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33 This chapter will discuss the common types of automated weather observation systems in use at general aviation airports today. The word âautomatedâ is significant, as the FAA does sponsor various programs that incorporate human observers in weather reporting. However, these pro- grams are more typical at air carrier airports than in general aviation, and their use is also being phased out. Thus, only the use of automated systems is discussed here. For automated weather observation and reporting systems that are commonly found at gen- eral aviation airports, the systems can be grouped into two distinct categories: those that are approved to provide federally certified weather data, and those that are not. To say that weather data is âcertifiedâ means that it is reported in accordance with the Federal Meteorological Handbook No. 1 (FMH-1). The FMH-1 handbook defines the surface observa- tion standards for all federal agencies that collect and report surface observations. FMH-1 also specifies that each agency should provide a manual of specific guidance for systems for which they are responsible. For the FAA, this guidance is found in four main documents: â¢ FCM-S4-1994âFederal Standard for Siting Meteorological Sensors at Airports â¢ Order JO 6560.20CâSiting Criteria for Automated Weather Observing Systems (AWOS) â¢ Order JO 7900.5DâSurface Weather Observing â¢ AC 150/5220-16EâAutomated Weather Observing Systems (AWOS) for Non-Federal Applications FAA-Approved Automated Weather Reporting Systems For automated equipment approved by the FAA to provide certified weather data, systems fall under two categories: ASOS and AWOS. A typical ASOS installation is shown in Figure 6, and a typical AWOS III installation is shown in Figure 7. Weather reporting by certified weather systems, and most noncertified systems, includes a mixture of units of measurement from both the International System of Units (SI), colloquially the metric system, and those from the United States Customary System (USCS). For example, temperature is measured in Celsius, which is an SI unit. Altitudes and cloud height are measured in feet, a USCS unit. Visibility is measured in miles or fractions of miles, which are USCS units. Wind speed is reported in knots, a unit which is not a traditional unit in either the SI or USCS systems but is the International Civil Aviation Organization (ICAO) standard for aviation speed measurement. One knot is approximately 1.15 miles per hour. Atmospheric pressure is measured in USCS units as inches of mercury (in-HG) and is reported as the altimeter setting in aviation. When conditions warrant, a variation of the barometric C H A P T E R 4 Existing Technologies
34 Airport Surface Weather Observation Options for General Aviation Airports pressure altitude called the density altitude may also be reported. Density altitude is pressure altitude corrected for a nonstandard temperature. In laymanâs terms, it is the elevation an air- craft âfeelsâ that it is flying at. In aviation, the standard pressure and temperature are 29.92 inches of mercury and 15 degrees Celsius, but only at sea level. As altitude increases, the standard temperature drops approxi- mately 2 degrees Celsius for every 1,000 feet. Without delving into the mathematics, the effect Source: Delta Airport Consultants, Inc. Figure 6. ASOS Roanoke-Blacksburg Regional Airport. Source: Delta Airport Consultants, Inc. Figure 7. AWOS III â New Kent County Airport.
Existing Technologies 35 is that temperature (and pressure) deviations from standard conditions at a given altitude will create conditions in which the aircraft will perform as if it is at a different altitude. One example provided by AOPA shows that for a field elevation of 7,000 feet at a temperature of 18 degrees Celsius (and standard pressure), the density altitude is 9,040 feet. Pilots would need to adjust their performance calculations for this higher elevation. Density altitude calculations can be critical in high, hot environments, especially those com- pounded by shorter runways. High density altitudes can greatly increase both takeoff and landing distances and can also decrease climb performance, especially in the piston engine aircraft typical of general aviation. Density altitude can also report as below the field elevation in particularly cold environments, in which case aircraft performance will be better than expected. Automated Surface Observing System (ASOS) The creation of the ASOS in the 1980s was a joint effort by the NWS, the FAA, and DOD to automate the recording and dissemination of weather data. The ASOS program ran from the first installation in 1991 to the last installation in 2004. According to NOAA, there are currently 903 ASOS stations in the United States. While not every ASOS system is located at an airport, most of them are. ASOS systems are maintained by the NWS to the standards required by the FAA for them to be eligible to report data to the WMSCR. Sensors An ASOS station contains the following sensors per JO 7900.5D: 1. Wind speed and direction 2. Visibility 3. Precipitation identification (type, rate) 4. Cloud height indicator (ceilometer) 5. Temperature and dew point 6. Atmospheric pressure (barometer) 7. Precipitation accumulation (heated tipping bucket) 8. Freezing precipitation (optional) 9. Thunderstorm reporting (optional) These sensors record data continuously. However, the reports issued by the ASOS include manipulation of this data via time-averaging algorithms from the ASOS processor. For example, the wind speed reported by an ASOS is actually the average speed over the preceding 2 minutes. And this 2-minute average comprises the preceding 24 consecutive 5-second averages, with each those 5-second averages determined from measurements taken every second. All other sensors have their data time-averaged via their own relevant methodology. Siting Requirements Siting requirements for an ASOS are found in FCM-S4-1994. The sensor suite of an ASOS is typically placed at a single location within a 20- by 30-foot rectangular area. The pressure sensor is often located remotely, within a nearby building and co-located with the system acquisition control unit (ACU). The entire system must be outside of all airport object-free areas, safety areas, obstacle-free zones, and any location that would impact an instrument procedure. The system is preferably placed from 1,000 to 3,000 feet down-runway from the primary runway threshold. Furthermore, FCM-S4-1994 contains guidance indicating the system is to be located outside of the FAR Part 77 Primary Surface and Transitional Surface.
