Identifying low reflection amplitude and low level phase noise points for permanent scatterer (PS) interferometry Yitzhak August, Dan G. Blumberg, Stanley R. Rotman Ben-Gurion University of the Negev, Earth and planetary image facility lab, Beer-Sheva, Israel Abstract — The PSI (Persistent Scatterers Interferometry) method relies on identifying a small group of scatterers that maintain high phase reliability over a relatively long period of time. This study demonstrates a new algorithm to identify natural PSC (persistent scatterer candidates) targets in non- inhabited areas. The application of our PSC selection process is conducted for a natural arid scene as opposed to the more common use of the PS technique, which is done mostly for urban areas with structures exhibiting strong reflection (manmade objects). We present a novel robust method to identify PSC in open fields and in places of low backscattering (natural areas). Our method is based on the amplitude time history signature of each point. The main difference between urban areas and open field areas is the low reflectance and less deterministic behavior of the scatterer; hence it is a challenge to detect these low reflection and stable points. Conventional methods for PSC detection require a preprocessing with fine calibration and are mainly suitable to use in urban areas, but may fail when used in the open fields. One of the advantages of our method is the use of a simple process of calibration which is based only on the flight geometry and gain factors without any auxiliary data or assumptions. Consider a vector consisting of the measurement of a PS point as a function of time. We can express this signal as an amplitude times a phase. The amplitude differs between PS points; however potential PS points should correlate spatially and temporally in terms of the phase, independent of their amplitude. Our method improves locates several candidate points with a narrow phase distribution and thus, enables the location of PSCs in natural open areas. Index Terms — Calibration, Identifying Persistent Scatterers, Interferometry, InSAR. I. Introduction The PS technique provides a way to estimate the fine topographic changes and the motion of objects and structures. In contrast to the classical SAR interferometry technique, the PS technique relies only on a small group of pixels. The PS technique relies on a group of point targets whose amplitude and phase are stable as a function of time (i.e. invariant of look angle (baseline), soil moisture content and are geometrically and physically stable). Point-like target scatter is a scatter that dominates the scattering from a resolution cell, it was shown that for high reflection target the internal pixel phase noise is low [1]. The first notification of this kind of points was noted by Hanssen and Usai [2]. It was shown that some certain man made targets exhibited high coherence, even when using interferograms with long time spans and large spatial baselines (spans of look angles). Using those low level phase noise pixels provides the key for atmospheric phase estimation and fine topographic change estimation. By searching for pixels that include a single dominant stable scatterer, a network of so called Persistent Scatterers (PS) can be established. One of the main difficulties and challenges of using and applying the PS method is the first selection of permanent scatterer candidates group (PSC) (i.e.), the process of selecting an elementary resolution cell which presents a reflection from a stable point scatter. The process of point searching needs to be applied in a very careful way. Selection of un-stable points or non-point scatter will lead to a process failure or to high errors in the topographic change estimation. In general, a persistent scatterer point can be metallic structures or a corner reflector, a corner of a building or a solid rock formation in rural areas. Different ways for PSC detection were presented in [3]-[4] and mainly were used in populated areas and urban areas. In this paper we present a new strong and reliable method to identify permanent scatter candidates in open fields and in places of low backscattering areas (natural areas). We call this method Amplitude Time Signature (ATS). II. ATS algorithm for PSC selection Our method is based on the amplitude time history signature (ATS) of each point (voxel). The application of the PSC selection process is present to be used in natural regions where most of the areas not cover with manmade objects. The process of selecting PSC is based on the pixel absolute reflectance signature over a time series. The stage of PSC searching contains three main parts. The first part is calibration of the SAR image data, this stage is based on the ESA presented algorithm [5] but with some simplifications. Our calibration process is done for the geometric properties of the image acquisition i.e., only calibration on the spatial domain is done. We exclude the time dependence part of the calibration from our process; the process of the PSC model deals with this part. The second part is the estimation of the PSC amplitude time signature model. In this step we use the Principal Component Analysis (PCA) method in order to estimate the signature of PSC ATS. The PSC model in this part is the product of the SAR system changes over time, some geometric contribution, the atmosphere and also other unpredictable changes over time. The amplitude signature of the PSC is estimated using only a small and high reliable group of points in the set of images.