Atmospheric correction for Interferometric Synthetic ApertureRadar (InSAR) technique is a set of different methods to remove artefact displacement from an interferogram caused by the effect of weather variables such as humidity, temperature, and pressure. An interferogram is generated by processing two synthetic-aperture radar images before and after a geophysical event like an earthquake. Corrections for atmospheric variations are an important stage of InSAR data processing in many study areas to measure surface displacement because relative humidity differences of 20% can cause inaccuracies of 10–14 cm InSAR due to varying delays in the radar signal. Overall, atmospheric correction methods can be divided into two categories: a) Using Atmospheric Phase Screen (APS) statistical properties and b) Using auxiliary (external) data such as GPS measurements, multi-spectral observations, local meteorological models, and global atmospheric models.
On the other side, atmospheric noise might have some value for atmospheric research in meteorology because atmospheric artefacts signals are related to water vapour in the troposphere. The spatial resolution of the InSAR map for C-band satellites like Sentinel-1 without multi-looking is around 20 meters. That means InSAR can measure Precipitable Water Vapor (PWV) in the atmosphere in a 20m grid over hundreds of kilometres, which is much denser than other methods such as GNSS and space-borne passive sensors. However, the long revisit time of Sentinel-1 (temporal resolution, 12 days) at the moment is the main disadvantage of this technique from the meteorologists' side. Nevertheless, using the capability of InSAR to measure PWV in high spatial resolution is interesting for meteorological research.
What is InSAR? InSAR can provide accurate (millimeter-level) ground displacement fields for large areas over hundreds of kilometres. This technique uses two synthetic aperture radar images of the same area acquired at different times to measure surface motion between those times. Nevertheless, the result of interferometric synthetic-aperture radar in interferogram form includes actual displacement and other effects. Hence, these other effects must be calculated and removed from interferograms to achieve an accurate result of real ground displacement. Some of these errors have more influence, such as orbital errors, topographic effects and atmospheric artefacts. There are several methods to remove all these noises until a reasonable and acceptable amount except atmospheric effects with high accuracy. Thus, a significant challenge for the InSAR community remains atmospheric artefacts. Sometimes especially in areas with high humidity, the effect of atmospheric noise is much larger than geophysical events and prevents surface displacement detection.
Atmospheric noise
In radar satellites, microwave signals are reflected off a persistent scatter in a target area, and their two-way travel time is measured by satellites. Water vapor in the troposphere and free electrons in the ionosphere affect the propagation of microwave signals through the atmosphere because the different refractive index in these layers affects the speed of propagation.
Ionosphere
Ionospheric phase noise, which occurs more apparent with larger wavelengths, such as P or L-band radars, is a consequence of variations in free electron density in the 100–1000 km altitude along the travel path. Radar satellites with large wavelengths like ALOS Polsar1/2 (L-band, λ=~24 cm) and NISAR are more vulnerable to ionospheric delays. However, this noise is roughly less effective in C-Band (λ=5.6 cm) and X-band (λ=3.1 cm) SAR satellites such as Envisat, RADARSAT, ERS1/2 and TerraSAR-X and usually is negligible. There are a couple of methods to remove the Ionospheric noise artefact: