Continuous wave (CW) millimeter-wave radars have gained the interest from industry for accurate short distance measurement. In particular, six-port radar (SPR) architecture presents advantages such as high resolution combined with low-power consumption and low-cost implementation. A WR15 waveguide six-port interferometer is designed and realized for accurate phase-shift measurement in the millimeter-wave regime for harsh environment conditions. This latter is combined with dedicated calibration algorithms to provide measurements performance in terms of sensitivity and accuracy. Experimental results are exemplary shown in this paper with mean phase-shift error of 2.2 degrees at the test frequency 60 GHz.