36 Airport Surface Weather Observation Options for General Aviation Airports The sensor suite should be located away from cultivated land to avoid impacts from airborne debris affecting sensor readings. Some sensor mounting heights may be increased to keep them sufficiently above the average snow depth. The wind sensor is to be mounted 30 to 33 feet above the average surrounding ground eleva- tion, and also at least 15 feet above any object within a 500-foot radius of the sensor. This results in a requirement to control and protect 18 acres of property to support the wind sensor. The wind tower is located at least 10 feet from the remaining sensors and is not considered an obstruction to them. The visibility sensor is mounted 10 feet above ground and requires a clear view for 300 feet within the northern octant. The northern octant is that portion of ground centered on true north and subtending an angle of 45 degrees, as shown in Figure 8. The ground within 100 feet of the visibility sensor should also be maintained with vegetation under 10 inches, and the area is to be free from the impacts of strobe lights as well as localized smoke or fog that are not typical of the airport environment. The cloud height sensor is also to be protected from the effects of strobe lights and mounted a minimum of 4 feet above ground, or higher as needed to guard against the effects of snow. Temperature and dew point sensors are mounted 5 feet above ground and in free-circulating air away from buildings, cooling towers, or artificial heat sources. Pressure sensors should be located so that they are not affected by artificial airflow and should be mounted at least 1 foot above average snow depth. The pressure sensor may be located with the equipment in the field or co-located with the ASOS ACU, which may be located in a building. Each site contains two or three sensors at the installation site, and data is time-averaged from the multiple sensor inputs. Any metal obstructions located adjacent to a lightning sensor must be positioned a minimum distance away from the sensor in the amount of two times the height that the obstruction extends above the sensor. Precipitation type sensors differentiate between types of precipitation (e.g., rain or snow) and are mounted 10 feet above ground or at least 6 feet above average snow depth. Local terrain should be relatively flat. Precipitation occurrence and freezing rain sensors should be mounted at least 6 feet above ground or 4 feet above the average snow depth. The sensors should not be shielded by close-in buildings or obstructions. Source: Delta Airport Consultants, Inc. Figure 8. Visibility sensor.
Existing Technologies 37 For an airport with a glide slope installation, the entire ASOS may be co-located behind the glide slope shelter installation, per Figure 1 in FCM-S4-1994, as duplicated here in Figure 9. From a practical standpoint, locating the wind sensor often proves to be the controlling factor in siting an ASOS, because it is the tallest object and also requires the largest area of protected and controlled ground around it. Typically, if the wind sensor is suitably protected, all other sensors may be installed without additional accommodation, assuming environmental factors such as strobe lighting or farming activity are not issues. An airport with an ASOS is required to maintain the surrounding land in such a way that it meets the siting requirements of FCM-S4-1994. This may result in expanded coordination efforts and costs should airport development impact the siting requirements of the ASOS. Data Reporting ASOS information is disseminated in multiple ways. The system broadcasts locally on a discrete VHF frequency which is designed to be receivable by aircraft out to 25 miles below 10,000 feet, although local terrain and environmental conditions may affect this. The system is also typically accessible via telephone line. At airports with an Automatic Terminal Information Service (ATIS), the ASOS information may also broadcast over the ATIS frequency. Additionally, ASOS systems report to the WMSCR, so their information is available in the NAS through a multitude of data platforms that access the WMSCR. The transmission of ASOS data to the WMSCR is standardized to the METAR format. ASOS METARs are issued hourly at 55 minutes past the hour. ASOS are installed in four levels, as follows: 1. Service Level D: Minimal acceptable level of service. Completely automated, with no human augmentation by a human observer. This is the level of system typical of general aviation airports where there are no NWS or FAA weather observers on site. 2. Service Level C: A human observer, usually ATC personnel, may add information to the broadcast. This system also includes backup weather equipment in the event the observer determines the ASOS is reporting incorrectly. Source: FCM-S4-1994. Figure 9. ASOS location at glideslope.
38 Airport Surface Weather Observation Options for General Aviation Airports 3. Service Level B: All elements of Level C, but augmentation is beyond the reporting ele- ments of the ASOS. Typically included at small-hub airports or those with special weather conditions. 4. Service Level A: All elements of Level B, but augmentation level is even higher. Typical at major hubs and high-volume airports with special weather conditions. For those ASOS locations staffed by a weather observer, the ASOS can also issue an Aviation Selected Special Weather Report (SPECI). A SPECI is an off-schedule METAR and would be issued when certain conditions begin to change rapidly. The specific threshold conditions that may generate a SPECI are listed in detail in FMC-1, Section 2.5.2. The general conditions that may generate a SPECI include: â¢ Large wind shifts in speed or direction in a short period of time â¢ Visibility decreases below a threshold â¢ Tornado, funnel cloud, or waterspout â¢ Thunderstorm begins â¢ Hail, freezing precipitation, or ice pellets begin â¢ Squalls â¢ Ceiling decreases below a threshold â¢ Volcanic eruption â¢ Aircraft mishap â¢ Other issue deemed by the observer to be critical Ownership ASOS systems may be owned by the NWS, the FAA, or DOD but are maintained by the NWS. The NWS is responsible for inspection and maintenance of the systems and performs these functions to meet standards set by the FAA. Cost Because ASOS systems are federally owned and operated, an airport with an ASOS on site will not incur any costs associated with its operation or maintenance. Because ASOS stations are no longer installed as of 2004, cost of acquisition is not discussed here. The NWS does occasionally upgrade components, but these costs are internal to the NWS. If an airport operator allows airport development to affect the siting criteria of the ASOS, the airport may incur costs associated with relocating the equipment. Automated Weather Observing System (AWOS) An AWOS system that is properly installed, commissioned, and maintained can legally pro- vide certified reporting of weather conditions to the NAS. AWOS systems are similar to ASOS systems in that they both are automated, and they report much of the same data. They may even appear visually similar in layout and type of equipment. AWOS systems, however, do differ from ASOS systems in some ways. The installation of ASOS systems ended in 2004 and the program has no plans to resume. Conversely, AWOS systems continue to be installed every year. Of significance to airport opera- tors is that if they wish to install an AWOS at their facility, it will be installed under the FAAâs non-federal program. Non-federal means the system is not owned or operated by the federal
