relative and smooth. Any independent source of information, such as adding ALIGN to your

SPAN solution, provides one more way to help improve your solution.

providing signals with their location and time. Using these signals and solving for time, a receiver

is able to determine the user’s precise latitude, longitude and altitude. For more in-depth

information, see our book on GNSS (available here

GNSS computed positions are typically very accurate and available. However, there are

locations where GNSS does not work well, or at all, such as in ‘urban canyons’ or in tunnels

in velocity, combined with the direction (attitude), the INS determines its velocity and direction.

Summing the velocity leads to position.

In this way, the INS determines the change in position, velocity and attitude. Because the

changes measured by the INS are relative, a starting point, called the initial alignment, is

needed. GNSS can easily provide the starting position and velocity, and the INS can detect the

roll and pitch from the ‘strong’ signal of gravity. The most difficult component is heading, as

computed relative to true North and is available on an ALIGN enabled SPAN system.

For more information on dual antenna products from NovAtel, visit our website here

The process to determine heading for an INS is difficult. In high grade systems, the gyroscopes

are sensitive enough to detect the spin of the Earth

North. From the initial alignment, changes in heading are continually added to the solution to get

the current heading. In systems that are less sensitive, an external source is required to provide

the initial alignment.

providing an initial alignment for the attitude solution.

The most difficult attitude component to determine in a GNSS + INS solution is heading

(azimuth) because the rotation of the Earth is small compared to the noise and errors in

gyroscope measurements. Roll and pitch can be determined with greater ease, as they depend

on the very strong ‘signal’ of gravity. Adding an ALIGN solution improves the ongoing

the latest release of SPAN firmware, the difference between the ALIGN heading vector and the

SPAN heading is now constantly estimated and refined, allowing the system to make better use

of the heading update. Also, the ALIGN solution is now used when the system is in a turn,

increasing the availability and usefulness of the external heading update.

As with all systems that involve multiple inputs, the system is greater than the sum of the parts.

The ALIGN solution is only one component of the SPAN heading. While the ALIGN solution is

very accurate (improving with increased antenna separation), the additional need to

mathematically transform the ALIGN solution into the INS computational frame, along with the

GNSS measurement noise in the ALIGN solution, requires careful weighting of the additional

information in the SPAN solution.

‘see’ the error in the offset from the ALIGN solution to the INS heading, allowing the system to

make better use of the ALIGN heading information.

Low dynamic situations (slow velocities, few turns) are a challenge for inertial navigation

systems. This is especially true for lower cost systems. Lower dynamics reduce the ability of the

navigation filter to estimate the errors in the system because there are fewer independent

observations. Adding ALIGN to your system provides an additional, independent observation

source of the most difficult to estimate parameter (heading) of a GNSS + INS system.

The figure below shows how ALIGN helps in very low dynamic situations. This dataset came

from a test situation that is very difficult for a lower grade IMU. The error was estimated relative

to a navigation–grade, post-processed inertial reference.

determined from the results of the IMU input into the SPAN system.

Because these two headings come from different sources, they behave differently. The INS

system will not wander, but will have sharper, discrete jumps as new measurements are made.

Be sure to use the covariance information, available in the INSCOV or INSATTX logs to assess

the quality of the solution.

While ALIGN benefits all SPAN systems; the effect of that benefit is most noticeable with lower

grade IMUs, such as the SPAN-CPT. The ALIGN solution helps to restrict the drift that the

provide a better chance for the system to improve its accuracy.