How Airport Radio Detection and Ranging Works & It’s Usage

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The Radar stands for Radio Detection and Ranging. It is having been in use since World War II and is currently an integral part of civilian aviation across the globe.

There are two types of airport radar systems: primary and secondary.

The radar operation principle

The radar uses the pulse technique, whereby radio waves are transmitted in very short bursts. These bursts are known as pulses. The duration of each pulse is called pulse width/length. As these pulses travel at the speed of light, in a short time multiple pulses can be produced.

PRI = 1/PRF So, if the PRF is 1000 pulses in one second, the PRI is equal to PRI = 1/300

= 0.00333 seconds.

This means the interval between each pulse is 0.00333 seconds. Because it is hard to work with such small numbers, when it comes to radar, we normally use microseconds. So, the 0.00333 seconds becomes 3,330 microseconds.

The radar works on the echo principle. The radar which is the transmitter sends out the pulse. This pulse is then reflected by the aircraft back to the radar. As there is some energy loss in the process, the range of the radar is proportional to the fourth power of range:

The primary radar

The primary radar is the most basic form of radar. As said before, the range is based on the echo principle, while the bearing of the object or the aircraft is calculated based on the searchlight principle.

To explain how the echo principle works, we can think of trying to figure out the distance of an aircraft. So, you point the radar at the aircraft, and it takes 200 microseconds for the pulse to return. Given that the pulse travels at the speed of light, which is 300,000,000 m/s, we can use the speed equation to find the distance.

Distance = Speed x Time= 300,000,000 x 200/ (1000,000 x 2)

So, the distance of the aircraft from the radar is 30,000 m or 30 km.

The bearing is derived by knowing the direction the radar is pointed to, pretty much like a searchlight.

The primary radars are self-sufficient in that they can lock a target without asking for permission. The radar does not need a response from the target. Hence, they are still used in the military.

The Secondary Surveillance Radar (SSR)

The pulse sent out by the SSR is known as an interrogation signal. The interrogator or the radar sends the signal on a carrier frequency of 1030 Mhz, while it receives the signal from the transmitter (aircraft transponder) on 1090 Mhz. The transponder, on the other hand, receives on a frequency of 1030 Mhz and sends the signal back on 1090 Mhz.

The side lobes are wasted energy, and if an aircraft tries to reply within the side lobes, incorrect bearing read-outs will be given. So, pulse P2 is made such that its intensity is higher than the strongest side lobe. The P2 pulse is radiated out in all directions, whereas P1 and P3 are radiated in the direction of the antennae. The P2 pulse is generated by fixed antennae which are located near the main radar.

When an aircraft is in a side lobe, the P2 pulse is stronger than the P1 and P3 pulse. And when within the main lobe, the P2 pulse is weaker than the P1 and P3 pulse. This way, the aircraft does not respond to the interrogation when P2 is stronger than P1 and P3.

How the aircraft is identified is by entering a code on the transponder. When giving the clearance for the flight, the controller gives the pilot a transponder code. This code must be then entered by the pilot into the transponder.

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