AWOS Tolerances, Components, And Accuracy

| April 3, 2021 | By: Severe VFR
Close up view of AWOS Station

Many pilots rarely stop to think about the accuracy of the weather reports given to them. 

Yes, we have all been victims of a bad TAF that either grounded us for no good reason or resulted in us being up in the air when we really shouldn't have been. 

But I am not talking about a forecast; I am talking about the real-world weather reports given by automatic reporting stations such as AWOS and ASOS systems. 

For simplicity, we will only be discussing the components and their accuracy for the AWOS system, which the FAA primarily operates. However, the FAA has created a Non-Federal Program for non-federal operators such as U.S. territories, states, local governments, companies, and individuals. 

The FAA has also created an advisory circular listing the requirements for these non-federal AWOS installations to meet operational criteria and operational accuracy, to be considered an approved source of aviation weather information.

Non-federal and federal AWOS installations most likely have similar requirements and accuracy. However, I could not find information regarding federal AWOS systems regarding the accuracy of a federally owned and operated AWOS system and its components. So I can not make this a statement of fact. 

Nevertheless, the non-federal program's information should give a very close estimate of an AWOS stations' accuracy. 

Determining Accuracy

The FAA measures most AWOS equipment's accuracy by calculating the Root Mean Square Error (RMSE) of the individual components. The RMSE measures the error of a model in predicting quantitative data by comparing the instrument's output value to the true value of the parameter being measured with the following equation:

Close up view of AWOS Station

RMSE is a measure of error, or in our case, it is the measure of error between the real-world value and the measured value. When instruments are tested for accuracy, multiple tests, or comparisons, are performed. The data collected is then used to determine an RMSE value for the individual components. If the RMSE value is determined to be acceptable, the instrument is deemed accurate and installed as part of the AWOS.

Operational Tolerances

Before diving into the accuracy of AWOS components, I think it would be fair to discuss what conditions AWOS equipment must be designed to withstand. It would not be fair to judge the ability or accuracy of equipment that makes up the AWOS if it were to be operating in an environment it was never designed to operate within.

All AWOS equipment should demonstrate proper function when operating in the following conditions:

  • Site Elevation: 100ft below sea level to 10,000ft above sea level 
  • Temperature: -35 to +55 degrees Celsius (Can be modified to -55 to +55 degrees Celsius for airports with extreme conditions)
  • Relative Humidity: 5% - 95% over the entire temperature range
  • Windspeed: Up to 85 knots
  • Hail: Up to a half-inch diameter
  • Freezing Rain: Buildup of half an inch per hour, lasting for 1 hour with 40-knot winds
  • Rain: Up to 3 inches per hour with 40-knot winds

As long as the following conditions are not exceeded, then the AWOS should be able to function accurately and properly. 

Instrument Accuracy

If the environmental conditions match the operational tolerances that an AWOS is designed for, then the data outputted by the AWOS must meet the following performance standards for the following equipment installed on an AWOS:

Mechanical Windspeed and Direction Sensor

The mechanical windspeed sensor should respond to wind speeds as low as 2 knots and continue to respond up to a maximum wind speed of 85 knots. The sensor should have an accuracy of ± 2 knots up to 40 knots. Above 40 knots, the sensor should have an RMSE of ± 5%. 

The mechanical wind direction sensor should be aligned to true north and have a range of 1-360° in azimuth. The sensor must begin to output wind direction data when the wind velocity is equal to 2 knots and must have an accuracy of ± 5° RMSE.

Ultrasonic Windspeed and Direction Sensor

Ultrasonic wind speed and direction sensors are allowed to be installed instead of a mechanical wind speed and direction sensor. 

The ultrasonic wind speed sensor should respond to wind speeds as low as 1 knot and continue to respond up to a maximum wind speed of 85 knots. An accuracy of ±1 knot is expected for wind speeds up to 40 knots. Above 40 knots, the ultrasonic sensor should have an RMSE of ± 3%. 

