Part Part 19 - Part 139 (Aerodromes) Manual of Standards 2019 - COMMUNICATION, NAVIGATION, SURVEILLANCE (CNS) AND METEOROLOGICAL (MET) FACILITIES | Zoe
CHAPTER 19 COMMUNICATION, NAVIGATION, SURVEILLANCE (CNS) AND METEOROLOGICAL (MET) FACILITIES
Note CNS and MET facilities at an aerodrome may include all or any of the following:
(a) Radio navigation facilities, including the following: ILS, DME, VOR, NDB, GBAS;
(b) surveillance sensor sites, including radar, automatic dependent surveillance – broadcast (ADS-B) and multi‑lateration systems;
(c) air-ground and point-to-point communications systems including radio bearer systems and satellite communications sites;
(d) air traffic services centres;
(e) ATC towers;
(f) meteorological facilities including transmissometers.
19.01 General
(1) An aerodrome operator must refer to the CNS or MET service provider, for a hazard and impact assessment, any application for development mentioned in sections 19.04 to 19.15 of this MOS that would be:
(a) within the aerodrome boundary; and
(b) near, or likely to affect, an existing CNS or MET facility.
Note 1 The assessment by the CNS or MET service provider will need to determine if the building restricted area for the relevant facility would be infringed by the development, and what hazards may result if the development application proceeds.
Note 2 CASA recommends that the aerodrome operator monitor, and advise the relevant CNS or MET service provider of, any application for development mentioned in sections 19.04 to 19.15 of this MOS that would be outside the aerodrome boundary, but near, or likely to affect, an existing CNS or MET facility.
(2) An aerodrome operator must consult the relevant CNS or MET service provider to ensure that:
(a) adequate provision is made in establishing restricted areas near the CNS or MET facility; and
(b) there is adequate control of vehicles and aircraft operations near the CNS or MET facility.
Note Control measures may include the use of markers or signage as agreed between the CNS or MET service provider and the aerodrome operator.
(3) An aerodrome operator must consult the relevant CNS service provider:
(a) to determine the distance that aerodrome roadways, runways, taxiways and any public roads provided by the aerodrome operator may be constructed from a radio navigation facility; and
(b) to ensure that unauthorised personnel and vehicles are kept clear of the facility; and
(c) to ensure as far as possible that grazing livestock are kept clear of the facility; and
(d) to ensure as far as possible that any suitable materials provided by the aerodrome operator are used in constructing the facility’s perimeter fencing and warning signs, bearing in mind that non-metallic materials are preferred.
(4) An aerodrome operator must consult the relevant surveillance sensor sites service provider:
(a) to determine the distance that aerodrome roadways, runways, taxiways and any public roads provided by the aerodrome operator may be constructed from the surveillance sensor sites; and
(b) to ensure as far as possible that unauthorised personnel and vehicles are kept clear of the facility; and
Note Control measures may include the use of markers or signage as agreed between the CNS service provider and the aerodrome operator.
(c) to ensure as far as possible that suitable materials provided by the aerodrome operator are used in constructing the site’s perimeter fencing and warning signs, bearing in mind that non-metallic materials are preferred.
(5) An aerodrome operator must not proceed with any development proposal mentioned in subsection (1) that is assessed by the CNS or MET service provider at the aerodrome as:
(a) not meeting the standards in this Chapter for the CNS or MET facility (as the case requires); or
(b) likely to create a hazard to aircraft; or
(c) likely to impact adversely on the operation of the CNS or MET facility;
unless:
(d) a safety case demonstrates how any hazard or adverse impact may be mitigated; and
(e) in light of the safety case, the applicable CNS or MET service provider agrees to the development proposal in writing, with or without conditions.
(6) If there is a line of sight requirement:
(a) for ATC facilities under Part 172 of CASR; or
(b) for ARFF facilities under Subpart 139.H of CASR;
then the aerodrome operator must actively preserve the required line of sight unless otherwise approved in writing by CASA.
19.02 Maintenance
(1) An aerodrome operator must document procedures for the maintenance of the areas around CNS and MET facilities on the aerodrome.
(2) The procedures for ground maintenance around the facilities must include consultation with the service provider to ensure that maintenance activities do not interfere with operation of the aid or facility.
