Using the internal GNSS/IMU
Below we cover the basics of the internal GNSS/IMU for CANedge variants that support this.
Table of Contents
Install the device
The CANedge incl. GNSS/IMU relies on an active GPS antenna in order to provide valid GPS-based signals. The GPS antenna should be installed with clear visibility to the sky.
To make use of IMU data (e.g. acceleration rates), you must also ensure that the CANedge itself is installed and oriented correctly. See the Docs (Configuration/GNSS).[1]
DBC decode the data
The internal GNSS/IMU data is recorded in ‘CAN Channel 9’, an internal pseudo channel. To decode the raw GNSS/IMU CAN frames, use the DBC file from the Docs (Internal signals section). The DBC file is also included below.
Note that most software tools (asammdf GUI, Python API, …) support loading multiple DBC files, so that you can in parallel load DBC files relating to CAN Channel 1 and/or 2.
Reduce file size
By default, all GNSS/IMU messages are broadcast at the maximum frequency. This can quickly increase your log file size and we recommend that you filter/prescale the data to only record what you need at the frequency required. You can do so via the CAN INTERNAL section of the Configuration File. For details on filters/prescalers, see our section on reducing file size.
Enable sensor fusion
If your CANedge is installed in an automotive you can enable sensor fusion. This will greatly enhance accuracy, reduce signal noise and even let you retain a fix in GNSS hostile areas such as parking lots, mines, tunnels etc.
To enable sensor fusion in your Configuration File, selected the advanced view and set the ‘Dynamic platform model’ to Automotive and ‘Sensor fusion’ to Enabled. For most use cases, you can leave the IMU-mount alignment angles at their default values (with ‘Method’ set to Manual)[2].
In addition, you should align the device correctly as described in the CANedge1 Docs GNSS alignment section. In simple terms, this can be achieved in a car by installing it in your trunk with the DB9 connectors pointing towards the left wheel.
Use internal GPS time to sync CANedge RTC
The CANedge real-time clock (RTC) can be synchronized via a CAN message. This minimizes drift and can be useful e.g. in ensuring that the RTCs of multiple CANedge units stay in sync.
The CANedge Docs contain a detailed example on how to configure this. For a ‘quick start’ you can use below partial config file, which you can load via the partial config loader tool. This syncs the CANedge RTC based on the internal GPS Epoch signal when TimeConfirmed is true[3].
FW 01.08, RTC CAN sync 'partial config'
| [1] | Note that sensor fusion is only supported in automotive applications. Some messages, such as Attitude (roll, pitch, heading), are only valid when sensor fusion is active. |
| [2] | The Docs describe how you can manually adjust and/or estimate the IMU alignment angles. This is relevant if your device cannot be installed as per the default alignment and/or if you want to ensure that your installation is as accurate as possible. This is more advanced and not necessary in most use cases. Note that setting the IMU-mount alignment method to Estimate will result in the device only recording IMU adjustment angles and no other GNSS/IMU data - see the Docs for details. |
| [3] | We recommend that you also downscale the GnssTime message within the CAN INTERNAL tab by adding a Time prescaler with a value of 60000. This reduces the frequency of the message to one message per minute |