Earth's circumference mile in the seventeenth century and the metre in the eighteenth. Earth's polar circumference is almost exactly 40,000 km because the metre was originally calibrated on this measurement (1/10-millionth of the distance between the poles and the equator), which is almost exactly 21,600 nautical miles (being defined as one minute of a degree – i.e. 360 multiplied by 60). It has been described as "the single most important measurement" of the Earth, assuming the Earth to be a sphere (the Earth is an technically an ellipsoid, albeit with just 0.3% flattening). Earth's circumference lines are the foundation of the coordinate
Inertial measurement unit GPS-aided navigation. Micro-PNT integrates a highly accurate master timing clock into an IMU (Inertial Measurement Unit) chip, making it a "TIMU" ("Timing & Inertial Measurement Unit") chip. So these TIMU chips for Micro-PNT have an integrated 3-axis gyroscope, a 3-axis accelerometer, and a 3-axis magnetometer. Together with the integrated highly accurate master timing clock, it simultaneously measures the tracked movement and combines that with timing from the synchronized clock. With sensor fusion, it does absolute position tracking, all without external transmitters or transceivers. A very wide variety of IMUs exists, depending on application types, with performance ranging: The accuracy of