GB2510617A - Assessment of runway visual range - Google Patents

Abstract

Runway visual range (RVR) is often assessed using visibility sensors located adjacent to the runway but some distance from the edge. The invention employs the principles of forward and/or backward light-scattering geometry using the runway light fittings as light sources, enabling RVR to be assessed actually on the runway. The runway lights L1, L2, L3 have photosensors incorporated in them to measure forward scattered light or back scattered light. The large number of sensors on the runway itself gives spatial and temporal information of visibility on the runway which is used to assess RVR along the length of the runway. The runway centre lights or edge lights may be used.

Description

Sensor System for Assessing Visual Range on a Runway.

This invention relates to a method for assessing visibility along a runway by measuring light scattered from the beams emitted by runway light fittings.

Runway Visual Range (RVR) is an assessment of the visual guidance available to pilots of aircraft using a runway for take-off or on the approach to landing and during deceleration after landing. RVR is a very important weather parameter for airport operations since it can preclude aircraft from operating in adverse conditions. Also, critical values of RVR are used to define operational limits e.g. for CAT 1, CAT II and CAT III (a,b,c) categories of Instrument Landing System (ILS) assisted operations.

Annex 3 of the Convention on International Civil Aviation stated the definition of RVR as: The range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line.' The Eighth Air Navigation Conference (Montreal 1974) modified this definition to: Since, in practice, RVR cannot be measured directly on the runway a RVR observation should be the best possible assessment_of the range over which the pilot of an aircraft on the on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line RVR observations may be made by human observers but the International Civil Aviation Organisation (ICAO) Document 9328 Manual of Runway Visual Range Observing and Reporting Practices' recommends the use of instruments to assess RVR (Instrumented RVR, or IRVR) for CAT I operations and IRVR assessments are compulsory for CAT II and CAT III operations.

The instruments currently used to assess IRVR may be transmissometers' or forward scatter meters'. Transmissometers measure the optical transmittance of the atmosphere over a defined baseline distance (normally around 15 to 20 metres). Forward scatter meters generate a beam of light and measure the amount of light scattered from that beam by aerosol particles, water droplets, snowflakes or hailstones. Forward scatter meters measure light scattered through an angle of less than 90 degrees (usually in a defined sample volume of a few litres). For other meteorological purposes, sensors which measure light scattered by an angle greater than 90 degrees (and often close to 180 degrees) and known as backward scatter meters' may be used to assess visibility.

The drawbacks of current methods of assessing IRVR are that only a few instruments are employed, positioned along the length of a runway (normally close the touchdown zone, mid-point and stop-end) and the instruments must be positioned at some distance (more than 80 metres) from the runway centreline (ICAO, 2005) to one or both sides of the runway. However, visibility (or atmospheric transmittance) can often change significantly over distances much shorter than the spacing between conventional instruments, and shorter than their distance from the centreline.

Also, the calculation of TRVR from instrument measurements requires an assessment of the background illumination level in order to determine the illumination threshold for which pilots can distinguish the contrast between lights or markings and the background. This requires either an assumption of the likely background illumination level or, preferably, the use of one or more background luminance meters' to measure the level of background lighting. However, there is some evidence that background luminance meter readings can be affected by other airfield lighting, not necessarily associated with the background light level close to the runway (which is appropriate for assessing IRVR).

The limitations of current methods of assessing TRW may have contributed to the serious incident which occurred at Nairobi Airport in Kenya on 2yth April 2008 which involved an Airbus A340 aircraft encountering an area of much lower visibility (thicker fog) along the runway than simultaneously reported by the single transmissometer adjacent to the touchdown zone, some distance away from the incident site. This resulted in the pilots losing sufficient visual guidance and the aircraft departing from the runway (AAJB, 2009).

Whereas current methods of assessing IRVR employ a few individual sensors situated adjacent to a runway, this invention is a method to employ the principle of scatter meters for sensors to measure light scattered from the beams produced by the runway light fittings.