Ultrasonic wind direction sensors should be aligned to true north. The sensor should begin to output wind direction data when the wind velocity is equal to 1 knot and possess an accuracy of ± 3° RMSE. 

Temperature Sensor

The temperature sensor should be thermally isolated to measure the outside air temperature with an accuracy of 1°F RMSE for the entire temperature range of the sensor, with a maximum error of 2°F. 

Humidity Sensor

The humidity sensor should not become damaged if the sensor were to become excessively wet from precipitation or moisture absorption after a loss of power. The sensor should return to normal operation without damage or human intervention within 30 minutes after experiencing an excessively wet condition or after power is restored.

Humidity sensors must be accurate within 5% of the real-world value. 

Dewpoint Sensor

If the dewpoint sensor is a dewcell, similar to the temperature sensor, the sensor should not become damaged if the sensor were to become excessively wet from precipitation or moisture absorption after a loss of power. The sensor should return to normal operation without damage or human intervention within 30 minutes after experiencing an excessively wet condition or after power is restored.

The dewpoint sensor has a wide range of allowable accuracies depending on the outside dry bulb temperatures.

With an error not to exceed 2°F (RMSE) dew point

30°F temperature; 80, 90, 100 percent relative humidity

60°F temperature; 80, 90, 100 percent relative humidity

90°F temperature; 80, 90, 100 percent relative humidity

With an error not to exceed 3°F (RMSE) dew point

30°F temperature; 15, 45, 75 percent relative humidity

60°F temperature; 15, 45, 75 percent relative humidity

90°F temperature; 15, 45, 75 percent relative humidity

120°F temperature; 15, 40 percent relative humidity

With an error not to exceed 4°F (RMSE) dew point

-30°F temperature; between 65 and 95 percent relative humidity

0°F temperature; 25, 60, 95 percent relative humidity

+20°F temperature; 25, 60, 95 percent relative humidity

Pressure Sensor

Two or three pressure sensors are provided for each AWOS system. Each sensor should be able to operate at a higher pressure equal to standard atmospheric pressure at 100ft plus 1.5 inches of mercury (30.03 + 1.5 = 31.53 inHg), as well as a low pressure equal to standard atmospheric pressure at 10,000ft minus 3.o inHg (20.58 - 3.00 = 17.58 inHg). 

Each sensor should be capable of measuring the pressure range at any fixed location of +1.5 to -3.0 inHg from the standard atmospheric pressure at that location.

The accuracy should be ±0.02 inHg RMSE at all altitudes from -100 to +10,000 feet mean sea level (MSL), with a maximum error of 0.02 inHg at any one pressure.

Each sensor must be stable and remain accurate within 0.02 inHg RMSE for at least 6 months, with the maximum allowable error of 0.02 inHg.  

Cloud Height Sensor

Cloud height sensors should have a range of at least 12,500ft. They should provide an output of at least three cloud layers when surface visibilities are equal to or greater than 1/4 mile. 

Under laboratory conditions, the sensor should be accurate within 100ft or 5 percent, whichever is greater. 

The sensor should provide an output at least once every 30 seconds. However, to extend sensor life, the sampling rate may be reduced to provide at least one sample every 3 minutes when no cloud, obscuring phenomena aloft, or CH/VV values are detected during the preceding 15 minutes. 

The ceilometer should be able to remain clear of snow with a rate of 2 inches per hour for 1 hour at a temperature of 20°F, as well as remaining clear of ice for 60 minutes under conditions of freezing rain with a maximum accumulation rate of a half-inch per hour of clear ice. 

Visibility Sensor

The visibility sensor should be capable of determining visibilities from less than a quarter-mile to 10 miles. The accuracy of the visibility sensor depends on the reference visibility: 

1/4 through 1-1/4 miles: ± 1/4 mile

1-1/2 through 1-3/4 miles: + 1/4, -1/2 mile

2 through 2-1/2 miles: ± 1/2 mile

3 through 3-1/2 miles: + 1/2, -1 mile

4 and greater than 4 miles: ± 1 mile

Precipitation Accumulation Sensor

Precipitation includes all types of precipitation in liquid freezing, frozen, and in combinations of all three.

