Difficulty with 3D curve fitting

I want to fit this data with my own custom function, but it is showing "NaN computed by model function, fitting cannot continue.Try using or tightening upper and lower bounds on coefficients with very close value." so I am looking for trajectory fitting. Can somebody will help me with this.

[Walter Roberson]

I am having difficulty with my eyes, so there is no way I can possibly read the model expression in that captured image. Some of it is just plain blurred and I don't think it is entirely my eyes either.

Please post the actual model expression as text.