This would enable assessment of IRVR on the runway and along its length. The sensors monitor scattering of the actual light output from runway fittings (forward and/or backward scattered light), automatically compensating for variation in output due to ageing, contamination, electrical supply variation, etc. Also, the use of a larger number of sensors statistically reduces the error associated with the assessment and provides more information about spatial variation of visibility over a much smaller scale than is possible using current methods. It is also possible to obtain information about background light levels in the actual vicinity of the runway lights and/or markings which provide guidance to pilots, which enhances the calculation of RVR.

The main advantage of this method is that it allows assessment of IRVR actually on the runway and along its length.

Document 9328 (TCAO, 2005) describes many factors which can adversely affect the assessment of TRVR, such as contamination and ageing of lights which reduce their actual output, differences between actual and assumed light intensity, errors in measuring background luminance, differences in background luminance on the runway from where it is measured, etc. The assessment also depends on the intensity selected for variable intensity runway lighting systems (often 1% to lOO% in defined increments, selectable by Air Traffic Control personnel) and many conventional TRVR systems require the actual current supplied to the lighting electrical circuit(s) to be monitored directly (rather than assuming the selected setting is actually being applied to the lights), adding complexity to the lighting system. In this method, sensors measure light scattered from the beams emitted by the actual runway lights and the background luminance level in their vicinity, so factors such as those mentioned above are automatically included in the assessment of RVR.

Although the accuracy of instrument types currently employed is generally considered to decrease from transmissometers to forward-scatter-meters to backward-scatter-meters, the use of an array of multiple sensors in this (scattering) method can be used to statistically reduce errors in the overall assessment of IRVR. The calculation algorithm can use forward and/or backward scattering measurements and can also incorporate information from a conventional sensor present weather' to optimise the assessment. Also, knowledge of the variation in visibility with distance along the runway is much improved, which is important in the case of spatially inhomogeneous fog and fog patches.

Although some conventional sensors employ near-infra-red radiation sources, the use of light within the visible spectrum allows the sensors in this method to most accurately simulate human perception of visibility.

Conventional instruments normally perform measurements at a height of 2.5 metres above ground whereas shallow fog is often less than 2 metres deep so can exist on the runway undetected. This method automatically measures any obscuration of the runway lights.

Whereas conventional instruments must be taken out of service for regular calibration, this method allows simple visual calibration by manual observation along the runway and calibration can be completed at any time with no loss of serviceability.

The invention will be described by reference to the accompanying drawings: Figure 1: when a beam of light passes through the atmosphere some of the light is scattered out of the beam by atmospheric aerosol particles such as dust or water droplets.

The graph shows the angular distribution of scattered light corresponding to a visibility of one kilometre, taken from Dietze, 1957. (The synoptic definition of fog' is visibility less than one kilometre.) Figure 2: A schematic diagram showing the principle of operation of a transmissometer', an instrument which measures the transmittance of a light beam over a path through the atmosphere. T' is the light source, or transmitter, and R' is the light receiver. The distance between T and R, the baseline Tb', is usually around 15 to 20 metres (although sometimes a mirror is used in a folded-baseline' configuration, with T and R located in the same unit).

Figure 3: A schematic diagram showing the principle of operation of a forward scatter meter'. Again, T' is the light source and R' is the light receiver. There is no direct light path between T' and R' but light scattered from the beam emitted from T' is collected at R'. The distance between T' and R' is usually around one to two metres, with a scattering angle of around 30 to 50 degrees (the sample volume' of atmosphere is shown in black).

Figure 4: A schematic diagram showing the principle of operation of a backward-scatter meter'. Again T' is the light source and R' is the light receiver, with no direct optical path between them. Light scattered backwards' from the sample volume (shown in black) is collected by the receiver R'. The distance between T' and R! is usually small so that the scattering angle is close to 180 degrees (the distance T-R and scattering angle are exaggerated in the diagram for clarity).

Further details of transmissometers and scatter meters can be found in ICAO, 2005.

Figure 5: Isocandela contours for a runway centre line light (new light at maximum intensity setting) with 30 metre longitudinal spacing showing the position of pilot's eyes in the beam at various eye heights and distances, taken from ICAO, 2005.

Figure 6: Plan view of a typical runway light fitting, inset into the runway surface. The diameter is usually around 200 to 400mm. The apertures A' (shown cross-hatched) contain an optical system (prism(s) and/or lens(es)) which emit a light beam along the runway direction with an angular distribution similar to that shown in figure 5. The example shown provides single bi-directional beams although other configurations can produce one or more beams in one or both directions along the runway or taxiway.