(3) All ground maintenance carried out around CNS and MET facilities must be in accordance with the instructions provided by the CNS and MET service provider.
19.03 Installation requirements
Unless it is for air navigation purposes, equipment and installations must not be located in an area to which Chapter 6 or Chapter 7 applies.
CHAPTER 19
Division 2 CNS facilities
19.04 Protection of VOR facilities
Note There are 2 types of VOR, the Conventional VOR (CVOR) and the Doppler VOR (DVOR). The antenna for either type of VOR may be mounted directly on the ground or on an elevated counterpoise structure. Typically the DVOR type has 48 antennas equally spaced on the circumference of a circle approximately 13.5 m in diameter, and one antenna at the centre of the circle. The DVOR is less susceptible than the CVOR to scalloping effects caused by reflective objects or structures in its vicinity.
(1) For an elevated CVOR, an aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the CVOR:
(a) any proposal for development within 200 m of the CVOR antenna;
(b) any proposal for development between 200 m and 1 500 m from the centre of the CVOR antenna that exceeds an angle of elevation of 1.5° measured from ground level at the CVOR antenna.
(2) For a ground-mounted CVOR, an aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the CVOR:
(a) any proposal for development within 200 m of the CVOR antenna;
(b) any proposal for development between 200 m and 1 500 m from the centre of the CVOR antenna that exceeds an angle of elevation of 1.0° measured from ground level at the CVOR antenna.
(3) For an elevated DVOR, an aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the DVOR:
(a) any proposal for development within 100 m of the DVOR antenna;
(b) any proposal for development between 100 m and 1 500 m from the centre of the DVOR antenna that exceeds an angle of elevation of 2.0° measured from ground level at the centre of the DVOR antenna.
(4) For a ground-mounted DVOR, an aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the DVOR:
(a) any proposal for development within 150 m of the DVOR antenna;
(b) any proposal for development between 150 m and 1 500 m from the centre of the DVOR antenna that exceeds an angle of elevation of 1.5° measured from ground level at the centre of the DVOR antenna.
(5) If the type of VOR facility is not known, the aerodrome operator must contact the navigation service and facility provider responsible for the facility for further information.
19.05 Protection of DME facilities
(1) If a DME antenna is co-located with a VOR, localizer or glide path facility, the building restricted area of the co-located facilities must be considered as the DME building restricted area.
(2) For a DME that is not co-located as mentioned in subsection (1), an aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the DME:
(a) any proposal for development within 100 m of the DME antenna that would infringe a horizontal plane located 4 m below the centre of the DME antenna;
(b) any proposal for development between 100 m and 1 500 m from the DME antenna that would exceed an angle of elevation of 2.0° measured from the same horizontal plane beneath the DME antenna.
19.06 Protection of instrument landing systems (ILS)
(1) An aerodrome operator must consult with the relevant CNS service provider for an ILS to establish and define a building restricted area that ensures no component of the ILS is adversely affected by:
(a) electromagnetic interference; or
(b) the presence or construction of buildings; or
(c) the presence of temporary or permanent structures.
(2) An aerodrome operator must refer any development proposal that infringes the building restricted area of a localizer, a glide path or a localizer far field monitor for assessment by the responsible navigation service and facility provider for the ILS.
Note: 1. Electromagnetic interference (EMI) can be produced by a variety of sources including, for example, power lines, substations, electric welding, radio frequency (RF) welding, RF curing and some industrial-scientific-medical equipment.
Note: 2. Buildings and other structures can reflect ILS signals in unwanted directions, causing interference and distorting the information provided to aircraft.
Note: 3. For aerodrome planning, CASA recommends that aerodrome operators should consult the relevant navigation service and facility provider to ensure adequate provision is made for ILS installations and the necessary critical and sensitive areas.
(3) An aerodrome operator must consult with the relevant navigation service and facility provider to establish and define appropriate:
(a) ILS critical areas for each ILS installation; and
(b) ILS sensitive areas for CAT I, SA CAT I, CAT II, SA CAT II and CAT III ILS installations.
Note 1 The size and shape of a critical or sensitive area depends on the characteristics of the particular ILS system and the configuration of the particular environment.
Note 2 A critical area may be separately established for vehicles and aircraft of particular sizes.
