Runway Visual Range
(RVR) is a term used in aviation meteorology to define the distance over
which a pilot of an aircraft on the centreline of the runway can see the
runway surface markings delineating the runway or identifying its centre
line. RVR is normally expressed in feet or metres.
RVR is used as one of the main criteria for minima on instrument
approaches, as in most cases a pilot must obtain visual reference of the
runway to land an aircraft. The maximum RVR reading is 2,000 metres or
6,000 feet, above which it is not significant and thus does not need to be
reported.
RVRs are provided in METARs and are transmitted by air traffic
controllers to aircraft making approaches to allow pilots to assess whether
it is prudent and legal to make an approach.
RVR is also the main criteria used to determine the
category of visual aids that are installed at an airport.
The International Civil Aviation Organisation ICAO
stipulates that for RVR values above 550 m, CAT I lighting
shall be installed, if RVR is between 300 m and 549 m then
CAT II lighting is required. CAT IIIa is installed for RVR
values between 175 m and 300 m. CAT IIIb is required for
RVR values between 50 m and 175 m while there is no
RVR limitation for CAT IIIc visual aids.[citation needed]
Originally RVR was measured by a person, either by
viewing the runway lights from the top of a vehicle parked
on the runway threshold, or by viewing special angled
runway lights from a tower at one side of the runway. The
number of lights visible could then be converted to a
distance to give the RVR. This is known as the human
observer method and can still be used as a fall-back.
Today most airports use Instrumented Runway Visual
Range or IRVR, which is measured by devices called
(forward scatter meters) which provide simplified
installation as they are integrated units and can be
installed as single unit(s) at a critical location along the
runway or transmissometers which are installed at one
side of a runway relatively close to its edge. Normally
three transmissometers are provided, one at each end
of the runway and one at the mid-point. In the US,
Forward Scatter RVRs are replacing transmissometers
at most airports. According to the US Federal Aviation
Administration: "There are approximately 279 RVR
systems in the NAS, of which 242 are forward scatter
NG RVR Systems and 34 are older Transmissometer
Systems."
A transmissometer is an instrument for measuring the extinction
coefficient of the atmosphere, and for the determination of visual
range. It operates by sending a narrow, collimated beam of energy
(usually a laser) through the propagation medium. A narrow field of
view receiver at the designated measurement distance determines
how much energy is arriving at the detector, and determines the
path transmission and/or extinction coefficient. Atmospheric
extenction is wavelength dependent phenomenon, but the most
common wavelength in use for transmissometers is 550 nm, which is
right in the middle of the visible waveband, and allows a good
approximation of visual range.
Transmissometers are sometimes referred to as telephotometers,
transmittance meters, orhazemeters.
The term transmissometer is also used
by oceanographers and limnologists to refer to a device for
measuring the optical properties of natural water. In this context, a
transmissometer measures thetransmittance or attenuation of
incident radiation from a light source with a select wavelength, often
660 nm, through a defined cell volume.
EMOR - Extended MOR Technology
Latest generation transmissometer technology
makes use of a co-located forward scatter visibility
sensor on the transmitter unit to allow for higher
accuracies over an Extended Meteorological Optical
Range or EMOR. After 10,000 meters the accuracy
of transmissometer technology drops off and it is at
higher visibilities that forward scatter visibility sensor
technology is more accurate. The co-location of the
two sensors allows for the most accurate technology
to be used when reporting on current visibility. The
forward scatter sensor also enables auto-alignment
and auto-calibration of the transmissometer device.
Unidades
Una candela se define como la intensidad
luminosa de una fuente de luz monocromatica de
540 THz que tiene una intesidad radiante de
1/683 vatios por estereorradián, o
aproximadamente 1.464 mW/sr. La frecuencia de
540 THz corresponde a una longitud de onda de
555 nm, que se corresponde con la luz verde
pálida cerca del límite de visión del ojo. Ya que
hay aproximadamente 12.6 estereorradianes en
una esfera, el flujo radiante total sería de
aproximadamente 18.40 mW, si la fuente
emitiese de forma uniforme en todas las
direcciones. Una vela corriente produce con poca
precisión una candela de intensidad luminosa.