19. Navigation Sensors
19-6
19.2 Source of Position
The figure below shows how source for position is chosen. The Kalman filter uses all position sensors which have
either primary or secondary as input for its calculation. DGPS and SYLEDIS position sensors are considered more
accurate than other position sensors.
The latitude and longitude position is shown at the upper right corner on the ECDIS display, and in the example
below the position source is DGPS. Other indications which may be displayed in the position area are as follows:
• NO POSIT: Appears (in red) when no dead reckoning sensors are chosen and valid.
• DEAD REC: Shown (in red) when position source is dead reckoning.
• DGPS, GPS: LORAN, DECCA or SYLEDIS: Name of position source.
• LOST DGPS: Shown (in red) when the user-selected DGPS device has lost the DGPS signal.
• FIL LOST DGPS: Shown (in red) when the Kalman filter is in use and the user-selected DGPS device has lost
the DGPS signal.
• FILTER HIGH: Shown when the position source is the Kalman filter with high-precision position sensor.
• FILTER: Shown when position source is the Kalman filter without high-precision position sensor.
• BAD FILTER: Shown (in red) when position source is the Kalman filter and the filter has excluded at least one
of the chosen sensors because of poor accuracy.
KALMAN filter
GPS in diff. mode
or SYLEDIS
selected as
PRIMARY
GPS in diff. mode
or SYLEDIS
selected as
SECONDARY
Any other than
GPS in diff. mode
or SYLEDIS
selected as
PRIMARY
Any other than
GPS in diff. mode
or SYLEDIS
selected as
SECONDARY
Position
used by
the system
Selected and position exists
Selected and position exists
Selected and position exists
Selected and position exists
Not
selected
Backup
Dead Reckoning
Selected and
position exists
Not selected
or no position
Not selected
or no position
Not selected
or no position
Not selected
or no position
Selected and
no position
If the system changes the source of position because of lost sensor data or the differential mode is changed on a
DPGS device, the system immediately generates the alarm “2009 Position source change”. If the Kalman filter is
used, the system generates the alarm “2010 Filter: Pos source change” after a time out period of 90 sec.
If a chosen DGPS sensor changes its operation mode and its status is primary or secondary, then the system
generates the alarm “4027 DGPS pos. source change” (4027 for first sensor, 4028 for second, etc.). These alarms are
not generated if the user has chosen Pri no alarm or Sec no alarm.
If source of position includes a position sensor which is in datum mismatch condition, the latitude, longitude and
chosen datum are displayed in red.
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