Appendix E
ASOS Assessment
The committee examined the Automated Surface Observing System (ASOS) as part of its effort to assess the effectiveness of the federal government's existing institutional arrangements to plan and direct improvements in the aviation weather system. As mentioned in Chapter 3 (page 26), the committee identified 10 areas of ongoing concern. These concerns are as follows:
1. Availability of the data communications system. The Federal Aviation Administration (FAA) has been working since 1990 to deploy the Automated Weather Observing System (AWOS) Data Acquisition System (ADAS) that enables ASOS units to deliver their data to the national aviation weather data collection system.1 Until they are connected to a data communications system such as ADAS, the FAA can not commission the ASOS units that have been installed. As a result, as of December 1994, hundreds of the FAA ASOS units had been installed but had not been commissioned (see Table 3-1, page 26).
2. Interpretation of ASOS readings. Automated systems are not a one-for-one replacement for human observers. Automated systems sense and report meteorological conditions differently than human weather observers. For example, a human weather observer assesses ceiling and visibility by looking at the entire visible sky at a single point in time. ASOS, on the other hand, assesses ceiling by observing a small section of the sky directly overhead and integrating over time, and the ASOS visibility sensor measures conditions between a transmitter and receiver that are less than 3 feet apart. Although these two methods "are fundamentally different, they yield similar results under most [but not all] conditions" (NWS, 1995). Users who have relied on human weather observations need to understand these differences in order to properly interpret ASOS data. Training of pilots and controllers to appreciate these differences has not kept pace with the installation of ASOS units and has resulted in user skepticism about the ASOS program and confusion about ASOS unit reliability and accuracy. (Lack of familiarity with ASOS readings can sometimes lead users to conclude incorrectly that an ASOS unit is malfunctioning.)
3. Instrument performance and system development testing. The National Weather Service (NWS) tested ASOS technology as it was developed. In addition, the initial deployment of 55 ASOS units was followed by about a year of on-site test and evaluation. In general, ASOS units have performed in accordance with design specifications. However, the NWS has had to modify many ASOS units after installation to correct deficiencies with the rain gauge, anemometer, and data-reporting algorithms that were not detected during initial system testing. It takes up to a year to implement software changes to modify the algorithms and, as a result, users sometimes perceive that the NWS is not responsive in correcting deficiencies that they report (Kisner, 1995). In fact, the NWS stopped commissioning its ASOS units for several months during late 1994 and early 1995 to correct deficiencies in installed units and to improve the ability of the NWS logistics system to supply spare parts and complete ASOS repairs in a timely fashion. In addition, early in 1995 the FAA stopped commissioning its ASOS units to conduct a 6-month performance standardization test at about 25 ASOS sites. The FAA took this action in response to concerns expressed by air traffic controllers about ASOS performance.
4. Completeness of the weather observation. As appropriate, manual weather observations contain remarks regarding approaching thunderstorms or fog, cloud type (which is especially important to pilots of small aircraft flying in mountainous regions), and other phenomena that are difficult if not impossible to detect with automated systems. These remarks provide additional detail and add to the usefulness of the observation for aviation. ASOS weather observations, however, do not include these types of comments.
Video cameras could provide remote users with a panoramic view of meteorological conditions at ASOS sites. Cameras could also enable remote users to investigate the accuracy of questionable or inconsistent ASOS
readings (such as snow when temperatures are significantly above freezing). There is currently no plan for widespread deployment of video cameras to augment ASOS or AWOS units. However, the NWS is field testing video cameras in Utah and Arizona: forecasters at two NWS offices are using video cameras to provide them with a "human presence" at two remote ASOS-equipped airports. Initial feedback from NWS staff regarding the utility of these cameras is favorable.
ASOS units also have no instruments to detect or report the presence of tornadoes, thunderstorms/lightning, virga (i.e., precipitation that evaporates before it falls to the ground), volcanic ash, or runway visual range (RVR). As a result, forecasters and other users who desire this information must find other means of obtaining it.
