Radar Pulse Techniques: Range and Timing

Basic Radar Principles

RADAR (Radio Detection and Ranging) is a system that uses electromagnetic waves to detect objects and determine their distance and direction.

Primary vs Secondary Radar

• Primary Radar: Emits pulses of electromagnetic energy that reflect off objects (echoes). It measures the time it takes for the signal to travel back and forth to calculate distance.
  • Examples: Airborne Weather Radar (AWR), Primary Surveillance Radar (PSR), Surface Movement Radar (SMR).
• Secondary Radar: Uses an interrogator (ground) and transponder (air) system. The transponder receives the signal and emits a stronger coded reply.
  • Examples: SSR (Secondary Surveillance Radar), DME.

Pulse Parameters

PRF (Pulse Repetition Frequency)

It is the number of pulses transmitted per second (pps).

• Relation to Maximum Range: The PRF determines the theoretical maximum unambiguous range. The radar must wait for the pulse to return from the maximum distance before sending the next one.
• Formula: $ \text{Maximum Range (km)} = \frac{300,000}{2 \times \text{PRF}} $ (Where 300,000 is the speed of light in km/s).
• The lower the PRF, the longer the listening time and therefore the greater the maximum range.

Pulse Width

It is the duration of the transmitted pulse.

• Relation to Minimum Range: The radar cannot receive while transmitting. Therefore, the pulse width determines the minimum distance at which an object can be detected (blind spot).
• Range Resolution: A shorter pulse allows better distinction between two objects close together in the same direction (better resolution).

Power

• Peak Power: It is the maximum instantaneous power during the pulse duration. It determines the maximum detection range (ability to penetrate the atmosphere and return).
• Average Power: It is the energy averaged over the entire cycle (including the silence time). It is much lower than the peak power.

Applications

• ATC (Air Traffic Control):
  • Primary Radar: To see aircraft without transponders and weather.
  • Secondary Radar (SSR): To identify aircraft, obtain altitude and squawk codes.
• Airborne:
  • Weather Radar (AWR): Detection of precipitation and turbulence.
  • Radio Altimeter: Measurement of height above terrain.
  • DME: Distance measurement (pulse technique).
  • TCAS: Collision avoidance (based on SSR).