Existing Technologies 39 government but is installed, maintained, and operated under a series of rules which allow it to report data to the federal government (i.e., to the NWS, NADIN, and WMSCR). Specific requirements for the non-federal program are discussed in the following sections. Sensors For both federal and non-federal AWOS systems, there are several performance levels. The performance level of each category of AWOS is defined in AC 150/5220-16E, Section 1.2: 1. AWOS A: Altimeter 2. AWOS A/V: Altimeter, visibility 3. AWOS I: Wind speed, direction and gusts; temperature; dew point; altimeter; density altitude 4. AWOS II: All items from an AWOS I, plus visibility 5. AWOS III: All items from an AWOS II, plus precipitation accumulation (rain gauge), cloud height, sky condition 6. AWOS III P: All items from AWOS III, plus present weather identification 7. AWOS III T: All items from an AWOS III, plus thunderstorm/lightning detection 8. AWOS III P/T: All items from an AWOS III, plus present weather identification, thunderstorm/ lightning detection. 9. AWOS IV Z: All items from an AWOS III P/T, plus freezing rain detection 10. AWOS IV R: All items from an AWOS III P/T, plus runway surface condition 11. AWOS IV Z/R: All items from an AWOS III P/T, plus freezing rain detection, runway surface condition An AWOS transmits data that is updated every minute. The reported data is not an exact mea- surement of each sensor at the time a broadcast is heard. Instead, like an ASOS, the AWOS pro- cessor averages instantaneous measurements over time intervals and via algorithms as stipulated by the FAA to produce the reported condition data. Also, not all conditions may be reported at all times. Precipitation will not be reported if there is none, and density altitude is only reported when it is more than 1,000 feet above field elevation. Siting Requirements Siting requirements for the installation of AWOS systems are found in Order JO 6560.20C. The sensor location criteria explained in Chapter 3 of the order is nearly identical, in both word- ing and requirements, to that specified for ASOS systems in FCM-S4-1994. For all practical purposes, the siting requirements for AWOS and ASOS system sensors are the same. However, anyone looking to site an AWOS should refer directly to Order JO 6560.20C, Chapter 3. In order to provide certified weather information, the AWOS must be located at the airport, with one exception. The pressure sensor for the altimeter setting for an AWOS A is permitted to be located off-airport, provided it meets certain other location criteria. For an offsite loca- tion, the location must be within 6 nautical miles of the runway landing threshold, and the elevation of the pressure sensor must be within 500 feet of the field elevation. Furthermore, the algorithm used to determine the altimeter setting must include input from a calibrated temperature source. When an instrument approach is relying on an altimeter source that is outside these location criteria, the approach may include a Remote Altimeter Source Setting (RASS) penalty. A RASS penalty will be shown on the instrument approach plate and takes the form of an increase in approach visibility, minimum descent altitude, or both. Not all approaches may include a RASS penalty; some may simply state the approach is not authorized without a local altimeter source.
40 Airport Surface Weather Observation Options for General Aviation Airports The use of a remote altimeter setting may be beneficial if the local pressure sensor is out of service, or to initiate an instrument approach at an airport without any on-field weather report- ing. The RASS penalty is a function of the distance of the source from the airport, the difference in elevation of the source and the airport, and other local factors that the FAA has determined may influence pressure variations. In general, the larger the lateral or vertical distance from the airport to the remote source, the larger the penalty. AWOS systems have more flexibility with respect to location than an ASOS, because they come in configurations that do not always require a wind sensor. AWOS A and AWOS A/V systems may be more easily located, because they do not have the restrictions associated with a wind sensor. The AWOS I is the first system with a wind sensor, and so, generally, upgrading from an AWOS I to any higher system will not create any new siting issues because meeting the wind sensor criteria usually means all other sensor siting criteria can be met. The siting criteria for an ASOS and an AWOS I system or higher are, for all practical purposes, the same. Some of the case studies for the Virginia Department of Aviation AWOS project describe examples in which the siting criteria could not be met perfectly; these situations were coordi- nated with the FAA and still approved for installation and commissioning. Airports with siting challenges should coordinate their specific issues with the FAA Non-Fed coordinator as early in the process as possible. Approved Manufacturers The FAA maintains a listing of manufacturers whose equipment has undergone the FAAâs certification process to determine that their equipment meets the performance standards required for reporting approved, certified weather to the NAS. The FAA Non-Fed office provides the current listing, dated June 27, 2018, of certified system manufacturers. This listing is shown in Table 8. Model All Weather , Inc. Optical Scientific, Inc. (formerly Belfort Instrument Co.) Mesotech International Vaisala, Inc. AWOS A AWOS A/V AWOS I AWOS II AWOS III AWOS III P AWOS III T AWOS III P/T AWOS IV Z AWOS IV R AWOS IV Z/R Table 8. FAA-approved AWOS manufacturers.
