Dual GNSS RTK Compassing¶

Overview¶

RTK Compassing (Dual GNSS) is a system that determines heading by use of two GNSS receivers and antennas. It replaces the need for magnetometers which can be problematic in the presence of ferrous materials (e.g. steel) and EMI generating circuits (e.g. electric motors and drivers).

Heading Accuracy¶

The generalized heading accuracy under ideal conditions is shown in the following plot.

Dual GNSS heading accuracy vs baseline

Recommended Minimum Baseline¶

The recommended minimum baseline (distance between dual GNSS antennas) is 0.25 meters for multi-band GNSS compassing. The solution can operate at shorter baseline distances but is less robust and more susceptible to getting caught in a local minimum which may not converge to the correct heading.

Antenna Orientation¶

Important

It is recommended that both GNSS antennas be identical and have the same physical orientation relative to each other (i.e. the antenna cable should exit in the same direction on both antennas). This will ensure best RF phase center alignment and heading accuracy. The actual RF phase center is often offset from the physical center of the antenna case.

Mismatch	Match	Match

Rugged GNSS Antenna Ports¶

Rugged

On the Rugged IMX, the MMCX ports labeled 1 and 2 are the GNSS1 and GNSS2 antenna inputs.

Dual Antenna Locations¶

The location for both GNSS antennas must be correctly specified by the user in the DID_FLASH_CONFIG variables within 1 cm accuracy:

DID_FLASH_CONFIG.gnss1AntOffset[X,Y,Z]
DID_FLASH_CONFIG.gnss2AntOffset[X,Y,Z]

These values describe the distance of each GNSS antenna from the IMX Sensor Frame origin in the direction of the Sensor Frame axes. The Sensor Frame is defined using DID_FLASH_CONFIG.sensorConfig.

coordinate_frames

Validate GNSS Antenna Offsets¶

To validate the GNSS antenna offset values, inspect the INS heading (DID_INS_1.theta[2]) and verify that it points in the forward direction of the sensor frame (and the platform). Note that the RTK compass heading reported in DID_GNSS2_RTK_CMP_REL.baseToRoverHeading always reflects the raw heading from the GNSS1 antenna to the GNSS2 antenna and does not change with the antenna offsets; the INS heading, however, incorporates the antenna offsets and orientation to rotate this RTK compass heading into the sensor frame. Because of this, the antenna offsets can be adjusted and re-validated immediately by simply watching the INS heading update.

Example Antenna Configurations¶

The following are examples that illustrate what the GNSS antenna offsets should be for two different antenna configurations.

Drone¶

Drone Example

DID_FLASH_CONFIG.gnss1AntOffset[0] =  0.0
DID_FLASH_CONFIG.gnss2AntOffset[1] = -0.3   (negative direction of Y axis)
DID_FLASH_CONFIG.gnss2AntOffset[2] =  0.0

DID_FLASH_CONFIG.gnss2AntOffset[0] =  0.0
DID_FLASH_CONFIG.gnss2AntOffset[1] =  0.3
DID_FLASH_CONFIG.gnss2AntOffset[2] =  0.0

Automobile¶

dual_gnss_antenna_example_car

DID_FLASH_CONFIG.gnss1AntOffsetX = -0.5 (negative direction of X axis)
DID_FLASH_CONFIG.gnss1AntOffsetY =  0.5
DID_FLASH_CONFIG.gnss1AntOffsetZ = -0.5 (negative direction of Z axis, above IMX)

DID_FLASH_CONFIG.gnss2AntOffsetX = -1.5 (negative direction of X axis)
DID_FLASH_CONFIG.gnss2AntOffsetY =  0.5
DID_FLASH_CONFIG.gnss2AntOffsetZ = -0.5 (negative direction of Z axis, above IMX)

GNSS Antenna Ports¶

The following table explains how ports A and B on the Rugged IMX map to GNSS antennas 1 and 2.

Ports	Rugged IMX	IMX Module
GNSS 1 antenna port	A	1
GNSS 2 antenna port	B	2

Setup¶

Step 1 - Specify Offsets for Both Antennas¶

Refer to the Dual Antenna Locations section for a description of the GNSS antenna offset.

DID_FLASH_CONFIG.gnss1AntOffsetX = ?
DID_FLASH_CONFIG.gnss1AntOffsetY = ?
DID_FLASH_CONFIG.gnss1AntOffsetZ = ?

DID_FLASH_CONFIG.gnss2AntOffsetX = ?
DID_FLASH_CONFIG.gnss2AntOffsetY = ?
DID_FLASH_CONFIG.gnss2AntOffsetZ = ?

Using EvalTool - select Data Sets -> DID_FLASH_CONFIG and set gnss1AntOffset[X,Y,Z] and gnss2AntOffset[X,Y,Z] with the GNSS antenna offsets.

Using CLTool - run the CLTool using the following options replacing the [OFFSET] with the GNSS antenna offsets.

-flashconfig=gnss1AntOffsetX=[OFFSET] 
-flashconfig=gnss1AntOffsetY=[OFFSET] 
-flashconfig=gnss1AntOffsetZ=[OFFSET]
-flashconfig=gnss2AntOffsetX=[OFFSET] 
-flashconfig=gnss2AntOffsetY=[OFFSET] 
-flashconfig=gnss2AntOffsetZ=[OFFSET]

Step 2 - Enable GNSS Dual Antenna¶

Set the RTK_CFG_BITS_ROVER_MODE_RTK_COMPASSING (0x00000004) bit of RTKCfgBits.

DID_FLASH_CONFIG.RTKCfgBits |= RTK_CFG_BITS_ROVER_MODE_RTK_COMPASSING       // |= 0x00000004

Using EvalTool - go to Settings -> RTK -> Rover Mode, set the dropdown menu to GNSS Compassing, and press the Apply button.

Using CLTool - run the CLTool using the -flashconfig=RTKCfgBits=0x4 option to enable GNSS Dual Antenna.

RTK Compassing Fix Status¶

INS and GNSS Status Flags¶

The RTK compassing fix status can be identified using the valid bit in the INS and GNSS status flags.

DID_INS_1.insStatus & INS_STATUS_RTK_COMPASSING_VALID               // INS status - RTK heading is valid and aiding INS heading.
DID_GNSS1_POS.status & GNSS_STATUS_FLAGS_GNSS2_RTK_COMPASS_VALID        // GNSS status - RTK heading is valid and available in DID_GNSS2_RTK_CMP_REL.

RTK compassing fix is indicated when the RTK-Cmp radio button turns purple in the EvalTool INS tab.

Progress and Accuracy¶

The base to rover heading accuracy indicates how much error is in the base to rover heading (RTK compassing heading). The ambiguity resolution ratio, arRatio, is a metric that indicates progress of the solution that ranges from 0 to 999. Typically values above 3 typically indicate RTK fix progress.

DID_GNSS2_RTK_CMP_REL.baseToRoverHeadingAcc         // (rad) RTK compassing accuracy
DID_GNSS2_RTK_CMP_REL.arRatio                       // Ambiguity resolution ratio

The DID_GNSS2_RTK_CMP_REL status can be monitored in the EvalTool GNSS tab.

Stationary Application¶

For RTK compassing stationary application, enabling the STATIONARY INS dynamic model (DID_FLASH_CONFIG.dynamicModel = 2) is recommended to reduce heading noise and drift. This will reduce heading error during RTK compassing fix or loss of fix. See INS-GNSS Dynamic Model and Zero Motion Command for details.