The NWS did not give ASOS a lightning-detection capability because it anticipated that information on lightning and thunderstorm activity would be available from other sources such as WSR-88D weather radars and the National Lightning Detection Network, which provides near-real-time locating data on cloud-to-ground lightning. The FAA intended to transmit lightning data to federal ASOS units via ADAS. ASOS units would then use this information to report the presence of nearby lightning as if they had their own lightning detection instruments. However, the ADAS acquisition contract issued by the FAA did not include the capability to transmit this data to ASOS units. In October 1994 the FAA issued an engineering change request to modify the ADAS contract to provide this capability, and the FAA anticipates that ADAS will provide ASOS with lightning data by the summer of 1996.
The NWS and FAA are modifying existing ASOS units to include a freezing rain sensor. Deployment of the freezing rain sensor started in 1995 following successful field tests during the winter of 1994–1995.
RVR is a measure of how far down the runway an approaching pilot can see runway markings (during the day when good visibility prevails) or runway lights (at night and during the day when visibility is poor). RVR is a better measure of operational conditions at airports than is visibility. ASOS does not have an RVR sensor, and this shortcoming has been a contentious issue between the FAA and users such as general aviation pilots and air carriers. Although the FAA does not believe that Federal Aviation Regulations require RVR, the FAA is working to develop a new generation of automated RVR sensors. These sensors will replace existing sensors at 258 airports in the United States (Miles et al, 1995).
5. Equal or better level of service. "FAA policy is that the performance of combined automated systems must be equal to or better than manual observation capability" (FAA, 1994). Many users, however, do not believe that ASOS meets this standard at airports that previously had human weather observers. To some extent, this perception reflects a lack of consensus between the FAA and users about what data surface observations should include. For example, air carriers did not realize that ASOS units would not provide RVR until the first ASOS units were being installed. As late as April 1995, the FAA was still working with representatives of user groups and other interested parties to establish service standards for automated surface weather observations. These standards would specify the following:
- weather elements to be reported;
- reporting accuracy for each element;
- percent availability for each element; and
- maximum service outage time for each element (FAA, 1995).
6. Augmentation of ASOS observations. Human weather observers must augment ASOS readings at locations, such as major airports, that require complete weather observations. As mentioned above, the FAA and NWS are developing automated instruments to address user needs for information on thunderstorms/lightning, freezing rain, and RVR. However, there are currently no plans to automate observations of tornadoes, virga, volcanic ash, or cloud type. It takes five meteorological technicians to provide 24-hour coverage at a single site where augmentation is needed and no other certified weather observers are available. This represents labor costs on the order of $120,000 per year. It is not yet clear how the FAA and NWS will satisfy long-term requirements for human augmentation to meet the needs of aviation weather users, including their own staffs. For example, some tower controllers believe that additional automated instruments should be developed to take over part of the augmentation responsibilities that the FAA has assigned to them.
7. Access to ASOS data. Comparisons of ASOS and human weather observations in an operational setting indicate that the weather conditions reported by ASOS units show more variability than human observations. This is primarily caused by fluctuations in meteorological conditions, which ASOS units diligently measure and report as they continuously monitor the weather. In contrast, human weather observers make hourly observations unless there is a major change that warrants making a special weather observation. In any case, the variability of ASOS observations means that users typically require access to a series of ASOS observations to develop a good understanding of the weather. Currently, up-to-the-minute ASOS readings are available only by telephoning individual ASOS units or by listening to local ground-to-air
radio broadcasts. Air carriers, however, would like to have easy access to all reported ASOS data, just as many NWS offices do. This would allow them to graph ASOS data and discern trends more easily than is possible from hourly and special readings. Tower controllers, on the other hand, prefer to receive just the hourly and special observations so they do not have to recheck their ASOS displays every time a pilot asks them for a current weather observation (Ramsey and Burgas, 1995; Clark, 1995).
