Crystal clear

摘要

MAGNETIC COMPASSES have guided sail-ors for centuries, but a compass tells you only in which direction you are pointing, not whereabouts you are. A new form of magnetic navigation being developed by the United States Air Force Research Laboratory (AFRL) can do better. It employs magnetometers made from tiny diamonds to indicate an aircraft's precise location over Earth's surface. It is so accurate that it might supersede gps for aerial navigation. A conventional compass aligns with the magnetic field generated by convection currents within Earth's liquid-iron core. This field is, itself, approximately aligned with the planet's axis of spin, and thus with the geographical poles. But there are fainter terrestrial magnetic fields around as well. These are embedded in the rocks of Earth's crust and have a pattern sufficiently elaborate and distinctive as to be unique from place to place. In principle, therefore, they can tell you where you are. The trick is to reduce that principle to practice.