Filter Initialization

Upon start-up or after a filter reset command is issued, the Navigation Filter will require valid initial conditions to be provided before it will run. The running mode of the filter, and the available outputs, depend on the initialization conditions selected to initialize the filter. See Filter States for more information regarding initial conditions and filter mode.

Filter initialization is configured through a single command: Initialization Configuration (0x0D,0x52) command. The following discussion explains how to use each parameter in this command.

 

WARNING: In instances where the 3DM-CX7-INS may experience large time jumps after the Navigation Filter has been initialized (i.e. acquiring a PPS fix), filter instabilities may occur. It is recommended that a Reset Navigation Filter (0x0D,0x01) command is performed if a large system time jump is experienced after filter initialization. To check the PPS status, monitor the Time Sync Status (0xA0, 0x02) and verify the Time Sync Boolean is true.

 

Wait for Run

The user may elect to have the Navigation Filter begin running as soon as the selected initial conditions are available or remain idle until a run command is issued. This option is set through the Wait For Run Command parameter.

 

Initial Condition Source

The initial conditions include an initial attitude (pitch, roll, and heading), initial velocity, and an initial position estimate. The user may elect to have some of these values provided automatically by the 3DM-CX7's on-board sensors, or they may choose to provide some or all of them manually or from external aiding measurements. The sources of these initial conditions – manual (user supplied) or automatic (from internal measurements) – are set through the Initial Cond Src parameter.

 

Sources for Auto-Initialized Conditions

The following describes the auto-initialization sources for the initial conditions:

Pitch/Roll

If pitch/roll is set to auto-initialize, the 3DM-CX7 will use its own accelerometer measurements for pitch/roll initialization.

Tip: For best performance, initialize pitch and roll while the device is stationary. Linear acceleration will introduce errors into this attitude measurement.

Auto-Heading Alignment Selector

If a user would like to auto-initialize heading, they have two sources. A user can opt to use one source, or two sources through the Auto Heading Alignment Selector:. The sources are: GNSS kinematic alignment, and magnetometer alignment. Each auto-heading source is described in more detail in the following section.

Heading

The sources for auto-initialization of heading are:

  • Kinematic alignment: Heading is initialized from the GNSS velocity vector. Once the vehicle reaches a velocity threshold of 2 m/s, heading is initialized in the direction of travel. This method makes the assumption that the vehicle's direction of travel is in the positive X direction of the vehicle frame, and is violated by moving in reverse.Special care must be taken to accelerate in the X direction only until the velocity threshold is reached especially when using this alignment method on vehicles that have the ability to move perpendicular to the vehicle X axis.

  • Magnetometer: Heading is initialized from a magnetometer measurement. Special care must be taken if this method is to be used. See Magnetometer Warning for details.

  • External Heading: Heading is initialized from an external heading input provided once at least every 5 seconds (preferably more often).

Position/Velocity

If set to auto-initialize, the external GNSS position and velocity is used for initialization.

Manually Initialized Conditions

Manual estimates of the Initial Heading, Initial Pitch, Initial Roll, Initial Position, and Initial Velocity can be entered in their respective fields in the Initialization Configuration (0x0D,0x52) command. If a user is providing manual position/velocity estimates, they need to specify the reference frame in the Reference Frame Selector.

The following discussion addresses both the -AR and the -AHRS versions of the 3DM-CX7. The two variants differ in some respects because the -AHRS has a magnetometer which, by default, it uses to estimate absolute heading. The -AR variant does not have a magnetometer.

IMPORTANT: Features requiring a magnetometer are only available on device models with magnetometers. See Product Description to verify that your device has a magnetometer.

 

NMEA over GPIO Initialization

See NMEA Input page for instructions on initialization with NMEA.

 

Initializing Pitch and Roll

The 3DM-CX7 auto-initializes pitch and roll every time the device is powered on, reset, or the Reset Navigation Filter (0x0D,0x01) command is issued.

Tip: For best performance, initialize pitch and roll while the device is stationary. Linear acceleration will introduce errors into this attitude measurement.

 

Initializing Relative Heading

Relative heading is available on all 3DM-CX7s. When configured to estimate relative heading, every time the 3DM-CX7 is powered on, reset, or the Reset Navigation Filter (0x0D,0x01) command is issued the 3DM-CX7 will provide relative heading measurements defining its current heading as 0.

Relative heading can be updated through the Set Initial Heading Control (0x0D,0x03) command. This command will define the 3DM-CX7's current heading as the written heading value.

TIP: To confirm that heading measurements are relative verify that the 3DM-CX7 reports that it is in vertical gyro mode. See Filter States for more information on filter modes and how to determine which mode the 3DM-CX7 is in.

3DM-CX7-AR

The 3DM-CX7-AR provides relative heading by default. No configuration is required.

3DM-CX7-AHRS

The 3DM-CX7-AHRS can provide relative heading if its magnetometer is disabled as an aiding measurement. The 3DM-CX7-AHRS defaults to having the magnetometer enabled as an aiding measurement.

 

Initializing Absolute Heading

TIP: To confirm that heading measurements are absolute verify that the 3DM-CX7 reports that it is in AHRS mode. See Filter States for more information on filter modes and how to determine which mode the 3DM-CX7 is in.

3DM-CX7-AR

The 3DM-CX7-AR estimates absolute heading when it is provided an absolute heading measurement through the External Heading Update (0x0D,0x17) command or the External Heading Update With Time (0x0D,0x1F) command at least once every 5 seconds.

3DM-CX7-AHRS

The 3DM-CX7-AHRS uses its internal magnetometer to produce absolute heading estimates in relation to magnetic north by default. No configuration is required.

If the magnetometer is disabled, the 3DM-CX7-AHRS estimates absolute heading when it is provided an absolute heading measurement through the External Heading Update (0x0D,0x17) command or the External Heading Update With Time (0x0D,0x1F) command at least once every 5 seconds.

External heading updates can also be used when the magnetometer is enabled. External heading updates will override the magnetometer. If an external heading update is not provided, the 3DM-CX7-AHRS will fall back on the magnetometer.

NOTE: To input external heading measurements, external heading must be enabled as an aiding source through the Aiding Measurement Control (0x0D,0x50) command.

 

Configuring the Magnetometer

The magnetometer can be disabled or enabled as an input to the estimation filter on the 3DM-CX7-AHRS through the Aiding Measurement Control (0x0D,0x50) command. It is enabled by default.

true north vs. magnetic north

Magnetometer-aided heading defaults to an absolute heading in relation to magnetic north. A user can get absolute heading estimates in relation to true north by entering a declination for the application space that the device will be operating in. To accomplish this, a user must use the Magnetic Field Declination Source Control (0x0D,0x43) command to set the declination source to manual and enter a declination.

Declinations for a given area can be easily determined through internet searches. Declinations must be entered in radians.