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

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 dBm 2 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 dBm 2 (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 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.

Geodetic SAR for Height System Unification and Sea Level Research—Results in the Baltic Sea Test Network

“…The lower precision of the DLR2 is due to the repaired electronics, which changed the electronic characteristics of this device. Similar findings on range delay variations of 1.2 and 2.1 m for this type of ECR are also reported in [10].…”

“…The computation of range and azimuth residuals with respect to the surveyed ECR coordinates, applying the corrections for Sentinel-1 systematic effects, tropospheric and ionospheric delays and solid Earth tides. Note that the analysis is performed with the surveyed ECR origin coordinates corrected for the geometric ECR phase center offsets specified in the ECR manual for ascending and descending passes, following the methods proposed in [10]. The ECR origin coordinates are observed during installation with differential GNSS and are considered to be accurate within 5 cm or better depending on the instrumentation and observation time.…”

A low-cost active reflector and a passive corner reflector network for assisting landslide monitoring using multi-temporal InSAR

“…However, this problem can be reduced using a calibration curve obtained estimating a temperature-dependent factor which allows the partial removal of this seasonal variability [19,20]. Nevertheless, the AR used in this study provided an acceptable performance for the concerned application.…”

Investigating radar cross sections of passive radar reflectors for C-band and X-band SAR satellite missions

“…Depending on the capacity of radar targets either to generate radar signals or just reflect ones generated by remote sources, radar reflectors can be either active or passive. Active radar reflectors are currently evolving, and the scientific literature of the last decade shows their potential [3][4][5][6][7][8][9] . Because of their low cost of construction and maintenance, passive artificial corners are even more diffused as radar reflectors in SAR applications.…”