Filter Initialization

The following discussion addresses both the -AR and the -AHRS versions of the 3DM-CV7. 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.

 

 

Initializing Pitch and Roll

The 3DM-CV7 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-CV7s. When configured to estimate relative heading, every time the 3DM-CV7 is powered on, reset, or the Reset Navigation Filter (0x0D,0x01) command is issued the 3DM-CV7 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-CV7's current heading as the written heading value.

TIP: To confirm that heading measurements are relative verify that the 3DM-CV7 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-CV7 is in.

3DM-CV7-AR

The 3DM-CV7-AR provides relative heading by default. No configuration is required.

3DM-CV7-AHRS

The 3DM-CV7-AHRS can provide relative heading if its magnetometer is disabled as an aiding measurement. The 3DM-CV7-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-CV7 reports that it is in AHRS mode. See Filter States for more information on filter modes and how to determine which mode the 3DM-CV7 is in.

3DM-CV7-AR

The 3DM-CV7-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-CV7-AHRS

The 3DM-CV7-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-CV7-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-CV7-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-CV7-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.