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25 kHz channel spacing environment, an effective adjacent
channel rejection characteristic of 60 dB or better is assumed.
This assumption will result in a geographical separation
distance D between VHF VOLMET ground transmitters
derived as follows (nautical miles may be substituted for
kilometres):
D = (dl + d2) km
where
dl = distance between aircraft and wanted ground station
= radio horizon + 27.8 km (15 NM)
and
d2 = distance between aircraft and unwanted ground
station
= 24.1 km (13 NM).
(See Figure A-7.)
6.2 Where it is necessary to take account, on a regional
basis, of receivers not specifically designed for 25 kHz channel
spacing and used in a 25 kHz channel spacing environment, an
effective adjacent channel rejection characteristic of the
receiver of the order of 40 dB is assumed. This assumption
will result in a minimum geographical separation distance D
between VHF VOLMET ground transmitters derived as
follows:
ATT A-7 111 1/01
Annex I0 - Aeronautical Telecommunications Volume V
D = (dl + d2) km
where
dl = distance between aircraft and wanted ground station
= radio horizon + 27.8 km (15 NM)
d2 = distance between aircraft and unwanted ground
station
= 240.9 km (130 NM).
6.3 Application of the above criteria in the case of aircraft
altitudes of 13 500 m (45 000 ft) and 20 000 m (66 000 ft)
results in the following separation distances:
Receiver dl d2 D
rejection km km km
Altitude characteristic (NM) (NM) (NM)
13 500 rn 60 dB 491 (265) 24.1 (1 3) 515 (278)
(45 000 ft)
13 500 rn 40 dB 491 (265) 241 (130) 732 (395)
(45 000 ft)
20 000 rn 60 dB 619 (334) 24.1 (13) 643 (347)
(66 000 R)
20 000 rn 40 dB 619 (334) 241 (130) 860 (464)
(66 000 ft)
6.4 The above criteria are based on the following
additional assumptions:
1 ) Effective radiated power: an ERP of 100 W for the
ground stations.
Note.- I f an ERP of 20 W is assumed, this would
result in sepai-ation distances for 13 500 m (45 000 ji) of
472 km (255 NM) for 60 dB receiver adjacent channel
rejection and 572 km (309 NM) for 40 dB receiver adjacent
channel rejection.
2) Interfering signal strength: if the received signal
strength is in excess of the free space propagation value,
then the maximum value will not exceed the free space
value by more than 5 dB over average earth. This
condition is satisfied when transmitters of 20 W ERP or
more are used in conjunction with a receiver adjacent
channel rejection of not less than 35 dB. Thus, the
minimum distance for d2 can be derived from a
consideration of receiver muting level, receiver adjacent
channel rejection and transmitter ERP.
7. RF - ~haractehsticsfo r digital VHF systems,
interference immunity performance
7.1 Receivingfirnction - interfeference immunity performance.
The standard measurement technique for digital systems
provides that the desired signal field strength be doubled, and
that the undesired signal be applied in increasing levels until
the channel performance, that is the specified error rate,
degrades to a value equal to the value found at the specified
receiver sensitivity.
For the VDL, the effect of the measurement technique is
that the desired signal strength is increased from 20 microvolts
per metre to 40 microvolts per metre. Then the undesired
signal on the adjacent or any other assignable channel is raised
to the specified level higher than the desired signal until the
specified error rate is exceeded.
Care should be taken to ensure that on-channel noise power
is not included in the measurement of the undesired signal.
7.2 Assignment criteria. Assignment of frequencies for
VHF digital link should take into account the VDL RF system
characteristics in order to avoid harmful interference to or
from co-channel and adjacent channel assignments, in keeping
with regionally and nationally agreed spectrum management
policies.
ATT A-8
Attachment A Annex I0 - Aeronautical Telecommunications
Figures A-8 to A-15. Propagation curves for standard
atmosphere (301) for frequency of 127 MHz
ESSAII.T.S.A.-1966 Propagation Model
These curves labelled "5 per cent time availability" represent only a
statistically expected value; i.e., a probability of 0.05 that a particular
situation will result in the specified field strength or greater during 5 per
cent of the time.
The parameters used to develop these curves include:
1) frequency of 127 MHz;
2) horizontal or vertical polarization;
3) smooth earth with land or sea surface;
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