The Inertial Labs AHRS-II-P can be integrated with an external GNSS receiver. This integration allows for increasing AHRS-II dynamic accuracy. Using an external GNSS receiver gives the following advantages to AHRS-II operation:
- Stable Heading source.
- Accurate orientation calculation at maneuvers of the carrier object.
- Using a PPS signal from the GNSS allows the synchronization of AHRS-II calculations and data output with highly accurate GNSS timing.
- Provide accurate position and velocity.
Also, using external GNSS receiver data allows the automatic update of the magnetic declination value that is used for true heading calculation. The external GNSS receiver should be preconfigured before integration to output data that the AHRS-II unit can accept. There are the following variants of the supported data from external receivers:
- Binary messages from NovAtel receiver.
- Binary messages from u-blox receiver.
- Binary messages from Septentrio receiver.
- NMEA messages from any GNSS receiver.
The best variant is binary data, which are more accurate, quicker, and more complete than NMEA messages. The AHRS-II unit should also be preconfigured for operations with an external GNSS receiver. Adjustments can be made using GUI software or by command. The article contains instructions for both methods: using GUI and commands.
The AHRS-II configuration using the AHRS-II GUI
Connect the AHRS-II to the computer and power it on. Choose the serial port number to which the device’s main COM1 port is connected and set the baud rate manually, or click the “Auto” button to automatically detect the unit’s baud rate in the “Test options” menu of the AHRS-II GUI. Use the “Read parameters” item under the Parameters menu to read the parameters from the connected device memory. An information window with “Parameters were successfully read” will appear. Open the “Devices options” menu of the AHRS-II GUI, “External sensors” tab, and set COM protocol, receiver data type, port bps, and data rate. The values should be the same as those in the preconfigured external GNSS receiver. The adjustments of the AHRS-II in the “Devices options” menu of the AHRS-II GUI should be as in Figure 1. Press the OK button and use the “Write parameters” item under the Parameters menu to apply changes. An information window with “Parameters were successfully written” will appear.
The AHRS-II-P configuration by commands
The following process demonstrates the required sequence of commands to configure the AHRS-II-P to accept data from an external GNSS receiver. Connect the AHRS-II-P to the computer and power it on. Open any serial port terminal, choose the serial port number to which the device’s main COM1 port is connected, and set the baud rate manually.
The first command that we recommend sending is Stop, which has a constant structure so that the AHRS-II-P switches to idle mode and is ready to accept commands from the host computer:
AA 55 00 00 07 00 FE 05 01
AA 55 – Header, 00 – Message type, 00 – Message identifier, 07 00 – Message length, FE – Stop command code, 05 01 – Checksum.
The second command is LoadAHRSIIpar_RAM to set the AHRS-II port type to input data from the external GNSS receiver:
AA 55 00 00 09 00 B0 1A 00 D3 00
AA 55 – Header, 00 – Message type, 00 – Message identifier, 09 00 – Message length, B0 – Code for LoadAHRSIIpar_RAM command, 1A- Parameter code, 00 – Parameter value COM port type, D3 00 – Checksum.
The response to the LoadAHRSIIpar_RAM command should be as follows:
AA 55 01 B0 09 00 00 00 00 BA 00
AA 55 – Header, 01 – Message type, B0- Message identifier, 09 00 – Message length, 00 – Data list error, 00 00 – Reserved, BA 00 – Checksum. If all bit values are 0 for the Data list error, there is no error in the parameter code, payload, and parameter values. A value of 1 for any of the bits indicates the errors shown in Table 6.44 of the AHRS-II ICD.
The third command is LoadAHRSIIpar_RAM to set the external GNSS receiver’s data type, baud rate, and data rate:
AA 55 00 00 0B 00 B0 35 04 09 0A 07 01
AA 55 – Header, 00 – Message type, 00 – Message identifier, 0B 00 – Message length, B0 – Code for LoadAHRSIIpar_RAM, 35 – Parameter code, 04 – Parameter value u-blox bin data type(depends on the receiver), 04 – Parameter value for 460800bps data rate, 0A – Parameter value for 10 Hz data rate, 07 01 – Checksum.
The response to the LoadAHRSIIpar_RAM command should be as follows:
AA 55 01 B0 09 00 00 00 00 BA 00
AA 55 – Header, 01 – Message type, B0- Message identifier, 09 00 – Message length, 00 – Data list error, 00 00 – Reserved, BA 00 – Checksum. If all bit values are 0 for the Data list error, there is no error in the parameter code, payload, and parameter values. A value of 1 for any of the bits indicates the errors shown in Table 6.44 of the AHRS-II ICD.
The last command is SaveAHRSIIpar_FLS, which has a constant structure. Save command would save all current device configurations from the RAM to nonvolatile memory:
AA 55 00 00 07 00 B2 B9 00
AA 55 – Header, 00 – Message type, 00 – Message identifier, 07 00 – Message length, B2 – SaveAHRSIIpar_FLS command code, B9 00 – Checksum.
The response to the SaveAHRSIIpar_FLS command should be as follows:
AA 55 01 B2 08 00 B9 00 74 01
AA 55 – Header, 01 – Message type, B2 – Message identifier, 08 00 – Message length, B9 00 – Checksum of the SaveAHRSIIpar_FLS command, 74 01 – Checksum.
The AHRS-II-P operation with an external GNSS receiver
Once both devices are successfully preconfigured, it is necessary to use the INS GUI software for working with devices. Please contact the Inertial Labs support team at support@inertiallabs.com, and we’ll provide the latest INS GUI. Connect the RS232 serial port of the external GNSS receiver to the COM2 port of the AHRS-II-P unit, connect the AHRS-II-P COM1 serial port to the host computer, and power the AHRS-II on. The AHRS-II-P is ready for operation with an external GNSS receiver.
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