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We give an overview of past work using synthetic aperture radar (SAR) interferometry (InSAR) to detect, observe and characterize sinkholes, present the advantages and disadvantages of using satellite-borne SARs for this application, and discuss the availability to the community of free and open SAR data. In particular, we discuss the planned NASA-ISRO Synthetic Aperture Radar (NISAR) mission, a joint venture between the National Aeronautics and Space Administration (NASA) and the Indian Space Research Organisation (ISRO) currently being built for launch in 2022. Introduction Because of the prevalence of rock susceptible to dissolution, natural sinkholes can be found on all the continents except Antarctica (Ford and Williams, 2013). In fact, it is estimated that up to 40% of the contiguous US is susceptible (Weary and Doctor, 2014) and the at-risk area is even larger when cavity collapse in general, including that unrelated to dissolution of rocks, is considered (e.g., collapse of mines, pipes, and lava tubes). The spatial scale alone of the area to be covered makes groundbased monitoring difficult, encouraging the application of remote sensing methods to sinkhole location and assessment. Indeed, one can argue that even many groundbased methods are ‘remote’ in that they seek to identify underground caverns from the surface. However, here we consider a different scale of operation and take up the topics of how well sinkholes can be identified and studied from space and what instruments are available to use for the purpose. No Earth-observing spaceborne instruments currently exist that can identify subterranean caverns directly, so all sensors detect ground surface conditions that could be indicative of a sinkhole, and rely on ancillary data and in situ measurements for validation. Sinkhole mapping and change tracking using remote sensing methods can be done based on several identifying features, all of which are also used in field surveys. Historically topographic survey methods have been used most commonly to identify areas of subsidence, with surface elevation, change in elevation, and shape of the depression as the primary sinkhole indicators. These remain the most commonly used sinkhole indicators accessible with remote sensing. Soil moisture, standing water and vegetation type, particularly that which is different from the surrounding area, are secondary indicators also measurable to some extent with remote sensing. Additionally, because sinkholes form where rainwater or snowmelt percolates through the soil, their presence can sometimes be inferred from the water flow pathways, particularly from changes in the presence or absence of standing water without an apparent surface run-off path (e.g., Hofierka et al., 2018). Although several remote sensing instruments are available (radar, lidar (Light Detection and Ranging), and optical/infrared imagers), lidar is now the most frequently used remote sensing technique for sinkhole identification. Unfortunately, no existing spaceborne lidar has the needed resolution for this purpose. A future lidar-based land surface topography mission with ground spacing of 1-m to 5-m was identified as a high priority in the 2017 Decadal Survey undertaken by the National Academy of Sciences but was acknowledged to be not yet technically feasible (NAS, 2018). This leaves radar as the most promising candidate for measuring the topographic indicators of sinkholes from space. Today there are a number of spaceborne synthetic aperture radars (SARs) with spatial resolution sufficient to be of use for some of the larger sinkholes, at least, and more are planned for the future, including a NASA Earth observing SAR mission scheduled for launch in 2022. These instruments and their suitability are the topics of the following sections. SAR Applied to Sinkhole Study Overview SAR imaging instruments available in the civilian sector in general are not able to achieve the decimeter spatial resolution of optical imagery. The finest available SAR Cathleen E. Jones Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109 USA, cathleen.e.jones@jpl.nasa.gov

Capabilities, limitations and opportunities for studying sinkholes using synthetic aperture radar interferometry