Existing Technologies 41 Discussions with Vaisala indicate that its AWOS A/V is actually certified as an AWOS II, so the system includes all the sensors and capabilities of an AWOS II. Where a system has not been certified by the FAA, dissemination of that data in any format must first be prefixed with the word âadvisoryâ (AC 150/5220-16E, Section 1.2.f). Because only those systems rated AWOS III and higher are permitted to transmit to the WMSCR, there are at present only two manufacturers producing equipment that is permitted to transmit to the WMSCR. Systems certified in the AWOS A or AWOS A/V category may still have utility for official flight purposesâfor example, for providing an approved altimeter source for instrument approaches or approved visibility information for certain Part 135 operations. However, dissemination of this information on a national scale is not possible through the WMSCR. This information can still be transmitted locally, such as through an airport website. Data Reporting AWOS information is disseminated in multiple ways. The system ideally broadcasts locally on a discrete VHF frequency available to aircraft. The range of reception for these transmissions varies with aircraft altitude, terrain, weather, antenna type, and other factors and is generally at least 10 miles in the worst cases, but it may exceed 25 miles. The AIM notes in section 4-3-16 (p. 4-3-32) that reception should be available within 25 miles below 10,000 feet. Systems are also typically accessible via telephone line. An AWOS may also transmit over a shared UNICOM frequency or the voice output channel of a navigational aid (NAVAID). Transmission over a shared frequency is not ideal, but may be required in areas where fre- quency congestion is an issue. This was partially addressed years ago with frequency splitting to the 0.005 megahertz (MHz) level, but additional splitting is unlikely, given the limitations of so many GA radios. When an applicant applies for an AWOS, he or she must coordinate the frequency assignment with the FAA Non-Fed coordinator, who will facilitate the frequency assignment with the Federal Communications Commission (FCC). Individual airports may also elect to display their weather data (both certified and noncertified) on a website. This may be accomplished through several commercially available services that facilitate this. Noncertified weather must always be prefaced with the word âadvisory.â Display of weather data in this fashion typically means it will be available only to those looking for it on the website; the data may not be actively reported to other flight planning software from a website. The company All Weather, Inc. (AWI), which is one of the two approved manufacturers for certified AWOS III systems, maintains a website at http://usawosweb.com/main that lists data from participating AWI customer systems. This website may not list all AWI sites, and it does not list non-AWI systems. The website also is not tied to any other NAS reporting tool. Finally, the website displays the following note: âFAA Disclaimer: The data displayed is for advisory purposes only and is not to be used for flight planning or operations.â Transmission of AWOS data to the WMSCR is completed through a NADIN data link, which is discussed in the following. System Commissioning and Agreement Even though they do not own the weather reporting systems, the FAA still regulates the instal- lation, performance, and maintenance requirements of non-federal equipment because it is
42 Airport Surface Weather Observation Options for General Aviation Airports reporting into FAA-controlled systems. These standards are detailed in FAA Order 6700.20Bâ Non-Federal Navigational Aids and Air Traffic Control Facilities as well as in AC 150/5220-16E. This regulation happens in three ways. First, the weather reporting system must be from one of the approved manufacturers on the list maintained by the FAAâs Non-Fed office. Second, prior to allowing the system to broadcast to the public, the owners of the system must certify that they will operate and maintain the system in a manner consistent with FAA require- ments. These requirements are detailed in FAA Order 6700.20B and they are established when the system owner signs the required Memorandum of Agreement (MOA) and the Operations and Maintenance Manual (OMM) with the FAA. These agreements ensure the owner and FAA have a mutual understanding of the requirements to operate and maintain the system. Third, once the MOA/OMM is signed, the system must then be commissioned by the FAA. In this process, the FAA technician will verify that the system siting requirements have been met and that all equipment is performing within the required tolerances. The technician will also verify that all components are produced by an approved manufacturer. A system must be commissioned in order to support instrument approach procedures, pro- vide certified weather for aviation purposes, and report to the WMSCR. Commissioning is required for all levels of AWOS in order for the data to be certified. WMSCR Data Transmission The transmission of AWOS data to the WMSCR differs from that of an ASOS. For AWOS sys- tems, only those systems rated as AWOS III or higher are permitted to report to the WMSCR; an AWOS III is functionally equivalent to an ASOS. This limitation on reporting to the WMSCR is codified in AC 150/5220-16E, Section 1.3. AWOS reporting to the WMSCR is made in METAR format at a rate of three times per hour. In order to be eligible to transmit to the WMSCR, the AWOS must meet these additional requirements: 1. Equipment has been provided by an approved manufacturer. 2. Equipment is installed per the siting criteria in Order JO 6560.20C. 3. System is commissioned by the FAA. 4. System is maintained per FAA requirements. With FAA commissioning complete, the system is eligible to transmit data to the WMSCR and on to the broader NAS. In order for a non-federal AWOS of eligible system type to transmit to the WMSCR, the owner of the AWOS must engage an eligible third-party firm to facilitate the transmission. This third-party firm completes the transmission through a subscription to the WMSCR. The FAA maintains a list of FAA-approved third-party service providers to accom- plish this. The current list on the FAAâs website includes: â¢ All Weather, Inc. (AWI) â¢ anyAWOS â¢ DBT Transportation Services, LLC â¢ Remote Systems Integration â¢ National Association of State Aviation Officials â¢ University Research Foundation Only approved providers can facilitate the WMSCR connection because the process requires interaction with computer gateways which ultimately have access to closed FAA systems.
