This study focuses on the analysis of the ground effect in counter-rotating coaxial rotors. To investigate the aerodynamic performance of a coaxial rotor system, the aerodynamic loading is measured for different rotor vertical spacing, rotational speed, and height above the ground. To link aerodynamic loading with flow topology, velocity fields in the rotor slipstreams are measured with particle image velocimetry (PIV). A semi-empirical model is additionally proposed to complement existing ground-effect theories from the literature by accounting for the effects of rotor spacing, ground proximity, and rotor-to-rotor aerodynamic interactions. The results of the performance analysis show that the ground effect is more pronounced in coaxial configurations than in single rotors, especially at minimum spacing and height, where the thrust increases about twice the corresponding value of single rotors. The analysis of the PIV velocity fields reveals how the inflow to the bottom rotor accelerates the downstream flow, increasing the flow rate and further reducing the induced velocity near the ground. As the rotor spacing increases, these interactions weaken, causing the aerodynamic loading to converge to that of a single rotor at a spacing around 90 % of the rotor radius. The proposed model inspired by experimental data provides a robust framework for predicting coaxial rotor performance near the ground. It also allows integration of the ground effect model into UAV control strategies for improved flight stability and safety.