Synthetic Aperture Radar · An Antenna Perspective

The synthetic-aperture antenna principle

The synthetic-aperture antenna is the principle behind Synthetic Aperture Radar (SAR). A SAR carries a real antenna, typically around twelve metres long, on a satellite, and by combining the echoes collected along its orbit it synthesises an effective aperture kilometres in length, giving significantly higher resolution than the real antenna could achieve alone. The antenna principle is described below.

Open the design tool ↓
Core Principle · Synthetic Aperture Antenna

A radar looking straight down cannot separate points mirrored left and right of its ground track: their echoes return with identical delays. By looking off to one side instead, each point lies at a distinct distance, so its echo arrives at a unique delay.

Flyby: echoes collected while the target is in the beam
elements: 0 · Lsyn = 0 m
Synthetic beam sharpens as the array grows
azimuth resolution δaz = La/2

A SAR looks to one side as it flies along its orbit. Each pulse acts as one element of an array, and combining the echoes coherently builds a synthetic aperture Lsyn kilometres long, giving an azimuth resolution far finer than the real antenna could reach alone.

Interactive Synthetic Aperture Radar Tool
Side-looking geometry · beam on the ground
Far-field radiation pattern G(θ,φ)
Antenna La, Le, fc
Wavelength λ fc5.55 cm
Aperture area A La·Le9.6 m²
Aperture eff. η fixed0.72
Gain G A, λ, η44.5 dBi
Az beam θaz La, λ0.23°
El beam θel Le, λ3.5°
Azimuth res δaz La only6.0 m
Orbit h, θi
Slant range R h, θi855 km
Effective velocity v h7.50 km/s
Slant range res δR B only1.50 m
Coupled antenna × orbit
Ground swath Le, λ, R, θi99 km
Az footprint La, λ, R3.7 km
Synth aperture Lsyn La, λ, R3.95 km
Tdwell Lsyn, v0.53 s
NESZ all−22 dB
Min antenna area v, λ, R, θi3.5 m²

It is assumed that the aperture excitation is uniform (no amplitude tapering), so the pattern's first sidelobes sit at −13.2 dB. Applying a real taper would lower those sidelobes and widen the main beam slightly.