The strength of the radar signal reflected back to the satellite. Surfaces that strongly reflect radar (such as buildings, rocks, and metal structures) appear bright in amplitude images, while surfaces that absorb or scatter radar (such as smooth water or dense vegetation) appear dark.

A satellite pass where the spacecraft is travelling from south to north. Over the UK, ascending passes typically occur during evening hours. The satellite views the ground from a westward-looking angle.

The portion of the radar signal that is reflected back towards the satellite after hitting the Earth's surface. The amount of backscatter depends on surface roughness, moisture content, and the angle of the terrain relative to the radar beam.

The spatial separation between the satellite's position during two different acquisitions. A perpendicular baseline that is too large reduces the quality of the interferogram, while a baseline of zero would provide no topographic sensitivity. Optimal baselines depend on the application.

A radar frequency range with wavelengths of approximately 4–8 cm, used by Sentinel-1 and other satellites. C-band radar is sensitive to small-scale surface features and vegetation but can lose coherence in heavily vegetated areas over time.

A measure of how similar the radar signal is between two acquisitions, expressed as a value between 0 and 1. High coherence (close to 1) indicates a stable surface where reliable displacement measurements can be made. Low coherence occurs when the surface has changed significantly – for example, due to vegetation growth, ploughing, or construction activity.

A three-dimensional representation of terrain elevation. InSAR processing uses a DEM to remove the topographic contribution to the radar phase, isolating the signal caused by surface displacement.

A satellite pass where the spacecraft is travelling from north to south. Over the UK, descending passes typically occur during morning hours. The satellite views the ground from an eastward-looking angle.

Movement of the ground surface or a structure, measured in the line-of-sight direction between the satellite and the target. Displacement towards the satellite is typically shown as negative (uplift), while displacement away from the satellite is positive (subsidence).

A map of phase differences between two radar images acquired at different times. The characteristic coloured fringe patterns in an interferogram represent cycles of displacement, with each complete colour cycle (fringe) corresponding to half the radar wavelength of movement – approximately 2.8 cm for Sentinel-1.

A radar technique that compares the phase of two or more SAR images to measure surface displacement or generate elevation models. InSAR can detect movements of just a few millimetres over large areas.

A radar frequency range with wavelengths of approximately 15–30 cm, used by satellites such as ALOS-2 and NISAR. The longer wavelength penetrates vegetation canopy and maintains coherence better than C-band in forested or agricultural areas, making L-band well-suited for monitoring natural terrain, landslides, and tectonics. However, it is less sensitive to small-scale surface features and structures.

The direction along which the satellite measures displacement, pointing from the ground target towards the satellite. Because the satellite views the Earth at an angle (typically 30–45° from vertical), line-of-sight measurements capture a combination of vertical and horizontal motion.

A radar frequency range with wavelengths of approximately 30–100 cm. P-band can penetrate dense vegetation and shallow subsurface layers, making it valuable for biomass estimation and subsurface mapping. P-band is not typically used for InSAR displacement monitoring.

Radar target that maintains stable reflective properties over time, such as buildings, infrastructure, or exposed rock. Persistent scatterers provide the most reliable long-term displacement measurements and are the basis of PS-InSAR time series techniques.

The position of the radar wave within its cycle at the moment it returns to the satellite, measured as an angle between 0° and 360°. Small changes in the distance between satellite and ground cause measurable phase shifts, which form the basis of InSAR displacement measurement.

The process of converting wrapped phase values (which cycle between 0° and 360°) into continuous, absolute displacement measurements. This is a critical and computationally challenging step in InSAR processing.

A system that transmits radio waves and detects their reflections to determine the location and properties of objects. Unlike optical sensors, radar provides its own illumination and can operate regardless of sunlight or cloud cover.

The acquisition of information about the Earth's surface from a distance, typically using satellites or aircraft. Remote sensing techniques include optical imaging, radar, and thermal sensors.

The interval between successive satellite passes over the same location. Sentinel-1's revisit time is 12 days per satellite (6 days with both satellites operational). Shorter revisit times allow more frequent monitoring and improve time series analysis.

A radar imaging technique that combines multiple radar pulses acquired as the satellite moves along its orbit to synthesise a much larger antenna, achieving high spatial resolution. SAR produces detailed images of the Earth's surface regardless of weather or daylight conditions.

The size of the smallest feature that can be distinguished in an image, determined by the pixel size on the ground. Sentinel-1 provides spatial resolution of approximately 5 × 20 metres in standard Interferometric Wide swath mode.

How frequently observations are made over the same location. Also referred to as revisit time. Higher temporal resolution enables detection of rapid changes and provides more data points for time series analysis.

The processing of multiple SAR acquisitions over an extended period to track how displacement evolves over time. Time series techniques such as Persistent Scatterer InSAR (PS-InSAR) and Small Baseline Subset (SBAS) can measure displacement rates of a few millimetres per year.

A radar frequency range with wavelengths of approximately 2.5–4 cm, used by satellites such as TerraSAR-X, COSMO-SkyMed, ICEYE, and Capella Space. The shorter wavelength provides higher spatial resolution and sensitivity to fine surface details, but coherence degrades more rapidly over vegetated areas. X-band is well-suited for monitoring urban environments and infrastructure.