We demonstrate that in-situ spectroscopic ellipsometry can be used to measure the height evo- lution of nanostructures during low energy ion sputtering of GaSb. A graded anisotropic effective medium approximation is used to extract the height from the optical measurements. Two different growth regimes have been observed, first exponential, then followed by a linear regime. The linear regime is not expected by the traditional sputtering theories. The in-situ results correspond well to ex-situ AFM measurements.