Analysis of internal timings and clock rates of TerraSAR-X

Abstract

Due to the indirect measurement principle of radar on base of signal travel time, a very precise calibration of the sensor's internal clock with regard to clock rate and potential time offsets relative to the Coordinated Universal Time is an essential prerequisite for accurate pixel localization in Synthetic Aperture Radar. Giving special considerations on this aspect, we developed two algorithms that improve the already high localization accuracy of TerraSAR-X: The either one very precisely determines the true oscillator clock rate, while the other one effectively refines the accuracy of time annotation. Experimental analyses evaluate the obtained improvements in localization accuracy from both techniques.

Measurements on the Absolute 2-D and 3-D Localization Accuracy of TerraSAR-X

“…We may already excluded some potential causes: The stability of the sensor internal clock rate governing all timings in the SAR payload is routinely monitored in the TerraSAR-X mission and in this way known with nine digit accuracy (the technique is described in Assuming that the trend in the range measurements is real, finding a possible cause for it is subject of ongoing investigations. We may already excluded some potential causes: The stability of the sensor internal clock rate governing all timings in the SAR payload is routinely monitored in the TerraSAR-X mission and in this way known with nine digit accuracy (the technique is described in [27]). In consequence, this error contribution cannot exceed 3 mm and is therefore below the amount of the observed trends.…”

“…The lower performance of the azimuth localization accuracy is limited by the annotation precision (18.6 microseconds step width) of the raw data acquisition time. Even if it is derived from very accurate pulse per seconds (PPS) information of the on-board GPS receivers [27], the information is stored in the SAR payload with an insufficient number of digits. We strongly recommend future SAR missions to provide a finer time annotation.…”

“…We strongly recommend future SAR missions to provide a finer time annotation. For TerraSAR-X we found a way to overcome the truncated timing by applying a post processing of the azimuth timing [27]. Since May 2014, this post processing is part of the operational TerraSAR-X SAR processor and lowers the quantization error of the azimuth timing by about one order of magnitude.…”

“…The estimate for the clutter contribution results from Equation (3), while the estimates for the measurement accuracy of tropospheric and ionospheric delay are taken from the respective data products. Considering the methods discussed in [27], the assumed uncertainty in the azimuth timing, taken from L1B annotation of operational TerraSAR-X products, amounts to about 1 microsecond, corresponding to about 7 mm azimuth error. The Analog to Digital Converter (ADC) sample rate of both TerraSAR-X sensors (about 330 MHz) is operationally monitored to ascertain that the difference between its actual and annotated value does not exceed a margin of 1 Hz.…”

“…2018, 10, x FOR PEER REVIEW 12 of 21 [27]). In consequence, this error contribution cannot exceed 3 mm and is therefore below the amount of the observed trends.…”