GNSS and inertial navigation systems (INS) are frequently combined to optimize navigational assurance in manned and unmanned systems such as airplanes, spacecraft, land vehicles and marine vessels. They are complementary technologies because while GNSS/GPS systems provides an absolute position in the world, INS measures your orientation and movement within it.
But how does the fusion of two different sensor technologies build a more robust and resilient system?
Using Hexagon | NovAtel’s SPAN GNSS+INS technology as a case study, we’ll outline how these sensors complement each other and further refine accuracy and reliability in high-precision applications.
The purple path indicates a combined GNSS+INS solution that retains accuracy even when satellite signals (grey path) are interrupted. The GNSS-only path (orange) and INS-only path (blue) both lose accuracy from the system’s true path (green).
Why combine GNSS and INS?
While GNSS and GPS receivers compute your position in relation to satellites, inertial navigation enables measurement of your velocity, attitude and any heave motion. When combined, the two navigation techniques enhance each other. An inertial measurement unit (IMU) allows a GNSS receiver to maintain position when satellite signals are interrupted. In contrast, the absolute position and velocity accuracy of GNSS helps compensate for potential IMU errors like positional drift. NovAtel explains how these two technologies complement each other in episode five of their Introduction to GNSS webinar series.
For example, GNSS can provide a user in a moving vehicle with its location. If the GNSS tracking is interrupted due to multipath or potential interference, the user cannot be sure of their location. When these algorithms include inertial measurements, the user’s positioning solution can estimate its trajectory and location based on its present speed, attitude and direction.
See the above diagram, where the moving vehicle’s positioning becomes inaccurate and unreliable when satellite signals are lost. By using a combined GNSS+INS solution (seen in purple), the system maintains its accuracy.
NovAtel’s OEM7 family of GNSS receiver boards and combined GNSS and IMU enclosures use SPAN GNSS+INS technology to fuse these measurements and optimize their positioning solution.
Building a robust positioning system
Different GNSS and INS architectures enable varying levels of integration: loosely coupled, tightly coupled and deeply coupled. These levels of integration describe the level of robustness within the positioning system.
Performance and system robustness increase as GNSS andINS measurements become more deeply coupled and integrated.
With no integration, GNSS and INS operate independently without blended measurements. When loosely coupled, GNSS positioning and velocity measurements help the IMU compute an inertial trajectory. This process merges GNSS signal processing with IMU estimates in an INS filter. However, there is no feedback between the INS and GNSS solutions to further refine measurements.
A tightly coupled architecture combines raw GNSS measurements with raw IMU data, enabling the GNSS receiver to compute a position with fewer than the minimum four GNSS signals. There is feedback between the GNSS and INS solutions, enabling continuous operations through GNSS interruptions. With this solution, users can continue working even in areas with partially obstructed GNSS signals.
The most robust GNSS+INS system possible is within a deeply coupled architecture. With this integration level, a user receives the benefits of continuous operation and 3D positioning in a tightly coupled system while also achieving rapid signal reacquisition when a GNSS signal is lost and a high resistance to interference and jamming.
Spoofing describes when your GNSS position is intentionally altered. A deeply coupled GNSS+INS solution is harder to fool because it can continue operating through GNSS interruptions.
Building a resilient positioning system
Jamming and spoofing both require interference to overwhelm a GNSS system. Once the signal lock is lost, the user no longer has a reliable and trustworthy position – their system may become jammed, or the satellite signals may produce a false and spoofed position. However, the more resilient a positioning system, the better it can withstand interference, jamming and spoofing attempts. Learn more about how to identify and protect against interference in this GPS World article.
A deeply coupled GNSS+INS solution enables a user to rely on inertial measurements to supplement partial GNSS signals. Continuous operations are possible, even when faced with interference. When the interference ends, a deeply coupled system reacquires its satellite tracking much quicker than a loosely coupled system.
SPAN GNSS+INS technology uses a deeply coupled architecture that is inherently resistant to interference. NovAtel’s family of OEM7 receivers supports SPAN technology, as well as the extended GNSS Resilience and Integrity Technology (GRIT) firmware suite.
Resilient and robust GNSS+INS technology
GNSS and INS do more than complement each other; they enhance their positioning solution’s accuracy, robustness and resiliency. Users with manned and unmanned aerial vehicles working in surveying, defense, transportation or agriculture rely on sensor fusion innovations like SPAN GNSS+INS technology for continuous operations. The 3D positioning and orientation provided along with the built-in resiliency of a deeply coupled solution benefits the user further in these applications.
NovAtel’s OEM7 GNSS receiver boards integrate easily into any positioning system, bringing SPAN GNSS+INS technology support with them. GNSS+INS enclosures with combined receivers and IMUs like the PwrPak7D series, CPT7 or MarinePak7 combine OEM7 receivers with high-accuracy IMUs for a ready-to-install GNSS and INS positioning and navigation solution.
To learn more about NovAtel’s positioning solutions, SPAN technology and interference mitigation tools, reach out to their sales team.
All images: NovAtel
This page was produced by North Coast Media’s content marketing staff in collaboration with NovAtel. NCM Content Marketing connects marketers to audiences and delivers industry trends, business tips and product information. The GPS World editorial staff did not create this content.