Figure 7: A cross-section through the typical light fitting shown above in figure 6, along the line X-X in that figure. The line X-X in this figure also shows the level of the runway with most of the light fitting body sub-surface. A' again shows the optical system, in this case producing a beam of light directed towards the right of the page.

Figure 8: Shows a typical four-aperture inset runway light fitting modified so that the A's are the usual light-emitting sources but the B's are light receiving sensors.

Figure 9: A schematic diagram illustrating how the forward scatter' principle can be applied to runway lights. The light-beam source T' and the scattered-light receiver R' are both incorporated into adjacent lights (normally spaced 15 or 30 metres apart).

Figure 10: An illustration employing the forward scatter' principle of figure 9 extended to an array of runway lights. [1, [2, [3, etc are adjacent lights located along a runway.

Figure 11: A schematic diagram showing how the backward scatter' principle can be applied to runway lights. This uni-directional light is modified so that the aperture A' emits a light-beam along the runway (towards the right of the page) while the aperture B' contains a sensor to detect light scattered in a backward direction (the sample volume' is shaded black).

Figure 12 An illustration employing the backward scatter' principle of figure 11 extended to an array of runway lights (for this example the centre line lights are spaced at 15 metre intervals and the edge lights at 30 metres, with the size of the light fittings enlarged for clarity).

The brightness of runway lights can be controlled by pulse width modulation' of the electrical supply to the light emitter (for example, modulation of the voltage or current supplied to a light-emitting diode ([ED), gas-discharge or incandescent lamp). Figure 13 shows the light output as a function of time (both arbitrary units) for a high-intensity light setting (where the light is on' for most of the time), whereas figure 14 shows a low-intensity setting where the light is on' for a much shorter time than it is off'.

Figure 15: An illustration of the variation of scattered-light detector output with time (again arbitrary units) for runway light at high-intensity in poor visibility (e.g. fog). The level G' corresponds to the scattered light intensity when the light is on' (for backward scatter light) or when the adjacent light is on' (for forward scattered light) and the level H' corresponds to the background' light level when the appropriate light is off'.

Figure 16: Illustrates the detector output for high-intensity light setting when the visibility is much higher than in the case of figure 15. In this case, both the scattered-light peak intensity] and the background' light level K' are much lower (since the amount of light scattered, both forwards' and backwards', is much lower in good visibility).

Figures 17 and 18 correspond to figures 15 and 16, respectively, for the case of low-intensity light setting in poor and good visibility (respectively).

Runway Visual Range (RVR) is defined as the range over which a pilot can see runway lights or runway surface markings (ICAO, 2005). RVR is assessed by calculation based on Koschmeider's law or Allard's law taking into account prevailing conditions. Together with an instrument measurement of Meteorological Optical Range (NOR), calculations also require measurement or estimation of the background light level. Further details of assessment of RVR can be found in ICAO, 2005.

The intensity of a beam of light passing through the atmosphere reduces as the distance from the source increases, mainly due to scattering and absorption by aerosols. Scattering is the dominant process for fog, rain and snow (which are the most prevalent weather conditions causing reduced visibility), although absorption contributes in dust, haze and smoke.

Figure 1 shows the variation of scattered light intensity with angle from the original beam (in this example for a visibility of one kilometre; the synoptic definition of fog' is visibility less than one kilometre). It can be seen from the graph that a large proportion of the light is scattered forwards i.e. close to the original beam direction, the intensity of scattered light decreases to a minimum around 110-120 degrees from the beam then increases again towards 180 degrees. Light scattered through an angle less than 90 degrees is said to be forward scattered' whereas light scattered through more than 90 degrees is said to be backward scattered'.

Figure 2 illustrates the principle of operation of a transmissometer', an instrument which measures the transmittance of a light beam over a path through the atmosphere. T' is the light source, or transmitter, and R' is the light receiver. The distance between T and R, the baseline If, is usually around 15 to 20 metres (although sometimes a mirror is used in a folded-baseline' configuration). Measurement of the amount of light received at R compared with the intensity of the beam emitted from T enables calculation of meteorological optical range' (NOR) which is used in the calculation of RVR. Transmissometers are relatively expensive to install and operate and require very stable bases for the light transmitter and light receiver units (or reflector unit) to preserve the optical alignment.

