# What is Tracking?

For most Australians tracking conjures up the image of an Aboriginal tracker following the tracks left by a lost person stumbling through the bush. For a CRO Tracker it meant being able to find and follow a spacecraft and determine its position in space.

When you first hear an aeroplane in the sky, your two ears work together to help you to find the direction from which the sound is coming. You can sometimes improve the acquisition by making your ‘antennas’ larger - cupping the palms of your hands behind your ears. Once your eyes see the aeroplane you lock onto the image and the sound is no longer necessary for you to continue tracking. You could, if you were knowledgeable about compass angles, estimate the horizontal angular position (bearing or azimuth angle) and the vertical angular position (elevation angle) of the target but estimating distance would not be as easy.

A ground station, tracking a spacecraft, requires exactly the same type of information but with much more precision. It also needs to measure the distance or range of the spacecraft and its speed, the rate at which the range is changing – the shorthand for which is ‘range rate’. The set of measurements of angle, range, and range rate accumulated while the spacecraft passes over the station is used to calculate its future orbital path. The network can then predict when the spacecraft will pass over other tracking stations around the world later on in its orbit.

Curiously, the tracking network would need to measure an Apollo spacecraft’s velocity more precisely than its position; the velocity to an accuracy of better than 50 cm/sec and the distance to better than 1 km in Earth orbit and 10 km in lunar orbit. This degree of precision is equivalent to one-fortieth the thickness of a typical sheet of paper, as it would appear when viewed edge-on at arm’s length.