Section 171.47 - Performance requirements(a) The Instrument Landing System must perform in accordance with the "International Standards and Recommended Practices, Aeronautical Telecommunications, Part I, Paragraph 3.1" (Annex 10 to the Convention on International Civil Aviation) except as follows:(1) The first part of paragraph 3.1.3, relating to suppression of radiation wholly or in part in any or all directions outside the 20-degree sector centered on the course line to reduce localizer does not apply.(2) Radiation patterns must conform to limits specified in 3.1.3.3 and 3.1.3.4, but this does not mean that suppression of radiation to the rear of the antenna array to satisfy difficult siting positions (as per 3.1.3.1.4) is not allowed. For example, if a reflector screen for the antenna array is required to overcome a siting problem, the area to the rear of the localizer may be made unusable and should be so advertised.(3) A third marker beacon (inner marker) is not required.(4) The frequency tolerance of the radio frequency carrier must not exceed plus or minus 0.002 percent.(b) Ground inspection consists of an examination of the design features of the equipment to determine that there will not be conditions that will allow unsafe operations because of component failure or deterioration.(c) The monitor is checked periodically, during the in-service test evaluation period, for calibration and stability. These tests, and ground checks of glide slope and localizer radiation characteristics, are conducted in accordance with FAA Handbooks AF P 6750.1 and AF P 6750.2 "Maintenance Instructions for ILS Localizer Equipment" and "Maintenance Instructions for ILS Glide Slope Equipment".(d) Flight tests to determine the facility's adequacy for operational requirements and compliance with applicable "Standards and Recommended Practices" are conducted in accord- ance with the "U.S. Standard Flight Inspection Manual", particularly section 217.Doc. No. 5034, 29 FR 11337, Aug. 6, 1974, as amended by Amdt. 171-9, 38 FR 28557, Oct. 15, 1973