Vaisala Transmissometer LT31
RVR with New Eyes
Accurate and reliable determination
of the Runway Visual Range (RVR)
reduces airport downtime and
improves safety of operations.
The Vaisala Transmissometer LT31
is the qualied observer of RVR
providing correct visibility data
without interruptions and with
minimal maintenance.
Measurement Range
from 10 to 10 000 m
The LT31 provides the most
accurate and reliable solution for the
automatic measurement of RVR. The
Meteorological Optical Range (MOR)
measurement range is from 10 to
10 000 meters, which covers the full
required RVR range (CATI ... CATIIIb),
as well as the range required for
Aeronautical Visibility (the ICAO
dened visibility). The accuracy of
the LT31 meets the ICAO and WMO
requirements.
This full measurement range is
achieved with a single baseline
system, which makes measuring
easier and more economical.
The Vaisala Transmissometer LT31 enables the accurate and reliable single baseline
measurement for CATIIIb category airports.
www.vaisala.com
White LED Provides Wide
Spectrum Light Source
The LT31 incorporates a white
LED as a light source. White light
is needed for the best accuracy
in transmittance measurement.
The WMO recommends the use
of a wide spectrum (white) light
sources (e.g. lasers or colored LEDs)
for transmissometers as narrow
spectrum light source will cause
measurement error with some
weather phenomena.
Features/Benefits
Single baseline for the
▪
MOR measurement range
10 ... 10 000 m
State-of-the-art white light
▪
source
Automatic alignment with
▪
quality control as a standard
feature
Automatic calibration as a
▪
standard
Internal back-up battery
▪
Compatible with Vaisala
▪
Transmissometer MITRAS and
SKOPOGRAPH II Flamingo
Window contamination
▪
compensation
Meets the ICAO and WMO
▪
requirements for RVR and
visibility
Based on decades of field
▪
experience
Sophisticated self-diagnostics
▪
Automatic Calibration with Integrated
Forward Scatter Sensor
Calibration of a transmissometer is traditionally
based on human observations. A reliable and accurate
calibration has required very high visibility, stable
conditions, and skilled and well trained personnel.
The Vaisala patented automatic calibration method for
transmissometers is based on an integrated forward
scatter sensor/present weather sensor. The system
automatically detects drift and adjusts the sensor
settings accordingly. Weather conditions do not need
to be as good as for manual calibration; the LT31
automatically recognizes suitable conditions.
Automatic Fine Alignment
One of the major error sources in transmittance
measurement is alignment drift. Checking and adjusting
the alignment has also required skilled and trained
personnel.
To maintain the measurement accuracy, the LT31
performs an automatic optimization of the alignment.
The alignment quality is also continuously evaluated
without any human intervention.
The integrated forward scatter sensor.
The automatic ne alignment also provides easy handling
of the LT31 during installation. The alignment quality
during harsh weather conditions is ensured by a double
mast construction. The outer tube works as wind and
solar radiation shield. The inner support structure is
separated from thermal and mechanical stress caused by
solar radiation and wind load.
Contamination Reduction
In general, precipitation leads to an increased amount of
window contamination. The LT31 has long and narrow
weather protection hoods that reduce the amount of
window contamination caused by precipitation.
For wind driven precipitation or dust, the instrument is
equipped with a powerful blower. The blower creates
an air curtain in front of the instrument window. The
air curtain is specially designed not to disturb the
measurement path and cause the measurement errors that
have been a problem with conventional blower designs.
Automatic Window Contamination
Compensation
Window contamination is a signicant source of error
in transmissometers. Maintaining high accuracy has
required frequent cleaning. However, the effects of
contamination can be automatically compensated if the
window transmittance can be measured accurately.
In the LT31, window contamination is also compensated
for by the most accurate method: by measuring the
transmittance directly through the window glass.
Extensive Self-diagnostics
The LT31 has a sophisticated self-diagnostics that
provides detailed status information for all functional
units. In addition, this feature helps to locate possible
technical failures. It records a history of signicant
operational situations, warnings, and alarms from the
instrument.
Internal Back-up Battery
The LT31 can be equipped with an internal back-up
battery. This feature provides steady data availability
during short power breaks, e.g. while back-up generators
are started.
True window contamination
compensation based on V-shaped
windows.
Airport runway lights at night.
Present Weather Reporting
The optional present weather reporting from RVR sites
gives a full picture of the prevailing weather conditions
within the entire airport area. The arrival of weather
fronts and the presence of local showers can easily
be monitored due to the multiple present weather
observation points. The integrated present weather sensor
provides the type and intensity of precipitation for METAR
and local weather reporting purposes.
Background Luminance Sensor LM21
The optional LM21 sensor offers the means for measuring
the ambient light level or background luminance in RVR
applications. The background luminance sensor is used
for measuring the background against which the runway
lights or runway markings are seen.
420
1022
2685
Technical Data
Performance
MEASUREMENT RANGE
Baseline (m) MOR range (m) Transmittance range
30 10 ... 10 000 <0.01 % ... 100 %
50 25 ... 10 000 <0.2 % ... 100 %
75 37.5 ... 10 000 <0.2 % ... 100 %
Transmittance measurement resolution: 20 bit
Accuracy according to ICAO and WMO specifications
for RVR and Visibility
Light source light emitting diode (LED), white
Window contamination
compensation
automatic, direct window
transmittance measurement
Calibration automatic with integrated forward scatter sensor
Alignment automatic fine alignment with servo mechanism
Environmental
Temperature range -40 ... +60 °C
Humidity 0 ... 100 %RH
Wind 60 m/s
EMC IEC/EN 61326-1
Electrical safety IEC/EN 60950-1
Data messages
LT31 standard, incl. full status report
MITRAS standard message (for replacement)
SKOPOGRAPH II Flamingo standard message (for replacement)
Present weather reporting (optional)
Identifies 7 different types of precipitation (rain, freezing rain,
drizzle, freezing drizzle, mixed rain/snow, snow, ice pellets)
Reports 49 codes from WMO4680 code table
Dimensions
Dimension in mm
depth 1022 mm
width 420 mm
height 2685 mm
Electrical
Power 100/115/230 VAC, ±10 %, 50-60 Hz
Power consumption max. 800 W with all options
Interfaces
data RS232/RS485/modem (optional)
maintenance RS232
Back-up battery (optional) typical back-up time 1 h at +20 °C
Obstruction light (optional)
Mechanical
Single baseline for full measurement range
Baseline length 25 ... 75 m (30 recommended)
Measurement height 2.5 m
Weight
Transmitter unit LTT111 85 kg
Receiver unit LTR111 82 kg
Window blower
For more information, visit
www.vaisala.com or contact
us at sales@vaisala.com
Easy installation
Installation of transmissometers has always been a time
consuming task, even for trained personnel. During raw
alignment of the instrument heads, the LT31 assists the
installation via a service terminal and by giving acoustical
guidance through beepers. The final alignment is carried out
fully automatically.
Ref. B210416EN-C ©Vaisala 2010
This material is subject to copyright protection, with all
copyrights retained by Vaisala and its individual partners. All
rights reserved. Any logos and/or product names are trademarks
of Vaisala or its individual partners. The reproduction, transfer,
distribution or storage of information contained in this brochure
in any form without the prior written consent of Vaisala is strictly
prohibited. All specifications — technical included — are subject
to change without notice.
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