian-accessible
signals employed so-called Selective
, which intentionally encoded inaccuracies of up to 100 meters
into GPS transmissions. This made GPS effective enough for long-distance
navigation (usually by air) but not precise enough for ground-level street
directions (or surgical-strike munitions targeting).

On May 1, 2000, the U.S. government disabled Selective
Availability, removing that intentional variable from the system and making
possible all those wonderful GPS-driven toys and pastimes we’ve since come to
know and love.

Yet even though the government has disabled Selective
Availability and compensated for ionospheric interference, another phenomenon
consistently puts the onboard GPS atomic clocks 38 microseconds out of sync
with their earthbound counterparts, leading some to suggest that the basis of
the next-generation GPS system needs to be on very different communications


What physical phenomenon causes the atomic clocks aboard
Global Positioning System satellites to consistently read 38 microseconds out
of sync with their earthbound counterparts, leading some to suggest that the
next-generation GPS should take this factor into account?

The culprit is none other than Einstein’s theory of
relativity (which, at this point, really appears to be more of a fact than a

GPS satellites make two complete intermediate circular
orbits of the Earth every day, maintaining a constant velocity (about 4 km/s)
relative to the Earth. This velocity—although representing only 0.0013 percent
of the speed of light—nonetheless presents measurable effects of relativity
within the context of GPS tracking.

In order for GPS systems to be accurate within one meter,
the satellites’ internal clocks must be accurate to within four nanoseconds
(which is why constantly cross-synchronized atomic clocks are necessary). Thus,
the 38-microsecond variance between the ground and orbital GPS chronometers
requires an explicit compensation within the entire GPS network.

Some have suggested that the basis of the next-generation
GPS deployment be on relativistic principles, rather than simply compensate for
relativistic effects. The fact that several other organizations and nations are
developing competitors for GPS—including the European Union’s Galileo network
and Russia’s GLONASS—may spur U.S. developments in this direction.

For those of you concerned that GPS is a military-grade
targeting system just waiting for someone to exploit it, worry not. Knowing a
precise set of geometric coordinates and being able to develop a guidance
system that can hit precise coordinates are two very different technical tasks.

Besides, the U.S. Air Force has proven techniques for
so-called navigational warfare, which
involves the selective jamming of GPS signals of specific combat theaters. And,
if worse comes to worst, the government can toggle on Selective Availability
again, virtually at will. That’s not just a techno-tactical advantage—that’s
hard-core Geek Trivia.

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Geek’s online journal of rants, opinions, crazy ideas, half-baked notions,
bizarre concepts, wild schemes, and trivial observations unfit even for Geek

The Quibble of the Week

If you uncover a questionable fact or debatable aspect of
this week’s Geek Trivia, just post it in the discussion area of the article.
Every week, yours truly will choose the best post from the assembled masses and
discuss it in the next edition of Geek Trivia.

This week’s quibble comes from the April 19 edition of Geek
Trivia, “Stationary
TechRepublic member NickNeilsen
busted me for mislabeling the specific component on the Soyuz 11 return capsule
that failed, leading to the death of the crew.

“The ‘control valve’ was actually a pressure
equalization valve designed to allow fresh air into the Soyuz cabin at
altitudes below four to five kilometers. Now, if you are really good, you can
tell the rest of your readers what caused the valve to fail.”

Before I even had a chance to respond, member #1 Kenster busted out the following
citation from official Soviet documentation.

“‘At approximately 723 seconds after retrofire, the 12
Soyuz pyro cartridges fired simultaneously instead of sequentially to separate
the two modules. . . the force of the discharge caused the internal mechanism
of the pressure equalization valve to release a seal that was usually discarded
pyrotechnically much later to adjust the cabin pressure automatically. When the
valve opened at a height of 168 kilometers, the gradual but steady loss of
pressure was fatal to the crew within about 30 seconds.'”

Such quibbling dedication warms this Trivia Geek’s heart.
Keep up the good work, and keep those quibbles coming.

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The Trivia Geek, also
known as Jay Garmon, is a former advertising copywriter and Web developer who’s
duped TechRepublic into underwriting his affinity for movies, sci-fi, comic
books, technology, and all things geekish or subcultural.