National Weather Service United States Department of Commerce

                                                                          Sensor Suite Design

The ASOS uses a variety of sensors to obtain a comprehensive and real time observation of the weather conditions a particular location. Standard ASOS sensor layout can be seen below:

                                                                   Barometric Pressure Sensor

Atmospheric pressure is the most important surface weather element for aircraft operations since it provides the means of establishing the height of an aircraft above the ground. It is the only element that cannot be directly observed or qualitatively sensed by the observer or pilot. As a result, pressure has always been carefully measured and the operational sensor routinely compared to some reference standard.

All the currently computed pressure elements will continue to be reported by the ASOS with the same or higher level of precision as the human report. The pressure parameters available from ASOS are:


Sensor Pressure

Altimeter Setting

Pressure Remarks

Sea-Level Pressure

Density Altitude

Pressure Altitude

Pressure Change/Tendency

Station Pressure


Because accurate pressure is critical, three separate and independent pressure sensors are used at towered airport locations. At other locations, two pressure sensors are used. The ASOS algorithm compares the pressure sensors’ readings and issues a pressure report only when there is acceptable agreement between at least two sensors.


                                                          Pressure Sensor

The ASOS pressure measurement instrument consists of redundant digital pressure transducers, which use capacitive sensors, one side of which is permanently evacuated to a vacuum to make it a barometric pressure sensor. Advanced microcomputer electronics and sophisticated firmware provide reliable performance. The barometers are located on a tray at the bottom of the ACU and are exposed to the ambient air pressure. In cases when the ACU is installed in pressurized buildings, this exposure is through a port connected to an outside static pressure vent. Figure 7 shows the pressure sensors in the ACU. The specified operational characteristics for these sensors are: n Range: 16.9 - 31.5 inches of mercury.

Accuracy: ± 0.02 inches of mercury
Resolution: 0.003 inches of mercury (measurement); 0.005 inches of mercury (reporting)


                                           Pressure Strengths and Limitations

The pressure sensors are the most reliable and accurate sensor in ASOS. The only limitation (if one can call it that) is that pressure remarks will be reported more often in automatic ASOS METAR messages than in manual METAR messages simply because of the continuous weather watch which ASOS provides.


                                             Ambient/Dew Point Temperature

To determine dew point temperature, a mirror is cooled by a thermoelectric or Peltier cooler until dew or frost begins to condense on the mirror surface. The body of the mirror contains a platinum wire RTD, similar to that used for ambient temperature. This RTD assumes the mirror’s temperature, which is held at the dew point temperature. When this condition occurs, the mirror’s surface is in vapor pressure equilibrium with the surrounding air (i.e., has reached the saturation vapor pressure). The temperature required to maintain this equilibrium is, by definition, the dew point temperature. 

Optical techniques are used to detect the presence of surface condensation. Within the hygro thermometer, a beam of light from a small Light Emitting Diode (LED) is directed at the surface of the mirror at a 45 degree angle. Two photo-resistors are mounted to receive the reflected light. The “direct” sensor is placed at the reflection angle and receives a high degree of light when the mirror is clear. The indirect sensor is placed to receive light scattered when the mirror is clouded with visible condensation, (i.e., dew or frost formation). 

In normal operation, a feedback loop controls an electric heat pump running through a cooling-heating cycle, which cools the mirror until dew or frost is formed; it then heats the mirror until the condensate (dew or frost) is evaporated or sublimed. This cycle nominally takes about 1 minute to complete. 

As the mirror’s cloudiness increases, the “direct” sensor receives less light and the “indirect” sensor receives more light. When the ratio of indirect to direct light reaches an adaptive criterion value, the mirror is considered to be at the dew point temperature. The adaptive criterion value (ratio of indirect to direct light) is adjusted once a day to compensate for residual contamination on the mirror due to dust and other airborne particulates.

Since a clean mirror needs relatively less indirect light to determine when dew has formed than a dirty mirror, the mirror is heated once a day to recalibrate the reference reflection expected from a dry mirror. This procedure compensates for a possible dirty or contaminated mirror and redefines adaptive criterion value used to determine when dew or frost has occurred. This once per day recalibration nominally takes about 15 minutes. 

