Aircraft Systems

6General Principles of Navigation

6.1 Coordinate Reference System for the Earth

Navigation is the process of moving from one’s current position to another specific point on the surface of the Earth that is not in view. Early explorers would follow sequences of topographical features, producing maps as they went but navigation across large featureless areas such as oceans or deserts requires a global coordinate system. It was realized from ancient times that the Earth is a sphere with the earliest recorded proposal around 500 BCE by Pythagoras. When the cause of lunar eclipses was identified around 430 BCE by Anaxagoras, the curved shadow of the Earth on the moon provided direct visual evidence of the spherical shape of the planet. Around 250 BCE, Eratosthenes estimated the circumference of the Earth based on the difference in the elevation of the Sun at the same time on the same day of the year at Syene (modern‐day Aswan) and Alexandria in Egypt, which are around 800 km apart. His value was within around 10% of the modern‐day accepted value of 40 075 km around the equator.

6.1.1 Latitude and Longitude

Having established the Earth is a sphere, the most convenient coordinate system is spherical polar coordinates (Figure 6.1) and since the radius coordinate is constant, it can be omitted so that just two angles are required to specify any point on the surface. The polar angle ϕ, referred to as latitude is measured relative to the equatorial plane while the azimuthal angle,