19. Navigation Sensors
19-13
19.7.2 Gyro-associated alerts
When no gyro value is received from gyro selected by "First priority gyro selection", alert AL 10 (Missing working
gyro) is generated at the Autopilot FAP-2000.
When no gyro value is received from a gyro not
selected by "First priority gyro selection", alert AL 11 (Reference
gyro missing or Compass comparison alert) is generated at the Autopilot FAP-2000. Note that alert AL 11 is
activated also if the deviation between Gyro 1 and Gyro 2 is too large (default is 12º).
ECDIS generates alert 705 for missing Gyro 1 and alert 706 for missing Gyro 2. If the deviation between gyro 1 and
gyro 2 is over 5º, then ECDIS generates the alert 404 Gyro data unreliable.
19.8 Filter Operation
The ECDIS has an automatic multi-sensor Kalman filter for position. The filter can calculate estimated position
using all operative position sensors.
To view position sensor status, put the cursor on the Position indication, confirm that Sensors appears in the mouse
functions area then push the right mouse button.
Operator can manually include or exclude sensors. Remaining sensors (= sensors with any selection other than OFF)
are included in the filter processing. The purpose of this filter is to continuously monitor the position given by each
individual sensor to determine if the sensor is reliable or not. Also selected gyro and log sensors are included into
this analysis. The filter knows the behavior of different types of sensors (Loran, Decca, GPS, DGPS, etc.). The
result of the processing is a smoothed position that is used as final position of the ship. The filter eliminates all
"position jumps" in the final position even if there is a position jump in a sensor. (A position jump is caused by line
change in Decca, loss of differential signal with DGPS, etc.)
The filter can be selected or deselected by switching it ON or OFF. When switching filter state from ON to OFF,
new start position of the filter is set as an average value of the enabled sensors (sensors with any selection other than
OFF). Change the filter state from OFF to ON, and the filter continues processing from current position.
When the filter is selected, alerts are generated if the system does not have a valid position available. No alerts are
generated for position jumps of the position sensors selected for use with the filter as long as the filter is able to
estimate a reliable position based on selected position sensors. If there is a change in the availability of the sensor
selected with the Kalman filter and if the filter cannot continue the same level of position accuracy, then the system
generates the alert "410 Filter: POSN source CHG". This change of available sensor may or may not affect the
operation of the filter.
The filter requires at least one position sensor and one course sensor (i.e., gyro) selected and valid in order to
operate. The filter has one main alert "400 Filter: POSN unreliable". This alert means that the filter is not able to
calculate the position from the connected sensors. This can happen if there is only one position sensor connected and
the position generated by it becomes unreliable or if there are more position sensors but they are deviating too much.
The alert can appear also if the speed/course sensor (Log or Gyro) is found to be unreliable. Note that the filter
indicates unreliable position sensor by showing position in red.
If the filter is used without automatic route steering then, in case of alert 400, only the filter itself is set in the OFF
state. After generating the alert 400, the system will use its normal method to select position source from available
sensors. Normally, this causes a jump in the position. After the alert 400 is generated, the operator should evaluate
which sensor caused the problem, disable the bad sensor then turn the filter ON again.
The Kalman filter is designed to support automatic route steering and thus it requires that you have some speed. If
the speed is less than the value set in the installation parameters (for example, 4 kn), the system generates the alert
"408 Filter: SPD below 4 kn". If you don't use automatic route steering, the system also selects the filter as OFF
after generating the alert 408. With automatic route steering, the system keeps the filter selected, but repeats alert
408.
The system can check that each selected position sensor is within the position discrepancy limit set by the operator.
(For further details, see section 19.5 "Position Discrepancy Alert".) If a position sensor has position discrepancy
active, it is automatically excluded from the Kalman filter. This feature enables the operator to control how much
discrepancy there can be between the Kalman filter position and any position sensor.
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