Ionospheric and Tropospheric Impact for InSAR Time Series Analysis in the Central Andes: A Case Study from Northwestern Argentina

The ionosphere, located 50 km above the Earth's surface is characterized by ionization processes that can significantly impact electromagnetic signals within the microwave wavelength range. The magnitude of the impact depends on the density of free electrons, which have daily and seasonal oscillations but are also tied to the 11-year solar activity cycles. Radar signals are delayed after interacting with free electrons and ions, and the magnitude of such delay is inversely proportional to the radar frequency. Thus, sensors operating in the longer wavelength L-band are more affected than those operating in the C-band. However, even C-band interferograms can be significantly affected if the region is close to the geomagnetic equator.

The Central Andes in Northwestern Argentina, being in proximity to the geomagnetic equator, offer an excellent setting to study the impact of the ionosphere on interferograms. Its low vegetation cover results in highly coherent interferograms, and predominantly dry conditions at high elevations lead to small tropospheric disturbances.

We employ the split spectrum technique extended to time series analysis to identify interferograms that are impacted by ionospheric contributions. Subsequently, we apply statistical methods to those time series to recognize acquisitions more likely to be contaminated by the ionosphere.  The magnitude of ionospheric contribution is compared to tropospheric delay. We demonstrate the impact of the high-solar activity on interferograms by correlating our time series of ionospheric delay to sunspot activity and total electron content maps. The analysis of Sentinel 1 C-band data from both ascending and descending tracks reveals a more significant contribution in ascending passes in response to the daily cycle of free electron density. These findings prove the relevance of the ionosphere as source of disturbance in interferograms from Sentinel C-band, particularly for studies at the regional scale.