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 radar systems. The primary radar and the secondary radar.

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.

The time between two pulses is called Pulse Recurrence Interval (PRI) and the frequency or the number of pulses emitted in one second is known as the Pulse Recurrence Frequency (PRF). And thus, the mathematical relationship between PRI and PRF can be written as:


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:

Radar power = Range^4


The primary radar works on echo principle. Photo: Oxford ATPL

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.

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Primary radar

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)

= 30,000 m.

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 Secondary Surveillance Radar (SSR) does not rely on the reflected signal from the target. So, it does not need a lot of power. It sends a pulse, and this pulse must be received, accepted, and then sent back by the target. So, it requires positive cooperation from the target. For this to work, the target is required to carry a piece of equipment called a transponder.

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.

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Secondary Surveillance Radar

An SSR. Many times, the SSR is mounted on top of the primary radar. Picture:

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The interrogation consists of two main pulses named P1 and P3. If operated under mode A, the time interval or period between the pulses is 8 microseconds, and if done under mode C, the interval is around 21 microseconds. There is also one other pulse generated called the P2. This pulse is formed 2 microseconds after P1. The reasoning behind this pulse is for side lobe suppression. The problem with that radar is that it creates side lobes with one main lobe.

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.

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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.

When the aircraft responds to the interrogation, two framing pulses called F1 and F2 are generated, which are 20.3 microseconds apart. Within these two pulses, a possible 12 pulses can be generated depending on the code.


Over 12 pulses can be generated within the two main framing pulses. Photo: Oxford ATPL

The codes can be represented with the letters A, B, C, and D, as shown in the table below:

Screenshot 2022-09-27 at 15.42.45

Photo: Anas Maaz

The table shows that numbers 1,2 and 4 are assigned for each letter. Each of these numbers is a pulse. In the case above, the code is 7777 and all 12 pulses are generated. The example below shows how code 7644 is generated.

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Photo: Anas Maaz

Here a total of 7 pulses is generated. When there is a zero in the code, a pulse is not generated. If you think about it, each digit can be put 8 times (including zero) on each position in the transponder. And if you multiply 8 x 8 x 8 x 8 you get 4096. And this is the total number of codes that is possible.

Mode A and C

Mode A and C are two modes of transponder operation. Mode A is an interrogation to identify the aircraft and mode C is used to get the altitude of the aircraft.

The mode A and mode C interrogation is differentiated by the time frame between the pulses P1 and P3. When the time between P1 and P3 is 8 microseconds, it is a mode A interrogation and when the time difference is 21 microseconds it is a mode C interrogation.

When a mode C interrogation is made by the radar, the aircraft transponder generates a code that gives the altitude of the aircraft referenced to a pressure of 1013 hPa. This is independent of the pressure setting on the altimeter by the pilot of the aircraft. The mode C can broadcast altitudes up to 128,000 ft.


Mode A and C provide both aircraft identification and altitude data. Photo: Oxford ATPL

Mode S

The mode S is a mode that overcomes some limitations of an SSR which uses modes A and C. The SSR suffers from errors like garbling whereby when two aircraft can give out overlapping replies. Mode S can be used to avoid such issues.

The ‘S’ in mode S stands for selective addressing. Each aircraft is assigned a 24-bit ICAO address by the registered state. A total of 16 million codes is possible with mode S compared to just 4096 codes possible with mode A and C. This is a huge advantage.

One of the best things about mode S transponders is that they are still able to respond to mode A and C interrogation and work on the same frequency band.

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Mode A, C and S works on same frequencies. Photo: eurocontrol.ent

How mode S functions

The radar or the interrogator initially sends an all-call signal with its IC (Interrogator Code) to the aircraft. Once the aircraft is within the coverage of the radar, it receives this all-call interrogation.

The transponder of the aircraft then generates an all-call reply containing the 24-bit address of the aircraft with the IC of the interrogator. This is sent to the radar, which then acquires the target aircraft.

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Mode S acquisition and lock out. Photo: ICAO.

The radar then sends a signal to the aircraft, called a roll-call signal which tells the transponder to not responds to all-call signals from the radar using the same IC. The aircraft transponder hence ignores all-call signals from the IC radar for a total of 18 seconds, before it is reset to ensure that the radar can lock out a second time if required. However, this does not become an issue for the radar because the radar only uses all-call signals to lockout new aircraft entering its domain, not those that have already been locked out.


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