Despite the old Air Force photograph, I grew up in an environment where one of my first stops in the morning was to a weather office where someone briefed me about what to expect and handed me a few pages tailored for my operation. In the civilian world, if you bothered, you could get similar service by picking up the phone. But these days its all on line. Much better.
Everything here is from the references shown below, with a few comments in an alternate color.
d. Automated Surface Observing System (ASOS)/Automated Weather Observing System (AWOS) The ASOS/AWOS is the primary surface weather observing system of the U.S. (See Key to Decode an ASOS/AWOS (METAR) Observation, FIG 7−1−7 and FIG 7−1−8.) The program to install and operate these systems throughout the U.S. is a joint effort of the NWS, the FAA and the Department of Defense. ASOS/AWOS is designed to support aviation operations and weather forecast activities. The ASOS/AWOS will provide continuous minute-by-minute observations and perform the basic observing functions necessary to generate an aviation routine weather report (METAR) and other aviation weather information. The information may be transmitted over a discrete VHF radio frequency or the voice portion of a local NAVAID. ASOS/AWOS transmissions on a discrete VHF radio frequency are engineered to be receivable to a maximum of 25 NM from the ASOS/AWOS site and a maximum altitude of 10,000 feet AGL. At many locations, ASOS/AWOS signals may be received on the surface of the airport, but local conditions may limit the maximum reception distance and/or altitude. While the automated system and the human may differ in their methods of data collection and interpretation, both produce an observation quite similar in form and content. For the “objective” elements such as pressure, ambient temperature, dew point temperature, wind, and precipitation accumulation, both the automated system and the observer use a fixed location and time-averaging technique. The quantitative differences between the observer and the automated observation of these elements are negligible. For the “subjective” elements, however, observers use a fixed time, spatial averaging technique to describe the visual elements (sky condition, visibility and present weather), while the automated systems use a fixed location, time averaging technique. Although this is a fundamental change, the manual and automated techniques yield remarkably similar results within the limits of their respective capabilities.
1. System Description.
(a) The ASOS/AWOS at each airport location consists of four main components:
(1) Individual weather sensors.
(2) Data collection and processing units.
(3) Peripherals and displays.
(b) The ASOS/AWOS sensors perform the basic function of data acquisition. They continuously sample and measure the ambient environment, derive raw sensor data and make them available to the collection and processing units.
2. Every ASOS/AWOS will contain the following basic set of sensors:
(a) Cloud height indicator (one or possibly three).
(b) Visibility sensor (one or possibly three).
(c) Precipitation identification sensor.
(d) Freezing rain sensor (at select sites).
(e) Pressure sensors (two sensors at small
airports; three sensors at large airports).
(f) Ambient temperature/Dew point temperature sensor.
(g) Anemometer (wind direction and speed sensor).
(h) Rainfall accumulation sensor.
(i) Automated Lightning Detection and Reporting System (ALDARS) (excluding Alaska and Pacific Island sites).
3. The ASOS/AWOS data outlets include:
(a) Those necessary for on-site airport users.
(b) National communications networks.
(c) Computer-generated voice (available through FAA radio broadcast to pilots, and dial-in telephone line).
NOTE−Wind direction broadcast over FAA radios is in reference to magnetic north.
4. An ASOS/AWOS report without human intervention will contain only that weather data capable of being reported automatically. The modifier for this METAR report is “AUTO.” When an observer augments or backs−up an ASOS/AWOS site, the “AUTO” modifier disappears.
5. There are two types of automated stations, AO1 for automated weather reporting stations without a precipitation discriminator, and AO2 for automated stations with a precipitation discriminator. As appropriate, “AO1” and “AO2” must appear in remarks. (A precipitation discriminator can determine the difference between liquid and frozen/freezing precipitation).
Photo: ASOS, from noaa.gov
[ASOS User's Guide, ¶2.1.1] The ASOS sensors continuously sample and measure the ambient environment, derive raw sensor data and make them available to the collocated DCP. These raw sensor data include visibility extinction coefficients, ceilometer cloud hits, freezing precipitation resonant frequencies and other sensor data. These data are processed as preliminary input into the observation algorithms. The ASOS consists of the following basic complement of sensors:
[AIM, ¶7-1-12.d.5.] There are two types of automated stations, AO1 for automated weather reporting stations without a precipitation discriminator, and AO2 for automated stations with a precipitation discriminator. As appropriate, "AO1" and "AO2" must appear in remarks. (A precipitation discriminator can deter- mine the difference between liquid and frozen/freezing precipitation).
Figure: Forward scatter visibility sensor, from ASOS User's Guide, figure 13
[ASOS User's Guide, ¶4.2]
Most airports will have their visibility based on the sensor next to the touchdown zone of the primary instrument runway.
The sensor isn't looking away from the airport at all, it is sampling the obscuration at its location.
The "advantage" cited is that it will serve the primary instrument runway best and that its readings will agree with the transmissometer. But of course that means it may not be best when evaluating other runways.
Aeronautical Information Manual
Automated Surface Observing System (ASOS) User's Guide, National Oceanic and Atmospheric Administration, March 1998
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