Direction determination

At the very end of the 19th century, in response to the development of battleships with large traversable guns that affected magnetic compasses, and possibly to avoid the need to wait for fair weather at night to precisely verify one's alignment with true north, the gyrocompass was developed for shipboard use.

Its major disadvantage is that it depends on technology that many individuals might find too expensive to justify outside the context of a large commercial or military operation.

It also requires a continuous power supply for its motors, and that it can be allowed to sit in one location for a period of time while it properly aligns itself.

Because of the Earth's axial tilt, no matter what the location of the viewer, there are only two days each year when the sun rises precisely due east.

In either hemisphere, observations of the night sky show that the visible stars appear to be moving in circular paths, caused by the rotation of the Earth.

This is best seen in a long exposure photograph, which is obtained by locking the shutter open for most of the intensely dark part of a moonless night.

Near the end of the 20th century, the advent of satellite-based Global Positioning Systems (GPS) provided yet another means for any individual to determine true north accurately.

The government agencies responsible for the satellites continuously monitor and adjust them to maintain their accurate alignment with the Earth.

In contrast with the gyrocompass which is most accurate when stationary, the GPS receiver, if it has only one antenna, must be moving, typically at more than 0.1 mph (0.2 km/h), to correctly display compass directions.

The exact latitudes and longitudes of the antennas are determined, which allows the cardinal directions to be calculated relative to the structure of the vehicle.

These wavelengths are nevertheless used for marine radio navigation as they can travel very long distances "over the horizon", which is valuable for ships when the line-of-sight may be only a few tens of kilometres.

For aerial use, where the horizon may extend to hundreds of kilometres, higher frequencies can be used, allowing the use of much smaller antennas.

It is estimated that the UK's advanced "huff-duff" systems were directly or indirectly responsible for 24% of all U-boats sunk during the war.

Modern systems often used phased array antennas to allow rapid beamforming for highly accurate results, and are part of a larger electronic warfare suite.