References 121 Regarding local polynomial approximation, one could question if choices of basis function other than polynomials can give better performance. In fact, using Fourier-based vectors would create a link between this approach and the Fourier-based array interpolation in Section 3.5.1. This combination could lead to a local Fourier-based model with potentially better performance, although at the expense of greater computational cost. Finally, there is a need to develop tools for predicting performance of the different approaches, in terms of a statistical analysis for a given (known) model and methods for predicting performance using available data only. Robust testing based on the Bootstrap (Zoubir and Boashash, 1998), which is closely related to cross-validation, is a natural choice for this task. Acknowledgement The authors would like to thank Professor Visa Koivunen of Helsinki University of Technology for clarifications regarding implementation of the manifold separation approach. REFERENCES Agrawal, A., Jablon, A., 2003. A calibration technique for active phased array antennas. In: IEEE International Symposium on Phased Array Systems and Technology, pp. 223­228. Allen, J., 1964. Array antennas: new applications for an old technique. IEEE Spectr. 1 (11), 115­130.