AMOSSG/3-SN No. 11

20/1/02

Revised
AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

THIRD MEETING

Miami, 22 to 25 January 2002

Agenda Item	5:	Review of the requirements for meteorological observations and reports at aerodromes contained in Chapter 4 of Annex 3

INTEGRATION CONSIDERATIONS FOR RVR AND VISIBILITY

(Presented by E. Robinson)

1.	INTRODUCTION

1.1	Most automatic weather observing systems (AWOS) today use forward scatter meters (or transmissometers) to measure the atmospheric extinction coefficient which is used to determine and report Meteorological Optical Range (MOR). Koschmieder's equation is used to determine MOR. During the day this MOR value is representative of human visibility. At night the MOR value is also representative of human visibility in areas that are well illuminated, however MOR tends to underestimate the visibility that a human would estimate in areas that are not well illuminated. WMO-No. 8 (CIMO Guide), par. I.9.2.5 states, "during hours of darkness, the observer's estimates of visibility were about 30 percent higher than instrumental measurements of MOR". Under night conditions the contrast threshold of the eye changes from the identification of dark objects against a light background to the identification of light sources against a dark background. Since visibility in METAR reports is used to determine minima for aviation this difference may create operational issues for pilots when using MOR.

1.2	In Runway Visual Range (RVR) systems, both Koschmeider's and Allard's equations are used where Allard's equation takes the intensity of point light sources (runway lights) into account and is used to determine a representative visual range which is appropriate for a specific runway, or more specifically, for a given point on a specific runway (e.g. touchdown zone). The intensity of the light sources and the illumination threshold of the eye must be known for a representative determination of visual range when using Allard's equation. A background luminance sensor is also used to correct the illumination threshold of a human eye for a measured background luminance.

1.3	The purpose of this paper is to initiate discussions on the implications and the benefits of integrating RVR and MOR systems and requirements, where it is feasible. It also assumes that the reporting of visibility at night from AWOS systems, based on MOR measurements, will evolve in the future through CIMO to deal with this 30 percent difference with observers at night.

2.	METEOROLOGICAL VISIBILITY

2.1	MOR represents the optical depth of the atmosphere or the length of path in the atmosphere required to reduce the luminous flux in a collimated beam to 5% of it's original value for the measured extinction coefficient. Solving Koshmeider's equation for a contrast threshold of .05 results in MOR = -
ln(0.05)/ 3/ (where represents the measured extinction coefficient). This is an objective physical measurement that can be well calibrated. Both forward scatter meters and transmissometers are currently used as sensing elements for the determination of the atmospheric extinction coefficient value that is used in this calculation.

2.2	As stated in WMO No.8, par. I.9.2.2; "For daytime observations, visual estimates of visibility gives a good approximation of the true value of MOR." This requires black, or near black objects at many different distances. If a visual range at night for aeronautical purposes (VR-Special) was to be defined and determined from a measured MOR by a full AWOS (e.g. where no observer inputs are available), as well as reporting MOR as the AWOS does today, the values of the light sources would need to be known or estimated and the dark adaptation of the human eye must be assumed to be complete and stable. Note: There are large uncertainties in this VR-Special process that would need careful consideration and well informed users. In RVR applications, runway lights are of known intensity and are controlled to a high standard, although there is some uncertainty due to aging, dirt and power, among other issues.

2.3	Discussions in various subcommittees at the COM/MET/OPS meeting (1990) had concluded that a 1000 candela light should be used for this purpose. CIMO however is strongly against this value due to its extreme intensity and WMO recommends 100 candelas for this purpose. There would need to be an agreed standard for this value for use internationally.

2.4	With the use of a single forward scatter meter it is necessary to place the sensor at the best suitable location, as it depends on the layout and size of the airport, to measure a representative value of atmospheric extinction coefficient.

3.	RUNWAY VISUAL RANGE

3.1	There are well-documented ICAO standards for the placement of forward scatter meter sensors and for the algorithms that are required for the calculation of RVR data. Generally, the forward scatter meters used for RVR operate on the same principles as those used on an AWOS for MOR; in many cases they are the identical sensors. Background luminance sensors are used with RVR systems and the light source intensity value for nighttime visibility is derived from the active runway lamp settings. Some RVR systems do not identify what the actual runway lamp settings are but instead use the maximum lamp setting as a constant in the calculation. This is a safe practice as long as the runway lights are set to the maximum value under low visibility conditions, which is the normal practice. This simplifies the RVR equipment since lamp setting input determination is not required.

