-
 300 m ahead of the threshold for Touchdown zone RVR, 

-
 300 m before the runway end for Roll- out RVR and 

-
 1000 to 1500 m from threshold for Mid RVR.
RVR definition considers pilot’s eye to be at 15 ft height
above runway surface. The light path from runway markings
(at 0 ft) to the pilot’s eye (at 15 ft) has a mean height of 7.5 ft.
Hence, transmissometers are positioned at 6 to 10 ft height
above the runway.
The RVR Computation considers:

-
 the atmospheric transparency.

-
 the background luminance.

-the intensity of runway lights

6.8.3 Transmissometers.
The transmissometers consists of
-
 a projector which emits light pulses,

-
 a receiver having photo-detector that converts light pulses into electric pulses and amplifies them.

-
 a background luminance sensor

-
 a recorder

-
 an RVR computer which accepts receiver output, luminance sensor output and intensity of runway lights to compute the RVR

-a RVR display unit.

The projector, receiver and luminance sensor are located at runway site. RVR recorder is kept in Met section. The RVR computer and display units are housed in a cabinet and kept in the ATC tower.
The projector and the receiver are installed at runway site at a ‘base line’ distance of 150 m normally and can be reduced if needed. The lowest RVR given by transmissometers is normally twice the base line distance. 
In a Reflecting Transmissometer the projector and receiver are combined and the other element is a retro reflector. The two elements are placed at half the required distance called the folded base line.
6.8.4 Automatic Visual Range Assessor. (AVRA)
This is a system with features same as transmissometers, but in this receiver sends digital signals to a central processor in the Met Briefing Room where RVR and system status are displayed and recorded. RVR and status are also relayed for display in the ATC.
******* Chapter 7 - LOW VISIBILITY PROCEDURES (LVP)

7.1  Criteria for Implementing Safeguarding Procedures (SP)
Safeguarding Procedures shall be initiated when the RVR is less
than 1200m and visibility is forecast to deteriorate to 800m or
less.
The cloud ceiling is 400ft and forecast to fall to 200ft or less.

7.2  Criteria for Implementing Low Visibility Procedures (LVP)
Low Visibility Procedures shall be implemented when:
a)  either the Touch Down RVR or MID RVR is less than 800m, or,
b) the cloud ceiling is less than 200 ft; and
c)  SP have been completed and the airport is safeguarded.
7.3 Implementation of Safeguarding Procedures (SP) On the receipt of outlook for Low Visibility the Watch Supervisory Officer (WSO) will inform the Airport Manager, Apron Controller, Communication/Technical Supervisor and the Aerodrome Tower Controller. On receipt of the above information the above mentioned agencies will take action for proper planning for activation of LVP. When all the concerned agencies have completed their necessary actions they shall report to WSO (ATC) that their SP are completed and the airport is safeguarded for CAT II/IIIA operations.
　
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