Figure 3 illustrates the principle of operation of a forward scatter meter'. Again, 1' is the light source and R' is the light receiver. There is no direct light path between T' and R' but light scattered from the beam emitted from T' is collected at R'. The distance between T' and R' is usually around one to two metres, with a scattering angle of around 30 to 50 degrees (the sample volume' of atmosphere is shown in black). The amount of scattered light received at R varies inversely with visibility and its measurement enables calculation of MOR. Forward scatter meters (FSM5) are much less costly to install and maintain than transmissometers. They require a rigid, precision construction to preserve the optical alignment, although the whole instrument can be mounted on a single frangible pole.

Figure 4 illustrates the principle of operation of a backward-scatter meter'. Again T' is the light source and R' is the light receiver, with no direct optical path between them. Light scattered backwards' from the sample volume (shown in black) is collected by the receiver R'. The distance between T' and R' is usually small so that the scattering angle is close to degrees (the distance T-R and scattering angle are exaggerated in the diagram for clarity). Measurement of the amount of scattered light received at R enables calculation of MOR. Backward scatter meters (BSM5) also require rigid construction and can be mounted on a single frangible pole. The cost associated with the installation and operation of BSM5 is of the same order as for FSM5.

Transmissometers, FSM5 and BSM5 all require a stable, well-defined source beam of light and a stable light receiver with well-defined optical acceptance geometry. A background luminance sensor (BLS) is also required to quantify the background illumination level to enable calculation of RVR.

The essence of this patent application is that runway light fittings also provide a stable source of light with a precisely-defined beam pattern. Such fittings are mounted in physically stable bases with precision alignment to preserve the optical geometry of the light beams (and hence the guidance path provided to pilots approaching or departing from the runway).

The light scattered from the beams provided by runway lights can, using the operating principles of FSM5 and BSM5, be used to measure MOR and background luminance level.

The runway light fittings can provide physically stable bases for both the emitted light beams and the photo-sensitive receivers.

Figure 5 shows a diagram of isocandela contours for a runway centre line light (new light at maximum intensity setting) with 30 metre longitudinal spacing, also showing the position of a pilot's eyes in the beam at various eye heights and distances, taken from ICAO, 2005. The light beam is concentrated in a narrow cone' subtending a fairly small solid-angle and the intensity decreases rapidly with increasing angle from the beam-centre (which slopes upwards away from the source at an angle around 3.5 degrees above the runway surface).

Similar diagrams are used to define the beam pattern of runway edge lights which have a precise toe-in' angle relative to the runway direction (the angle varies with width of the runway in order to provide the correct guidance).

Although the following description is based on flush-fitting LED-type runway lights, the same principle could be adapted for other types of fittings e.g. elevated runway edge fittings, tungsten or halogen light units, etc. A plan view of a typical flush-fitting (or inset) runway light fitting is shown in figure 6 (diameter is usually around 200 to 400mm). The apertures A' (shown cross-hatched) contain an optical system (prism(s) and/or lens(es)) which emit a light beam along the runway direction with an angular distribution similar to that shown in figure 5. The example shown provides single beams along the runway in both directions. Other configurations are available which produce one or more beams in one or both directions along the runway or taxiway.

Figure 7 shows a cross-section through the typical light fitting shown in figure 6, along the line X-X in that figure. The line X-X in this figure also shows the level of the runway, the light fitting is inset into the runway surface with most of the body sub-surface and the top protruding around 12-25mm above the surface. A' again shows the optical system, in this case producing a beam of light directed towards the right of the page.

In order to measure light scattered from the main emitted beams, fittings can be modified to include one or more photo-sensor receivers. For example, figure 8 shows a typical four-aperture inset runway light fitting modified so that the apertures marked A' are the usual light-emitting sources but the apertures marked B' contain light receiving sensors.

