Kalman Filter - Furuno FCR-2107 Series Operator's Manual

15. Navigation Sensors
Alert related to SOG, COG, speed, heading, ROT, drift and docking speed components
It is possible that the operator has not chosen any speed or heading sensors, or that the chosen sensors do not have
any valid values. This kind of a situation is critical for the system, because it cannot even perform dead reckoning.
When no speed source is available, the system generates the alert "401 No speed available". When no heading
source is available the system generates the alert " 402 No heading available".
The system may find that SOG and/or COG from the sensors are unreliable, at which time it generates the alert "413
SOG&COG unreliable". Note that the source of SOG and COG is not only speed and course sensors, but the source
can also be position-fixing equipment.
Verify and choose valid sensors in the SPD/CRS and POSN pages of the Sensors dialog box for SOG and COG used
by the system.
15.6

Kalman Filter

The chart radar 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 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, alerts are generated if the system does not have a valid position available. No alerts 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 alert "410 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 alert "400 Filter: Pos 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. 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 choose position source from available
sensor. 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 kt), the system generates the alert
"408 Filter: Speed below 4 kn". If you don't use automatic route steering, the system also chooses the filter as OFF
after generating the alert 408. With automatic route steering the system keeps the filter chosen, but repeats alert
2008.
The system can check that each chosen position sensor is within position discrepancy limit set by the operator. (For
further details, see section 15.4 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.
15-10