Existing Technologies 43 The approved third-party service provider system allows an extra level of security to prevent unauthorized access to safety data or corruption of that data. It is notable that not all owners may elect to establish a WMSCR connection for their certified AWOS III (or better). As one example, Clarion County Airport (KAXQ) has an AWOS III P that is certified but does not report to WMSCR. This airport was contacted about the reason for this, and it is generally budget related. There is a cost for the WMSCR connection, discussed elsewhere in this report. There are many other eligible systems across the country that also are not reporting to WMSCR. However, there is no search tool to segregate them on the FAAâs website, and they are best found through trial-and-error searching. So long as an owner has an AWOS commissioned and fulfills the requirements of the MOA/ OMM, the system information will continue to be certified by the FAA. However, revocation of certified status could occur for any of the following reasons: â¢ Surrounding development violates siting criteria. â¢ System components are replaced with noncertified parts. â¢ Inspections and maintenance procedures are not implemented. â¢ Inspections and maintenance procedures are not properly documented. â¢ The system falls into disrepair. A revocation of certified status will automatically revoke WMSCR access. The above condi- tions would generally become known as a result of increased complaints from users about system status, from an FAA Form 7460 airspace filing for noncompatible development, or during an annual FAA inspection where siting violations or noncompliant parts or sensor performance metrics are observed. Ownership AWOS systems may be owned by the FAA, an airport, a state aviation agency, another public agency, or even privately. When a system is owned by any organization that is not the federal government, it is said to be a non-federal AWOS. Note that there is often a difference between who paid to install the AWOS and who the owner of the system is. An airport may utilize FAA or state funds in whole or in part to install a system, but the airport could be, and usually is, still the owner of the system. The owner of the AWOS is responsible for its operation and maintenance (O&M). The spe- cific requirements for ongoing O&M will be included in the MOA/OMM issued at the commis- sioning. They will, however, be in alignment with the requirements stated in AC 150/5220-16E, Section 4.3, summarized as: 1. Triannual preventive maintenance checks (once every 4 months) 2. Annual preventive maintenance verification check 3. Triennial verification check (once every 3 years) There are requirements with respect to personnel who conduct these inspections. Airport maintenance personnel may qualify to conduct the triannual inspections by taking a training course in accordance with Appendix 2 of AC 150/5220-16E, which details the requirements of the FAA Authorized Maintenance Equipment Training Program. The system owner may also contract out to a firm or individual who is qualified to perform the triannual checks. The annual and triennial verification checks are performed by FAA personnel. In addition to ensuring that the scheduled inspections occur, a system owner is also respon- sible for maintaining the area around the system in accordance with the siting standards. These
44 Airport Surface Weather Observation Options for General Aviation Airports responsibilities include keeping grass cut to acceptable lengths, ensuring no obstructions to the sensors are created, and protecting against noncompatible land uses that may affect the system. Cost The most common type of AWOS installation is the AWOS III or some variant thereof (P, T, or P/T). Costs to install such a system may vary highly, depending on a number of factors: â¢ Engineering and procurement costs â¢ Site access to power and communications â¢ Site grading â¢ Obstruction removal and tree clearing â¢ Ground access A review of the statewide project in Virginia that installed 17 AWOS III systems under a single contract throughout 2013 and 2014 showed an average cost per system of $140,000. This cost is for construction only and does not include engineering for design, procurement, administra- tion, testing, or inspection of the construction. The projectâs construction-only costs per site ranged from a low of $115,000 to a high of $175,000, excluding site clearing costs. Many sites required little to no additional clearing, but one site required clearing 11 acres of trees at an additional cost of $110,000. When added to the cost of that site, it brought the construction total to $265,000. This work was bid in 2012 and included all 17 sites, which provided the contractor a large economy of scale. Adjusting for inflation and market conditions, the current planning-level construction-only cost to install an AWOS III is projected at $250,000. This would be for a site with level ground, no obstructions, ownership of all required land, and convenient access to power. Local conditions requiring special access, land acquisition, clearing, or special power installation would increase this cost. Costs relating to engineering, procurement, and construction administration of the system should be added to the construction costs. These will vary based on local capabilities and require- ments and on project complexity but may exceed $50,000. Lesser AWOS systems may be installed for as little as $20,000 for an AWOS A, up to $75,000 for an AWOS A/V. The single most expensive component of an AWOS system is consistently reported to be the ceilometer (cloud height sensor), for which airports have reported paying anywhere from $35,000 to $45,000 in recent years. All systems from the AWOS A through the AWOS II do not include a ceilometer. Thus, reporting to the WMSCR requires a system that has this most expensive component. With an average power draw of approximately 300 watts, an AWOS III will use approximately 2,600 kWh of electricity per year, which is about one-quarter of the usage of an average U.S. household. This is equivalent to $300 to $500 per year, depending on location in the country. Required triannual inspections may be completed internally by the owner if personnel can be trained for this. Otherwise, many owners employ a contract firm to perform this work. The cost of annual maintenance contracts will vary based on geography, proximity to other systems, and other factors. Costs of $3,000 to $6,000 per year, or more, have been reported during interviews with airports. The cost for a telephone landline is estimated at $35 per month, or about $400 per year. Finally, the connection to the WMSCR must be contracted to an approved third-party firm. These costs may also vary, but a planning value of $100 per month is reasonable based on feedback from owners. Table 9 summarizes approximate costs for installing an AWOS III. The FAA inspection procedures mandate inspections of different levels on both a triannual basis (three times per year) and on a triennial basis (once every 3 years). The triennial inspection
Existing Technologies 45 items would be performed at one of the regularly scheduled triannual inspections already noted in Table 9. Repair costs are not included in this summary, as these are dependent on operating envi- ronment, quality and consistency of maintenance, general upkeep, and chance. Discussions with one FAA-authorized service technician indicated he charged a flat rate for a service visit, plus an hourly rate for time on site, plus the cost of parts. Such a visit could cost anywhere from $1,000 for a short visit requiring minor adjustments up to tens of thousands of dollars if an expensive part requires replacement. Automated Weather Sensor System (AWSS) This was a short-lived program conceived to develop an advanced AWOS system that pri- oritized ruggedness, reliability, and serviceability. Ultimately only 17 systems were installed in 2005, and none since then. These systems perform a combination of the functions of an AWOS III and an AWOS IV Z, and are certified to report to the WMSCR. However, given the small number of systems installed, and that they are no longer produced, they are of limited relevance to the general aviation community. Noncertified Systems By default, unless a system is certified by the FAA and subsequently commissioned to report data to the NAS, it is considered to be not certified. Any information broadcast from these non- certified systems must be preceded by the term âadvisory,â either in print or verbally. This is commonly conveyed as âadvisory weatherâ or âadvisory conditions.â Advisory weather cannot be used to make official decisions, such as using an advisory altimeter to initiate an instrument approach or using advisory visibility and cloud heights to determine VFR conditions or to meet certain Part 91 or Part 135 operations requirements. The FAA siting guidelines for certified systems ensure that outside influences on the data are minimized and that data is taken and reported in the same manner at all locations. Conversely, there are few regulations detailing the siting requirements for noncertified systems, so they could be reporting data that is free from artificial influences or data that is being compromised by unknown factors, but there is no way for a user, or the FAA, to verify this. Component One-Time Cost Annual Recurring Cost Engineering and procurement $50,000 Equipment and installation $250,000 Triannual maintenance $5,000 WMSCR connection $1,200 Telephone landline $400 Power $400 Total: $300,000 $7,000 Note: Installation presumes a site requiring no clearing, land acquisition, or significant site work. Table 9. Summary of costs to install an AWOS III.