(4) The aerodrome operator must liaise with the relevant navigation service and facility provider to ensure that the boundaries of each ILS critical area are marked by suitable signs and visual markers to prevent unauthorised access from vehicles and persons.
(5) The aerodrome operator must place signs at each road access point to an ILS critical area to warn drivers and pedestrians against entering the area without authority.
(6) The aerodrome operator must not permit:
(a) vehicles and plant to enter, or remain in, an ILS critical area while the ILS is in use; or
(b) construction access or variation to such access within an ILS critical area unless the access or variation has been coordinated with the relevant navigation service and facility provider.
(7) When vehicle access to an ILS critical area is required for a particular purpose, an aerodrome operator must:
(a) liaise with the navigation service and facility provider for the ILS to be temporarily withdrawn from service, unless otherwise authorised by ATC; and
(b) arrange for notification via ATC or via a NOTAM (as applicable) to inform pilots of the temporary withdrawal.
Note 1 Access to the ILS critical area would come under control where ATC is provided.
Note 2 An example of a particular purpose is grass cutting or other vegetation management activities.
(8) If low-visibility procedures are in effect at an aerodrome, the aerodrome operator must not permit vehicles or plant to enter, or remain in, an ILS critical area unless ATC has given the operator specific clearance for the vehicles or plant to enter or remain.
19.07 Protection of marker beacons
For marker beacons, an aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the beacons:
(a) any proposal for development within 5 m of the marker beacon antenna;
(b) any proposal for development between 5 m and 50 m from the marker beacon antenna that exceeds an angle of elevation of 50° measured from ground level at the base of the marker beacon antenna.
19.08 Protection of non-directional beacons (NDBs)
(1) For NDBs, an aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the NDB:
(a) any proposal for development within 60 m of the NDB antenna;
(b) any proposal for development between 60 m and 300 m from the centre of the NDB antenna that exceeds an angle of elevation of 5° from ground level at the centre of the NDB antenna.
Note Within 60 m of the centre of the NDB antenna, generally there should be no obstacles except for essential infrastructure such as the equipment building, antenna support towers and site fencing, and vegetation should be kept below a height of 0.6 m. Overhead low-voltage power lines and telephone lines serving the NDB should be kept at least 60 m from the centre of the NDB antenna. Power and telephone lines should be underground within 60 m of the centre of the NDB antenna.
(2) The following are not permitted over or on the earth mat area associated with the NDB:
(a) ploughing;
(b) livestock (other than sheep).
19.09 Protection of ground-based augmented systems (GBASs)
(1) For a GBAS, the aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the GBAS:
(a) any proposal for development within 200 m of the VDB antenna;
(b) any proposal for development between 200 m and 3 000 m from the VDB antenna that exceeds an angle of elevation of 0.9° measured from ground level at the base of the VDB antenna.
(2) For the RSMU antenna, the aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the antenna:
(a) any proposal for development within 155 m of the RSMU antenna;
(b) any proposal for development between 155 m and 3 000 m from the RSMU antenna that exceeds an angle of elevation of 3° measured from ground level at the base of the RSMU antenna.
19.10 Protection of radar sensor sites
(1) For any site within the aerodrome boundary that is used to monitor a primary, secondary or en route radar, the aerodrome operator must refer for assessment by the navigation service and facility provider any proposal for development that is:
(a) within 500 m of the site’s monitoring antenna, limited to a sector ± 45° either side of the line of sight to the radar; and
(b) 4 m or more above the height of the site monitoring antenna.
Note 1 All buildings and fences within and below the 4 000 m limit degrade the performance of the sensor. If possible, these obstructions should be avoided. Reflection minimisation techniques may be used to reduce the interference to a minimum, for example, RF traps, use of trees and shrubs, no fences tangent to the sensor.
Note 2 For further background details on the “area of interest” for radar monitoring sites, see Guideline G of the National Airport Safeguarding Framework, as in force or existing from time to time and freely available on the CASA website.
(2) For power lines and electrical transmissions within the aerodrome boundary which may cause interference with radar surveillance sensors, the following restrictions apply:
(a) all overhead power lines within 1 km of a radar surveillance sensor must be aligned radially from the radar or be located at least 10° below horizontal from the antenna;
(b) no radio interference emitters (for example, welders and electrical transmission lines) having any component of transmission in the radar frequency band are permitted within 2 km of a radar surveillance sensor;
(c) no overhead electrical transmission line with a line capacity mentioned in a row of column 1 of Table 19.10 (2) is permitted within the distance from a radar surveillance sensor mentioned in the same row in column 2.
