From IMU to GNSS/INS
Within each series, Xsens offers several product variants. Each variant is based on a firmware version which enables different functionalities. The figure below summarizes the functionality of each variant.
From IMU to GNSS/INS
IMU
The IMU variant is an Inertial Measurement Unit that measures 3D acceleration and 3D rate of turn with the addition of 3D magnetic field data and, depending on the product, barometric pressure. It does not fuse sensor data to deliver orientation estimates. The IMU can also be configured to output data generated by the strapdown integration algorithm (orientation increments ∆q and velocity increments ∆v).
VRU/AHT
The Vertical Reference Unit (VRU), also referred to as Active Heading Tracker (AHT) adds the first layer of algorithms which uses gravity as a reference for roll and pitch calculations. Essentially it delivers the same data as the AHRS, except for the yaw. The yaw estimate of a VRU/AHT product is unreferenced, which means that it is computed without any geographic/magnetic reference, though still superior to just gyroscope integration (e.g., when using the gyro bias estimation techniques). All data outputs from the IMU are also available in this product version. The AHS feature is also available in this product variant (see also chapter Additional setting options and features).
AHRS/MRU
This is the full Attitude and Heading Reference System (AHRS), also referred to as Motion Reference Unit (MRU) in marine applications. It provides various outputs: roll, pitch and heading (magnetic referenced yaw). In addition, all functionalities of the IMU and VRU are also available in this product variant. As of firmware version 1.4.1, the Xsens Avior and Xsens Sirius also support Heave data output.
GNSS/INS
The GNSS/INS variant is a product with an interface to an external or internal (RTK) GNSS receiver as well as a barometer. It provides roll, pitch, yaw/heading, as well as 3D position, 3D velocity and time data. In addition, all data outputs of the IMU, VRU/AHT and AHRS are also available in this product variant.
RTK GNSS/INS
The RTK GNSS/INS variant extends the GNSS/INS with Real-Time Kinematic (RTK) positioning. It uses correction data (RTCM) from a base station or an NTRIP service. With these corrections the GNSS receiver resolves carrier-phase ambiguities, which improves position accuracy from meter level to centimeter level when an RTK fixed solution is available. Without corrections, or when the receiver has only an RTK float solution, the product falls back to its standard GNSS accuracy. The RTK receiver is either integrated in the product or connected externally, depending on the product. In addition, all data outputs of the IMU, VRU/AHT, AHRS and GNSS/INS are also available in this product variant.
Dual Antenna RTK GNSS/INS
The dual antenna RTK GNSS/INS variant uses a GNSS receiver with two antennas mounted at a known distance apart (the baseline). From the relative position of the two antennas, the receiver computes a GNSS heading, which the sensor fusion algorithm uses as a direct heading reference. This gives a true-north referenced heading that doesn't depend on the magnetic field. It also doesn't require the vehicle to move or accelerate, so heading is available even at standstill or at low dynamics. This makes it well suited for applications with magnetic disturbances, or where accurate heading is needed right after start-up. Correct heading output requires configuring the antenna baseline and lever arms. Dual antenna heading can be combined with RTK positioning. In addition, all data outputs of the IMU, VRU/AHT, AHRS, GNSS/INS and RTK GNSS/INS are also available in this product variant.