The precipitation amount is a measure of the liquid or liquid-equivalent amount.

Two types of precipitation sensors are used: The tipping bucket rain gauge and the precision scale.

The tipping bucket collects rain in .01 inch increments and then tips over to empty the bucket. Light precipitation is defined as one but not more than two bucket tips in 10 minutes. Moderate precipitation is defined as more than two, but not more than five tips in 10 minutes. Heavy precipitation is defined as more than five tips in 10 minutes.

The precision scale, or All-Weather Precipitation Accumulation Gauge (AWPAG), measures the weight of the precipitation and reports in inches of rain. 

Both sensors must be capable of estimating the rate of precipitation from .01 - 5 inches per hour, with an RMSE accuracy of .002 inches per hour, or 4% of the actual value, whichever is greater. 

Present Weather Detector/Sensor 

The present weather detector sensor indicates the type of precipitation occurring. It should detect any precipitation type (Liquid, freezing, frozen, or a combination of all three). 

The sensor may qualify if it only can identify rain, drizzle, and snow. Other types of precipitation would be listed as unknown. This is due to sensor technology not yet being capable of identifying ice pellets and hail. 

The present weather detector sensor should identify the type of precipitation when the precipitation rate equals or exceeds .002 inches per hour with the following accuracy.

Within the temperature range of:

28°F to 38°F: Identify precipitation type correctly as:

Rain: 90 percent of the cases.

Drizzle: 80 percent of the cases.

Snow: 90 percent of the cases.

Less than 28°F: Identify precipitation type correctly as snow in 99 percent of cases.

Greater than 38°F: identify precipitation type correctly as:

Rain: 99 percent of the cases.

Drizzle: 90 percent of the cases

Only one type of precipitation should be reported, with the following priority order:

  1. Snow.
  2. Rain.
  3. Drizzle.
  4. Precipitation unidentified.

Thunderstorm Detection Sensor

The thunderstorm detection sensor can either be a stand-alone sensor or be connected to the thunderstorm detection network.

Both types should detect the presence of a thunderstorm within 30 nm of an airport and provide data about the location of the thunderstorm to the AWOS system once every minute. 

The thunderstorm detection sensor should identify and detect 90% of all thunderstorms within 10nm of the airport. The location RMSE for thunderstorms within 10nm of the airport should not exceed 3nm. 

 The sensor should also detect 80% of strikes between 10nm and 30nm of the airport. The location accuracy of lightning strikes between 10nm and 30nm from the airport must have an RMSE no greater than 6nm. 

No more than 2% of all thunderstorms reported by the sensor should have been caused by sources other than a naturally occurring thunderstorm. 

Freezing Rain Sensor

The freezing rain sensor detects the occurrence of freezing rain. This sensor should report freezing rain when .01 inches of freezing rain has accumulated. To meet operational standards, the sensor should correctly detect freezing rain 95% of the time. 

Frost should not cause false alarms on the freezing rain sensor. At temperatures above 40°F, or when there is no precipitation, the false alarm rate should not exceed .1%.

When the precipitation is snow, the false alarm rate should not exceed 1%. 

Runway Surface Condition Sensor

The runway surface condition sensor provides real-time information on runway conditions to alert the pilot if the runway is wet or if there are possible icing conditions. 

This sensor should be capable of detecting three-runway conditions:

  1. Dry Runway: No perceptible Moisture
  2. Wet Runway: Visible Moisture on the runway surface
  3. Freezing Conditions: Pavement temperature below freezing with moisture present on the surface

The temperature component of the runway surface condition sensor should be accurate within ± 1°F within the temperature range of 25° to 35°F. During testing, the sensor should be accurate at least 80% of the time in each of the three conditions. 

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