THE NEXT GENERATION IN VISIBILITY SENSORS OUTPERFORM BOTH TRADITIONAL TRANSMISSOMETERS AND FORWARD SCATTER SENSORS Steve Glander: Senior Sales Engineer All Weather, Inc. 1165 National Dr. Sacramento, CA USA 95834 sglander@allweatherinc.com Abstract The purpose of this paper is to introduce the next generation visibility sensor that combines the performance advantages from both a transmissometer and also from a forward scatter sensor while still retaining the reliability and ease of maintenance from a single mountable device. Experience shows that using the measurement of light from a direct transmission transmissometer over short distances, up to 2000 meters, gives a more accurate result than the use of a forward scatter measurement. However, above 2000 meters the differences in accuracy decrease until the forward scatter measurement becomes more accurate than direct transmission. The Dual Technology sensor from AWI uses both direct transmission and forward scatter to give greater accuracy during all around visibility conditions. FORWARD SCATTER, FOUR HEADS WITH DUAL TECHNOLOGY Verses TWO HEADS Better detects real-world visibility conditions 2 heads can measure visibility for: fog snow rain haze (depends) 4 heads with dual technology can measure visibility for: fog snow rain haze smoke dust sand
Why? With a forward-scatter sensor the scattered signal is assumed to be proportional to the extinction coefficient. The validity of the estimate depends upon the physical properties of the particles in the light path. Those physical properties are; a) Particle density b) Particle scatter function (i.e. the angular distribution of scattered light) c) Particle absorption Particle density. Since both the forward-scatter sensor signal and the extinction coefficient are proportional to the particle density, variations in particle density cannot affect the validity of the forward-scatter sensor measurement. Particle scatter function. The response of a forward-scatter sensor depends upon the fraction of light scattered into the range of angles detected. Since particles of different type have different scatter functions, the ratio of scattered signal to extinction coefficient can depend upon the type of scattering particles. One way of addressing the problem is to select a scattering angle where scatter function is as closely proportional as possible to the extinction coefficient for the weather phenomena you are concerned about. Therefore a fixed angle between the emitter and detector cannot adequately measure all types of particles. Particle absorption. A forward-scatter sensor cannot detect particle absorption. To better understand why a 2 headed design can only measure fog, snow and rain visibility degradation and not haze, smoke, dust, and sand visibility a more thorough explanation of scattering properties is needed. Fog. Fog has a relatively narrow scattering peak in the forward direction, very little scatter at 90 degrees, more scatter at 180 degrees and small maxima at the rainbows angles. The amount of scattering in the range 30 to 50 degrees is roughly independent of the drop-size distribution and, therefore, forward-scatter sensors operating at an angle in this range have provided the most consistent performance. Snow. Snow has a much more slowly varying scatter function than fog; light scatters more uniformly at all angles. At an angle of approximately 40 degrees, fog and snow have the same ratio of scattering to extinction coefficient. Rain. Rain has a even narrower forward-scatter peak than fog. The peak is so narrow that it may not significantly affect human vision and may not be detected by a transmissometer. Consequently, a forward-scatter sensor may underestimate the visibility by rain up to a factor of two relative to a transmissometer. Since rain that is not mixed with fog is rarely heavy enough to reduce the visibility substantially, this issue has not received much attention in the design of forward-scatter sensors.
