r way to resolve errors and further finesse your position is by using additional sensors like inertial navigation systems.
In this episode, I’ll explain inertial navigation systems, sensor fusion, and how additional systems work to complement GNSS positioning. If you’d like to read ahead about these technologies, download our free book, An Introduction to GNSS.
What are Inertial Navigation Systems?
An inertial navigation system uses rotation and acceleration information to understand your movement in a three-dimensional space. It calculates your position, velocity, and attitude in relation to an external reference. The system uses inertial measurement units (IMU) to calculate your precise relative movement over a period of time. An IMU includes a gyroscope and accelerometer on each of the three orthogonal axes. These sensors measure rotational and linear acceleration, so that the INS can use these measurements to determine position, velocity, and attitude, including roll, pitch, and azimuth.
But, IMUs still need an external reference in order to determine your location on the Earth. The INS on its own can only provide measurements in reference to itself. In order to understand where it is situated in 3D space, it requires an external reference. That is where GNSS comes in.
Combining GNSS and INS
Both GNSS and INS positioning have benefits and limitations. As we’ve seen in past episodes, GNSS positioning delivers latitude, longitude, and height positioning nearly anywhere in the world, but you need to have direct line of sight to at least four satellites. You can have meter to centimeter-level accuracy, but that varies and is even disrupted as a result of signal blockages, multipath, and atmospheric delays.
INS calculates the change in position based on its own direction and orientation measurements, but requires an external reference for those measurements to be applicable to the environment. As with any system, errors are present; for INS, sensor errors inherent to the IMU can cause position drift over time and 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