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NWS, the FAA and the Department of Defense.
ASOS is designed to support aviation operations and
weather forecast activities. The ASOS 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 transmissions on a discrete VHF
radio frequency are engineered to be receivable to a
maximum of 25 NM from the ASOS site and a
maximum altitude of 10,000 feet AGL. At many
locations, ASOS 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 at each airport location
consists of four main components:
(1) Individual weather sensors.
(2) Data collection package(s) (DCP).
(3) The acquisition control unit.
(4) Peripherals and displays.
(b) The ASOS 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
collocated DCP.
AIM 2/17/05
7−1−24 Meteorology
2. Every ASOS 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.
3. The ASOS 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” shall appear in
remarks. (A precipitation discriminator can determine
the difference between liquid and frozen/freezing
precipitation).
NOTE−
To decode an ASOS report, refer to FIG 7−1−7 and
FIG 7−1−8.
REFERENCE−
A complete explanation of METAR terminology is located in AIM,
Paragraph 7−1−31, Key to Aerodrome Forecast (TAF) and Aviation
Routine Weather Report (METAR).
8/5/04 AIM
Meteorology 7−1−25
FIG 7−1−7
Key to Decode an ASOS (METAR) Observation (Front)
AIM 8/5/04
7−1−26 Meteorology
FIG 7−1−8
Key to Decode an ASOS (METAR) Observation (Back)
8/5/04 AIM
Meteorology 7−1−27
e. TBL 7−1−1 contains a comparison of weather
observing programs and the elements reported.
f. Service Standards. During 1995, a government/
industry team worked to comprehensively
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