Small aerosol particles (haze having particle size less than the wavelength of light). The scatter function for particles with diameter less than the wavelength of light varies significantly with wavelength, but varies much less with angle than for larger particles. The difference results in greater scattering relative to the extinction coefficient at the angles used for forward-scatter sensors. Some of this difference may be compensated for by the absorption that may be produced by such phenomena. Thus, the proportionality between scattered light and extinction coefficient will be different from that of fog and will depend upon the wavelength selected for the measurement. The wavelength and scatter function effects result in approximately equal haze and fog forward-scatter sensor calibrations for human vision (centered in the green) if red light is used for the instrument. Absorbing particles (smoke, dust, and sand having particle size greater than the wavelength of light). Since a forward-scatter sensor cannot measure absorption, the resulting measurement is an overestimate of visibility Therefore, the relationship of a measurement by a 2 head forward-scatter sensor to the extinction coefficient depends on the nature of the phenomenon reducing the visibility. Not so with the AWI Dual Technology 4 headed design. With four heads the sensor actually performs two direct and two scatter measurements for each reported visibility measurement. Particles that absorb and do not scatter light are detected by the direct transmission measurement making it possible to measure visibility during haze, smoke, dust, and sand conditions. In effect the 4 headed design is both a short baseline transmissometer and a forward scatter sensor. Window contamination correction Any visibility sensor must be designed to fully compensate for the errors introduced into the measurement process by environmentally induced factors. These factors include dirt deposited onto the sensor windows, buildup of residues from exhaust from jets or automobiles, blowing snow or rain, blockages in the optical path due to spider webs or other insect phenomena, blockages due to blown leaves or grass, and condensation beyond the ability of the heaters to remove. Some of these factors are temporary, such as blowing snow, rain or condensation. However, the remaining factors continually build up between cleaning cycles. Of special note is mud and dirt blown into the air by jet exhausts and deposited on sensor window surfaces. This problem is exacerbated by wide body aircraft with high bypass turbo fan engines and by periods of rain. It has been reported that Transmissometers currently
installed at major airfields suffer this contamination problem and require constant cleaning to alleviate it. The same contamination will affect forward-scatter sensors A 2 headed forward-scatter sensor with horizontal optics cannot meet the required FAA 90- day maintenance cycle for window cleaning (less than 10 percent window loss in 90 days). A second related problem is how to detect when a sensor window is clogged with snow and therefore blocks the detection of scattered light; a clogged sensor will report good visibility when the visibility may be bad. By redesigning the 2 headed sensor from horizontal optics to the 45 degree look-down geometry, many of the its environmental issues have been improved. Blowing snow is less likely to clog the windows Dirt is less likely to contaminate the windows Any heat generated by sensor heads or hoods is above the scatter volume and will therefore not affect the fog in the scatter volume Today 2 headed designs generally use internal scattering measurements to detect window contamination. The receiver window is illuminated at a 45 degree angle with an additional LED while the transmitter window is viewed at a 45 degree angle with an additional photodiode. This approach has two limitations; 1) it cannot detect snow clogging away from the window, and 2) it generates some interesting problems when water droplets are on the windows. With the 4 headed design using two transmitters and two receivers each receiver looks directly at one transmitter and sees scattered light from the other transmitter. The transmitters are operated alternately to give direct and scattered signals from each receiver. It the four signals are combined, the resulting extinction coefficient becomes independent of all sources of drift, including window contamination. Not only can it determine presence; it can determine the absolute degrading effect of the contamination on the measurement of light. Once this level is determined, a compensation factor is added into the algorithm and a true relative visibility is reported. The 4 headed design reliably detects snow clogging; if only one of the two transmitter or receiver heads is clogged then it can still operate in a reduced reliability mode equivalent to a normal 2 headed design. The 4 headed design is also much more reliable than a conventional sensor. Should one of the sensor heads fail, in a conventional two headed sensor, the system would be inoperative. In the 4 headed sensor if one head fails the other three will continue to operate with no degradation in accuracy. Any visibility sensor that doesn t compensate for the environmental factors cannot, reliably, produce visibility measurements without constant inspection and cleaning of the sensor windows and optical path. Data is available, which indicates that cleaning the windows of an uncompensated sensor will have to occur at a far greater frequency that the ninety days usually recommended. Even with this constant cleaning, there is no guarantee that an uncompensated sensor is reading correctly from one moment to the next. Environmentally induced degradation can occur at any time, including right after cleaning.
