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level or should slope away from the station (at a downgrade not exceeding 4 per cent) to a distance of at least 300 m (1 000 ft)
and preferably to 600 m (2 000 ft) from the station. The site contours should be circular with respect to the antenna array to a
radius of at least 300 m (1 000 ft). The site should be as far removed from wire lines and fences as possible. The height of
wire lines and fences should not subtend a vertical angle of more than 1.5 degrees or extend more than 0.5 degree above the
horizontal as measured from the antenna array. These limits may be increased by 50 per cent for fences or lines which are
essentially radial to the antenna array or which subtend a horizontal angle of no more than 10 degrees. Single trees of
moderate size, up to 9 m (30 ft) in height, may be tolerated beyond 150 m (500 ft). No groups of trees should subtend a
23/11/06 ATT C-44
Attachment C Annex 10 — Aeronautical Communications
vertical angle greater than 2 degrees or be situated within 300 m (1 000 ft) of the station. Provisions should be made for
clearing trees to 600 m (2 000 ft) if it should prove necessary. No structures should subtend a vertical angle greater than
1.2 degrees or be situated within 150 m (500 ft) of the station. Wooden structures with negligible metallic content and with
little prospect of future metallic additions may subtend vertical angles up to 2.5 degrees.
3.2.2 In mountainous terrain, a mountain-top site will often be preferable. The site should be on the highest accessible
hilltop or mountain, the top of which should be graded flat to a radius of at least 45 m (150 ft). On such sites, the antenna
system should be installed approximately a half wavelength above ground level in the centre of the graded area and the
transmitter building should be beyond the graded area, far enough down the slope to be below optical line of sight from the
antenna array. No ground, trees, power lines, buildings, etc. between 45 m (150 ft) and 360 m (1 200 ft) should be within
optical line of sight of the antenna array.
3.3 Determination of the effect of
“polarization error” on VOR accuracy
3.3.1 As it is not possible to specify as yet the maximum permissible value of the vertically polarized component of the
radiation from the VOR, certain flight tests are necessary to determine the effect on the bearing indication accuracy due to the
presence of the “polarization errors”.
3.3.2 Three methods are available to determine the effects of “polarization errors”:
a) 30-degree wing rock;
b) flying 8 tracks over a ground checkpoint;
c) flying a circle at 30-degree bank.
The first of these methods is designed to measure the polarization errors which occur when an aircraft rolls while flying a
given VOR radial. The second method measures the polarization error for eight different aircraft headings when the aircraft is
not banked. The third method measures the polarization errors, for all aircraft headings, with the aircraft banked at 30 degrees.
The flight tests are as follows:
3.3.3 30-degree wing rock. The aircraft is flown on a constant heading towards the VOR station and is banked slowly
from plus 30 degrees to minus 30 degrees. The course deviation indicator current is recorded and converted into degrees of
course displacement.
3.3.4 Eight tracks over a ground checkpoint. The aircraft is flown over a specific ground checkpoint on eight different
headings displaced by 45 degrees. The course deviation indicator current is recorded and the recording is marked when the
aircraft is over the checkpoint. The indicated bearing on each heading is compared with the indicated bearing when the
aircraft is heading towards the VOR station and is over the checkpoint.
3.3.5 Circular flight with 30-degree bank. The aircraft is first headed towards the VOR station over a ground
checkpoint. From this point, it is flown in a circle at constant 30-degree bank. The course deviation indicator current is
recorded while the aircraft is flying this circle and converted into degrees of error from the bearing indicated at the beginning
of the procedure when the aircraft is over the checkpoint. The change of bearing of the aircraft with respect to the VOR
station must be subtracted from the course deviation error. The resultant, after receiver error has been eliminated, is assumed
to be polarization error.
3.3.6 The polarization tests may conveniently be conducted at an altitude of 300 m (1 000 ft). Flight tests in 3.3.4 and
3.3.5 may be employed with respect to a checkpoint which is approximately 33.4 km (18 NM) from the VOR.
ATT C-45 23/11/06
Annex 10 — Aeronautical Communications Volume I
3.4 Criteria for geographical separation of VOR type facilities
3.4.1 In using the figures listed in Table C-3, it must be noted that these are derived from the agreed formulae in
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