how to select proper parameters for "opt.StartPoint" ?

I want to fit some data as below code witch generate from curve fitting tool:

[xData, yData] = prepareCurveData( x, y );
ft = fittype( 'c*x^(a*sin(x*b)^2)', 'independent', 'x', 'dependent', 'y' );
opts = fitoptions( 'Method', 'NonlinearLeastSquares' );
opts.Display = 'Off';
opts.StartPoint = [0.452987881098739 0.698037618662922 0.104515161828401];
[fitresult, gof] = fit( xData, yData, ft, opts );

How can I know the best start points?

Answer 1

[Matt J]

Assuming b is known, the log-model

log(yData)=log(c) + a*(log(x)*sin(x*b)^2)

is a linear model in a and log( c ). Therefore, if you know a decent guess for b, you can use a linear fit (which doesn't require an initial guess) to develop initial guesses for the other parameters.

[hamzah almadani]

thank you for your answer, but how I can solve the equation ?

[Matt J]

For example,

 p=polyfit( (log(x)*sin(x*b)^2) , log(yData), 1 );
 a=p(1);
 c=exp(p(2))