Figure 9 is a schematic diagram illustrating how the forward scatter' principle can be applied to runway lights. The light-beam source T' and the scattered-light receiver R' are both incorporated into adjacent light fittings (normally spaced 15 or 30 metres apart). This idea is developed in figure 10 which shows how the forward scatter' principle of figure 9 can be extended to an array of runway lights. Li, L2, L3, etc are adjacent lights located along a runway (the lights can be either on the centreline or along the edge, or both can be used).

Each light produces a well-defined beam (directed upwards at a small angle, towards the right). Light scattered from the beams by aerosols is measured by photo-sensors located in adjacent fittings and the information can be used to assess RVR along the length of the runway.

Figure 11 shows a schematic diagram illustrating how the backward scatter' principle can be applied to runway lights. This uni-directional light is modified so that the aperture A' emits a light-beam along the runway (towards the right of the page) while the aperture B' contains a sensor to detect light scattered in a backward direction (the sample volume' is shaded black). This could be similarly incorporated into bi-directional light fittings. This idea is extended in figure 12 which indicates how the backward scatter' principle of figure 11 can be extended to an array of runway lights (for this example the centre line lights are spaced at 15 metre intervals and the edge lights at 30 metres, with the size of the light fittings enlarged for clarity).

The photo-sensor apertures denoted B' in figures 8 and 11 can employ an optical system to define the required scattered-light acceptance geometry for either forward-scattered or backward-scattered light or both. An electronic or mechanical solar shutter' can also be incorporated to protect sensors from direct sun.

The brightness of a runway light system can be varied in order to provide optimum visual guidance to pilots in various conditions of ambient light and visibility (see ICAO 2005 for further details). The light setting in use is normally expressed as a percentage of the maximum output of the light units employed.

The brightness of runway lights is controlled by modulation of the electrical supply (voltage and/or current) supplied to the light source. The example shown below and in the following figures is for LED light sources which are usually controlled by pulse width modulation' (PWM), although sinusoidal or thyristor waveforms could equally well be used e.g. for gas-discharge lamps, etc. Figures 13 and 14 show PWM-controlled LED light output for high and low intensity settings, respectively.

Figure 13 shows the light output as a function of time (both arbitrary units) for a high-intensity light setting (where the light is on' for most of the time), whereas figure 14 shows a low-intensity setting where the light is on' for a much shorter time than it is off'. The frequency of on/off switching is high enough that the human eye is unaware of the flashing' (due to persistence of vision) but the total light energy received by the eye is integrated over time and perceived as relative brightness.

Figures 15 to 18 illustrate the scattered-light detector response for high and low intensity sethngs in both good and poor visibility. The time axis in each diagram corresponds to the time axis of the light source in figures 13 and 14 (the detector response is virtually in phase with the PWM of the source) but the relative intensity axes are quite arbitrary (for illustration).

Figure 15 shows the variation of detector output with time for runway lights at high-intensity in poor visibility (e.g. fog). The level G' corresponds to the scattered light intensity when the light is on' (for backward scatter light) or when the adjacent light is on' (for forward scattered light) and the level H' corresponds to the background' light level when the appropriate light is off'.

Figure 16 illustrates the detector output for high-intensity light setting when the visibility is much higher than in the case of figure 15. In this case, both the scattered-light peak intensity] and the background' light level K' are much lower (since the amount of light scattered, both forwards' and backwards', is much lower in good visibility).

Figures 17 and 18 correspond to figures 15 and 16, respectively, for the case of low-intensity light setting in poor and good visibility (respectively). Again, the detected levels of scattered light (L and N) and of background light (M and P) are higher in conditions of poor visibility and lower in good visibility.

The levels of peak scattered light detected (G, J, L and N) can be used to estimate MOR whereas the levels of light detected during the light-off period (H, K, M and P) can be used to estimate the background luminance level. Both measurements can then be incorporated into the calculation of RVR.

The forward scatter and backward scatter geometries can be used independently or in combination.

The calculation algorithm can be further refined by incorporating information from one or more conventional present weather sensors' to allow for variation in the scattering profile of different aerosols e.g. mist, fog, rain, snow, dust, smoke, etc. As a general scientific principle, increasing the number of measurements made of a specific parameter reduces the error in the estimate of its value. Employing an array of runway lights (with appropriate sensors for the scattered light) means that a large number of measurements of MOR (and RVR) can be made in the runway environment. Although each measurement may be less precise that individual measurements made by a smaller number of conventional instruments, the overall assessment of MOR (and RVR) can be statistically more precise.