The ASOS hygro thermometer meets all NWS specifications for measuring range, accuracy, and resolution. The specifications for accuracy are given in Root Mean Square Error (RMSE) and Maximum (MAX) Error. 

The RMSE for Dew Point Temperature is given as a range of values and is dependent on the Ambient Temperature minus the Dew Point Temperature value (i.e., Dew Point Depression [DD]). The low end of the RMSE and MAX Error range is for a DD of 0°F; the high end of the Error range is for a DD of 63°F.


Both the manual and automated temperature sensors directly measure the ambient dry-bulb and the dew point temperatures. The hygro thermometer used in the ASOS is a modern version of the fully automated “HO-83” hygro thermometer, first used operationally in 1985. This instrument uses a platinum wire Resistive Temperature Device (RTD) to measure ambient temperature and a chilled mirror to determine dew point temperature. 

Both the manual and automated temperature sensors directly measure the ambient dry-bulb and the dew point temperatures. The hygro thermometer used in the ASOS is a modern version of the fully automated “HO-83” hygro thermometer, first used operationally in 1985. This instrument uses a platinum wire Resistive Temperature Device (RTD) to measure ambient temperature and a chilled mirror to determine dew point temperature. 

The RTD operates on the principle that electric resistance in a wire varies with temperature. This RTD is located in the stream of aspirated air entering the sensing unit and assumes the ambient air temperature. 



                                                 Wind Speed and Direction:

The rotating cup anemometer and the simple wind vane are the principal indicators of wind speed and direction. Until the mid 1940s, the electrical contacting anemometer was the standard wind measuring instrument. Since then, the “F420” series of instruments have become the standard for wind measurement in the U.S. A basic system of this series consists of a cup-driven Direct Current (DC) generator with an output calibrated in knots and a vane coupled to an indicator by means of a DC synchro-system. The ASOS uses a modern automated version of the F420, in which electro-magnetic signals generated by the rotating cup and wind vane are directly converted into reportable values by ASOS. 

                                                                Wind Sensor:

The ASOS wind sensor employs a “light chopper,” electro-optical method to determine wind speed and convert it to appropriate electro-magnetic signals. Wind sensor measurements conform to the Range, Accuracy, Resolution specifications described in the table below. In addition, the sensor’s starting threshold for response to wind direction and wind speed is 2 knots. Winds measured at 2-knots or less are reported as calm.



Wind Direction:
ASOS reports a 2-minute average of 5-second average wind directions once a minute (i.e., 24 samples each minute) for distribution through the OMO and computer-generated voice messages. The current 2- minute average wind direction is updated on selected OID screen displays once every minute and included in the transmitted METAR/SPECI messages. The direction from which the wind is blowing is reported to the nearest 10 degree increment (e.g., 274 degrees is reported as 270 degrees). Wind direction is reported relative to true north in the METAR/SPECI message, in the daily/monthly summaries, and on all video displays. Wind direction is reported relative to magnetic north in the computer-generated voice messages, and on the OID “AUX” data display screen.


Wind Speed:
A 2-minute average is updated once every 5 seconds and is reported once every minute in the OMO and computer-generated voice messages, and included in the METAR/SPECI message and various OID screen displays. 


Wind Gust:
This is a basic component of wind character and is updated every 5 seconds. It is appended to and reported with the basic wind observation only when appropriate conditions for reporting wind gusts exist. Wind gust information is included in the current OMO, computer-generated voice messages, the METAR/SPECI, and OID displays. 


Wind Shift:
This remark is reported in the OMO and the METAR/SPECI when appropriate.


Variable Wind Direction:
This data element is reported in the OMO and the METAR/SPECI when appropriate. 


