This work addresses the study and calculation of a link budget of a Frequency Modulated Continuous Wave (FMCW) radar altimeter to understand the feasibility and limitations of implementing this system on a CubeSat. Special attention will be given to the radar equation, and all associated factors (radar cross-section (RCS), antenna gain...) as these will be the basis of the entire link budget calculation. Altimetry sensors are an essential part of Earth observation satellites (such as for oceanographic studies), and several satellites are equipped with radio/radar altimeters. However, those types of equipment are very large and have high power demands, making its implementation practically impossible for CubeSats Thus, the novelty of this paper is the design of a compact Radar Altimeter based on FMCW architecture for a CubeSat.