The complexity of the atmosphere renders the modelling of the atmospheric delay in multi temporal InSAR difficult. This limits the potential of achieving millimetre accuracy of InSAR-derived deformation measurements. In this paper we review advances in tropospheric delay modelling in InSAR, tropospheric correction methods and integration of the correction methods within existing multi temporal algorithms. Furthermore, we investigate ingestion of the correction techniques by different InSAR applications, accuracy performance metrics and uncertainties of InSAR derived measurements attributed to tropospheric delay. Spatiotemporal modelling of atmospheric delay has evolved and can now be regarded as a spatially correlated turbulent delay with varying degree of anisotropy random in time and topographically correlated seasonal stratified delay. Tropospheric corrections methods performance is restricted to a case by case basis and ingestion of these methods by different applications remains limited due to lack of their integration into existing algorithms. Accuracy and uncertainty assessments remain challenging with most studies adopting simple statistical metrics. While advances have been made in tropospheric modelling, challenges remain for the calibration of atmospheric delay due to lack of data or limited resolution and fusion of multiple techniques for optimal performance.
Nairobi, Kenya’s capital city, is one of the fastest-growing cities on the continent. The rapid expansion of human activities has resulted in the overexploitation of natural resources, such as water. In the past, Nairobi had been identified as a vulnerable area to environmental hazards, such as land subsidence. Due to the lack of a functioning deformation-monitoring system in Kenya, the subsidence in Nairobi has yet to be empirically quantified. In this paper, we report the results of the first InSAR-based spatial assessment of land subsidence in Nairobi. Our analysis indicates both localized and regionalized subsidence in several locations in the west and north west of Nairobi. The largest deforming unit in Nairobi’s western part is subsiding at approximately 62 mm/yr. Land subsidence can be attributed to groundwater overexploitation because it coincides with regions with the highest decline in groundwater levels. However, subsidence can also be attributed to consolidation associated with rapid urbanization in other areas such as east of Nairobi. This evaluation corroborates previous hydrogeological investigations which indicated that Nairobi was at risk of subsidence, contributing to flooding in some residential areas. The findings will help guide future decision-making in several agencies as well as provide an effective tool for planning mitigation measures to prevent further subsidence. A few of these include regulating borehole drilling, planning of roads and buildings, and locating groundwater observation wells. In addition, the observed significant land subsidence stresses the need for an updated geodetic reference system. Since Nairobi plays a significant role in the economy of Kenya, the effects of subsidence may be devastating and it is imperative that steps are taken to minimize their impact.
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