                                                               Precipitation Accumulation


These improvements have resulted in the ASOS HTB becoming a very capable liquid precipitation accumulation gauge in all but the most extreme heavy rainfall events. However, some deficiencies still remain in its ability to fully measure precipitation accumulation during the cold-season LEFP events. Consequently, the ASOS HTB is primarily used to measure liquid accumulation. Alternative solutions are being pursued to provide LEFP information. These solutions include: (1) Provision of separate LEFP reports through existing manual supplementary observing networks from event-driven Supplementary Data Observations and schedule-driven Supplementary Climate Data reports, and (2) Development of a follow-on All-Weather Precipitation Accumulation Gauge for ASOS.


ASOS HTB 6 Primary Components:


  • A wind shield that surrounds the HTB and protects it against blowing snow from falling into the HTB collector funnel (the wind shield is installed on the ASOS HTB in climates where the snowfall is > 20% of the annual precipitation accumulation).
  • A 12-inch diameter collector funnel. 
  • A pivoting dual chamber tipping bucket. This bucket tips when one chamber is filled with 0.01 inch of liquid precipitation, thus emptying the contents into a drain pan and exposing the other chamber to the precipitation gathered by the collector funnel n An electronic switch which counts the number of tips per minute. 
  • A drain pan and a drain tube. 
  • Heating elements to prevent freeze-up during cold weather. 

The HTB has 2 heating elements. One heating element is wrapped around the underside of the collector funnel, and the other around the drain tube. Each heater is separately thermostatically controlled to maintain a temperature of 40°F. A master thermostat regulates electric power to both heating elements. Power is turned on when the temperature falls below 40°F and is turned off when the temperature falls below -20°F. Power is not turned on again until the temperature rises above -12°F. Power is turned off when the temperature is at or above 40°F. The HTB has a precipitation accumulation range of 0 to 10.00 inches per hour, a resolution of 0.01 inch (i.e., one tip) and an accuracy of ± 0.02 inch or 4% of the hourly total (whichever is greater).


                                                                  Present Weather Sensor

Accurate liquid-equivalent precipitation accumulation measurements are essential for hydrological, flood forecasting, and agriculture applications. For aviation purposes, freezing or frozen precipitation accumulation measurements provide a quantitative dimension to the qualitative detection and reporting of freezing or frozen precipitation by other ASOS sensors.

In the early development of an automated precipitation accumulation gauge, it was recognized that automated measurement of liquid and LEFP each presented a unique challenge, so a separate specification was written for each type of precipitation. The automated Heated Tipping Bucket (HTB) technology from the 1970s was adopted and modified to meet these needs. Over the years, many improvements were made and incorporated into ASOS. Early versions of the heated gauge applied excessive heat creating excessive evaporation and the under-reporting of the liquid-equivalent mass. 

The current version of the HTB gauge applies less heat over a longer heating cycle, thus yielding a more accurate mass measurement of frozen precipitation. Changes in the tipping bucket inner design also have improved overall performance in liquid precipitation events.

The Precipitation Identification sensor (PI), better known as a Light Emitting Diode Weather Identifier (LEDWI), differentiates rain from snow and determines the intensity of the precipitation. 

The LEDWI contains a coherent light transmitter (i.e., there is a continuous relationship among the various phases of the light waves within the beam) and a photodiode receiver. The transmitter and receiver are mounted on a cross arm 10 feet above the ground or base of the platform. They are equipped with heated lens hoods, face directly at each other, are separated by a distance of 2 feet and are oriented in a north-south direction with the receiver looking north. 

The transmitter generates a coherent Infrared (IRED) light beam, 50 millimeters in diameter, aimed directly at the receiver. The receiver lens is masked with a narrow 1 millimeter horizontal slit aperture through which the transmitter light beam passes before it is focused by the lens and impinges on the photodiode. The narrow aperture makes the receiver more sensitive to beam fluctuations caused by particles down to the size of a small raindrop (0.04 inch diameter). 

The transmitter generates a coherent Infrared (IRED) light beam, 50 millimeters in diameter, aimed directly at the receiver. The receiver lens is masked with a narrow 1 millimeter horizontal slit aperture through which the transmitter light beam passes before it is focused by the lens and impinges on the photodiode. The narrow aperture makes the receiver more sensitive to beam fluctuations caused by particles down to the size of a small raindrop (0.04 inch diameter). 