Table 19.10 (2) Minimum overhead electrical transmission line distance from aerodrome boundary
Line capacity
Distance
2 kV – 22 kV
400 m
22 kV – 110 kV
1 km
Above 110 kV
2 km
19.11 Protection of wide area multilateration (WAM) and automatic dependent surveillance – broadcast (ADS-B) sensors
For WAM and ADS-B sensors, the aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the WAM or ADS-B sensor:
(a) any proposed development within a 100 m radius from the antenna base that would exceed the height (the mentioned height) that is 1 m below the sensor antenna base (the building restricted area critical zone) but excluding buildings, trees, power and telephone lines, and fences that do not project above the mentioned height;
(b) any proposed development between 100 m and 1 500 m of the sensor that would exceed an angle of elevation of 1° measured from the centre of the sensor (the building restricted area sensitive zone).
Note Large obstructions, for example, multi-storey buildings, steel bridges and wind turbines, are potential sources of interference to correct operation. For a new installation, CASA recommends that the installation should be kept at least 1 500 m clear of these types of structures.
19.12 Protection of VHF/UHF communication facilities
Note Reliable VHF/UHF communications require a clear line-of-sight path between the base station and aircraft and vehicles using the facilities. The construction of buildings or towers can prevent reliable communications. The Area of Interest for the VHF/UHF facilities includes all developments between 100 m and 2 000 m that exceed a height of 10 m above ground level at the base of the VHF/UHF antenna.
(1) For VHF/UHF communications facilities, the aerodrome operator must refer the following for assessment by the navigation service and facility provider responsible for the facilities as illustrated in Figure 19.12 (1):
(a) any proposed development, pole, antenna, welding, surveying, construction or crane operation within 100 m of the centre of the VHF/UHF antenna (the restricted area);
(b) any proposed development between 100 m and 600 m from the centre of the antenna that would exceed an angle of elevation of 1.1° from 10 m above ground level (measured at the centre of the antenna) and then extending out to a distance of 2 000 m from the centre of the antenna, including the area above the restricted area (the area of interest).
(2) If there is overlap between the restricted area and area of interest illustrated in Figure 19.12 (1), the restricted area takes precedence over the classification for all included areas.
(3) Within the area of interest, all developments, except those concerning light poles and crane operations, must be referred for assessment to the communications service and facility provider responsible for the system.
Shows VHF communication facility restricted area and area of interest.
Figure 19.12 (1) VHF facility restricted area and area of interest (illustrates matters)
19.13 Protection of HF communication facilities
Note For HF transmit antennas, to ensure that the HF antenna performance is not compromised, substantial structures are generally prohibited within the restricted area, however, simple vertical towers and masts of any height may be acceptable beyond 100 m from the centre of the antenna.
(1) For HF communications facilities, the aerodrome operator must refer the following for assessment by the communications service and facility provider responsible for the HF transmit antenna, as illustrated in Figure 19.13 (1):
(a) any proposed development, pole, antenna, welding, surveying, construction or crane operation within 100 m of the antenna (the restricted area);
(b) any proposed development between 100 m and 600 m from the centre of the antenna that would exceed an angle of elevation of 2.5° from 10 m above ground level at the centre of the antenna, including the area above the restricted area (the area of interest).
Shows HF communication transmit facility restricted area and area of interest.
Figure 19.13 (1) HF transmit facility restricted area and area of interest (illustrates matters)
(2) For an HF receive antenna, the aerodrome operator must refer the following for assessment by the communications service and facility provider responsible for the HF receive antenna, as illustrated in Figure 19.13 (2):
(a) any proposed development, pole, antenna, welding, surveying, construction or crane operation within 100 m of the antenna (the restricted area);
(b) any proposed development between 100 m and 600 m from the centre of the antenna that would exceed an angle of elevation of 2.5° from 10 m above ground level at the centre of the antenna, including the area above the restricted area (the area of interest).
Shows HF communication receive facility restricted area and area of interest.