8. Authority of controllers to override ASOS readings. Low cloud ceilings or low visibility can curtail or shut down airport operations. During these conditions pilots waiting to takeoff or land, as well as the air traffic controllers in the airport tower, are anxious to resume operations. In the past, as soon as conditions improved, the airport's human weather observer would issue an updated visibility observation that would permit the resumption of flight operations. However, if the official source of airport weather observations is an ASOS unit, then FAA regulations require that controllers wait until ASOS reports that conditions are acceptable (unless the unit is malfunctioning and they report it out of service, in which case it can not be used as a source of official observations until it is serviced). As a safety measure, the ASOS visibility algorithm is programmed to increase reported visibility gradually when observed visibility goes up. Thus, after a thunderstorm passes through an airport and visibility rapidly increases, it may take several extra minutes before ASOS reports the increase. In this situation, FAA regulations prohibit certified weather observers such as tower controllers from overriding ASOS readings to issue a higher visibility, even if they observe that prevailing visibility is obviously higher than the visibility being reported by ASOS. This problem does not occur frequently, but when it does occur, it has a big impact on airport operations. As a result, the National Air Traffic Controllers Association has asked the FAA to grant controllers the authority to override ASOS visibility during rapidly changing weather conditions. In fact, controllers already have the authority to override ASOS visibility when they observe a lower visibility than ASOS is reporting, but the FAA is reluctant to grant controllers the authority to override ASOS visibility when they observe a higher visibility.
9. Maintenance and backup. In order to reduce the frequency and length of interruptions in flight operations, it is essential to reduce the length of tune that ASOS units at airports are out of service due to malfunction and to provide for backup weather observers while ASOS units are out of commission. This is especially important at airports that have previously relied on human weather observers. In fact, the FAA will install redundant ASOS units at major airports to ensure that ASOS malfunctions do not shut down operations, and it will install back-up sensors in ASOS units at some other airports to increase the reliability of selected instruments. In addition, the NWS has agreed to provide ASOS corrective maintenance response times of 12–36 hours, depending upon airport size and operational tempo. The FAA will develop two classes of repair actions: routine and essential. Essential repairs will be called for if commercial aircraft cannot land or depart until the observation associated with the malfunctioning instrument is available (through augmentation, redundant backup, or repair). However, as of January 1995 (when over 600 ASOS units had been installed), the FAA and NWS had not agreed upon which weather parameters should be treated as essential, and the FAA had not yet tasked the ASOS manufacturer to start installing backup sensors. Air traffic controllers at some airports designated to receive ASOS units believe that the FAA and NWS should take more aggressive action to reduce the impact of ASOS malfunctions by installing more backup systems, arranging for alternative sources of observations, or reducing repair times (Leedom, 1995).
10. Site selection. At some locations, users are concerned that ASOS site selection has been driven more by the availability of electrical power and communications than by the ability to take weather observations that are representative of the local area. Because most ASOS sensors measure meteorological conditions just in their immediate vicinity, it is important to locate them at sites that are representative of meteorological conditions in the local areas that are of greatest interest to aviation (such as runways and airport approaches).
References
Clark, P. 1995. Automated surface observations: New challenges—New tools. Pp. 445–450 in Sixth Conference on Aviation Weather Systems, held January 15–20, 1995 in Dallas, Texas. Boston: American Meteorological Society.
FAA (Federal Aviation Administration). 1994. Aviation Weather System Plan . Washington, D.C.: FAA.
FAA. 1995. Summary notes on Workshop on Automated Observations Service Standards and Demonstration, January 31-February 1, 1995. Silver Spring, Maryland.
Kisner, S. 1995. Terminal forecasting in Kansas with ASOS. Pp. 476–478 in Sixth Conference on Aviation Weather Systems, held January 15–20, 1995 in Dallas, Texas. Boston: American Meteorological Society.
Leedom, G. 1995. Presentation to the ASOS Users' Forum at the Sixth Conference on Aviation Weather Systems. Dallas, Texas. January 19, 1995.
Miles, C., et al. 1995. New generation runway visual range system. Pp. 347–350 in Sixth Conference on Aviation Weather Systems, held January 15–20, 1995 in Dallas, Texas. Boston: American Meteorological Society.
NWS (National Weather Service). 1995. ASOS Aviation Demonstration and Evaluation Plan. ASOS Quality Assurance Working Group. Silver Spring, Maryland. March 31, 1995.
Ramsey, A., and B. Burgas. 1995. Comparability between human and ASOS ceiling/visibility observations. Pp. 470–475 in Sixth Conference on Aviation Weather Systems, held January 15–20, 1995 in Dallas, Texas. Boston: American Meteorological Society.