Because the slit is much wider than its height, the receiver is more sensitive to beam fluctuations induced by the vertical velocity component of particles passing through the beam than the horizontal component. Built-in sensor algorithms minimize the possibility of any false identification caused by greater sensitivity. 

As a particle of rain or snow passes through the coherent light beam, the particle creates a shadow that modulates the light, which then passes through the receiver’s horizontal slit aperture as a partially coherent (intermittently disrupted), collimated (parallel to the slit) beam. The shadow varies depending on the size and speed of descent of the particle as it falls across the receiver. 

When many particles fall through the beam, a scintillation pattern is created. The fluctuating beam pattern is sensed by a photo diode and amplified, creating a jumble of frequencies containing information on the size and speed of the falling particles. A spectral analysis reveals how much energy or power is contained in the various frequency bands. For example, a predominance of power in low frequencies from 75 to 250 Hz indicates snow. When energy is predominantly in a band from 1000 to 4000 Hz, the precipitant is almost certainly rain. The LEDWI registers rain and snow mixed as a “smearing” of the spectral power, which is usually reported by ASOS as unknown precipitation (UP). This analysis is the basis of the discrimination algorithm, which differentiates rain from snow. 

When the precipitation is not mixed, (i.e., pure rain or pure snow) the LEDWI can determine the intensity of the precipitation. The intensity is determined by the power of the signal return in the rain (1-4 kHz) or snow (75-250 Hz) portion of the power spectrum. The power return of rain is derived from the size and fall velocity of the particles whose size distribution correlates well with liquid water content. It is possible to accurately determine the rain intensity through an empirical relationship (the Marshall-Palmer distribution), which can distinguish light (up to 0.10 inch per hour), moderate (0.11-0.30 inch per hour), or heavy (greater than 0.30 inch per hour) intensities for rain. 

In the case of snow, it is again the size and fall velocity of the snowflakes that determines their size distribution. This correlates well with the rate of snow accumulation. Unlike rain however, the density of snow can vary significantly depending on whether the snow is “wet” or “dry,” and so the liquid content cannot be accurately determined.


Freezing Rain Sensor

The ASOS Freezing Rain (FZRA) Sensor is based on technology initially developed to detect icing on aircraft in flight. The sensing device consists of a small cylindrical probe that is electrically stimulated to vibrate at its resonant frequency. A feedback coil is used to measure the vibration frequency, which is proportional to the mass of the probe. Magnetostriction is a property of certain metals in which a change in the (axial) dimension of a body causes a change in magnetization. It is used in the ASOS sensor to drive the probe at a natural resonant frequency of 40kHz. The axial vibration is of such low amplitude that it cannot be seen or felt. The probe is oriented vertically to provide optimal uniform exposure to freezing precipitation regardless of wind direction. This position also prevents birds from alighting.

When ice freezes on the probe, the combined mass increases and the resonant vibration frequency decreases. There is a well defined relationship between the measured frequency and the ice accretion on the probe. The freezing rain instrument is sensitive enough to measure accumulation rates as low as 0.01 of an inch per hour. The freezing rain sensor continuously monitors the resonant frequency of the vibrating probe, obtains a sample once a second, and once each minute averages the results to update the probe’s current resonant frequency. When excessive freezing rain accumulates, (i.e., equal to or greater than 0.08 inch) the sensor goes into a heating cycle to melt the freezing rain from the probe and return it to the base resonating frequency. This process normally takes two to three minutes. During this time, the sensor status is set to “de-ice” and the output is not updated.


Sky Condition

Observers have used rotating beam ceilometers (RBC) and the newer laser beam ceilometers (LBC) for years to measure the height of clouds. Visual estimates were still needed to determine the amount of clouds. The challenge of automating the data from such sensors was not only to process the height accurately, but also to provide a representative description of the amount of cloud coverage. Because the atmosphere is normally in motion, it was found that processing the ceilometer signal through a sophisticated algorithm over a 30-minute time period provided an optimally representative and responsive observation similar to that depicted by an observer. To be sensitive to the latest changes in sky conditions, the most recent 10 minutes of the data are processed twice (double weighted).