46 Airport Surface Weather Observation Options for General Aviation Airports Similarly, whereas certified systems are commissioned to verify that their sensors are per- forming correctly and are then inspected triannually thereafter against set performance stan- dards, noncertified systems receive no official calibration and are not continuously checked against any benchmark. They may read erroneously from the initial installation or stray over time, but there is no official process to document, track, and correct this. Noncertified systems may also not time-average data collection reporting in the same manner as certified systems are required to. This is not to imply that a noncertified system cannot provide data that might be as accurate as that of a certified system, only that the lack of standards and testing makes it impossible to determine how accurate any one noncertified system is. The FAA does not maintain a list of performance standards to validate noncertified equipment. Noncertified systems may incorporate some pieces of certified equipment. And if the certified equipment is installed and commissioned, those certified portions of the data may report without the âadvisoryâ prefix and may be used for safety of flight information. An example of a common combination of certified and noncertified equipment is a certi- fied AWOS A/V co-located with a noncertified suite of sensors. These systems will open the data reporting with the visibility and altimeter information, followed by an advisory prefix for the remaining data, which often includes all or most of those remaining parts that constitute a complete AWOS III. However, these advisory conditions are not certified, as they are generated by equipment that has not been certified by the FAA. In this case, the visibility and altimeter readings are approved for use in flight planning, but the remaining advisory data is not. Such a system will not report any data to the WMSCR. An example of such systems in widespread use across the United States are those AWOS A/Vs manufactured by Optical Scientific, Inc. (formerly Belfort Instrument Co.). This company produces an FAA-certified AWOS A/V but commonly installs them with additional uncertified components to provide advisory weather. The companyâs website lists additional (noncerti- fied) sensors for wind, temperature, humidity, and ceilometer. Many of these systems have been installed under the product name DigiWx. The mix of certified and noncertified sensors is not always consistent. Furthermore, avail- able databases that report system types are not always consistent with how the system itself is reporting. Consider the following examples of hybrid certified/noncertified installations and how they report and how they are documented. The FAA website referenced in these examples is: https://www.faa.gov/air_traffic/weather/asos/. Despite the website address, this site reports every ASOS and every certified AWOS from AWOS A through AWOS IV Z/R in the FAA database. Example 1âBradford County Airport (KN27), Towanda, PA Both the FAA website and AirNav (www.AirNav.com) list this installation as an AWOS III. However, www.aviationweather.gov does not indicate any METAR reporting for this site. Calling the system via telephone provides a report that states: â¢ Visibility and altimeter â¢ Weather advisories â Wind â Ceiling â Temperature â Dew point
Existing Technologies 47 â Relative humidity â Density altitude â Condensation altitude This readback should be interpreted to mean the installation is providing certified visibility and altimeter information (AWOS A/V) but is co-located with a collection of noncertified equip- ment reporting other data. Note the reporting of relative humidity and condensation altitude, neither of which is reported in a certified AWOS. These advisory conditions indicate that this system is something less than an AWOS III, even though it reports all of the information an AWOS III reports (except for possibly precipitation, which was not reported during this testing). Because the installation is not an AWOS III, it is not included for WMSCR/METAR reporting. Its listing on the FAA website as an AWOS III is possibly an oversight, or possibly an indica- tion that the system was once a certified AWOS III but for some reason lost a portion of that certification. Example 2âGreenville Municipal Airport (K3B1), Greenville, ME Calling the system via telephone provides visibility and altimeter data followed by âweather advisoriesâ for wind, ceiling, temperature, dew point, relative humidity, density altitude, and condensation altitude. The FAA website, as well as AirNav, lists this as an AWOS A installation even though it is reporting certified visibility and altimeter data, thus meeting the criteria for an AWOS A/V. Example 3âBult Field (KC56), Monee, IL Calling the system via telephone provides visibility and altimeter data followed by âweather advisoriesâ for wind, ceiling, temperature, dew point, relative humidity, density altitude, and condensation altitude. AirNav lists this as an AWOS A/V, which is consistent with how the system reports. However, the system is not included at all in the FAA database (at https:// www.faa.gov/air_traffic/weather/asos/), which includes 63 stations of all types in the state of Illinois, including AWOS A and AWOS A/V systems. These examples are not provided to highlight discrepancies in databases but rather to high- light that when noncertified equipment is included, the documentation and reporting of these systems may not be consistent across all sources. Furthermore, the relationships between FAA and NWS databases is exceedingly complex. It is possible that equipment changes simply take time to be updated in all locations where data is published. See the case example of the Saline County Airport for an example of this. Automated UNICOM An Automated UNICOM is an FAA-approved method of disseminating airport information, including weather. This system is described in the Pilot/Controller Glossary (p. PCG A-15) of the AIM as follows: Automated UNICOMâProvides completely automated weather, radio check capability and airport advisory information on an Automated UNICOM system. These systems offer a variety of features, typi- cally selectable by microphone clicks, on the UNICOM frequency. Availability will be published in the Airport/Facility Directory and approach charts. Many forms of aviation VHF equipment can be controlled by executing a series of micro- phone clicks on a frequency, with pilot-controlled lighting perhaps the most common example. An Automated UNICOM utilizes this same technology to access a weather broadcast.
