Why does the frequency appear to increase as it approaches you? The cycles take progressively less time to reach you as the police car approaches. For example, radio waves transmitted by an X-band radar on a frequency of 8.5 gigahertz (GHz) transmit a breath-taking one billion cycles per second. Frequency is simply a measurement of how many cycles per second a particular wave performs. The circular movement of a wave from its peak to its trough and back to its peak is called a cycle. Why? The siren generates sound waves and, like radio waves, these have peaks and troughs. In reality, the frequency of the tone is unchanged as it leaves the siren, but from the observer’s perspective it appears to rise in tone as it approaches and fall in tone as it leaves. As it passes, the tone changes and now appears to fall. As the car drives towards you, the tone of its siren seems to rise. In the distance, you hear the police car’s siren. You stand at the kerb waiting to cross the road. What does this mean? The oft quoted analogy is a police car siren. In the early 19th century, Austrian physicist Christian Doppler deduced that, for an observer, a wave’s perceived frequency is dependent on two things – the speed of the frequency’s source and the speed of the observer. That is range taken care of, but what about the target’s speed? This brings us to our second law of physics. It is then just a matter of calculating how far a radar pulse can travel in 0.1ms, which is 16.2nm, and we can therefore deduce that the ship is 16.2nm from the radar. So all the radar needs to do is halve the time for the round trip. The radar does not need to know the distance of the round trip, just how far away the target is. Because we know the radar pulse travels at 161,595kps, in 0.2ms the radar pulse will have travelled 32.4 nautical miles/nm. The 0.2ms comprises the time taken for the transmission, collision, and echo round trip. The radar’s pulse takes 0.2 milliseconds to be transmitted, hit the ship, and be reflected at the antenna as an echo. Let us suppose we have a radar that has detected a ship. In that quick glimpse, your brain will subconsciously compute how far the door is from where you are standing.Ī radar works in much the same way. Switch it on for a fraction of a second and the light briefly illuminates a door in front of you. This is akin to being in a dark room equipped with a torch. Most radars send out pulses of radio waves. Firstly, radio waves travel at light speed – 161,595 knots, or 299,274 kilometres per second. Two fundamental laws of physics let a radar do its work. Your greeting will echo back to you from the walls. Stand in a tiled cellar or cave and shout ‘hello’. Radio waves are transmitted from an antenna, they collide with an object, properly known as a target, and are reflected back at the antenna as echoes. Like most great ideas, radar employs an elegantly simple principle. Active Electronically Scanned Array radars explained The US’s Raytheon AN/FPS-115 ballistic missile early warning radars use a phased array architecture.
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