Inertial navigation sensors for mobile mapping

Inertial navigation sensors for mobile mapping

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Khurram Shaikh
Khurram Shaikh
Institute Of Advanced Technology (ITMA),
University Putra Malaysia
[email protected]

Rashid Shariff
ITMA, University Putra Malaysia,
[email protected]

Hishamuddin Jamaluddin
Department of Biological & Agricultural Engineering,
Faculty of Engineering, University Putra Malaysia,
[email protected]

Farrukh Nagi
College of Engineering, University Tenaga National (UNITEN), Malaysia,
[email protected]

Shattri Mansor
ITMA, University Putra Malaysia
[email protected]

Global Positioning System (GPS) is the main tool being utilized for navigational purpose but it is dependent on the satellite signals. Unfortunately these signals may get lost due to the blockage by buildings and canopy of the trees and in case of interference and jamming of signals. Inertial Navigation System (INS) can address this problem and support the non-availability of GPS signal for a short time

Global Positioning System (GPS) is the most popular navigation system in use today. But its limitations open doors for an autonomous navigation system such as Inertial Navigation System (INS). A decade ago Inertial sensors were only limited to aerospace and military applications due to their high cost and restrictions by the government but now due to the reducing cost they are finding their way in civil applications such as, mobile tracking, precision surveying, and precision farming.

No one denies the accuracy of GPS with the increased constellation of satellites and improved methods of data processing but GPS is still dependent on the satellite signal. Today, the surveyors are having problems of accuracy in places where the signal gets lost due to blockage by buildings, canopy, and other obstructions. Many of the studies are being carried out to address the issue of signal loss. INS is one the most popular navigation systems that can provide an autonomous solution in case of signal loss. The output given by inertial sensors is in terms of accelerations and angles that need to be processed to get position information.

Inertial Navigation System
INS has been around since the mid of twentieth century and now gaining popularity due to technological advancements in micro-machined sensors that are reducing in size as well as cost with the passage of time. The good thing about it is its autonomous availability and high bandwidth. An INS is the combination of sensors (Accelerometers and Gyros) grouped together as one equipment. Accelerometer, as the name suggests, senses the acceleration along a single axis and Gyro senses the angular movements also along the single axis. Both, Accelerometers and Gyros operate on the inertial principles (Newton’s Laws of Motion) providing navigation solution and for this reason it is named as Inertial Navigation System. An INS contains three accelerometers (measure accelerations) and three gyros (measure angles). These Inertial sensors are mounted as three orthogonal axes in an Inertial Measurement Unit (IMU) {A black box that houses the inertial sensors} and output the accelerations (velocity changes over time) that can be integrated twice to get positions and Angles (Angular rates in case of rate sensors) along the same axes. Angular rate sensors are used to measure the rate at which a vehicle rotates around a given axis. The rotations around x, y and z-axis are termed as Roll, Pitch and Yaw respectively.

These rotations can be integrated once to get the angles as a function of time and velocity changes and are converted to navigation data by numerically integrating twice and transformed from Body to Navigation Coordinate system to give us positions in navigation frame that is relatively fixed.

Velocity
Position

The good thing about INS is its independent and jamproof navigation data, as compared to GPS that is dependent on satellite signal, however, INS accuracy degrades with respect to time making it a major drawback. Most of the errors in the INS are caused by sensor imperfections (instrumental errors), therefore, accuracy mostly depends on the type of sensors available. However, the cost of the INS is directly proportional to the accuracy, implying that high performance accurate sensors are still very expensive and limited to certain applications.

The cost of tactical and navigation grade sensors is still very high and beyond the reach for certain civil applications. The use of accurate sensors is limited to commercial and military applications. The first INS was built and based on mechanical gyros with very complex and power consuming architecture. Later on strapdown solutions have been realized by using modern integrated electro-mechanical or electro-optical sensors (Luethi, 2000). These strap down systems are mostly based on the MEMS (Micro Electro-Mechanical System) technology that is relatively inexpensive and compact. These cost-effective sensors, due to their short-term sustainability and complementary characteristics, are widely used in INS especially with GPS.

Strapdown Inertial Navigation Systems (SINS)
A strapdown system is a major hardware simplification of the old mechanical systems. The accelerometers and gyros are mounted in body coordinates and not mechanically moved. Instead, a software solution is used to keep track of the orientation of the IMU (and vehicle) and rotate the measurement from the body frame to the navigation frame. This method overcomes the problems of old mechanical (gimbaled) system, and most importantly reduces the size, cost, power consumption, and complexity of the system. (Walchko, 2002). Figure 1 is a functional block diagram of a strapdown INS mechanization, which describes the process flow of the accelerations to the position, velocity and attitude. The angles (Angular rates in case of rate sensor) given by the INS need to be converted to Euler angles to be used in Direction Cosine Matrix (Transformation Matrix).