Consequences of Long-Term ASAR Local Oscillator Frequency Decay - an Empirical Study of 10 Years of Data
Marinkovic, Petar1; Larsen, Yngvar2

Phase trends in the Envisat ASAR interferometric phase have been observed and reported by all groups working in the field of interferometric deformation monitoring. The effect of the reported phase ramps on the corresponding deformation estimates is a ramp of approximately 15 mm/year (from near to far range). Furthermore, this effect has been captured by all of the state-of-the-art processing algorithms. While different mitigation strategies have been proposed, mainly looking into the baseline errors, and despite a couple of unsuccessful attempts, the actual physical cause for this issue has not yet been found.

In this contribution we report on the physical explanation of this trend. We also propose a correction model and correction strategy for ASAR data. The correction completely resolves for any of the observed issues with the phase ramps, as well as, it significantly improves the geo-localization accuracy of the sensor.

In order to find the actual physical cause of these phase trends, we performed an extensive analysis of 10.000 (ten thousand) ASAR products, over a 10 years period, with good temporal and spatial sampling. Specifically, we performed (1) re-estimation of the system parameters from Level-0 products, (2) geo-localization of Level-1 products using corner reflectors, (3) stack interferometric analysis.

The analysis indicated a systematic, smooth, and correlated in time, slow decay of the sensorís Local Oscillator (LO) frequency with respect to its nominal value. This decay systematically effects, and degrade the performance, of all the levels of ASAR products; with the maximum impact in the highest resolution data (Level 1), and in the last part of the mission. This LO drift unequivocally explains the issues observed with ASAR data.

Nevertheless, the observed LO decay is possible to describe accurately by an analytic model. The derived model is validated extensively on different case studies and corner reflector sites.

We will present and in detail discuss the effects of the observed ASAR Local Oscillator drift. We will also elaborate on the derivation, validation, and performance of the proposed correction model for SAR and InSAR applications.