To be most responsive to operational needs, the ASOS ceilometer is located near the touchdown zone of the primary instrument runway at most airports. At large airports, a secondary Cloud Height Indicator (CHI) may be located elsewhere on the airport to provide additional information when there is a meteorological discontinuity. At small airports the ceilometer may be collocated with other sensors near a center-field location or touchdown zone, depending on local siting requirements.


Height Indicator (CHI) Sensor

The ASOS uses a laser beam ceilometer with a vertical measuring range of 12,600 feet and reporting range of 12,000 feet. The ASOS cloud sensor, or CHI, is a vertically pointed laser transmitter and receiver. Its operation is similar to radar in that the time interval between pulse transmission and reflected reception from a cloud base is used to determine the cloud height. Sophisticated time-averaging algorithms in the ACU are also used to interpret “cloud hit” information from the CHI and determine cloud height and amount.

The CHI reports will contain only opaque clouds. Moisture layers, or thin clouds detected by the CHI and considered too thin to be a cloud, will be reported as a restriction to vertical visibility or simply not reported. The reporting of vertical visibility is dependent on the thickness and density of the moisture layer. To correctly classify these signals received by the ceilometer, sensor software processes the data into three categories: “no hit,” “cloud hit,” and “unknown hit.”



A sky condition algorithm has been developed for use where cloud formation (or advection) typically occurs in (or from) a known location and results in significant concurrent differences in sky conditions over the airport. The meteorological discontinuity algorithm uses output from two CHI sensors. The primary sensor is sited near the touchdown zone of the primary instrument runway. The second CHI is typically sited 2 to 4 miles away from the primary sensor, upwind in the most likely direction of the advection, or closer to the fixed source of the unique sky condition. The second CHI serves to detect operationally significant differences in sky conditions. Information from the meteorological discontinuity sensor is included in the ASOS METAR under appropriate conditions described below. Data from the primary and meteorological discontinuity sensors are independently processed through the single sensor algorithm and then compared. Only data from the primary sensor is used in the body of the METAR.


Forward Scatter Visibility

The ASOS visibility sensor (Figure 13) operates on a forward scatter principle in which light from a pulsed Xenon flash lamp in the blue portion of the visible spectrum is transmitted twice a second in a cone-shaped beam over a range of angles. The projector and detector are protected with a lens hood and canted down at 15 degrees from the horizon to prevent snow blockage. 

The detector is oriented north to minimize sun glare, particularly from low sun angles near sunrise or sunset. To optimally balance the detection efficiency and differentiation ability of the sensor under varying conditions, a nominal 45 degree horizontal incident angle is set between the projector beam and the detector field-of-view within the sampling volume. This is achieved by offsetting the projector about 45 degrees to the left (i.e., northwest) of the detector. As a result, the projector beam does not directly impinge on the detector lens. Only that portion of the beam that is scattered forward by the intervening medium in the sampling volume is received by the detector (see Figure 14). The sensor sampling volume is 0.75 cubic feet and the sensor response time is 20 seconds. A measurement sample is taken every 30 seconds. Visibility sensor measurement accuracy is specified in reference to comparison with two NWS visibility standards and is summarized in Table 5. In this regard, the forward scatter sensor has shown excellent performance when compared with the “Optec” Transmissometer standards.

Visibility in METAR is reported in statute miles (SM). The reportable increments are: M1/4SM, (less than 1/4SM), 1/4SM, 1/2SM, 3/4SM, 1SM, 1 1/4SM, 1 1/2SM, 1 3/4SM, 2SM, 2 1/2SM, 3SM, 4SM, 5SM, 6SM, 7SM, 8SM, 9SM and 10SM. Note that visibilities between zero and less than 1/4 mile are reported as M1/4SM8 . Measured visibilities exactly halfway between reportable values are rounded down. Visibilities of 10 miles or greater are reported as “10SM.”