Mere detection of contaminated windows is not sufficient to achieve satisfactory sensor performance for several reasons. First, each detection will cause and unscheduled maintenance action. Only one such occurrence a year effectively reduces the MTBF to 8000 hours, considerably less that should be expected by a modern sensor. Second, dirt and jet fuel buildup on a window will cause an uncompensated sensor to be reporting in error beyond safety limits, even though the threshold of the window contamination sensor has not been exceeded. FOUR HEADS Verses TRANSMISSOMETERS Experience shows that using the measurement of direct transmission of light over short distances, up to 2000 meters, gives a more accurate result than the use of forward scatter measurement. However, above 2000 meters the differences in accuracy decrease until the forward scatter measurement becomes more accurate than direct transmission. This phenomena is due to either solid particle obstruction or the size of the moisture droplets causing the degradation in visibility. As explained above typical forward scatter sensors do not have the ability to operate with solid particle obstructions as they do not scatter light, but absorb it. Equally true is that when the obstruction is due to either large moisture droplets or smaller but denser droplets forward scatter sensors are overwhelmed by the excessive scatter. In these circumstances the measurement of direct transmission of light is more accurate. In circumstances where the obstruction is due to a less dense moisture droplet the forward scatter sensor is more accurate as the direct transmission of light fails to detect any significant obstruction. It is therefore true to state that Transmissometers are more accurate at lower visibility measurement or when solid particles, such as sand or dust are present and forward scatter sensors are more accurate at greater visibility measurement. As proof of this forward scatter sensors are regularly tested against reference transmissometers as part of their calibration and certification requirements, however, equally transmissometers are also tested against reference forward scatter sensors. As explained earlier the Dual Technology sensor from AWI uses both direct transmission and forward scatter, and therefore utilizes the advantages of both methods. Even more so when it is considered that each cycle of measurements contains two direct measurements and two scatter measurements. Another significant problem with Transmissometers is one of maintenance and alignment. In order to ensure the reliability of measurement over a distance of 50 or 75 meters, it is necessary to ensure that both the transmitter and the receiver are accurately mounted and that the windows are kept clean. It is also necessary to ensure that the transmitter lamp output is regularly checked to ensure it is not degraded. A decrease in light output can result in a false reading.
With the Dual Technology four head system, the whole system is mounted on a single pole that eliminates the problems of divergence between the transmitter and receiver. The system also does a continuous check on the state of the windows and the light output. As the system does two direct and two scatter measurements it is a simple matter to ensure that any decrease in light output is compensated for automatically to ensure the longest continuous operation without the need for human intervention. Conclusion Safety of flight demands that the most accurate and highest performing visibility sensor be used at airports. By use of a unique dual beam technique that reduces the problem from a critical absolute measurement to a self-adjusting relative measurement you can expect the sensor to: better detect real-world visibility conditions, and provide higher confidence in the reported measurements over all ranges of visibility About the product The All Weather, Inc Dual Technology Visibility Sensor is a unique, patented, device that has two emitters and two detector heads. It is based on a FSVS that has been used on the FAA AWOS, AWSS, Canadian RVR, non-fed AWOS and international AWOS. The extinction coefficient is measured with a dual beam VS at a pre-determined scatter angle. The scatter angle chosen results in excellent repeatability for low visibility fog and snow events when compared to transmissometers. The dual emitters and receivers provide immunity to optical contamination and result in a much more robust sensor than is possible with a traditional two-head design. The FSVS measures the optical clarity of the atmosphere and reports the optical extinction coefficient. An infrared diode illuminates the sample volume with amplitude-modulated, narrow-band, optical radiation centered at a wavelength of 865-nanometer. Optical energy scattered by interaction with particles in the sample volume is measured at the relevant scatter angle. The angle is selected because it provides linear scattered signal amplitude for the particle size and distribution pattern for the visibility conditions of interest, fog and snow. The direct transmission path is used to measure the degradation of light caused by solid particles, such as dust, smoke and sand. Solid-state silicon photo detector measures optical energy scattered from the sample volume. An optical interference filter limits the photo detector to a narrow band of energy centered at the 865-nanometer wavelength. Signal filtering after the photo detector identifies signals that are in phase with, and at the same modulation frequency as, the optical source. This synchronous lock-in detection technique provides an output signal proportional to the scattered optical energy and is unaffected by background light or noise created by other optical sources in the field of view of the photo detector. The output signal is a voltage relative to the extinction coefficient of the sample volume.