In the past ICAO Annex 3 SARPS recommended that a ceilometer be placed at the middle marker site 900-1200 m from the touch down zone for instrumented runways .

This had an advantage that power and comms were generally  already established at that site or were planned at that site.

This gave a reading of cloud base height at a crucial decision height on the glidepath

With addition of ILS and co-located DME, more and more aerodromes have no middle marker. The piece of land located 900 to 1200 m from the landing threshold may be outside the aerodrome airfield and a ceilometer installation may be impracticable, or very costly.

As a consequence in more and more cases an alternative location must be found.

A typical recommendation for siting the ceilometer would be :

“When instrumented systems are used for the measurement of the cloud amount and the height of cloud base, representative observations should be obtained by the use of sensors appropriately sited. For local routine and special reports, in the case of aerodromes with precision approach runways, sensors for cloud amount and height of cloud base should be sited to give the best practicable indications of the height of cloud base and cloud amount at the runway threshold in use. For that purpose, a sensor should be installed at a distance less than 500 m from the threshold. This distance can be extended up to 900-1200 m from the landing threshold in the axis of the approach end of the runway”

http://www.icao.int/safety/meteorology/amofsg/AMOFSG%20Meeting%20Material/AMOFSG.10.SN.012.5.en.pdf

The user should refer to the latest edition of Annex 3 to ensure compliance.