The Automated Surface Observing System (ASOS) is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), and the Department of Defense (DoD). The ASOS systems serve as the nation’s primary surface weather observing network. The ASOS is designed to support weather forecast activities and aviation operations and, at the same time, support the needs of the meteorological, hydrological, and climatological research communities.
The ASOS is a collection of electronic sensors providing information to a computer that generates weather observations for users. With a modern complement of weather sensors, ASOS expands the information available to forecasters and the aviation community. A basic strength of the ASOS observation is that critical aviation weather parameters are measured continuously updating observations every minute, 24 hours a day, every day of the year. The ASOS consists of the following basic set of sensors: wind speed and direction; ceilometer (cloud height indicator); visibility; precipitation identification and accumulation; freezing rain; pressure; and ambient temperature/dew point.
ASOS detects significant changes, disseminating hourly and special weather observations via the NWS and FAA communications network. The ASOS routinely and automatically provides computer-generated voice observations directly to aircraft in the vicinity of airports, at most locations, using FAA ground-to-air radio. The report is also available via a telephone dial-in port.
The ASOS will automatically report the following basic weather elements. When preselected weather element thresholds are exceeded a special weather report is transmitted.
Sky condition: cloud height and amount up to 12,000 feet above ground level
Visibility up to 10+ statute miles
Basic present weather information: rain, snow, freezing rain
Obscurations: fog, haze
Pressure: sea-level pressure and altimeter setting
Ambient temperature and dew point temperature
Wind: direction, speed, and character
Selected significant remarks including: variable cloud height; variable visibility; precipitation beginning and ending times; rapid pressure changes; pressure change tendency; wind shift; peak wind; etc.
Precipitation accumulation
The NWS and the FAA began commissioning the ASOS in 1992 as part of a joint Department of Commerce, Department of Transportation, and DoD ASOS Program. The commissioning process of an NWS or FAA sponsored ASOS begins after the ASOS is accepted by the government, i.e., the National Oceanic and Atmospheric Administration’s Systems Program Office and, in the case of NWS systems, when it is released to the NWS. During this process, NWS and FAA officials evaluate and document: (1) the performance of the ASOS; (2) the readiness of the operational and maintenance personnel to utilize and support the ASOS in routine operations; and (3) the release of notifications to the local meteorological/aviation communities.
As of July 31, 1997, there were 405 commissioned ASOSs, and 376 non-commissioned ASOSs that are in various stages of the commissioning process. Figure 1.1-1 graphically presents the commissioned; installed, pre-commissioned; installed, non-commissioned; procured, not installed; and future procurement, funded FAA and NWS ASOS sites, combined in the first pie chart and separated in the two smaller pie charts.
A government/industry team worked to comprehensively reassess the requirements for surface observations at the nation’s airports. That work resulted in agreement on a set of service standards as well as the FAA and NWS ASOS sites to which the standards would apply. The term Service Standards refers to the levels of detail in the weather observation at sites where there is a commissioned ASOS. The Service Standards define the four different levels of service (A,B,C, and D) and the methodology used to determine the appropriate service level for an airport. Table 1.1-1 outlines the four levels of Service Standards.
The Service Standards developed by the government/industry workgroup and reviewed annually supercede the FAA’s former commitment regarding "equal to or better" service for weather observations. The legislation on "equal to or better" service no longer exists.
The first category, Service Level D, defines the minimum acceptable level of service. It is a completely automated service in which the ASOS observation will constitute the entire observation, i.e., a human weather observer adds no additional weather information. This service is referred to as a stand alone D Site.
The second category, Service Level C, is an augmented service in which a human observer, usually a controller, adds information to the automated observation (augmentation). Service Level C also includes backup of ASOS elements in the event of an ASOS malfunction or an unrepresentative ASOS report. In backup, the human observer inserts the correct or missing value for the automated ASOS elements. Although normally provided under the Limited Aviation Weather Reporting Service process by air traffic controllers located in the control tower, this service may also be provided by Flight Service Stations, NWS observers or a Non-Federal Observer Program at selected sites.
Two categories of airports require detail beyond Service Level C in order to enhance air traffic control efficiency and increase system capacity. Service Level A, the highest and most demanding category, includes major aviation hubs and high traffic volume airports with average or worse weather. The other category of airports includes smaller hubs or special airports in other ways that have worse than average bad weather operations for thunderstorms and/or freezing/frozen precipitation, and/or that are a remote airport. These airports are referred to as Service Level B.