Improvement of antenna beam pattern performance for space-borne active SAR systems using error compensation and optimization

Abstract

Synthetic Aperture Radar (SAR) with an active phased array antenna can support user needs for diverse operation modes and images that require lots of antenna beam patterns with different shapes and steered angles in the along-track direction as well as in the across-track direction. In particular, the antenna beam patterns of the space-borne SAR system significantly impact on the key performance parameters of SAR such as NESZ (Noise Equivalent Sigma Zero), ASR (Ambiguity to Signal Ratio), and RA (Radiometric Accuracy) due to geometric factors and system constraints. In terms of design, verification and operation for the space-borne SAR antenna performance, the beam patterns can be categorized as following three types. The first one is design beam pattern. It can be obtained by beam pattern synthesis from the SAR mode design and performance analysis. The second one is measured beam pattern. After manufacturing antenna hardware, the design beam pattern should be measured in order to verify the antenna. The last one is model beam pattern. It is mathematical modeling to represent real radiated patterns of the active SAR antenna. In ideal antenna, the three beam patterns are identical. However, there are differences between the beam patterns due to error factors. Therefore, algorithm to minimize the differences is essential to achieve improvement of the beam pattern performance for the space-borne SAR system with the active phased array antenna.

In this dissertation, error compensation and optimization methods are proposed for improvement of antenna beam pattern performance of space-borne SAR system. The dissertation consists of following three main ideas.

First, the error factors defined from the differences between the design and measured beam pattern are identified and characterized. Based on the characterization data, the errors are classified as setting errors, offset errors, and errors due to temperature variation. Furthermore, the appropriate and effective compensation method is proposed. The proposed method is established with short measurement time and high accuracy via validation process. In order to implement and validate the proposed method, an active phased array antenna, consisting of 16 TRMs (Transmit Receive Modules) is manufactured. In addition, the validity of the proposed method is shown in terms of antenna pattern shape, relevant key parameters of SAR performance.

Second, a method which improves the antenna model accuracy is proposed. The antenna model accuracy can be defined as the differences between the measured and model beam pattern. Because the measured beam patterns (radiated patterns) of the active SAR antenna include error contributions caused by its hardware, the antenna model should take into account the errors in order to improve the antenna model accuracy. The proposed method can be utilized to achieve the high accuracy using optimization of the errors based on a convex algorithm. The proposed method is validated by measured results of a manufactured antenna that is composed of 32 TRMs and is also analyzed in terms of SAR performance.

Third, TRM failure compensation method is proposed using optimization approach. In case of space-borne active SAR system, beam pattern should be resynthesized to meet SAR performance, because TRMs is impossible to be repaired or replaced when the TRMs are degraded or failed. Differences between the model and design pattern can be caused by the TRM failure. A method to minimize the differences is proposed using error matrix optimization based on convex algorithm. Notwithstanding the TRMs failure, the proposed method can efficiently and accurately provide the resynthesized beam patterns for the required criteria.