A novel method to measure the concentration of ejecta generated by the impingement of rocket plumes on granular surfaces is presented. This method is based on the principle of millimeter wave interferometry, using a fully-integrated frequency modulated continuous-wave 60-64 GHz radar to measure the path-integrated ejecta concentration between the radar itself and a reflector. The instrument is capable of quantitative measurements of high path-integrated ejecta concentrations up to 0.317 ± 0.133 vol%.m, one order of magnitude higher than state-of-the-art optical methods, at a repetition rate of 10 kHz and independently of the particle size distribution of the material. The interferometer was calibrated using a particle shadow counting technique for path-integrated concentrations up to 0.0223 ± 0.0008 vol%.m (0.280 ± 0.010 × 109 particles.m−2 ) and demonstrated on a reduced-scale plume-surface interaction experiment using a 6.5 N cold gas thruster impinging on a bed of regolith simulant under lunar (6.67 Pa) and martian (800 Pa) ambient pressures. A glass microsphere regolith simulant with a mean particle diameter of 105 μm and a standard deviation of 17μm was used for calibration and demonstration. The instrument performed nominally in both demonstrations conditions, measuring a maximum path-integrated ejecta concentration of 0.0419 ± 0.0170 vol%.m (0.527 ± 0.214 × 109 particles.m−2 ) in the lunar case and 0.131 ± 0.053 vol%.m (1.65 ± 0.67 × 109 particles.m−2 ) in the martian case.