Phase-Offset Calculation for InSAR DEM Generation Without Using Ground Control Points

Synthetic Aperture Radar (SAR) is a microwave remote sensing sensor, which can be mounted either on spaceborne or airborne systems and allows night and day and all-weather imaging. Use of interferometric SAR systems (which consist of two SAR antennas pointing to the same area from slightly different view angles) allows the generation of Digital Elevation Models (DEMs) by means of the so-called SAR interferometry (InSAR) technique [1], [2], [3], which exploits the phase difference (interferogram) of the data relevant to the same ground area and received by the two different antennas. Since SAR interferograms provide the phase value restricted to the [-π, π] interval, Phase Unwrapping (PhU) procedures [1], [2] are required to recover the original phase difference containing the needed topographic information of the observed area. However, such PhU procedures provide the desired information up to a Phase Offset (PhOff) value, which is constant over the whole interferogram [1], [2] but needs to be calculated. To this aim, different algorithms have been developed, most of which make use of Ground Control Points (GCPs), such as corner reflectors, properly deployed over the observed scene. However, use of GCPs, which also involves proper measurement of their precise position (typically by means of GPS ground-stations), may be expensive in terms of costs and time, and in case of emergencies it may be difficult to be carried out. Accordingly, in several cases of practical interest, GCP-based algorithms for InSAR PhOff evaluation, although very accurate, cannot be applied. To circumvent this limitation, some methods have been proposed in the literature [4], [5], [6]. In this paper we present a novel algorithm for the InSAR PhOff calculation without using GCPs. It is based on the availability of an external DEM, whose resolution can be significantly lower than that of the SAR data we deal with. More specifically, we propose a two step algorithm. In the first step, we calculate the synthetic interferometric phase relevant to the external DEM in order to evaluate a least square (LS) estimation of the searched PhOff. To refine this first solution, in a second step we exploit the difference between the external DEM and the InSAR DEM generated according to the PhOff value roughly estimated at the first step. In order to compute the final InSAR DEM with the proposed algorithm, we thus basically need to apply twice the (straightforward) operation leading from the InSAR unwrapped phase to the DEM. This renders the proposed algorithm very easy to be implemented and (computationally) very efficient. The proposed procedure has been applied, in this work, on real data acquired, over different test sites, from the airborne OrbiSAR System [7] operating at X-band. The external DEM used in the presented test-cases is the SRTM (Shuttle Radar Topography Mission) one, whose resolution (90 m) is more than one order of magnitude worst than the one achievable with the used OrbiSAR sensor (which is of the order of one meter). To assess the accuracy of the reached results, these latter have been compared with those obtained via an accurate algorithm that makes use of GCPs (properly deployed in the considered test-areas) showing the effectiveness of our solution.