Maths

Hi,
I want to do a Great circle Distance calculation in a PIC18F252 using either www.CCSinfo.com compiler or www.PicBasic.org

I am no maths guru and am worried that neither of these compilers can correctly calculate the distance accurately, does anyone know differently?

Is the a I2C FPU that I can use, or a slave PIC that acts like a FPU?

Thanks