 Runway Visual Range (RVR) Systems
Description
The AWI Runway Visual Range (RVR) system combines a Model 8364-E Dual Technology Visibility Sensor with a Model M488104 Ambient
Light Sensor (ALS) to generate RVR parameters based on high accuracy extinction
coefficient and ambient light data.
Visibility Sensor
The FSVS measures the amount of light scattered by atmospheric particles
of fog, dust, or precipitation. The sensor provides a highly accurate measurement
of extinction coefficient from 0.15 to 600 miles-1. Extinction
coefficients of RVR interest are 2.6 to 600 miles-1. Unlike other
visibility sensors, the Model 8364 uses a unique dual sensor monitoring technique
that measures direct and scattered optical energy. Because the visibility
sensor does not depend upon absolute measurements, measurements are independent
of the effects of the environment, thereby maximizing accuracy, reducing recurring
calibration, and minimizing maintenance requirements.
NOTE: The ICAO (International Civil Aviation Organization) has determined
that Forward Scatter Visibility Sensors can be used for international RVR
applications instead of Transmissometers. Read
ICAO's RVR acceptance and independent test results.
FSVS Features Provide Reliable RVR Calculations
The FSVS consists of four sensor heads: two infrared emitters and two solid-state
photodetectors. The detectors measure the light transmitted directly through
the sample volume between the sensors, as well as the light scattered by particles
in the sample volume. Scattered light is measured at a preset detection angle
that provides the most linear intensity for particles of fog, dust, rain,
and snow.
The measuring process of the sensor cancels several variables during calculation
and ensures that the visibility measurement is not affected by contaminants
on the lenses, or by temperature effects on the emitters and electronics.
An optical filter allows the detectors to respond to only a narrow band of
light to ensure that the input measurement and the output signal are unaffected
by background light or light sources in the detectors' field of vision.
The four-head configuration of the FSVS allows the sensor to continue operating
with only three sensor heads if one of the heads should fail. Built-in test
(BIT) functions report the sensor head failure so that appropriate maintenance
can be scheduled. The BIT functions also monitor power supply voltages, heater
status, and indications of abnormal operation.
The FSVS's internal microprocessor functions as a control unit, directing
the operation of the sensor as well as performing the required calculations.
The data collected is an average value, with averaging intervals of 1, 3,
5, or 10 minutes. The control unit also performs the BIT functions, which
are run as a regular part of sensor operation.
Using the QMet Handheld Maintenance Terminal or a laptop computer with terminal
emulation software, the operator can select the output interval, and specify
the output format (extinction, coefficient, or visibility in miles or kilometers).
The FSVS has been specifically designed for reliable operation in harsh environments.
Heaters keep the lenses free of rain, snow, and ice buildup. The operating
temperature range is -55° to +55° C. Maintenance is limited to cleaning
the emitter and detector lenses, inspecting for damage, and performing calibration
checks.
Optional battery backup is available to provide continued operation of the
sensor in case of AC failure. This emergency backup does not power the sensor
heaters, but it will allow the FSVS to continue collecting data for up to
four hours.
The FSVS has demonstrated performance surpassing the requirements established
by the FAA for Automated
Weather Observing Systems (AWOS).
Ambient Light Sensor
The Model M488104 Ambient Light Sensor (ALS) plugs directly into the FSVS's
control unit, which provides power and signal processing for the sensor. The
ALS measures the luminance of a 6° field of view of the north horizon
sky at an elevation of 6°, and provides a highly accurate measurement
of ambient light from 2 to 40,000 candelas per square meter. The sensor's
color response curve follows that of the human eye.
The ALS comes standard with sensor body and hood heaters to allow operation
in snow and icing conditions.
ALS Features Ensure Accuracy of RVR Calculations
Accurate measurement of luminance under low ambient light conditions is critical
for Runway Visual Range (RVR) calculations. To ensure accuracy, the ALS periodically
performs an autozeroing of offset, which greatly reduces measurement errors
due to offset drift. Offset errors in systems that are not autozeroing may
become significant under low luminance measurement conditions.
The ALS is designed for installation at airports where aircraft
exhaust, dirt, and other debris can find their way to the optical
window and reduce light transmission. Without correction, reduced ambient
measurements will be reported by the ALS. The ALS corrects for this by using
a narrow-beam, high intensity green reference LED outside the ALS window and
in the sensor's field of view to provide a test signal for window contamination
monitoring and relative calibration. The relative calibration feature ensures
reliable performance even with surface contamination on the ALS's optical
window.
As with the Model 8364 FSVS, the Model M488104 has been specifically designed
for reliable operation in harsh environments. Heaters keep the optical window
free of rain, snow, and ice buildup in the operating temperature range of
-55° to +55° C. Maintenance is limited to cleaning of the optical
window.
Runway Light Setting Interface for Nighttime RVR
The Runway Light Setting Interface is used to calculate nighttime Runway
Visual Range (RVR). It consists of an interface to the runway lights setting
switches that detects the runway light intensity and communicates it to the
RVR computer.
RVR Computer
The RVR computer calculates average visibility and RVR using Allard's and
Koschmeider's laws. The computer can interface with from 1 to 18 RVR sensors.
AWI can also design custom systems; consult AWI for details. The RVR computer
is available as a standard PC or as an embedded microcontroller.
Displays for RVR Data
Several types of displays are available for viewing RVR data, including PC,
Flat Panel, EL, and LCD. For other display types, consult AWI.
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