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8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 1/21
Visibility andVisibility Measurement
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 2/21
Visibility
Definition: “the greatest distance at which selected objectscan be seen and identified”
human observation
• depends on
• the objects (size, colour, shape, …)
• background of the objects (brightness, colour, …)
• the observer (eyesight, alertness, …)
• the atmosphere (weather)
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 3/21
Uses of visibility information
• Operational use (“how far would I be able to see”)
• traffic
• air
• road
• sea / lake / river
•
other• recreation (?)
• Air mass characteristics (“what is the optical state of the
atmosphere”)• synoptic meteorology
• climatology
• telecommunication...
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 4/21
Standards and recommendations
WMO:
• standardization of internationally distributed weather
information• observation methods, measurements (accuracy, range),
reporting
• synoptic meteorology, climatology, aviation (see ICAObelow)
ICAO:
• observing and reporting practices specific to the needsof international aviation
•
all weather message and observation matters coordinatedwith WMO, ICAO does not act as an expert on meteorology
• some practical aspects defined by ICAO only (measurementsites, frangibility, …)
• Runway Visual Range is specific to aviation, ICAO has an
active role in RVR recommendations
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 5/21
Standards and recommendations
FAA:
• standards for some aviation related weather
observations & equipment in the USA• airport automatic weather stations (AWOS), RVR in the USA
• has no official status internationally, but is influentialdue to the role of a forerunner (and general US influence)
NOTE:
• WMO and ICAO do not have any executive power, theyonly issue recommendations and standards
• national organizations are responsible for the implementation• local rules and specifications take precedence
• WMO & ICAO do not issue any approvals for instruments
• no international standards exist for any other
applications than meteorology and aviation!
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 6/21
Visibility observations
• WMO recommends observing large black (dark) objectsagainst sky, e.g. mountains or tall buildings
• when the objects and background are always similar theobservations will be more consistent
• At night the observed objects have to be light sources(lamps)
• e.g. flight warning lights of towers or tall buildings
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 7/21
Day visibility
• Visibility during day is limited by the contrast between the
object and the background• contrast decreases when the distance to the objects gets
longer
• think about a foggy day: if you try to see objects which
are quite far away, you will only see uniform gray fog -the contrast of the objects has decreased so low thatthey can not be distinguished from the fog
• The formula for contrast visibility is called Koschmieder’s
law
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 8/21
Night visibility
• Night visibility is limited by the intensity of the light whichreaches the eye of the observer
• light intensity is attenuated by the atmosphere (fog,snow, …)
• If the intensity is too low, the observer’s eye will not detectthe light
• eyes adapt to darkness, the detection threshold dependson the general level of illumination (i.e. how dark thenight is)
• The formula for the visibility of light sources is calledAllard´s law
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 9/21
Problems of visibility observations
• Application: operational visibility (“how far would I be ableto see?”):
•
the real experienced visibility depends on the observer,the target and the background• consider e.g. a car driver: the “official” visibility observations
are made by a another person in a different place with adifferent target
• Application: optical properties of the atmosphere
• observed visibility is subjective, while the optical state ofthe atmosphere should be an objective, physical
quantity• night visibility is not well defined (or the definition is not
generally accepted)
• visibility observations are dependent on background
luminance (night / twilight / day)
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 10/21
Solution (?): MOR
• Meteorological Optical Range (MOR) has been defined bythe WMO as the basic parameter to express the optical stateof the atmosphere
• MOR can (and should) be used instead of human visibilityobservations (WMO recommendation)
• MOR corresponds closely with human visibilityobservations (but only day observations)
• definition based on contrast visibility (Koschmieder’s law)
• The word “visibility” is often used when talking about MOR
• e.g. FD12 Visibility Meter is actually a MOR meter...
• MOR has been defined as a purely physical quantity
• objective, can be measured
• Other, application specific visibility values can be derivedfrom MOR
• e.g. RVR
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 11/21
Definition of MOR
• MOR can be measured by measuring the attenuation of light
• attenuation is mainly caused by scattering, to a small
degree also by absorption (in smoke, dust, …)
100 %
intensity
5 %
intensity
MOR
MOR is defined as the distance where the intensity of a lightbeam has been attenuated to 5% of the original intensity
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 12/21
Runway Visual Range
• ICAO definition:
• “The range over which the pilot of an aircraft on thecenter of the runway can see the runway surface
markings or the lights delineating the runway oridentifying its center line”
• how far the pilot can see the runway lights or the runwaysurface when the aircraft is about to touch down?
