How does SAR achieve high resolution?

Range resolution is achieved the same way as any radar: by using a wide bandwidth signal. Delta_r = c / (2B). With B = 600 MHz bandwidth: delta_r = 0.25 m. Pulse compression (LFM chirp or phase-coded waveforms) enables wide bandwidth without requiring short, high-peak-power pulses. Azimuth resolution is the remarkable part. A real aperture radar's azimuth resolution is delta_az = R times lambda / D, where R is range and D is antenna diameter. At 600 km range (LEO satellite) with D = 10 m and lambda = 3 cm (X-band): delta_az = 1.8 km. Completely useless for imaging. SAR exploits platform motion to synthesize a large aperture. As the platform moves, each pulse is transmitted and received from a different position. By coherently processing all these returns (accounting for the changing range to each ground point), the radar effectively creates a virtual antenna as long as the distance the platform travels during the observation time. The synthetic aperture length L_sa = R times lambda / D. The synthetic aperture resolution is then: delta_az = R times lambda / (2 L_sa) = D / 2. This result is independent of range, which is counterintuitive and remarkable. A 5 m antenna achieves 2.5 m azimuth resolution whether at 100 km or 1000 km range.

What are the main SAR modes?

Stripmap: the antenna points at a fixed angle perpendicular to the flight path. The synthetic aperture forms naturally as the platform moves. Resolution: D/2 in azimuth. Swath width limited by PRF constraints. Standard workhorse mode. Spotlight: the antenna steers to keep illuminating the same target area as the platform moves. This increases the synthetic aperture length beyond the natural value, improving azimuth resolution beyond D/2. TerraSAR-X spotlight mode achieves 0.25 m resolution. Disadvantage: illuminates only a small area, not continuous strip. ScanSAR: the antenna beam is periodically steered to different sub-swaths, trading azimuth resolution for wider coverage. Each sub-swath is illuminated for a shorter time, so the synthetic aperture is shorter and resolution is coarser (typically 10 to 100 m). Coverage: 100 to 500 km swath. TOPS (Terrain Observation by Progressive Scans): similar to ScanSAR but steers the beam forward through each sub-swath, providing more uniform SNR and avoiding scalloping. Used by Sentinel-1 (IW mode, 250 km swath, 5x20 m resolution). Inverse SAR (ISAR): the target moves (rotates) while the radar is stationary. Used for ship and aircraft classification.

Interferometric SAR (InSAR) uses two SAR images of the same area taken from slightly different positions (or at different times) to measure surface elevation or deformation with millimeter precision. The phase difference between the two images is related to the path length difference, which depends on the surface height and the baseline (antenna separation). For repeat-pass InSAR: the same satellite images the same area on two different orbits (e.g., Sentinel-1: 6-day repeat). The phase change between passes measures surface displacement along the radar line of sight. Displacement precision: 1 to 5 mm for a single interferogram, sub-millimeter with time-series techniques (PS-InSAR, SBAS). Applications: earthquake deformation mapping (coseismic and postseismic), volcanic inflation and deflation monitoring, landslide detection and monitoring, glacier flow velocity measurement, urban subsidence monitoring (groundwater extraction, mining), and infrastructure monitoring (dams, bridges). For topographic InSAR: two antennas on the same platform (or tandem satellites like TanDEM-X) image simultaneously. The baseline creates a stereo geometry. SRTM (2000) mapped global topography at 30 m resolution using this technique. TanDEM-X achieved 12 m resolution global DEM with 2 m vertical accuracy.

Radar Imaging

SAR

/sar/ — Synthetic Aperture Radar

Coherent processing of returns during platform motion. Azimuth: δ_az = D/2 (range-independent!). Range: δ_r = c/(2B). Modes: stripmap (D/2), spotlight (sub-D/2, 0.25m), ScanSAR (wide swath), TOPS (Sentinel-1). InSAR: mm-precision deformation. PolSAR: polarimetric classification. X-band (0.25m), C-band (5m), L-band (foliage pen).

Az: D/2

Spotlight: 0.25m

InSAR: mm

Understanding SAR

SAR is one of the most remarkable applications of coherent signal processing. By exploiting platform motion and phase-coherent recording, it achieves resolution that would require an impossibly large antenna. A satellite at 600 km altitude can image the ground with sub-meter resolution using a 5 m antenna. The equivalent real-aperture resolution at that range would require a 10 km antenna.

SAR images through clouds, fog, rain, and darkness, making it invaluable for surveillance, environmental monitoring, and disaster response. InSAR adds the ability to measure surface deformation with millimeter precision, enabling earthquake and volcanic monitoring from space.

SAR Equations

Range resolution:
δ_r = c/(2B)
B=600MHz: δ_r=0.25m

Azimuth resolution (stripmap):
δ_az = D/2
D=5m antenna: δ_az=2.5m
(Independent of range!)

Synthetic aperture length:
L_sa = Rλ/D
R=600km, λ=3cm, D=5m: L_sa=3.6km

InSAR displacement:
Δd = λΔφ/(4π)
Δφ=1 rad, λ=5.6cm: Δd=4.5mm

SAR System Comparison

Satellite	Band	Resolution	Swath	Revisit
TerraSAR-X	X (9.65G)	0.25m	10km	11 days
Sentinel-1	C (5.4G)	5×20m	250km	6 days
ALOS-2	L (1.27G)	1-10m	25-350km	14 days
RADARSAT-2	C (5.4G)	1-100m	18-500km	24 days
TanDEM-X	X (9.65G)	1-12m	30-100km	11 days

Common Questions

Frequently Asked Questions

Resolution?

Range: δ_r=c/(2B). 600MHz BW: 0.25m. Pulse compression (LFM chirp). Azimuth: δ_az=D/2 (stripmap). Platform motion synthesizes aperture L_sa=Rλ/D. Resolution = Rλ/(2L_sa) = D/2. 5m antenna = 2.5m at any range. Spotlight: steers to extend L_sa, achieves <D/2.

Modes?

Stripmap: fixed beam, D/2 resolution, continuous strip. Spotlight: beam steers to target, sub-D/2, small area. ScanSAR: multi-swath, wider coverage, coarser (10-100m). TOPS: progressive scan, uniform SNR (Sentinel-1 IW: 250km, 5×20m). ISAR: target moves, radar static (ship/aircraft ID).

InSAR?

Two SAR images, phase difference → displacement. Δd=λΔφ/(4π). Sentinel-1 C-band: 1-5mm precision/interferogram. PS-InSAR/SBAS: sub-mm with time series. Applications: earthquakes, volcanoes, landslides, glaciers, subsidence, infrastructure monitoring. TanDEM-X: simultaneous stereo for topographic DEM (2m vertical).

Related Terms

← Sampling SAW →

Radar Imaging

Request a Quote

Need SAR system design, InSAR processing, or radar imaging analysis? Contact our team.

Get in Touch