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Inertial Sense Binary (ISB) Aiding IMX

The IMX INS/EKF can be aided directly with GNSS position and velocity data supplied as Inertial Sense Binary (ISB) messages from an external device, such as another Inertial Sense product, a companion computer, or a custom host generating GNSS solutions. Because the data arrives already formatted as native IMX Data Identifiers (DIDs), the IMX does not need to parse a third-party GNSS receiver protocol (uBlox, NMEA, Septentrio, etc.) — it simply accepts and fuses the incoming DID messages into the EKF.

ISB GNSS aiding data can be supplied over any of the IMX serial ports (Serial 0, Serial 1, or Serial 2). The DID_FLASH_CONFIG.ioConfig GNSS1 and/or GNSS2 source fields must be set to the corresponding serial port so the IMX knows which port to use for GNSS1 and GNSS2.

This is the same mechanism the GPX-1 uses internally to supply GNSS1 and GNSS2 data to the IMX-5. When DID_FLASH_CONFIG.ioConfig GNSS type is set to GPX, the IMX-5 requests this exact set of GNSS DID messages from the GPX-1 over ISB. Setting the GNSS type to ISB allows any external source capable of generating these same DID messages to aid the INS in the same way.

Configure the IMX for ISB GNSS Input

  1. Connect the external ISB GNSS source to Serial 0, Serial 1, or Serial 2 (3.3V TTL UART) on the IMX. See the PCB Module hardware page for pinout.
  2. Set the serial port baudrate to match DID_FLASH_CONFIG.serXBaudRate for the port used.

  3. Configure GNSS1 (and GNSS2, if using dual antenna heading) using the EvalTool GPS Setting tab or DID_FLASH_CONFIG.ioConfig:

    DID_FLASH_CONFIG Value
    GNSS1 Source Serial 0, Serial 1, or Serial 2
    GNSS1 Type ISB (Inertial Sense Binary)
    GNSS2 Source Serial 0, Serial 1, or Serial 2 (optional, for dual antenna heading)
    GNSS2 Type ISB (Inertial Sense Binary)

  4. Stream the required DID messages (below) from the external device to the IMX at the desired GNSS update rate. A 200ms (5Hz) period is typical and matches DID_FLASH_CONFIG.startupGnssDtMs.

  5. Ensure the status field of each DID_GNSS1_POS/DID_GNSS1_VEL/DID_GNSS2_POS/DID_GNSS2_VEL message accurately reports fix type, satellites used, and accuracy (see eGnssStatus). The EKF uses these status flags to determine whether the data is trustworthy enough to fuse.

Required GNSS DID Messages

The following DID messages are needed to properly aid the INS, matching the same set the GPX-1 provides to the IMX-5. Full field definitions for each DID are available in the DID Descriptions reference.

GNSS1 (required)

DID Description
DID_GNSS1_POS GPS 1 position data. Used directly by the EKF for INS position aiding.
DID_GNSS1_VEL GPS 1 velocity data. Used directly by the EKF for INS velocity aiding.

GNSS2 (required only if using a second antenna for heading/RTK compassing)

DID Description
DID_GNSS2_POS GPS 2 position data.
DID_GNSS2_VEL GPS 2 velocity data.
DID_GNSS2_RTK_CMP_REL Dual GNSS RTK compassing relative info (baseline vector/heading between GNSS1 and GNSS2 antennas). Used by the EKF to aid INS heading.

These messages are not required for basic position/velocity/heading aiding, but provide status, diagnostic, and accuracy information used by the EvalTool and logging tools.

DID Description
DID_GNSS1_SAT / DID_GNSS2_SAT Satellite information: identifiers, CNO, elevation/azimuth.
DID_GNSS1_SIG / DID_GNSS2_SIG GNSS signal information.
DID_GNSS1_VERSION / DID_GNSS2_VERSION GNSS receiver software/hardware version info.
DID_GNSS1_RTK_POS_REL / DID_GNSS1_RTK_POS_MISC RTK precision positioning relative and accuracy/diagnostic info for GNSS1.
DID_GNSS2_RTK_CMP_MISC RTK compassing accuracy/diagnostic info for GNSS2.

Electrical Interface

The external ISB GNSS source can be connected to Serial 0, Serial 1, and Serial 2 (3.3V TTL UART) on the IMX.