3.2	RVR systems use the background luminance sensor to determine the visual threshold of illuminance, or to select which curve should be used (e.g. bright day, normal day, intermediate, night), or to calculate this value (Et ) directly for the given background luminance (B). This is defined in the Manual of Runway Visual Range Observing and Reporting Practices, second edition – 2000 as;

Log (Et ) = 0.57 log (B) + 0.05 [log (B)] 2 – 6.66

3.3	It may be possible to improve AWOS nighttime visibility estimates by applying similar methods based on measured MOR values, measured background luminance and artificial light sources to determine visual range specials.

3.4	It would also be cost effective to extend the range of RVR (50m - 10 km) as proposed by the WMO.

4.	INTEGRATION CONSIDERATIONS

4.1	There could be significant operational benefits and financial savings to be realized if the navaid (RVR) and meteorological (MOR) equipment were designed in a common manner in the future. Some commercial forward scatter meter manufacturers are already moving in this direction with an optional background luminance sensor integrated into these sensors. In some countries these systems are still independent and the sensors are different which not only adds to the costs but also introduces uncertainty in measurements and calibration. (e.g. Canada uses a four head forward scatter meter for RVR applications to increase redundancy but the AWOS systems use standard two head forward scatter meters.)

4.2	Future RVR equipment could, for a very small incremental cost, determine MOR and VR Special values within the same equipment and, similarly, the meteorological AWOS MOR systems could also determine RVR values if these systems would be sited properly and if they could have access to runway lamp setting information, where it is used.

4.3	The impact of this could be less duplication of effort and the merging of installation and maintenance functions between the meteorological and aviation sectors. In aerodromes, which already have RVR systems, the AWOS could access the MOR and VR Special output data from this RVR equipment to provide MOR data at a significantly reduced cost since the duplication of the full visibility measurement system is not required. It is assumed that siting representativeness could be determined from a combination of these forward scatter meters.

4.4	Similarly, at aerodromes where only MOR equipment exists (e.g. smaller airports), with appropriate runway site selection for the forward scatter meter, a representative RVR value could be provided where it currently does not exist, for little cost. Arguably, a center runway siting for smaller airports may be representative for both MOR and RVR. The risk to aviation may also be reduced by adding an RVR value to METAR data where it would normally not be available. It is noted that the regulations are specific regarding RVR sensors at touchdown zones but at smaller airports these implications may be small or the use of center RVR at these sites could have specific procedures defined for these smaller airports. This is a critical issue.

Note.— The VR Special discussed in this revised paper was referred to as Aerodrome Visual Range (AVR) in my previous draft. Please disregard my Jan. 13, 2002 paper and the references to AVR. My thanks to Dr. J. P. van der Muelen, KNMI, for his well-considered response to my poorly presented ideas.

5.	PREVAILING VISIBILITY ISSUES

5.1	The representativeness of a single forward scatter meter has been the topic of much debate and there is now general acceptance that the downwind temporal integration of the atmospheric extinction coefficient does provide representative data most of the time.

5.2	Occasionally however, non-homogeneous atmospheric conditions introduce more uncertainty into this representativeness (e.g. patchy fog in one or more directions but not within the sensor's sampling volume or integration path). This leads to the emerging requirement for prevailing visibility estimates from AWOS sites. Considerations need to be given to the use of multiple forward scatter meters (e.g. spatially distributed) to meet this requirement. This will likely emerge from future CIMO work for the definitions and guidelines/standards for use.

5.3	At large airports, multiple forward scatter meters already exist to determine the RVR values at multiple points on several runways. The larger the airport, the more sensors exist and the better the information available for the determination of area representativeness. Hence both prevailing visibility and MOR & VR Specials could be determined by AWOS equipment in the future with this integrated configuration without the need for the AWOS to duplicate the sensors for this purpose. It is likely that some off airport sensors may also be required for the determination of prevailing visibility.

5.4	On smaller airports, the array of sensors that would be required by an AWOS for prevailing visibility could also be used to determine appropriate RVR values for all runways. Hence the duplication of sensors and systems would be reduced while operational aviation capabilities are enhanced.

6.	ACTION BY THE GROUP

6.1	The group is invited to consider and discuss these issues; and in particular the:

a)	feasibility and implications of VR-Specials;

b)	benefits of MOR & VR Special and RVR integration;

c)	implications of siting RVR (center runway) on single sensor, small airport applications, specifically when RVR does not exist;

d)	likely impacts of multiple sensor use for prevailing visibility estimates; and

e)	next steps to be taken, specifically with CIMO on prevailing visibility.

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