# Overview

In order to get from A to B, it is necessary to know where you're currently at and where you're heading towards. Earlier, the job of a controller depended uniquely on position reports from the pilots, but nowadays we use radar most of the time.

# Position Reference System

In the earth we often give positions as coordinates, but how do they work?

Well, the earth has been divided into 360 parallels of longitude (which express east-west position with respect to the zero-meridian) and 180 parallels of latitude (which express north-south position with respect to the equator). By using this system, it is possible to express ANY point on the surface of the earth, but it is not precise enough, which is why each of these parallels has been further divided into minutes and seconds, where 1 minute = 1/60th of a degree and 1 second = 1/60th of a minute.

It is interesting to know that one minute of latitude is equal to the distance of one nautical mile (nm), but one minute of longitude is not equal, as the circumference of the earth varies with latitude.

Question: How would you express the following as a position coordinate?: 59 degrees, 2 minutes and 12 seconds north of the equator and 32 degrees, 39 minutes and 55 seconds west of the zero-meridian. (Answer at the end)

# Directions

In aviation, all directions are expressed using the 360 degree system, where the horizon is divided in 360 equal parts which form a complete revolution. A direction due north is 360º, east is 90º, etc...

There is 2 different "types" of directions:

1. Heading: The direction towards where the aircraft nose is pointing.
2. Track: The direction towards which the aircraft is actually travelling after adding the effect of wind.

All directions are given relative to the North Pole, but which one? The magnetic north pole or the true north pole? All headings sshall be given relative to the magnetic north pole, as the heading indicator in aircraft will show the heading relative to the magnetic north, and not the true.

# Speed

In aviation, speed is generally measured in knots (kt). 1kt = 1nm/h. To measure it, aircraft use an airspeed indicator. A conventional airspeed indicator compares the pressure of static air with the pressure of the air forced into the pitot tube (a small tube mounted outside of the aircraft, pointing towards the direction of travel). As an aircraft climbs, the air density decreases, so less molecules enter the tube per unit time and make it show a lower speed. The speed measured with this device is called Indicated Airspeed (IAS). It is the most "relevant" speed to pilots, as it truly represents how the aircraft will behave in respect to stall speeds and other know characteristics.

The actual speed at which an aircraft is going through the air (or past the air molecules) is called True Airspeed (TAS).

Another way of measuring the speed of an aircraft is with respect to the ground. The speed at which an aircraft is moving relative to the ground below is called Ground Speed (GS).

Typically another expression for speed is used during cruise (more or less above FL280). This is Mach Speed. Mach is a quotient of the local speed of sound, meaning that Mach 1.0 is equal to the speed of sound.

Check out this tool here for experimenting with different types of airspeed conversions.