48 Airport Surface Weather Observation Options for General Aviation Airports The system is included with the noncertified systems because this shared-frequency function- ality is sometimes used to broadcast weather information from noncertified systems. Shared use of the UNICOM frequency may be beneficial, because there is competition for the limited VHF band dedicated for aviation uses. For certified AWOS systems, the FAA Non-Fed Frequency coordinator is responsible for obtaining a dedicated VHF frequency, which is the preferred method of transmission. In some locations these frequencies may be difficult to obtain due to frequency congestion and interfer- ence issues, but the FCC works with the FAA to accommodate the transmission of flight safety information as best it can and works to establish a dedicated frequency. An organization installing a noncertified system will not have the backing of the FAA; the organization may find it impossible to secure a dedicated broadcast frequency or may be unwill- ing or unable to meet the FCC license terms or pay the requisite fees to maintain the license. In these cases, the systems may utilize the existing UNICOM or Common Traffic Advisory Fre- quency (CTAF) frequency to report their weather. The DigiWx, SayWeather, and SuperAWOS systems are capable of reporting through an automated UNICOM, although they can also oper- ate on a dedicated VHF frequency on continuous broadcast. Transmission of weather through an automated UNICOM is not as seamless as through a dedicated frequency. For example, 47 CFR 86.219âAutomatic Operations notes that auto- mated UNICOM transmissions are to be as brief as possible and may never exceed 1 minute. However, while the weather information is broadcasting, other aircraft wishing to utilize the frequency will not be able to communicate. This could interrupt other pilots attempting to com- municate with UNICOM or with each other in the air (if on the CTAF) for extended periods of time. Some systems may incorporate technology that senses frequency congestion and abbrevi- ates the weather reporting. Further, 47 CFR 87.219 indicates that automated UNICOM may only transmit in response to an interrogation. In addition, automated UNICOMs may not provide weather at a facility where there is an FAA-certified automatic weather reporting station, unless the UNICOM itself is certified by the FAA. The CFR further states that if weather is provided by an automated UNICOM, the information must be preceded by the word âadvisory.â This seemingly creates a situation in which information obtained by an FAA-certified automated weather observing system must report as advisory if it is reporting through an automated UNICOM. Modular Automated Weather System The Modular Automated Weather System (MAWS) has been installed at a number of Alaskan airports by the NWS. The system provides advisory information, including ceiling and visibility. MAWS stations use sensors that are all certified for use in AWOS, but the data is not transmitted over VHF or via WMSCR, and they do not produce a METAR. The current list of MAWS units is available at the Alaska Aviation Weather Unit website, at https://www.weather.gov/aawu/stnlist. Weather Cameras A wholly different type of noncertified weather reporting system is weather cameras. There is no FAA-certified version of a weather camera that includes standards for placement, resolu- tion, functionality, coverage, reliability, or data dissemination. Due to this lack of standards, the current uses of cameras to document weather conditions for aviation purposes vary widely in their execution. The most comprehensive use of weather cameras for aviation purposes is in Alaska. It is also the only FAA-supported use of weather cameras. The website (https://avcams.faa.gov/index.php)
Existing Technologies 49 is run by the FAAâs Aviation Weather Camera Program Office, and displays a map titled âFAA Aviation Weather Cameras.â The site is an interactive map that allows users to click on a report- ing location to see near-real-time camera images taken from any of 233 locations across Alaska (as of February 2019) and 54 locations across British Columbia and Yukon Territory in Canada. The sites are not all at airports. For example, mountain passes and other critical locations are also included. The site also displays METARs from official FAA data sources (i.e., certified ASOS or AWOS) and displays advisory weather at many locations with a disclaimer noting that âAdvisory WX data comes from a non-FAA-certified Automated WX Source.â While this noncertified weather data is not being reported to the WMSCR, it is being reported on a federal government website, which is a departure from practices for data from the con- tinental United States. Furthermore, the information provided includes cameras described as either FAA cameras or third-party cameras. All camera data, regardless of its source, is described as supplementary. The âAboutâ section on the Alaska weather camera webpage (https://avcams. faa.gov/about.php) states: The camera information contained on this website is a designated FAA supplementary product. Camera images are generally updated every 10 minutes. The time of the last update is indicated on each image. Current site conditions may differ from displayed images due to a variety of reasons; i.e., rapidly changing conditions, image update frequency, optical distortion, etc. As a supplementary product, these images may only be used to improve situational awareness. They may not be used to comply with regulatory requirements; e.g., to determine weather minimums for IFR flight. METAR information is provided for planning purposes only. It is recommended that you contact a Flight Service Station for a complete pilot weather briefing and all pertinent NOTAM information by dialing 1-800-WX-BRIEF (992-7433). Additional information on the Alaska weather program is presented in Chapter 5, Case Examples. The Washington State Department of Transportation also runs a statewide network of aviation weather cameras. This information is noted on the departmentâs website (http:// www.wsdot.wa.gov/aviation/WebCam/) under an âAviation Disclaimer,â as follows: The airport web cameras are offered as a service for pilots to view current airport conditions. They should not take the place of a pilotâs responsibility for obtaining a full, pre-flight weather briefing prior to flight from their local FAA Flight Service Station. This disclaimer is consistent with the FAAâs position on the Alaska cameras that the camera information does not supplant the need to obtain a legal flight briefing on weather. The state website contains a clickable map to obtain the camera data, although its functional- ity differs from the FAAâs Alaska map. The site contains 43 camera locations. Selecting a location brings up a webpage for that camera, with anywhere from one to four views from the camera. There is also a description provided for where the cameras are mounted and which direction they are facing. The views are static views, not motion views, and each site may depict a differ- ent image-refresh rate, which is typically from 5 to 15 minutes. The King County International Airport/Boeing Field camera contains two views, which update every 30 seconds. If a certified weather source is available at the site, there is a link to the reporting for that system through the NWS (or sometimes a private third-party provider); but unlike the Alaska system, the weather information is not displayed directly on the camera website. Figure 10 shows a view from a representative camera at the Skagit Regional Airport (KBVS). The information provided on the cameraâs web page (https://wsdot.wa.gov/aviation/WebCam/ Skagit.htm) states: This web camera was purchased with WSDOT Aviation Security Grant Program funds and is operated courtesy of the Port of Skagit â Skagit Regional Airport.