Also, by employing an array lights with sensors along the length of the runway, a more detailed estimation of the spatial and temporal variation of visibility along the runway can be achieved. Since the light sources are the actual runway lights, the effects of contamination, ageing of the lamps, etc are automatically included in the measurements.

Such an array of lights with sensors would also enable continuous monitoring of individual light performance and/or failure. The photo-sensors can be protected from direct sun by employing electronic or mechanical solar shutters.

Calibration of the system can be simply achieved by manual observation (or photographic/video estimation) of RVR from any point on the runway in comparison with that calculated from the scattering measurements (as above).

Assessment of RVR using the method described above can overcome many of the drawbacks associated with conventional sensors and measurements.

References AAIB, 2009, UK Air Accident Investigation Bureau Bulletin: 11/2009 G-VAIR Dietze, G., 1957: Einfuhrung in die Optik der Atmosphare. Leipzig, Akad. Verlagsgesellshaft, Geest U. Portig K.-G-, 263pp (from Vogt, H., 1968: Journal of the Atmospheric Sciences, Vol. 25, 912pp).

ICAO, 2005: International Civil Aviation Organisation, Doc 9328, Manual of Runway Visual Range Observing and Reporting Practices (Third Edition)'.

ICAO, 2010, Annex 3 to the Convention on International Civil Aviation Meteorological Service for International Air Navigation (Seventeenth Edition)'.

Claims (8)

1. Claims 1. A method to assess runway visual range (RVR) using the runway light fittings as light sources and photo-sensors also located on the runway to measure scattered light.
2. 2. The scattered light measurements performed as in claim 1 may use either forward-scatter or backward-scatter geometry or a combination of both.
3. 3. Measurements of scattered light obtained as in claim 1 at different times during the modulation cycle of the runway light electrical supply can be used to ascertain peak scattering and background luminance levels for use in the calculation of RVR.
4. 4. Use of a large number of light fittings and photo-sensors distributed along the runway for measurements as described in claim 1 statistically reduces errors in RVR assessment and provides information about its variation along the runway.
5. 5. Using runway light fittings as the light sources for scattering measurements as in claim 1 automatically compensates for factors affecting RVR such as variations in light output due to ageing, contamination, supply variation, etc.
6. 6. RVR assessments made using the method described in claim 1 can be enhanced by incorporating information from one or more conventional present weather sensors'.
7. 7. Using runway light fittings as the light sources for scattering measurements as in claim 1 allows monitoring of individual light fitting performance or failure.
8. 8. The method described in claim 1 for assessing RVR can be calibrated by comparison with manual (or photographic or video) assessments of RVR.Amendments to the claims have been filed as follows Claims 1. A method to assess runway visual range (RVR) comprising using the runway light fittings as light sources and using photo-sensors also located on the runway with the light fittings, to measure scattered light, the scattered light measurements being used in a calculation of RVR.2. A method as defined in claim 1 wherein the scattered light measurements may use either forward scatter or backward scatter geometry or a combination of both.3. A method as defined in claim 1 wherein measurements of scattered light obtained at different times during the modulation cycle of the runway light electrical supply are used to ascertain peak scattering and background luminance levels for use in the calculation of RVR.4. A method as defined in claim 1 wherein the use of a large number of light fittings and photo-sensors distributed along the runway for measurements of scattered light can be used to statistically reduce errors in RVR assessment and provide information about its variation along the runway.5. A method as defined in claim 1 wherein the use of runway light fittings as the light sources for scattered light measurements will automatically compensate for cf variations in runway light output due to ageing, lens contamination and electrical supply variation, which affect the accuracy of assessment of RVR. 6. A method as defined in claim 1 wherein the accuracy of PAIR assessment can be O enhanced by incorporating information from one or more conventional present weather sensors' into the calculation of RVR.7. A method as defined in claim 1 wherein the use of runway light fittings as the light 0 sources for scattered light measurements allows monitoring of individual light fitting performance or failure.8. A method as defined in claim 1 wherein the assessment of RVR can be calibrated by comparison with manual (or photographic or video) assessments of RVR.