The benefits of GNSS INS integration

When the line of sight from GNSS equipment on the ground to satellites orbiting the Earth is blocked by obstructions such as trees or buildings, navigation becomes unreliable or impossible. That is where GNSS INS integration comes in handy.

What is an inertial navigation system?

An inertial navigation system (INS) is used to compute relative position over time from rotation and acceleration information provided by an inertial measurement unit (IMU). An INS uses the measurements from the IMU to provide position, velocity and attitude (roll, pitch and azimuth) calculations. When combining an INS with GNSS, the two navigation techniques augment and enhance each other. The relative position of the INS can be used to bridge through times when the GNSS solution is degraded or unavailable. This is known as "sensor fusion."

However, IMUs still need an external reference to determine your location on the Earth. The INS on its own can only provide measurements in reference to itself. To understand where it is situated in 3D space, an external reference is required. Enter GNSS.

Benefits of GNSS INS integration

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.

In addition, the ability of the INS to provide attitude determination is an important addition for several industries and applications, including marine, automotive and agriculture, and for activities involving aerial survey, mobile mapping, hydrography and autonomous vehicles.

Like all things, no system is perfect. The measurements provided by the IMU include several error sources. An uncorrected INS will drift from the true position quickly without a way to constrain the error growth. GNSS provides an external reference to the INS, allowing it to estimate the errors in the IMU measurements using a mathematical filter and mitigate their effect. Using GNSS provides an absolute set of coordinates for the initial start point, as well as continuous positions and velocities, which are used to update the INS filter estimates.

Altogether, GNSS and INS complement each other to make up for the other’s limitations. Metre- to centimetre-level accuracy can be achieved and further refined with a combined solution.

Types of GNSS INS integrations

Hexagon | NovAtel's SPAN technology describes our software that combines GNSS and INS positioning, using measurements from a GNSS receiver and IMU. GNSS and INS solutions can be combined and integrated at various levels, from loosely coupled, to tightly coupled, to deeply coupled. This coupling describes how integrated GNSS and INS technologies and measurements are in the solution.

Using GNSS positions and velocities to estimate INS errors is an example of a "loosely coupled" system. Essentially, the two components can operate independently but can communicate through a standardised interface. This type of coupling may be preferred in dynamic environments that have a strong need for adaptability and autonomy.

As the name implies, a "tightly coupled" or "deeply coupled" system has components that are strongly dependent on each other. Tight coupling has the advantage of better navigation accuracy and reliability but can be more challenging to maintain as a change to one component requires modifications to other parts of the system.

Regardless of how GNSS and INS are integrated, the two techniques enhance each other to provide a powerful navigation solution.

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