NEW FORWARD SCATTER VISIBILITY SENSOR FOR RVR APPLICATION
Tero Kähkönen
Vaisala Oyj, P.O.Box 26, 00421 Helsinki, Finland
Tel. +358 9 8949 2318, fax + 358 9 8949 2568
e-mai: tero.kahkonen@vaisala.com
1. INTRODUCTION
Forward scatter visibility measurement technology
has established itself alongside transmissometer
technology in safety-critical Runway Visual Range
(RVR) applications at airports. The International
Civil Aviation Organization's (ICAO) revised RVR
manual (ICAO 2000) has provoked international
interest in RVR systems based on forward scatter
visibility sensors.
The new design of the Vaisala FS11 Visibility
Sensor is centered on reliability and safety factors
that affect visibility measurement in the airport
environment.
The guiding principle of the instrument’s design
was to follow ICAO RVR and Federal Aviation
Administration (FAA) specifications, while making
full use of Vaisala's long experience in the field of
visibility measurement. This paper will provide an
overview of the instrument’s design and
performance.
Fig. 1. New Vaisala FS11 Visibility Sensor
2. TECHNOLOGICAL ADVANCES
2.1 Window contamination compensation method
Vaisala has developed a new method that measures the attenuation effect of window contamination and
compensates for it. This method lengthens the periodic maintenance interval between window cleanings and
ensures that the measurement accuracy is maintained throughout the maintenance interval.
The new method monitors the reflective properties of the window surface by sending the light beam into the
glass material at a specific angle. With clean windows, strong reflections occur at the interface of glass and
air. Contamination particles on the interface decrease the reflection effect and cause the light beam to scatter
and lose signal strength. Figure 2 illustrates this method.
The new method provides more sensitive and reliable measurement of window contamination than traditional
backscatter monitoring. Automatic compensation corrects the visibility measurement errors caused by
window contamination. The visibility measurement accuracy is maintained throughout the cleaning interval.
Figure 3 illustrates the sensitivity of the circuitry by showing the relationship between the measured
contamination signal and the transmittance of the window.
To ensure that the optical path is clear also in front of the window surface and hoods as well, a second
monitoring circuit has been implemented. This circuit transmits a light beam outside the window surface and
monitors the amount of backscatter signal. Possible obstructions in the light path are detected in the form of
an increased backscatter signal.
Lens
Reflecting light
beam
Transmitter LED
100%
80%
60%
40%
20%
0%
0%
10
%
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%
30
%
40
%
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%
60
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%
10
0%
Window
Window transmittance
Detector
Normalized contamination measurement signal
Fig. 2. Optical configuration of window
contamination measurement circuitry
Fig. 3. Relationship between the measured
contamination signal and the transmittance of the
window
2.2 Reliable operation in harsh weather conditions
The FS11’s low maintenance requirement is a result of the new window contamination measurement and
compensation system, its weatherproof head-down design and high-power heaters. The mechanical optical
head design and head-down configuration provide very efficient protection against all windblown particles.
The optical surfaces are even protected from particles flying horizontally. The high-power heaters, with
individual temperature monitoring and control, ensure that snow does not accumulate on the optical heads
even during severe snowstorms.
The open mechanical design minimizes the shadowing effect on the sample volume. This ensures that wind
direction related errors do not appear in the visibility measurement results. Also, the measurement sample
volume has been moved away from the parts that generate heat to ensure correct results in calm wind
situations.
According the ICAO RVR manual, a forward scatter meter may underestimate RVR in rain by up to a factor of
two if the rain events are not identified and the underestimation is not taken into account. The optical and
electrical design of the FS11 ensures reliable individual rain droplet detection from the measurement signal.
The effect of rain droplets on the extinction coefficient is calculated separately. This provides accurate results
during rain events.
Fig. 4. The head-down design of the optical heads
Fig. 5. Optical head with high-capacity heating
element
2.3 Safety and reliability
To fulfill airport frangibility requirements, the FS11 is equipped with an approved frangible composite mast
structure that is designed to break when subjected to collision forces. The mast construction has been impact
tested and verified to show compliance with ICAO year 2005 design criteria for frangibility.
