Inertial Navigation System (INS)

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An inertial navigation system (INS): While GNSS provides accurate positional information with relatively low integrity, INS offers position and attitude information subject to drift but with relatively high integrity. It is a navigation aid that uses a computer (Flight controller), motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate via dead reckoning the position, orientation, and velocity (direction and speed of movement) of the moving object without the need for external references.

An INS is a system with two main components (IMU and GNSS).  Inertial measurement units (IMUs) typically contain three orthogonal rate-gyroscopes and three orthogonal accelerometers, measuring angular velocity and linear acceleration respectively where as GNSS/USBL is used to measure the absolute position.

INS performs the positioning stability or helps to bridge positioning gaps which sometimes could not be done by a GNSS/USBL or either by the IMU alone.

This means that if a drone pilot planned a drone to be flown from A to B then it may be started from point A and flys with its designed speed but after a little while, it may drift a little bit  from the right direction because of the systematic errors in the INS.

Integrity and Positioning Accuracy: This drifting happens because of the low relative positioning capability from the GPS, which would be corrected by its relatively higher accuracy of absolute position information, but with relatively low integrity capability of GNSS/USBL units . Whereas cycle-slip and loss of lock errors from the GNSS/USBL can be corrected by the short term, relatively low positioning but relatively high integrity capability of INS.

inertial-navigation-system-4-730Compensates Each Others Deficiencies: As we attempted to show in the above mensioned paragraphs because the two technologies have complementary characteristics. These two units will compensate each other’s deficiencies.

Direction and Speed: The three-dimensional direction, speed/acceleration are determined by the IMU (Inertial Measurement Unit). Using these determined directions and speed/acceleration information the three-dimensional relative position is translated.

The Right Direction Maintained: Then using the latest GNSS/USBL position information and the relative position that is already translated, the computer system calculates the amount of drift value to maintain its best approximate direction again, and  in order for maintaining the right direction it  will take further process like filter setting and Vessel geometry calibrations .

IMU: So we can say IMU is the system which contributes relative position computation and the GPS/USBL error compensations over time to the overall system.

IMU Components:  IMUs now a day are built from three accelerometer and three gyroscopes to provide all rotation information of the drone.

Filter Setting and Vessel Geometry: However, in order to the INS achieve the accurate absolute positioning of itself having only the compensated absolute position of the drone that we have seen in the above section is not enough. But it requires the following two procedures in addition.

  1. Filter setting: This is the procedure of integrating the different parameters according to the platform type and its most probable behaviours.
  2.  Vessel Geometry: In this procedure, the two axises (the INS axis and the platforms axis) will be exactly aligned. This is done mechanically and the remaining offsets will be determined by using proper calibrating procedures.

Final Compensated Navigation Parameters: Inorder to reach the final compensated navigation parameters of the INS’s positions, in addition to the above procedures, the relative position between the GNSS/USBL and IMU must be determined as well.

The sources for the above notes are : GIM and Wikipedia
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