On the Efficacy of Compact Radar Transponders for InSAR Geodesy: Results of Multiyear Field Tests

Compact and low-cost radar transponders are an attractive alternative to corner reflectors (CRs) for interferometric synthetic aperture radar (InSAR) deformation monitoring, datum connection, and geodetic data integration. Recently, such transponders have become commercially available for C-band sensors, which poses relevant questions on their characteristics in terms of radiometric, geometric, and phase stability. Especially for extended time series and for high-precision geodetic applications, the impact of secular or seasonal effects, such as variations in temperature and humidity, has yet to be proven. In this article, we address these challenges using a multitude of short baseline experiments with four transponders and six CRs deployed at test sites in The Netherlands and Slovakia. Combined together, we analyzed 980 transponder measurements in Sentinel-1 time series to a maximum extent of 21 months. We find an average radar cross section (RCS) of over 42 dBm2 within a range of up to 15° of elevation misalignment, which is comparable to a triangular trihedral CR with a leg length of 2.0 m. Its RCS shows the temporal variations of 0.3-0.7 dBm2 (standard deviation), which is partially correlated with surface temperature changes. The precision of the InSAR phase double differences over short baselines between a transponder and a stable reference CRs is found to be 0.5-1.2 mm (one sigma). We observe a correlation with surface temperature, leading to seasonal variations of up to ±3 mm, which should be modeled and corrected for in high-precision InSAR applications. For precise SAR positioning, we observe antenna-specific constant internal electronic delays of 1.2-2.1 m in slant range, i.e., within the range resolution of the Sentinel-1 interferometric wide (IW) product, with a temporal variability of less than 20 cm. Comparing similar transponders from the same series, we observe distinct differences in performance. Our main conclusion is that these characteristics are favorable for a wide range of geodetic applications. For particular demanding applications, individual calibration of single devices is strongly recommended.