Meteorological Discontinuity Visibility Sensor

At some airports a second visibility sensor is placed where unique weather, not necessarily representative of the entire airport, may impact flight operations for a portion of the airport or a particular runway. The secondary meteorological discontinuity visibility sensor may be used to provide an early alert of deteriorating conditions, such as fog rolling in off a nearby bay or river. Whereas the primary visibility sensor is sited near the touch-down zone of the primary instrument runway, the second (meteorological discontinuity) visibility sensor is sited about 2-4 miles away in the most likely location for a meteorological discontinuity. 

The data from the primary and meteorological discontinuity sensors are independently processed through the single sensor algorithm and then compared. Only data from the primary sensor is used in the body of the METAR. 

If the primary visibility sensor is not operational, the visibility is not reported, a maintenance check indicator ($) is appended to the METAR, and no further comparisons are made. If the meteorological discontinuity visibility sensor is not operational, the remark “VISNO LOC” is added to the METAR, where LOC is the nominal location of the meteorological discontinuity visibility sensor (e.g., CF, RWY26L), a maintenance check indicator ($) is appended to the METAR, and no further comparisons made. 

If both sensors are operational, a meteorological discontinuity visibility remark is reported when the visibility measured by the meteorological discontinuity sensor is less than 3 miles and is also less than the visibility measured by the primary visibility sensor by one-half mile or more. When these conditions are met, a remark in the form “VIS VALUE LOC” is added to the METAR, where VALUE is the visibility reported by the meteorological discontinuity sensor and LOC is the nominal location of the meteorological discontinuity sensor. For example, a meteorological discontinuity visibility value of 1SM on runway 26L is reported as: VIS 1 RWY26L.


Single Site Lightning Sensor

Where required, the ASOS uses the Global Atmospherics Inc. (GAI) Model 924 single site lightning sensor as a source for reporting a thunderstorm. The ASOS Lightning Sensor (ALS) is installed at selected Service-Level “D” ASOS sites that do not have the FAA Automated Lightning Detection And Reporting System (ALDARS). The ALDARS is another source of lightning information provided through the National Lightning Detection Network. The ALS sensor is a single-point omnidirectional system that requires two criteria before reporting a thunderstorm: an optical flash and an electrical field change (radio signal), which occur within milliseconds of each other. The requirement for simultaneous optical and radio signals virtually eliminates the possibility of a false alarm from errant light sources. 

The sensor can detect cloud-to-ground and cloud-to-cloud strikes. All strikes are counted, but only the cloud-to-ground strikes are used to generate an estimate of the range. Cloud-to-ground strikes are grouped into three range bins: 0 to 5 miles, 5 to 10 miles, and 10 to 30 miles. Because the cloud-to-cloud detection is less efficient than cloud-to-ground detection, the ASOS considers cloud-to-cloud strikes to be within 5 miles. 

A cloud-to-ground strike is made up of one or more individual flashes. Within one flash, numerous discharges can occur; these individual discharges are called strokes. The sensor groups all strokes occurring within 1 second of each other into a single flash. The range of a cloud-to-ground strike is determined by the range of the closest stroke within a flash. 

The sensor automatically “ages” each lightning strike for 15 minutes. Because a thunderstorm is defined to be in progress for 15 minutes after the last lightning or thunder occurs, the sensor continues to report each strike in the appropriate bin for 15 minutes after it is first detected. A thunderstorm is determined to end when no strikes are detected within the last 15 minutes.


Data Collection Package (DCP): The data collection package receives all of the raw senor data from each individual sensor and transmits it to the acquisition control unit (ACU) for processing and distribution.

Acquisition Control Unit (ACU): The ACU, which is the central processing unit for the ASOS, is usually located inside a climate controlled structure, such as an observing office or control tower building. It ingests data from the DCP(s) and pressure sensors, and is capable of accepting information from the FAA New Generation Runway Visual Range (NGRVR) system.

The ACU performs final processing, formatting, quality control, storage and retrieval of the data, and makes ASOS data available to users through various outlets. A brief description of the various ASOS data outlets and data