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15. Navigation Sensors
15-2
POSN (Position) page
The field of a position sensor contains a label (here
DGPS-GLL and GGA+ZDA) that indicates the name of
the sensor; a status (primary/secondary/off) that
indicates if the sensor is used or not; position at conning
position and local datum; speed and course which has
(MAG) if the course is referenced to magnetic north. A
DGPS position sensor has additional text Diff, if
differential signal is in use. Latitude and longitude
values will appear in red and with additional text in the
following conditions:
• Kalman filter has excluded the sensor from its
estimated position. Additional text is "Excluded".
• Received position from position sensor is in
another datum as set to be received in the chart
radar. Additional text is "Datum".
• If position sensors have position discrepancy
active. Additional text is "Discrepancy".
Position sensors have priority, which is indicated as "primary" or "secondary". Only one sensor can be primary
while the others can be secondary or off. If a position sensor is changed from secondary to primary state and another
position sensor was chosen as primary, then that sensor previously selected to primary state is then automatically
selected to secondary state. When the position source is changed based on priorities and signal validity to another
position source, then you get the alert "409 Position source change". Note that alert 409 is active only when the
Kalman filter is off because the filter has its own process to calculate estimated position and in that process priority
given by primary and secondary is not used.
If a DGPS is chosen as primary or secondary, and its status changes from "no Diff" to "Diff" or from "Diff " to "no
Diff" the system generates the alert "727 DGPS pos. Source change" (alert numbers are from 727 to 732 depending
on position receiver).
"Pri no alert" and "Sec no alert" can be chosen from the DPGS box if you want to disable the alert "727 DGPS pos.
source change". These selections are intended for the area where the differential coverage is fading. One example is
if you sail along coastline but you are so far away that the differential signal changes its status very often.
The system generates the alert "738 Datum mismatch" (alert numbers are from 738 to 743 depending on the position
receiver) if output datum of a chosen position sensor (status other than Off) is changed from WGS-84 to another
datum or if chosen position sensor does not send datum message. Additional text "datum" appears, in red. Further,
lat/lon values for the position sensor turn red. If the position sensor is also used for system position, Kalman filter
and your ship's position (Lat/Lon values and datum used) are indicated in red. If output datum of a chosen position
sensor is changed from no datum message or another datum to WGS-84 and the operator has used the selection
Primary or Secondary, then the system generates the alert "744 Datum change" (alert numbers are from 744 to 749
depending on position receiver).
Note: The ability to check datum of position is a relatively new feature for position receivers. It was introduced in
standard IEC 61162-1 Ed2, published in July 2000. Only EPFS (for example GPS or DGPS) that has "IEC 61162-1
Ed 2 (2000-7)" indicated in their type approval certificate can support the ECDIS to detect datum mismatch.