require an absolute reference to correct for that drift. Lastly, accuracy can be limited by the quality of the IMU, as INS can only generate a position in relation to an externally provided point.
But when combining GNSS and INS, they seamlessly complement each other. While the GNSS position understands your location in the world, the INS solution understands how you move through it. When GNSS signals are disrupted, or line of sight is lost, INS navigation can extend accurate positioning until the signals return. INS can be used as a constraint to reacquire lost GNSS signals, or filter out poor quality signals altogether. And lastly, the position calculated by GNSS can be used as the required reference point for INS to function. Altogether, GNSS and INS complement each other to make up for the other’s limitations. Meter- to centimeter-level accuracy can even be validated and further refined with a combined solution.
GNSS+INS for Kinematic Applications
A GNSS+INS solution allows kinematic applications to become much more accurate, especially in challenging GNSS environments. In this diagram, you can see how a moving vehicle relying only on a GNSS solution would lose positioning due to signal blockage and obstructions. Using an INS only solution would cause your position to be unreliable due to position drift over time without an external reference.
But, a combined GNSS and INS solution allows the moving vehicle to stay in line with the “true path,” or the trajectory with as many minimized errors as possible, for the application.
Different Methods for Sensor Fusion
Combining GNSS and INS is an example of what is called Sensor Fusion. There are other technologies that can be combined in this manner to deliver more and more accurate solutions, or to provide different types of information that help paint a better picture about the application.
Sensor fusion can include the combination of GNSS and INS, used in NovAtel’s SPAN® technology. Sensor fusion also includes additional sensors like odometers, light detection and ranging, also known as LiDAR, and vision aided navigation.
NovAtel SPAN technology describes our software that combines GNSS and INS positioning, as well as the hardware built to measure GNSS signals and IMU measurements. GNSS and INS solutions can be combined and integrated at various levels, from deeply coupled, to tightly coupled, to loosely coupled. This coupling describes how integrated GNSS and INS technologies and measurements are in the solution. Our introduction to GNSS book goes into more detail about loosely coupled, tightly coupled, and deeply coupled solutions.
Odometers track the velocity of a ground vehicle and can offer an independent measurement of distance to the GNSS and INS solution, especially when GNSS signals are disrupted. This is especially useful in GNSS signal outages, such as a tunnel.
LiDAR systems use pulses of light to understand the solution’s surroundings. Each light pulse that is emitted from the LiDAR unit gets reflected from various surfaces, allowing the system to compute a range between the sensor and any surrounding objects.
By monitoring the solutions range to an upcoming point with a known position, LiDAR can help improve position accuracy in an obstructed area, such as an urban environment. This is also an example of combining various types of data, positioning and point cloud data, to provide meaningful information about the location.
Here’s an example of a point cloud data map from a mobile mapping application. The lasers on the LiDAR device mapped the ground, the location of the trees and bridge. Darker green indicates the closer distance to the bottom of the canyon, while objects become light green to orange the further away they were. As the vehicle moves through the point cloud, it can match its current range to various objects (trees, bridge, etc.) to a previous range to the same object and estimate the change in position over time.
Vision aided navigation is also called photogrammetry. This concept is very similar to LiDAR, except the system is now using cameras 