The PHI Instrument of the Solar Orbiter mission requires an Image Stabilization System (ISS) with a very high precision. A nanometric range solution based on a Limb Sensor has been successfully implemented. The main element is a four-quadrant photodiode sensor that provides a signal dependent on the position of the sun image on it. In order to reach the required precision, around 20 bits of resolution is necessary. As space qualified ADCs only reach 16 bits, an oversampling solution using Comb filters has been used. This solution has been compared with one based on a correlator tracker. The results of this comparison show that the main advantage is a reduction of the power consumption. On the other hand, the main disadvantage of this solution is that it does not allow the detection of absolute displacement of the solar surface.