• RVR can be a human observation (at CAT I airports), butinstruments are strongly recommended
• instrument RVR (IRVR) is required in many countries at
CAT I airports as well• Annex 3 wording has changed into a clearly stronger
recommendation in the latest edition
• human RVR can not meet e.g. the requirements for
updating frequency nor consistency
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 13/21
RVR system
• RVR is either the visibility of the runway surface (contrastvisibility = MOR) or the visibility of the runway lights
• RVR system calculates both and reports the highervisibility value as RVR
• MOR is used as the runway surface visibility
• MOR (visibility) sensor is needed
• Runway light visibility depends on:
•
runway light intensity• attenuation of light in the air (MOR)
• illumination threshold = human eye detection threshold(depends on the darkness of the night)
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 14/21
RVR system
• Runway light intensities are stored in the RVR computer
• the values stored are the nominal (100%) values
•
the air traffic controller may use lower than 100%intensities, a special device (RSI) is needed to monitor thesetting in use
• Attenuation of light is measured by a MOR (visibility) sensor
•
transmissometer of forward scatter sensor• The illumination threshold is estimated from the darkness ofthe night, which is measured by a background luminancesensor
• RVR computer calculates the RVR from the measured values(MOR and background luminance) and the stored lightintensity table
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 15/21
Standard visibility measurements - 1
WMO - synoptic meteorology
• the reported value should be MOR (day and night)
•
reporting range in SYNOP: below 50 m to over 70 km• not always covered completely by instruments or human
observations
• accuracy:
• requirement: +/-50m below 500m, +/-10% above 500m• “typical instrument accuracy”: +/-10 - 20%
• measurement height not specified, should berepresentative
• e.g. human eye height
• other heights may be needed due to environmentalreasons, e.g. heavy snow conditions
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 16/21
Standard visibility measurements - 2
ICAO - airport observations
• the reported value should be MOR
•
range: below 50 m - over 10 km• accuracy:
• “desirable”: +/-50 m up to 600 m, +/-10% between 600 mand 1500 m, +/-20% above 1500 m
• “attainable”: +/-50 m up to 500 m, +/-10% between 500 mand 2000 m, +/-20% above 2000 m
• measurement height not specified, should berepresentative
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 17/21
Standard visibility measurements - 3
ICAO - RVR
• range: below 50 m - 1500 m
•
locally in some countries RVR is measured up to 2000 m• accuracy:
• “desirable”: +/-10 m up to 400 m, +/-25 m between 400 mand 800 m, +/-10% above 800 m
• “attainable”: +/-25 m up to 150 m, +/-50 m between 150 mand 500 m, +/-10% above 500 m and up to 2000m
• measurement height not exactly specified, should berepresentative of pilot’s visibility from 5 meters height
• 2.5 m height often used (pilot at 5 m is viewing runwaylights on ground)
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 18/21
Measuring MOR
• Transmissometer: a direct measurement of the attenuation
• Transmissometer estimates the distance where the intensitywould reach 5% of the original value
100%
intensity
? %
intensity
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 19/21
Measuring MOR
• Scatter sensor: an indirect measurement of the attenuation
• The scatter sensor measures a part of the scattered light andestimates the amount of all scattered light -> attenuation
• the estimate can be accurate, if the weather type is known
• the accuracy of a scatter sensor may depend on weathertype
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 20/21
Transmissometer versus forward scatter
TRANSMISSOMETER:
• measurement principle the same as human vision
•
accurate in all weather• easy to calibrate
• limited dynamic range (2 decades)
• fail-safe: loss of signal corresponds to low visibility
• higher cost: two units (Tx and Rx)
• more difficult installation (heavy foundation, alignment)
• typically needs more maintenance (MITRAS not
necessarily)• larger sample volume - more representative and safer
measurement
8/11/2019 Visibility Training
http://slidepdf.com/reader/full/visibility-training 21/21
Transmissometer versus forward scatter
FORWARD SCATTER:
• measures visibility indirectly
•
needs corrections for linearity & precipitation• needs chain of calibrators and standards
• critical to manufacturing and calibration repeatability andtolerances
• large measurement range (3...4 decades)
• fail-unsafe: loss of signal corresponds to high visibility
• lower cost: single unit
• easy installation
• less maintenance
• small sample volume - less representative measurement