The MLS is a precision approach and landing guidance system which provides position information and various ground-to-air data. The position information is provided in a wide coverage sector and is determined by an azimuth angle measurement, an elevation angle measurement and a range (distance) measurement.

1. (a) An MLS constructed to meet the requirements of this subpart must include:

   1. (1) Approach azimuth equipment, associated monitor, remote control and indicator equipment.
   2. (2) Approach elevation equipment, associated monitor, remote control and indicator equipment.
   3. (3) A means for the encoding and transmission of essential data words, associated monitor, remote control and indicator equipment. Essential data are basic data words 1, 2, 3, 4, and 6 and auxiliary data words A1, A2 and A3.
   4. (4) Distance measuring equipment (DME), associated monitor, remote control and indicator equipment.
   5. (5) Remote controls for paragraphs (a) (1), (2), (3), and (4) of this section must include as a minimum on/off and reset capabilities and may be integrated in the same equipment.
   6. (6) At locations where a VHF marker beacon (75 MHz) is already installed, it may be used in lieu of the DME equipment.

2. (b) In addition to the equipment required in paragraph (a) of this section the MLS may include:

   1. (1) Back azimuth equipment, associated monitor, remote control and indicator equipment. When Back Azimuth is provided, a means for transmission of Basic Data Word 5 and Auxiliary Data Word A4 shall also be provided.
   2. (2) A wider proportional guidance sector which exceeds the minimum specified in §§ 171.313 and 171.317.
   3. (3) Precision DME, associated monitor, remote control and indicator equipment.
   4. (4) VHF marker beacon (75 MHz), associated monitor, remote control and indicator equipment.
   5. (5) The MLS signal format will accommodate additional functions (e.g., flare elevation) which may be included as desired. Remote controls for paragraphs (b) (1), (3) and (4) of this section must include as a minimum on/off and reset capabilities, and may be integrated in the same equipment.
   6. (6) Provisions for the encoding and transmission of additional auxiliary data words, associated monitor, remote control and indicator equipment.

3. (c) MLS ground equipment must be designed to operate on a nominal 120/240 volt, 60 Hz, 3-wire single phase AC power source and must meet the following service conditions:

   1. (1) AC line parameters, DC voltage, elevation and duty:

      120 VAC nominal value—102 V to 138 V (±1 V)* 240 VAC nominal value—204 V to 276 V (±2 V)* 60 Hz AC line frequency—57 Hz to 63 Hz (±0.2 Hz)* *Note: Where discrete values of the above frequency or voltages are specified for testing purposes, the tolerances given in parentheses indicated by an asterisk apply to the test instruments used to measure these parameters. Elevation—0 to 3000 meters (10,000 feet) above sea level Duty—Continuous, unattended

   2. (2) Ambient conditions within the shelter for electronic equipment installed in shelters are:

      Temperature, −10 °C to + 50 °C Relative humidity, 5% to 90%

   3. (3) Ambient conditions for electronic equipment and all other equipment installed outdoors (for example, antenna, field detectors, and shelters):

      Temperature, −50 °C to + 70 °C Relative humidity, 5% to 100%

   4. (4) All equipment installed outdoors must operate satisfactorily under the following conditions:

      Wind Velocity: The ground equipment shall remain within monitor limits with wind velocities of up to 70 knots from such directions that the velocity component perpendicular to runway centerline does not exceed 35 knots. The ground equipment shall withstand winds up to 100 knots from any direction without damage. Hail Stones: 1.25 centimeters ( 1/2 inch) diameter. Rain: Provide required coverage with rain falling at a rate of 50 millimeters (2 inches) per hour, through a distance of 9 kilometers (5 nautical miles) and with rain falling at the rate of 25 millimeters (1 inch) per hour for the additional 28 kilometers (15 nautical miles). Ice Loading: Encased in 1.25 centimeters ( 1/2 inch) radial thickness of clear ice. Antenna Radome De-Icing: Down to −6 °C (20 °F) and wind up to 35 knots.

4. (d) The transmitter frequencies of an MLS must be in accordance with the frequency plan approved by the FAA.
5. (e) The DME component listed in paragraph (a)(4) of this section must comply with the minimum standard performance requirements specified in subpart G of this part.
6. (f) The marker beacon components listed in paragraph (b)(4) of this section must comply with the minimum standard performance requirements specified in subpart H of this part.