Figure 19.13 (2) HF receive facility restricted area and area of interest (illustrates matters)
19.14 Protection of satellite ground stations (SGSs)
(1) An SGS antenna within an aerodrome boundary must have a clear line of sight to:
(a) the intended operating satellite on the geostationary arc; and
(b) an alternative satellite that may be used following a failure of, or to substitute for, the intended operating satellite.
(2) Unless otherwise advised by the SGS service and facility provider (the provider), the aerodrome operator must ensure that an SGS antenna is sited to allow alignment to any satellite located on the geostationary arc between 122 and 172 East longitude (corresponding to Asiasat4 and GE23).
Note This is the arc of potential interest to Airservices Australia and will result in an angle of elevation greater than 20° for any site in Australia.
(3) The provider must ensure that the minimum clearance, from the edge of the antenna to any proposed development referred for assessment under section 19.01, is as follows with reference to the bore sight of the antenna for all possible satellites, as shown in Figure 19.14 (3):
(a) 10° — for all electrically-passive items; and
(b) 30° — for any electrically-active items;
Note CASA recommends that minor obstructions such as guy wires should be avoided if possible but will usually have minimal impact on the performance of large antennas. Allowance should be made for aircraft flight paths which may intersect the antenna beam at close range. If possible, antennas should be located to ensure that aircraft do not cross through the bore sight when on the ground or at low altitudes.
(4) The aerodrome operator must refer any proposed development to the provider for assessment if the development would be:
(a) both:
(i) within 0 m and 30 m of the SGS; and
(ii) within ± 90° azimuth from True North centred on the SGS, regardless of the height; or
(b) all of the following:
(i) between 30 m and 150 m of the SGS;
(ii) within ± 90° azimuth from True North centred on the SGS;
(iii) greater than 10 m high when measured from the ground at the base of the SGS site.
Shows communications satellite ground station manned centre site.
Figure 19.14 (3) Communications satellite ground station manned centre site (shows matters)
19.15 Protection of microwave links
(1) Equipment microwave links associated with CNS within an aerodrome boundary must have clear line of sight to any object, structure or development which may then need to be referred for assessment, in order to transmit aviation information from remote stations back to ATC.
(2) The aerodrome operator must refer any proposed development to the microwave link system to the microwave service and facility provider for assessment if the development would be within 50 m of the line of sight mentioned in subsection (1).
(3) If the aerodrome operator does not know the location of microwave links, the operator must contact the system authority for information and guidance as to locations.
19.16 Siting requirements for CNS facilities
Note CASA recommends that for new and existing facilities, this section should be considered with the engineering, technical and instruction manuals associated with the required function and protection of the CNS facility.
(1) Subject to section 7.02, this section applies only in relation to the siting of CNS facilities within an aerodrome boundary.
(2) Unless required for air navigation or aircraft safety, equipment or an installation that could endanger an aircraft in the air or on the ground must not be located:
(a) on a runway strip; or
(b) in a runway end safety area; or
(c) on a taxiway strip; or
(d) within the distance specified in Row 2 in Table 6.53 (1)-2 as applicable for the relevant taxiway code letter; or
(e) in a clearway.
(3) Equipment or an installation required for air navigation or for aircraft safety, which is located on any of the following, must be frangible and mounted as low as possible to the surrounding ground level:
(a) that portion of a runway strip within:
(i) where the code number is 3 or 4 — 75 m of the runway centreline; or
(ii) where the code number is 1 or 2 — 45 m of the runway centreline;
(b) a runway end safety area, a taxiway strip, or within the distances specified in Table 6.53 (1)-2 as applicable for the relevant taxiway code letter;
(c) a clearway if it would otherwise endanger an aircraft in the air.
Note CASA recommends that any equipment or installation required for air navigation or aircraft safety which must be located on the non-graded portion of a runway strip should be regarded as an obstacle and should be frangible and mounted as low as possible.
(4) Unless its function requires it to be there for air navigation or aircraft safety, equipment or an installation must not be located within the following distances from a precision approach runway nominated as CAT I, II or III:
(a) 240 m of the end of the runway strip;
(b) where the code number is 3 or 4 — 60 m of the extended centreline;
(c) where the code number is 1 or 2 — 45 m of the extended centreline.
