CHAPTER 2

SENSOR EXPOSURE

2.1 GENERAL.

Sensor siting shall not violate runway or taxiway object free areas, runway or taxiway safety areas, obstacle free zones, or instrument flight procedures surfaces as defined in FAA Advisory Circular (AC) 150/5300-13, Airport Design, or FAA Handbook 8260.3, TERPS. Notwithstanding these constraints, the sensor exposure will strive to minimize or eliminate the effects of manmade or geographical obstructions. The tower used to mount the wind sensor is not considered an obstruction to the sensor collection system, but it will (with the exception of the temperature, dew point, and pressure sensors) be at least 10 feet (3 meters) away from the other sensors. Sensors should be located as far as practicable from cultivated land to reduce contamination by dust and dirt. It may be necessary to increase the heights of some sensors based on the average maximum snow depth for the location, which will be determined by averaging the maximum annual snow depths over the period of record.

2.2 PRESSURE SENSOR.

The pressure sensor will be installed on the airfield, usually in a weatherproof facility (building, shelter, enclosure, etc.). When the pressure sensor is vented to the outside, a vent header will be used. In most cases, internal venting of the pressure sensors may be satisfactory. However, if it is determined that internal venting will affect the altimeter setting value by ± 0.02 inches of mercury or more, outside venting will be used. A portable transfer standard will be used to resolve any questions regarding the need for external venting. Siting that will cause pressure variations due to air flow over the venting interface should be avoided. The venting interface will be designed to avoid and dampen pressure variations and oscillations due to "pumping" or "breathing" of the pressure sensor venting and porting equipment. Each sensor will have an independent venting interface from separate outside vents (if outside venting is required) through dedicated piping to the sensors. The sensors should also be located in an area free of jarring, vibration, and rapid temperature fluctuations (i.e., avoid locations exposed to direct sunlight, drafts from open windows, and air currents from heating or cooling systems). If the pressure sensors are sited outdoors, the height of the vent header shall not be less than one foot above the average maximum snow depth, or 3 feet (1 meter) above ground level, whichever is higher.

Pressure sensor derived values are of critical importance to aviation safety and operations. Great care shall be taken to ensure that pressure sensor siting is suitable and accurate. The field and sensor elevations above Mean Sea Level (MSL) elevation shall be determined to the nearest whole foot in accordance with agency procedures. The distance between the elevation of the pressure sensors and the field elevation will not exceed 100 feet (30 meters).

The above criteria are applicable to altimeter-only systems, except: (1) the pressure sensor will be installed within 6 miles (9.6 kilometers) of the instrument runway threshold, (2) a temperature correction is used in the algorithm to compute altimeter setting, and (3) the elevation difference between the height of the pressure sensors and the field elevation may be increased to 500 feet (150 meters).

2.3 CLOUD HEIGHT SENSOR.

The cloud height sensor will be mounted on a platform/pedestal with the sensor optics a minimum of 4 feet (1.2 meters) above ground level or above maximum snow depth, whichever is higher. The sensor should be located as far as practicable from strobe lights and other modulated light sources.

2.4 VISIBILITY SENSOR.

The visibility sensor will be mounted on a platform/pedestal as free as possible from jarring and vibration. Unless otherwise specified by the manufacturer, the receiver will be pointed in a northerly direction. The sensor should be located as far as practicable from strobe lights and other modulated light sources. It should neither be located in an area that is subject to localized obstructions to vision (e.g., smoke, fog, etc.) nor in an area that is usually free of obstructions to vision when they are present in the surrounding area. It will be mounted so the optics are 10 ± 2 feet (3 ± 0.6 meters) above ground or 6 feet (2 meters) above the average maximum snow depth, whichever is higher. Ten feet (3 meters) above the ground is the preferred height. The area within 6 feet (2 meters) of the sensor should be free of all vegetation and well-drained. Any grass or vegetation within 100 feet (30 meters) of the sensor should be clipped to a height of about 10 inches (25 centimeters). These precautions are necessary to reduce the probability of carbon-based aerosols (e.g., terpenes) and insects from interfering with sensor performance. In addition, backscatter-type sensors must have a clear area for 300 feet (90 meters) in the forward (north) octant. Some sensors may require additional clear areas. The clear line of sight requirement for the sensor optics will be as specified by the sensor manufacturer.

2.5 WIND SENSOR.

The wind sensors (wind direction and wind speed) will be oriented with respect to true north. The system software will be used to make required adjustments to magnetic north. The site should be relatively level, but small gradual slopes are acceptable. It will be mounted 30 to 33 feet (9 to 10 meters) above the average ground height within a radius of 500 feet (150 meters). The sensor height shall not exceed 33 feet (10 meters) except as necessary to: (a) be at least 15 feet (4.5 meters) above the height of any obstruction (e.g., vegetation, buildings, etc.) within a 500 foot (150 meters) radius, and (b), if practical, be at least 10 feet (3 meters) higher than the height of any obstruction outside the 500 foot (150 meter) radius, but within a 1,000 foot (300 meter) radius of the wind sensor. An object is considered to be an obstruction if the included lateral angle from the sensor to the ends of the object is 10 degrees or more.

