The satellite's real radar dish is physically small, so on its own it radiates a wide, poorly-focused beam — like trying to read fine print through fogged glass. Flying along the orbit, the radar fires a pulse every few metres and records the faint echo from each position, including how its phase shifts as the range to the ground point changes (the Doppler history). Combined coherently, those echoes behave exactly as if a single antenna spanning the whole flight segment had transmitted and received at once — a "synthetic" aperture far larger than the real dish.
δ_real = λ·R / D (real antenna alone)
δ_synth = λ·R / (2·L) (L = synthetic aperture length)
- Real antenna size (D) — the satellite's actual dish; bigger helps a little, but stays tiny compared to an orbital pass.
- Flight-path length (L) — how much of the orbit's pulses get coherently combined; resolution improves in direct proportion to L, not to D.
- Pulses combined — one marker per transmitted pulse along the growing synthetic-antenna line trailing the satellite.
Real-world relevance: this is exactly how satellites such as Sentinel-1 or Capella image the ground to metre-scale detail through cloud and darkness — the physical antenna never changes size, only the length of orbit used to synthesize it.