ILS Category 2 approaches use a Radio Altimeter (RA) derived DH (Decision Height). An RA measures the aircraft height directly above the surface immediately below the aircraft. Any sloping terrain, obstacles, etc. that might cause a rapid fluctuation or interference with a stabilized and smooth decrease in RA altitude (near the DH) could result in a less predictable/precise arrival at the DH.
Are there specific runway (displaced threshold or similar) design criteria that require the surface from which the RA takes its measurement at the DH point to be flat?