Radar Systems: Primary, Secondary and Airborne Weather Radar
Core concepts
Primary radar detects returns from targets using pulse timing to give range and antenna direction to give bearing; it needs no cooperation from the target. Secondary surveillance radar interrogates a transponder, which replies with identity (Mode A), pressure altitude (Mode C) or, in Mode S, a unique address and data. Range from SSR is slant range, so targets at altitude appear further away than their ground position. Airborne weather radar uses short wavelengths (X band, about 9 GHz) to detect precipitation; the display shows reflectivity, with gain and tilt controlling what is seen. Tilt management is critical: too much downward tilt shows ground clutter, too much upward tilt misses weather at range.
Traps in questions
Expect questions on: the difference between primary and secondary radar in terms of cooperation and range accuracy; the fact that Mode C gives pressure altitude, not height above ground; shadowing and attenuation behind heavy precipitation; the danger of painting a storm as weaker than it is because of attenuation; and the correct use of tilt to avoid ground returns. Another trap is that weather radar does not measure turbulence directly; it shows precipitation, and a rapid change in reflectivity gradient suggests hazard.
Rule of thumb
For weather avoidance, aim the tilt so the radar beam just touches the ground at the maximum display range; this gives the clearest picture of weather above the surface. Remember that heavy rain can hide a more intense cell behind it.