50 Airport Surface Weather Observation Options for General Aviation Airports The camera is mounted on the top of the Port of Skagit Administration/Airport Terminal building. The camera provides four viewsâRunway 11 (Northwest), Runway 29 (Southeast), Crosswind Run- way 4/22 (South), and Runway 11/29 Windsock (West). The images displayed should indicate the direction âNorthwest,â âSoutheast,ââSouth,â or âWest,â along with a time/date stamp. This camera operates 24 hours a day, with the images updating every 15 minutes. To report problems with the camera, please contact: Port of Skagit Administrative Office, at (360) 757-0011. All information subject to use disclaimer. Aviation Disclaimer: The web cam images are being provided as a service for pilots to view current airport conditions. They should not take the place of a pilotâs responsibility for obtaining a full, pre-flight weather briefing prior to flight from their local FAA Flight Service Station. Airport cameras are also installed by airports themselves. There are commercially available packages that provide plug-and-play functionality allowing an airport to output the camera infor- mation either to their local website or to national websites that may offer connectivity for a fee. The implementation methods of aviation weather cameras are diverse. Installation loca- tions on the airfield, camera types, viewing angles, clarity, update frequency, and several other performance metrics all vary from system to system. Furthermore, the camera systems in use predominantly transmit still images taken at fixed intervals and not streaming video. Thus, the age of the data may vary across platforms. Ease of access to the data also varies. Additionally, there is no central clearinghouse where nationwide camera data is available. Camera data (e.g., ceiling and visibility) is not certified data or approved for official flight planning decisions, but the manner and wording with which these restrictions are conveyed differs across delivery platforms. Weather cameras are discussed in Chapter 6 under Items for Future Research. Grant Eligibility The FAA supports the installation of AWOS systems through the AIP. Guidance on the eli- gibility of systems is provided in Order 5100.38DâAirport Improvement Program Handbook, commonly called the AIP Handbook. Only airports listed in the NPIAS are eligible to participate in the AIP Program. The AIP Handbook was updated with Change 1 in February 2019. Figure 10. WSDOT weather camera, Skagit Regional Airport.
Existing Technologies 51 AIP requirements for support of AWOS are contained in Table K-2 of the AIP Handbook. In order to be eligible for AIP reimbursement, the system must conform to the following general requirements: â¢ Must pass a Benefit-Cost Analysis (BCA) for AWOS III or higher (some exceptions apply, as discussed later). â¢ Equipment must be provided by an FAA-certified manufacturer for the level of system proposed. â¢ Sponsor must obtain a useable VHF frequency for transmission, if required. â¢ Must be commissioned by the FAA. â¢ If AWOS III or higher, must be connected to the WMSCR. â¢ Sponsor must maintain the system for the life of the equipment. [Equipment life was reported by one manufacturerâs representative as approximately 15 years.] Sponsors should consult Table K-2 in the AIP Handbook for a complete listing of the require- ments for an AIP-supported AWOS, as the requirements stated here are only a brief summary. The AIP Handbook indicates that the AWOS BCA requirements may be found in an FAA publication, âEstablishment and Discontinuance Criteria for Automated Weather Observing Systems (AWOS).â This publication dates from 1983 but is still the document currently refer- enced in the AIP Handbook. While the general guidance is that AWOS III or higher must pass the BCA, the AIP Handbook, Change 1, does allow for some exceptions to this. The Handbook waives the BCA requirement for those airports classified as primary airports, and for those listed as a national or regional airport in the FAAâs ASSET report. According to the 2012 ASSET report, Appendix B-1: Sum- mary by State, of the 3,303 National Plan of Integrated Airport Systems (NPIAS) airports in the United States, excluding territories and possessions, a total of 550 are classified as national or regional. In addition, the FAA Reauthorization Act of 2018 waives the requirement for a BCA if the AWOS will âassist an applicable airport to respond to regional emergency needs, including medical, firefighting, and search and rescue needs,â and if the system will not create any radio interference. However, this BCA exception only applies to states with a population density of fewer than 50 people per square mile based on the most current decennial census. According to the 2010 census, only 14 statesâAlaska, Colorado, Idaho, Kansas, Maine, Montana, North Dakota, Nebraska, Nevada, New Mexico, Oregon, South Dakota, Utah, and Wyomingâmeet this criterion. AWOS systems may also be installed and maintained under a variety of state programs. These programs vary widely across the country. Some states provide no support for weather station installation or maintenance, while others support the majority of operating costs; for example, Virginia supports 95% of scheduled maintenance as well as WMSCR connection fees. The case examples presented in this report discuss some of the funding scenarios provided by a variety of states. Additional sources of funding to support AWOS installations include the Bureau of Indian Affairs in Alaska, which used transportation funding through a native community to install an AWOS at the Chenga Bay Airport (PFCB) on Prince William Sound. Once certification of this system is complete, the availability of this data may increase access to this isolated island com- munity, which is not reachable by traditional surface access. Sponsors may also investigate funding partnerships with other stakeholders in the airport whose operations may benefit from having a weather system on the field. This could include a fixed-base operator (FBO), a Part 135 charter operator, or a tenant whose operations depend on real-time, accurate weather information located on the field.