BBC - h2g2 - Side Lobe Suppression

The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures

When transmitting from a rotating radar aerial, the main signal is a beam in front of the aerial, rather like a rotating searchlight. But, unlike a searchlight, lesser signals are emitted in other directions around the sides and back of the aerial. Any emissions not in the main interrogation beam are referred to as side lobes.

Why do Side Lobes Matter?

With primary radar, side lobes are not much of a problem. They are weak signals producing even weaker returns. The problem occurs with SSR (Secondary Surveillance Radar); if an aircraft transponder is in range of a side lobe it will reply. As this is a transmitted reply, not a reflection, it will be of the same amplitude as the main-beam replies. Unwanted replies from aircraft that are not in the main lobe will cause erroneous information to be displayed on the radar screen. An aircraft can appear to be at multiple locations, all at the same range but different directions. This is overcome by a technique called Side Lobe Suppression (SLS)...

How does SLS work?

The side lobes themselves are not suppressed, it is replies from side lobe interrogations that are suppressed. A radar interrogation signal consists of two pulses, designated P1 and P3¹. The spacing of P1 and P3 determines the mode of interrogation, hence the type of information in the reply. Transponders 'look' for a P1, P3 pulse pair at one the designate spacings (e.g. 3, 5, 8, 17 or 21μs²) which it recognises as a valid interrogation and transmits the appropriate response.

To prevent replies in the side lobes, a control pulse, P2, is transmitted from a near omnidirectional aerial, co-located with the rotating aerial. As would be expected, the signal strength from an omnidirectional aerial is the same in all directions. In this case, signal strength is reduced in the direction of the main beam. The strength or amplitude of P2 is much less than P1 and P3 pulses in the main interrogation beam. The P2 pulse is of an amplitude just bigger than the side lobes. The P2 pulse is transmitted 2μs after P1. If an aircraft transponder detects a pulse 2μs after the first pulse, that is equal or greater in amplitude than the P1 pulse, it knows that it is in a side lobe and suppresses replies even if a valid P3 is received. In the main lobe the P1 and P3 signal will be much larger than the P2 signal, so the transponder will know it is in the main beam and reply.