S-124 CG
Discussion paper 1 - Geographical filtering of NW - Area of selection for NWs to be notified Version 0 – 13/05/2015 – Yves Le Franc
Geographical filtering of NW - Area of selection for NWs to be notified
Users complain that they receive a lot of NWs which are not relevant to their area of navigation.
The source of the problem is the layout of the NWs listed by the current NW receivers. NAVTEX and
SafetyNET receivers receive NWs from stations which broadcast NWS within a very large area with
limited geo-filtering possibility:
NAVTEX
NWs broadcast in NAVTEX coverage area: radius 300-400 M from the station.
No geographic filtering on receiver other than the selection of the stations.
Example: NAVTEX stations in Mediterranean Sea (NAVAREA III)
SafetyNET
NWs broadcast in SafetyNET Satellite Ocean Region (Atlantic Ocean Region – East, Atlantic Ocean
Region – West, Indian Ocean Region, Pacific Ocean Region).
Geographic filtering on receiver: NAVAREA (area), coastal warning area, “user defined area” (circular
or rectangular) intersecting the ship location. In the expression “user defined area”, the user is MSI
provider.
The “user defined area” is a temporary geographic area. It is used occasionally, for example to
broadcast coastal warnings in an area where the NAVTEX station is down.
NAVAREAS with Inmarsat satellite ocean region coverage
With the current systems the end-user reads “general area” and “locality” in the NWs of the list to
see if the NW is of geographical interest.
Then, the idea appears to use “general area” to automate the selection of NWs to be notified to the
user.
Fixed predefined “general area” could be a practical way for coordinators and users to implement
the concept of “area of relevance” i.e. the area where ships must be informed of an event. One
could imagine that an S-123 data base of radio services contains the geographical limits of fixed
general areas and that the NW contains the Id of the general area concerned. Then, the user could
choose the general area(s) used to select the NWs to be notified.
But, the reality is that predefined general areas are not established everywhere, far from it, and
some discussions about the borders of NAVTEX services areas exist. Predefined general areas are
only suggested in S-53 Section 6 – part 1 - Standard Message Element Reference 1 – note I (use of the
established meteorological forecast areas), they are not universally defined. Thus, it seems that the
predefined general areas are maybe not the best way.
An alternative could be to implement the concept of “area of relevance” by coding the geographical
limits of this area within each NW. Then, the system aboard could select the NWs with an area of
relevance intersecting the ship’s location or the planned route.
In most of the cases, the coordinator would have to code a circular area centered on the location on
the event. The radius should be extended to consider the estimated speed of ships possibly
approaching, and the estimated time for anticipation by the ships.
If we consider ships speed = 20 knots, time for anticipation = 4 hours, then, the radius is 20x4 = 80 M.
The area is quite large and some ships within this area will be not concerned (eg not approaching).
The choice of the area of relevance would be uncomfortable for the coordinator. The assessment of
time for anticipation and of the speed of the ship is mainly in the hands of each user as the
assessment of the Time of CPA is.
In fact, the real need is not to associate an area to the NWs like in the current systems. The need is to
inform (notify) the user of the events which could impact his navigation, in time. For located events,
the main criteria seem to be the CPA and the time of CPA with a room (distance and time)
associated to the planned route. This mode of operation should be a requirement for the system
aboard (INS) in the future.
Notification of relevant NWs should be done by the INS provided that the potentially selected NWs
are known (received) by the system. It should be the case if the ship receives NWs located within a
large area as it is currently the case.
Then, “area of relevance” means no more “the area where ships must be informed of an event”. It
should mean “area where the NW must be broadcast”.
The risk with a mode of operation based on CPA is that some events located outside the buffer
associated to the planned route are omitted even if they have an impact on the ship navigation. This
is the case of some events which “radiate” from their location e.g. a major light extinguished
(range).
In these cases, the producer should encode the location of the event plus an “area affected” (eg
radius = the range of the light extinguished)1. The “area affected” will be taken into account instead
the location of the event for the INS geo-selection for notification. That is a disadvantage for the
coordinator.
A large minimal buffer should also be required, related at least to the general accuracy of NWs
location. Another solution is to encode the accuracy of location is each NW. This would be
demanding for the producer but it should be more correct.
Remarks:
The selection of NWs based on CPA and TCPA is for Notification to the user for located NWs. Other
functions could show other located NWs and NWs not located. For example, the display of NWs on
the ECDIS is not limited to the NWs selected by CPA and TCPA.
1
“area affected” (or “safety area”?) could be used also around a drifting hazard reported, an act of piracy
reported, etc.
The portrayal of NWs on the ECDIS display should reduce the problems issued from the layout of lists
of NWs. But the portrayal of NWs on the ECDIS display is not sufficient to address correctly NWs
aboard.
Some NWs are not located (eg significant malfunctioning of radio-navigation services). Of course,
they must be notified to the user without selection based on CPA and TCPA.
Comments:
1. Japan agrees with use of coding geographical limits of the area within each NW for setting “area of
relevance”. But it should be noted that some NW cannot be coded since they don’t have any
positions.
2. It is important to provide information on properly time, but it is also important for mariners to get
nautical information in advance on passage planning before departure.
3. In case of using CPA or TCPA, software will be more complicated and expensive, then systems will
be difficult to become popular among users.