Configuring Dual Antenna ALIGN with SPAN

The Dual Antenna ALIGN updates are used to help constrain the azimuth drift of the INS solution whenever possible. This is of the greatest value with lower-quality IMUs and in environments with low dynamics where the attitude error is less observable. Slow moving marine, agriculture and train applications are good examples of the intended use. By providing an external heading source, the solution drift can be constrained in these environments.

As with all ALIGN-capable products, the ALIGN solution is available from the GPHDT log and HEADING2 log. For INS heading (azimuth), use the INSATT log, the INSPVA log, or any other desired SPAN log containing azimuth.

To monitor the status of the ALIGN update being applied, observe bit 4 of the INS Extended Solution Status field of the extended INS logs (INSATTX, INSPOSX, INSPVAX and INSVELX).

Following the appropriate dual antenna configuration below, the SPAN system can be configured for different alignment routines depending on the motion conditions experienced during the alignment period. The different alignment routines are described in System Start-Up and Alignment Techniques.

Setup and Configuration

Dual Antenna ALIGN with SPAN can be configured on a dual-antenna receiver, e.g. OEM7720, or using two receivers. All dual-antenna receivers come with an ALIGN capable model. If using two receivers, the secondary non-SPAN receiver must be an ALIGN capable model. For additional information on models see the Model List for NovAtel (novatel.com/products/model-list-and-discontinued-products).

If using two receivers, both receivers should both be powered on, and a connection established between the SPAN receiver and the secondary non-SPAN receiver as shown in the example in Figure: SPAN – Two Receiver Dual Antenna Installation.

To enable the dual-antenna ALIGN solution to aid the INS alignment and provide heading updates, the offset between the antennas and the IMU must be known. This is achieved by entering lever arms to both antennas, using the SETINSTRANSLATION command.

To configure SPAN with ALIGN Aiding:

  1. Enter the lever arm from the IMU to the primary antenna.

    Abbreviated ASCII example:

    SETINSTRANSLATION ANT1 0.54 0.32 1.20 0.03 0.03 0.05

  2. Enter the lever arm from the IMU to the secondary antenna.

    Abbreviated ASCII example:

    SETINSTRANSLATION ANT2 0.54 2.32 1.20 0.03 0.03 0.05

    Alternately, the angular offset between the dual-antenna baseline (from primary GNSS antenna to secondary GNSS antenna) and the IMU frame forward axis can be entered directly via the ALIGN parameter of the SETINSROTATION command.

    It is recommended to enter the lever arms rather than entering the angular offset as the lever arms are easier to measure and will lead to better overall accuracy.

  3. Use the INSALIGNCONFIG command to configure the connection ports, baud, and heading rate.

    This command is not required for a dual-antenna receiver configuration, e.g. OEM7720, but can be used to adjust the heading rate – the port configuration fields will be ignored.

    Abbreviated ASCII example:

    INSALIGNCONFIG COM1 COM2 230400 5

For optimal performance of Dual Antenna with SPAN, an accurate ALIGN angular offset is required for each unique installation. This value can be calibrated should the entered ALIGN rotation value or value derived from the input lever arms be less accurate than 5 degrees. Refer to INSCALIBRATE command for details.

Optional HEADINGOFFSET Configuration

Optionally, the HEADINGOFFSET command can be used to rotate the ALIGN solution available in the GPHDT log and HEADING2 log. This is occasionally desired when the baseline does not line up with the direction of travel but is not required for use with SPAN. If this additional offset in the GPHDT log and HEADING2 log is desired, the following configuration details should be evaluated.

Most configurations of the HEADINGOFFSET command are not compatible with Waypoint Inertial Explorer and as such is not recommended for use if the data will be post processed. Contact support (see Customer Support) for more information.

The HEADINGOFFET command will apply a rotational offset to the fields in the HEADING2 log. This is handled differently depending on single receiver or dual receivers ALIGN setup, see below.

HEADINGOFFSET details when a dual-antenna receiver is used:

  • The HEADINGOFFSET command will apply a rotational offset to the fields in HEADING2 log. SPAN will automatically account for this additional rotation when applying the ALIGN update. As such, the provided ANT2 translation or ALIGN rotation should match the physical installation as per the standard instructions.

HEADINGOFFSET details when two receivers are used:

  • Option 1:

    Issue the HEADINGOFFSET command on both the primary and secondary receivers. The secondary receiver will apply the rotational offset to the fields in the HEADING2 logs being sent to the primary receiver. The primary receiver (running SPAN) will automatically account for this additional rotation when applying the ALIGN heading update.  As such, the provided ANT2 translation or ALIGN rotation should match the physical installation as per the standard instructions above.

  • Option 2:

    Issue the HEADINGOFFSET command on the secondary card where the HEADING2 log is computed. This will apply a rotational offset to the fields in the HEADING2 logs being sent to the primary card. The primary card (running SPAN) must be configured according to the offset orientation. This means that either the ANT2 lever arm or the ALIGN rotation must be adjusted and entered such that they account for the offset heading orientation.