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**tvauce** Thanks a lot for your time and consideration.

My problem is not estimating the two unknown constants Y and b since X is also unknown. I thus do not have a set of pair measured data (X,Z) to estimate Y and b, only Z is measured and available.

I have thought of the need for an iteration within each step. I have tried Recursive least square estimation method as well as Extended Kalman filter method but still have not find the answer.

If you have any other comments or suggestion, please let me know. Your time and kindness are highly appreciated.

Best regards