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of hours, to produce the WAFS forecasts.
2.2.2 If the primary operational workstation fails, the
forecaster would use a back-up workstation located on the
forecast floor. If the WAFC Washington SIGWX production
centre is out, WAFC London is called on to produce all of the
WAFC Washington SIGWX forecasts. These forecasts are then
switched back to the NWSTG with the routine WAFC London
SIGWX forecasts for dissemination via the International Satellite
Communications System (ISCS). If WAFC Washington
is affected by a massive failure, paragraph 5 describes the
action under "Total outage of a WAFC".
3. INTERRUPTIONS IN MESSAGE SWITCHING
3.1 WAFC London interruptions
Two message switches exist at WAFC London. In the rare
event of a failure at the primary switch, the back-up switch
would be used to send and receive data. The back-up message
switch can be immediately connected to the Washington and
Toulouse WMO global telecommunication system (GTS)
routes, and is permanently connected to the satellite distribution
system for information relating to air navigation
(SADIS) in the event of failure of the primary switch. In the
extremely unlikely event that WAFC London cannot connect
to the GTS, an integrated services digital network (ISDN)
circuit between Silver Spring and Exeter will be used to send
and receive data. This circuit is a physical back-up to the link
that is normally in place, and is used only when a major failure
occurs.
ANNEX 3 ATT C-1
Annex 3 - Meteorological Service for International Air Navigation Attachment C
3.2 WAFC Washington interruptions
An integrated services digital network ([SDN) circuit is
maintained between WAFC London and the ISCS uplink site.
In the rare event that the message switch at the NWSTG is out
WAFC London would be notified to begin sending WAFS
products on the ISDN circuit directly to the ISCS uplink site.
The ISCS satellites then broadcast these products normally. A
back-up message switch at another site is scheduled for
implementation in 2004.
4. INTERRUPTIONS IN SATELLITE UPLINK
AND SATELLITE DISTRIBUTION SYSTEMS
The operation of the satellite communications systems used to
provide the SADIS and ISCS broadcasts of the WAFS data is
outside the control of the two WAFCs. The reliability and
availability of these services is guaranteed by the commercial
operators, and not by the two WAFCs.
5. TOTAL OUTAGE OF A WAFC
5.1 WAFC London outage
It is considered extremely unlikely that WAFC London would
ever be totally out. A new state of the art infrastructure ensures
that both computers can operate completely independent of
each other. Separate primary and back-up power supplies are
furnished for each computer. The back-up procedures camed
out will depend on the nature of the outage. In a worst-case
scenario when the primary supercomputer becomes inoperable
and the WAFC Washington - WAFC London GTS link cannot
be regained, the ISDN circuit described in paragraph 4
would be used to send and receive data from Washington.
Thus, the failure of the primary supercomputer would not
affect the ability of WAFC London to disseminate WAFS
products on the SADIS broadcast or send them to Washington
for broadcast on the ISCS. In the event of the forecast office
at WAFC London becoming unserviceable, WAFC Washington
may be contacted to initiate back-up production of all the highlevel
and medium-level SIGWX forecasts routinely produced
by WAFC London. It should be noted that a back-up forecast
office is provisioned at WAFC London in a separate wing of
the building. Thus, it is considered highly unlikely that WAFC
London would be totally unable to produce their WAFS
SIGWX forecasts. The SIGWX forecasts would still be available
for SADIS uplink from either forecast office in such a
scenario assuming that either computer is available.
5.2 WAFC Washington outage
The WAFS product generation facilities are located at multiple
sites. Thus, WAFC Washington would never be completely
out. The back-up procedures camed out would depend on the
facility affected, and have been described above. Two independent
data paths exist fiom the message-switching centre. Thus,
a cable cut would not disrupt communications to or from the
switch. However, in a scenario where the Washington message
switch is completely out, the distribution of products to WAFC
London and to the ISCS is also severed. A separate location
for a back-up message switch is planned, but will not be
implemented until 2004.
6. ROUTINE AND BACK-UP SIGNIFICANT
WEATHER FORECASTS
Table C-1 summarizes the ICAO areas to be used by WAFCs
　
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