Three techniques for observing runway visual range (RVR) are currently in use:
- a. Transmissometer
The transparency of the atmosphere is measured in terms of the transmission factor over a horizontal distance between a light projector and a photo-sensitive receiver. The transmission factor is converted to RVR taking into account the characteristics of the runway edge and centre line
(if available) lights, background luminance, etc., available to a pilot for guidance, and the expected sensitivity of his eyes in the prevailing conditions of ambient illumination (day, twi-light or night).
b. Human observer
An observer counts the number of runway lights or day markers he can see from an observing position near the runway and this number is converted to RVR, making due allowance for the differences in light intensity, background, etc., from the different viewing positions of the observer and a pilot. Sometimes, where it is difficult to count runway lights, observations are made on a special row of runway of other lights, set up near the runway.
A television camera near the runway is directed at the runway (or special) lights or markers and the observer monitors the television receiver in the observing station.
Because of the trend to automation of aerodrome observations, transmissometer systems have become the principal means of observing runway visual range and now outnumber the human observer and television systems.
It should be noted that the length of the baseline (the distance between the light projector and the sensor) sets limits on the range of RVRs that can be measured.
For CAT II, IIIA or IIIB operations the baseline must be very short. At Schiphol Airport transmissometers with baselines of 82 and 12 metres are installed.
The human observer and television techniques as primary means of RVR assessment, but may be used as substitutes, are used very little.
In this system a camera is mounted at a height of 5 metres above the runway level and at a distance of 75 metres from the runway edge. It is directed at a series of lights, identical to the runway lights and placed on a line approximately parallel to the runway. An observer in the meteo-station, using a special lights switching technique, observes the RVR on a monitor.
1. Horizontal visibility aspects
2. Visual range
3. Observing techniques of RVR
4. Observing of RVR
5. Availability of RVR observations at ATS units
6. Reporting procedures of RVR reports
7. Accuracy of RVR reports
8. Slant visual range
9. The visual segment
10. Variation in visual segment
11. Flight visibility and vision at high altitudes