Intro to GNSS On-Demand Webinars

How do you measure a position through GPS satellites? What happens to a satellite’s signal as it travels toward the Earth? Which calculations or technologies do we use to generate the most accurate positioning possible?

As a leader in positioning and autonomy, Hexagon | NovAtel® wrote the book on GNSS. Now, we’re making that knowledge shareable far and wide in our webinar series.

Through the series, we follow a satellite’s signal through space, reception by antennas, computations by receivers, and applications across user equipment. We’ll explain how satellite infrastructure and atmospheric effects can contribute to positioning errors, and how technologies compensate for and resolve them.

We hope this series sparks your imagination! When you’re ready to learn more about GNSS technologies and its applications across industries, download our Introduction to GNSS book here.

Hexagon | NovAtel Enclosures and Post-Processing Software Product Manager Kiera Fulton describes how inertial navigation systems can be combined with traditional GNSS for enhanced positioning in episode five of our on-demand webinar series. She demonstrates how INS use inertial measurement units to understand motion, and outlines several methods of sensor fusion.


Hello, and welcome to episode five of NovAtel’s webinar series on An Introduction to GNSS. My name is Kiera Fulton; I’m the enclosures and post-processing software product manager at NovAtel, part of Hexagon. Thanks for joining me!

So far, the series has demonstrated and explained calculations and technologies behind GNSS, so you now should have solid foundational understanding of satellite positioning. Last episode, Jenn reviewed how we resolve positioning errors through GNSS equipment and infrastructure. Another 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 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 instead of laser beams. Usually, this kind of system takes the form of cameras to identify objects as reference points. The solution can then calculate relative position to those reference points, as well as a general heading to know what direction the solution is moving in. 

This is by no mean a comprehensive list of all the sensors that can be used to aid GNSS. The technologies listed here are just a sample of some of the more widely used techniques.


Sensor Fusion Across Industries

There are many different applications for a combined GNSS and INS solution, including automotive and agriculture. Not only can SPAN track the movement of a ground vehicle, but the trusted position can also be used to ensure it stays on a specified path. This is a fundamental technology used for autonomous cars and tractors.

In a marine environment, the right GNSS+INS solution would be able to compensate for heave motions from waves. For mobile mapping, INS sensors can monitor the subtle changes in position, velocity, and attitude on a small device carried by its user, while using LiDAR to collect 3D point cloud data about the user's surroundings. 

Sensor fusion with GNSS and INS technologies creates a comprehensive positioning solution that extends and verifies an accurate position in moving objects. They are very complementary technologies.