19. Navigation Sensors
19-3
POSN page
The field of a position sensor contains a label (here DGPS-1st and GPS+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 that
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 orange and with additional text in the following conditions:
Note: Magnetic north reference cannot be used with he Track Control System.
• 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 ECDIS. Additional text is "Datum".
• If position sensors have position discrepancy active. Additional text is
"Discrepancy".
Enable: Check to align chart, using the "drag and drop" feature.
Chart Align: Alignment is automatically selected if the operator has defined position
offset by using the chart align feature. The offset values are given in nautical miles and
degrees. If position alignment is used, the operator is reminded of it every 30 minutes by
the alert "411 ChartAlign: over 30 min".
DR: The system automatically selects dead reckoning if there are no valid and selected
position sources and if there are valid and selected speed and course sources. If the
system has selected dead reckoning, then the operator can enter a next WPT position for
the ship in the Lat and Lon fields.
Kalman filter: The Kalman filter calculates estimated position using all valid and
selected position, speed and course sensors. Select On to activate this feature.
SPD/CRS POSN
DR
LORAN
Off
°
kn
kn
kn
kn
DGPS-1st
NM
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 selected as primary, then that sensor previously selected to primary state is then automatically
selected to secondary state. When the position source is changed because of 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 selected 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 selected 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 selected position sensor (status other than Off) is changed from WGS-84 to another
datum or if selected position sensor does not send datum message. Additional text "datum" appears, in orange.
Further, lat/lon values for the position sensor turn orange. 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 orange. If output datum of a
selected 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) which has "IEC 61162-
1 Ed 2 (2000-7)" indicated in their type approval certificate can support the ECDIS to detect datum mismatch.
The system generates the alert "407 Position discrepancy" if there is discrepancy in position. Together with this
alert, the system shows the text "Discrepancy" in orange in the POSN page for a position sensor that has position
discrepancy active.
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