Filter Operation - Furuno FEA-2107 Operator's Manual

19. Navigation Sensors
19.7

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, place the cursor on the Position indication, confirm that Sensors appears in the
mouse functions area and 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 chosen 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 which 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 chosen, alarms are generated if the system does not have a valid position available. No alarms are
generated for position jumps of the position sensors chosen to be used with the filter as long as the filter is able to
estimate a reliable position based on chosen position sensors. If there is a change in the availability of the sensor
chosen with the Kalman filter and if the filter cannot continue the same level of position accuracy, then the system
generates the alarm "2010 Filter: Pos source change". 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) chosen and valid in order to operate.
The filter has one main alarm "2000 Filter: Pos unreliable". This alarm 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.
Alarm 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 alarm 2000, only the filter itself is set in the OFF
state. After generating the alarm 2000, the system will use its normal method to choose position source from
available sensor. Normally, this causes a jump in the position. After the alarm 2000 is generated, the user should
evaluate which sensor caused the problem. Disable the bad sensor and 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 kt), the system generates the alarm
"2008 Filter: Speed below 4 kn". If you don't use automatic route steering, the system also chooses the filter as OFF
after generating the alarm 2008. With automatic route steering the system keeps the filter chosen, but repeats alarm
2008.
The system can check that each chosen position sensor is within position discrepancy limit set by the user. (For
further details, see paragraph 19.5 "Position Discrepancy Alarm".) If a position sensor has position discrepancy
active, it is automatically excluded from the Kalman filter. This feature enables the user to control how much
discrepancy there can be between the Kalman filter position and any position sensor.
19-16