(5) Equipment or an installation required for air navigation or aircraft safety which is located on or near a strip of a precision approach runway nominated as CAT I, II or III must be frangible and mounted as low as possible if:
(a) for code number 4 and code letter F — it is situated on that portion of the strip within 77.5 m of the runway centreline; or
(b) it is:
(i) situated within 240 m of the end of the strip; and
(ii) it is situated within:
for code number 3 — 460 m of the extended runway centreline; or
(B) for code number 1 or 2 — 45 m of the extended runway centreline; or
(c) it penetrates any of the following:
(i) the inner approach surface;
(ii) the inner transitional surface;
(iii) the baulked landing surface.
CHAPTER 19
Division 3 Meteorological facilities
19.17 Protection of meteorological facilities
Note For background information, the following are set out in the MET section of the AIP, as in force or existing from time to time and freely available on the Airservices Australia website: the location and configuration of the Bureau of Meteorology weather information station sites, their phone numbers and VHF broadcast frequencies (if relevant). Instruments used to measure meteorological parameters are generally co‑located within an instrument enclosure which may contain a variety of instruments.
(1) This section applies only in relation to instrument enclosures within an aerodrome boundary.
(2) An aerodrome operator must refer any proposed development in or around a buffer zone or instrument enclosure to the provider of the meteorological facilities within the instrument enclosure for assessment of whether the development would adversely affect the operation of the facilities.
Note The buffer zones are described in section 19.18.
(3) Herbicides and weed sprays must not be used in the vicinity of an instrument enclosure if this would result in the natural vegetation within the enclosure being exposed to the herbicide or spray.
(4) For the buffer zone and the instrument enclosure, concrete or asphalt walkways:
(a) may be installed where the surface is likely to be rendered impassable or unsafe in wet conditions; and
(b) if installed, must be:
(i) no wider than 0.5 m; and
(ii) kept to a minimum consistent with reasonable accessibility to the instrument area.
(5) The instrument enclosure must not be artificially watered by a source located outside the enclosure.
19.18 Siting requirements for meteorological facilities
Note CASA recommends that for new and existing facilities, this section should be considered with the engineering, technical and instruction manuals associated with the required function and protection of the meteorological facility.
This section applies only in relation to the siting of meteorological facilities within an aerodrome boundary by the MET service provider as authorised under the regulations.
(2) The site of a standard instrument enclosure within an aerodrome boundary must:
(a) be at least a 16 m by 16 m square enclosure (instrument enclosure) in the middle of a 30 m by 30 m square buffer zone (buffer zone) aligned in the True North – South direction; and
(b) take account of the exposure requirements of the most sensitive instrument to be installed in the instrument enclosure and the purpose of the installation, as advised by the MET service provider.
(3) The instrument enclosure must be as follows:
(a) level;
(b) clearly defined;
(c) as densely covered with the natural vegetation typical of the surrounding area as is the surrounding area itself;
(d) maintained to a height of no more than 50 mm.
(4) The instrument enclosure must not be artificially watered.
(5) The outdoor instruments installed inside the instrument enclosure must be on a level piece of ground:
(a) whose surface is covered with short grass or is a surface representative of the locality; and
(b) surrounded by open fencing or palings to exclude unauthorised persons.
(6) The instrument enclosure area must not be top-dressed with any material that is inconsistent with the surrounding surface type.
(7) The buffer zone around an instrument enclosure must:
(a) be covered by the natural vegetation or ground cover of the region; and
(b) have the vegetation or ground cover maintained to a height of approximately 0.5 m.
(8) Any isolated obstruction under 15 m in height (for example, a mast, a post or a tree) must be at least 4 times the height of the obstruction distant from any point in the instrument enclosure.
(9) Subsection (7) also applies to the aerodrome weather station electronics unit, if installed.
(10) Subject to subsection (8), any obstruction that:
(a) is 15 m or more in height, or that is in the form of a shed or a similar built structure; and
(b) covers more than 45° of azimuth;
must be at least 10 times the height of the obstruction distant from any point in the instrument enclosure.
(11) Subsection (8) does not apply to an isolated mast which:
(a) is of a diameter or width so small that it does not obstruct direct solar radiation from falling on the instrument enclosure; and
(b) lies down-wind of the prevailing winds to mitigate the risk of producing rain shadows.