Exception: The height of a wind sensor installed on the Instrument Landing System (ILS) glide slope antenna tower or on a separate tower in area "A", Figure 1 will be reduced, as necessary, such that the height of the complete wind sensor installation (i.e., to include any required air terminal(s) and obstruction lights) does not exceed the height of the glide slope antenna installation. The minimum acceptable height for the wind sensor in this situation is 20 feet (6 meters). If side mounting (i.e., perpendicular to a tower) is necessary, a boom will be used to permit installation of the sensor at a minimum of 3 feet (1 meter) laterally from the tower. Side mounting is to be utilized only if top mounting is not practicable and the tower is of open design to allow for free air flow.

Figure 1 Precision Instrument Runway Siting

2.6 TEMPERATURE AND DEW POINT SENSORS.

The temperature and dew point sensors will be mounted so that the aspirator intake is 5 ± 1 feet (1.5 ± 0.3 meters) above ground level or 2 feet (0.6 meters) above the average maximum snow depth, whichever is higher. Five feet (1.5 meters) above ground is the preferred height. The sensors will be protected from radiation from the sun, sky, earth, and any other surrounding objects but at the same time be adequately ventilated. The sensors will be installed in such a position as to ensure that measurements are representative of the free air circulating in the locality and not influenced by artificial conditions, such as large buildings, cooling towers, and expanses of concrete and tarmac. Any grass and vegetation within 100 feet (30 meters) of the sensor should be clipped to height of about 10 inches (25 centimeters) or less.

2.7 LIGHTNING DETECTION (THUNDERSTORM) SENSOR.

The lightning detection (thunderstorm) sensor will be sited and mounted in accordance with the manufacturer's recommendations/specifications. For a single station sensor, metal obstructions will be no closer than two times their height above the sensor.

2.8 PRECIPITATION TYPE DISCRIMINATION SENSOR.

The precipitation type discrimination sensor detects precipitation and discriminates type (e.g., rain, snow). It will be mounted so that the optics are 10 ± 2 feet (3 ± 0.6 meters) above ground or 6 feet (2 meters) above the average maximum snow depth, whichever is higher. Ten feet (3 meters) above ground is the preferred height. If the system is double ended, the optical axis will be oriented generally north-south with the receiver facing north. The terrain between the receiver and transmitter should be relatively flat.

2.9 PRECIPITATION OCCURRENCE (YES/NO) SENSOR.

The precipitation occurrence sensor will be mounted in accordance with the manufacturer's specifications at a convenient height but not less than 6 feet (2 meters) above ground level or 4 feet (1.2 meters) above the average maximum snow depth, whichever is higher. Care must be taken to avoid shielding of the sensor by structures, buildings, and other obstacles.

2.10 FREEZING RAIN DETECTION SENSOR.

The siting requirements for the freezing rain sensor are the same as for the precipitation occurrence sensor.

2.11PRECIPITATION ACCUMULATION (LIQUID OR LIQUID EQUIVALENT) SENSOR.

The precipitation accumulation sensor will be mounted so that the orifice is horizontal and in an area where the terrain is relatively flat. The orifice is defined as the upper rim edge of the collector mouth. The height of the orifice will be as close to ground level as practicable. In determining the height of the orifice, consideration will be given to keeping the orifice above accumulated/drifting snow and minimizing the potential for splashing into the orifice. Surrounding objects will be no closer to the sensor than a distance equal to two times their height above the gage orifice. An object is considered an obstruction if the included lateral angle from the sensor to the ends of the object is 10 degrees or more. In order to reduce losses due to wind, an alter-type windshield is recommended to be installed on gages in areas where 20 percent or more of the annual average precipitation falls as snow. The surrounding ground can be covered with short grass or be of gravel composition, but a hard flat surface, such as concrete, gives rise to splashing and should be avoided. Separate sensors may be used to measure liquid and frozen precipitation accumulation (e.g., rain and snow) in which case the above criteria will be followed for each installation.

2.12SNOWFALL-SNOW DEPTH SENSOR.

The snowfall-snow depth sensor will be mounted at least 15 feet (4.5 meters) away from the wind tower over an area which would be expected to have snow cover and is representative of the area of interest. It will be mounted in accordance with manufacturer's specifications and recommendations.

2.13COMBINATION VISIBILITY, PRECIPITATION OCCURRENCE, AND PRECIPITATION ACCUMULATION SENSOR.

The siting requirements for the visibility sensor apply to this combination sensor or any other combinations of the precipitation parameters and visibility.

Cover
Table of Contents
Appendix
Return to Publications List