A wellhead system includes a wellhead assembly which is generally tubular, a landing string assembly, and an orientation sensor assembly which is mounted on the landing string assembly. The wellhead assembly is secured to a top of a borehole and has an orientation engagement part. The landing string assembly is lowered into the wellhead assembly to land in the wellhead assembly. The landing string assembly has an orientation part with an orientation feature. The orientation engagement part of the wellhead assembly corresponds to the orientation feature. As the landing string assembly is lowered into the wellhead assembly, the orientation feature engages with the orientation engagement part to rotate the landing string assembly about its longitudinal axis into a desired orientation relative to the wellhead assembly. The orientation sensor assembly measures an angular rotation of the landing string assembly about its longitudinal axis.