Abstract
Compact and low-cost radar transponders are an attractive alternative to corner reflectors (CR) for SAR interferometric (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. Here we address these challenges using a multitude of short baseline experiments with four transponders and six corner reflectors 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 degrees of elevation misalignment, which is comparable to a triangular trihedral corner reflector with a leg length of 2.0 m. Its RCS shows 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 corner reflectors 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 Swath (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.
Highlights
R ADAR transponders are active electronic devices that receive a radar signal, amplify it, and transmit it back to its source, such as a satellite carrying a Synthetic Aperture Radar (SAR) antenna. They can serve as a compact alternative to corner reflectors (CR) for precise SAR positioning [1], [2], SAR interferometry (InSAR), deformation monitoring over areas with few natural coherent scatterers [3], InSAR datum connection, and geodetic data integration to provide an absolute reference to the inherently relative InSAR measurements [4]
From the experimental results with four compact transponders manufactured by [5], installed at two different test sites, we conclude that they have an average Radar Cross Section (RCS) of 40-45 dBm2, which is comparable to a triangular trihedral corner reflector with a leg length of 2.0 m
The temporal standard deviation of the transponders’ RCS is up to 0.7 dB, which is more than two times the standard deviation observed for a corner reflector of equivalent RCS, considering the 0.25 dB radiometric stability of the Sentinel-1 SLC measurements [32]
Summary
R ADAR transponders are active electronic devices that receive a radar signal, amplify it, and transmit it back to its source, such as a satellite carrying a Synthetic Aperture Radar (SAR) antenna. They can serve as a compact alternative to corner reflectors (CR) for precise SAR positioning [1], [2], SAR interferometry (InSAR), deformation monitoring over areas with few natural coherent scatterers [3], InSAR datum connection, and geodetic data integration to provide an absolute reference to the inherently relative InSAR measurements [4].
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