A built-in UPS system has been implemented to ensure reliable and uninterrupted operation during mains
power outages.. The sophisticated self-diagnostics and modular design allow for very short mean time to
repair (MTTR). The measurement fork and background luminance sensor are separate, intelligent
instruments with their own processors and parameter memories. Both the visibility sensor measurement unit
and the background luminance sensor can be replaced quickly as pre-calibrated spare parts without on-site
calibration. This means very short down time in case of trouble.
2.4 Scientifically valid chain of calibration
The accuracy of the FS11 is ensured by a two-point calibration procedure. The first point is a zero scatter
signal, which is produced by blocking the receiver completely. The second point is a high scatter signal,
which is produced with a reference scatterer.
The scattering response of the calibration device can be traced to a reference FS11 visibility sensor, which is
in continuous operation at the Vaisala outdoor test field along with reference transmissometers and other
instrumentation. This FS11 reference unit has been calibrated by verifying its output values against those of
the Vaisala reference transmissometer under various fog and precipitation conditions.
The measurement accuracy of the FS11 in the field depends on how well the calibrated response of the
reference FS11 can be transferred to the field devices. To define the best possible configuration for the
reference scatterers, Vaisala undertook a development program that included extensive study of possible
sources of error that could appear during annual field calibration. The study exploited optical design
simulation programs that allowed the researchers to quantify the variations between the design parameters
relating to calibrator devices. Based on this study, it was found that the most stable and repeatable calibration
result could be achieved with a calibration method whereby two opaque glass plates of the same size are
placed in parallel, at a distance that is relatively wide in comparison to their diameter. This calibration method
is the most tolerant of small mechanical differences in plate locations and angles. The figure 7 illustrates the
design of the reference scatterer device. It also shows the mask plates that are used to verify the correctness
of the measurement beam geometry.
Reference transmissometers
FS11 sensors
Test field
A set of FS11 sensors is
calibrated against reference
sensors
Calibrator
This calibration is
transferred to production
units with a calibration
device
Production units
Fig. 6. Calibration chain of the FS11 Visibility Sensor
Fig. 7. FS11 with reference scatterer device
3. TEST RESULTS
Performance and verification testing of the FS11 was carried out over 2001-2002. Comparison tests were
carried out against the reference transmissometer, which verified that the new Vaisala FS11 Visibility Sensor
fulfills ICAO and FAA requirements relating to the RVR application.
Figures 8 and 9 provide an overview of the FS11’s measurement performance. The ICAO RVR manual
recommends that the box plot format be used for test result presentation. Therefore, the results below are
expressed as box plots which show the distribution of ratios of meteorological optical range (MOR) between
the test and reference sensor for different bins of MOR. MOR values are in the y-axis. The MOR ratios
between the FS11 and the reference transmissometer are in the x-axis. Figure 8 illustrates the concordance
of the FS11’s measurement with the reference transmissometer. Figure 9 illustrates the degree of
measurement consistency between two FS11 units.
Fig 8. Example of a box plot diagram from
comparison testing of the FS11 against reference
transmissometer
Fig 9. Example of a box plot diagram from the
comparison testing of two FS11units
4. CONCLUSIONS
The new Vaisala FS11 Visibility Sensor fulfills FAA and ICAO requirements for the demanding and safetycritical RVR application. Technological advances in mechanical design, a new measurement and
compensation method for window contamination, and calibration consistency ensure correct visibility
information and high data availability even during harsh weather conditions. The FS11 is also accurate in
conditions of high visibility, up to a range of 75 km, making it suitable for aeronautical and SYNOP visibility
measurement purposes.
5. REFERENCES
Manual of Runway Visual Range observing and reporting practices, Second edition-2000, Doc 9328-AN/908,
International Civil Aviation Organization
Runway Visual range System, Federal Aviation Administration Specification, FAA-E.2772, Oct 11, 1996