An Alternative Low Cost MEMS IMU/GPS Integration Scheme

This article investigates the use of artificial neural networks for developing an alternative integration scheme of low cost Microelectromechanical System (MEMS) Inertial Navigation System (INS) and Global Positioning System (GPS) for vehicular navigation applications. The primary objective is to overcome the limitations of current INS/GPS integration scheme and improve the positioning accuracy during GPS signal blockages. The results presented in this article indicated that the proposed technique was able to provide 47% and 78% improvement in terms of positioning accuracy during GPS signal blockages.

Introduction

With the evolution of modern computer technology in hardware and software, the fi eld of artificial intelligence has been receiving more attention in the development of new generation technology. Artificial intelligence (AI), also known as machine intelligence, is defi ned as the intelligence exhibited by anything manufactured (i.e. artificial) by humans or other sentient beings or systems (should such things ever exist on Earth or elsewhere) [Cawsey, 1999]. It is usually hypothetically applied to general-purpose computers. The term is also used to refer to the field of scientific investigation into the plausibility of and approaches to creating such systems.

Artificial intelligence has been verified as a successful and effective tool for providing solutions to certain engineering and science problems that can not be solved properly using conventional techniques [Cawsey, 1998]. The goal of applying artificial intelligent technologies is to provide intelligence and robustness in the complex and uncertain systems similar to those seen in natural biological species [Honavar and Uhr, 1994]. According to Russell and Norvig [2002], the techniques and the related research fi elds of artificial intelligence (AI) are given in Figure (1).

It is well known that Kalman filter approach has been widely applied as the core algorithm for INS/ GPS scheme for many navigation applications. Although it represents one of the best solutions for INS/ GPS integration applications, it has limitations in terms of model dependency, prior knowledge dependency, sensor dependency, and linearization dependency for general INS/GPS integrated navigation applications [Chiang, 2004]. Consequently, in order to overcome or reduce the impact of these limitations, several research works have been conducted to investigate possible alternative algorithms for INS/GPS integration scheme [see for example, Chiang, 2004]. The incorporation of Artificial Intelligence Algorithms (AIAs) for developing alternative INS/GPS integration scheme is fueled by the need for intelligent systems and the limitations with the current INS/GPS integration scheme.

Among artifi cial intelligent methodologies shown in the Figure (1), ANNs have been extensively studied with the aim of achieving human-like performance, especially in the field of pattern recognition and robot control and navigation [Mandic and Chambers, 2001]. ANNs are composed of a number of nonlinear computation elements which operate in parallel and are arranged in a manner reminiscent of biological neural interconnections. In addition, ANNs are designed to mimic the human brain and duplicate its intelligence by utilizing adaptive models that can learn from the existing data and then generalize what it has learnt [Ham and Kostanic, 2001]. Therefore, this article attempts to evaluate an alternative INS/GPS integration schemes developed by the authors for general land vehicular navigation and positioning applications using low cost MEMS INS.

Problem statement

According to Chiang [2004], the limiting factors of Kalman filter based INS/GPS integration are given in brief as follow;

Model dependency

Generally speaking, the development a model to be used in the Kalman fi lter starts with the construction of a full scaled “true-error model”, whose order is then reduced based on the prior knowledge and the insight gained into the physics of the problem, covariance analysis, and simulation [Salychev et al, 2000].

Typically, the dynamics model is based on an error model for three position errors, three velocity errors, and three attitude errors in an INS (the system error states). These errors are also augmented by some sensor error states such as accelerometer biases and gyroscope drifts, which are modeled as stochastic processes (i.e., 1st Gauss Markov process or random walk) [Rogers,2003]. In fact, there are several random errors associated with each inertial sensor. Therefore, it is usually diffi cult to set a certain stochastic model for each inertial sensor that works efficiently at all environments and reflects the long term behavior of the sensor errors. Hence a model-less navigation algorithm that can perform the self-following of the vehicle under all-conditions is required.

Prior knowledge dependency

As mentioned previously, some initial knowledge is required to start a Kalman fi lter, such as the state transition matrix (Fk,k-1), the measurements design matrix (Hk), the noise coefficient matrix (Gk-1 ), the system noise covariance matrix (Q) and the measurements noise covariance matrix (R). Among them, the Q and R matrices are the most important factors for the quality of the Kalman filter estimation for an INS/ GPS integrated system. Theoretically, the optimal Q and R matrices can guarantee the optimality of the estimation; however, tuning the Q and R matrices can be time consuming and it requires experience and background in both systems. Consequently, the requirement of human intervention for Q/R tuning is very high. In other words, the tuning process can be regarded as a special form of learning as it is usually done by an expert and needs time to obtain the optimal solution. Consequently, a new navigation algorithm that can reduce the level of human intervention and is capable of learning by itself to adapt the latest dynamic model is preferred.

Sensor dependency

The need to re-design algorithms based on the Kalman filter (i.e., states) to operate adequately and efficiently on every new platform (application) or different systems (e.g. switch from navigation grade IMU to tactical grade IMUs) can be very costly. In addition, the Q and R matrices tuning is heavily system dependent [Vanicek and Omerbasic, 1999]. As a result, a new navigation algorithm that is adaptable and can reduce the level of sensor dependency is highly desirable.

Linearization dependency

INS/GPS integration for land vehicular navigation is composed of non-linear dynamic in nature. However, since the principle of Kalman fi ltering is to estimate a linear dynamical model using a recursive algorithm along with certain stochastic information, the linearization of INS or GPS dynamics model is required [Brown and Huang, 1992]. However, the linearization process is usually a 1st order approximation process that results in deviations between the assumed “true error model” and the real “true error model”. As a result, a new navigation algorithm that is nonlinear in nature and can reduce the impact of linearization is preferred.
The impact of those limiting factors projects on the positional error during GPS outages. In other words, each of those factors contributes certain amount of positional error accumulation when the Kalman filter operates in prediction mode.

Objectives

The objectives of this article are to: (1) provide a brief review about the latest development of alternative INS/GPS integration scheme and (2) evaluate an alternative INS/GPS integration scheme developed by the authors for the use of a low cost MEMS INS/GPS integrated system.

Recent development of alternative INS/GPS Integration chemes

The primary objective of developing alternative INS/GPS integration scheme is to reduce the impact of remaining limiting factors and improve the positioning accuracy during GPS signal outages. The recent research activities involved with developing alternative schemes for general navigation applications fall into two categories:

Alternative fi lters

Xu [1996] suggested a new selflearning navigation fi lter associated with probability space and non- Newtonian dynamics. This new filter relied basically on the information contained in measurements on the vehicle: position fixes, velocities and their error statistics. Mohamed [1999] suggested Adaptive Kalman filter (AKF) based INS/GPS integration architecture. Fredrik et al., [2002] proposed a framework for positioning, navigation and tracking problems using particle fi lters (sequential Monte Carlo methods). It consisted of a class of motion models and a general non-linear measurement equation in position. Frykman [2003] suggested particle filters based aircraft integrated navigation with the utilization of INS and GPS. Shin and El-Sheimy [2004] suggested an UKF based INS/GPS integration scheme.

AIAs

Meng and Kak [1993] suggested a neural network-based navigation algorithm for a mobile robot. Townsend et al., [1994] proposed a Radial Basis Function (RBF) Networks approach for mobile robot positioning. Dumville and Tsakiri [1994] utilized a neural network to integrate DR and GPS for land vehicle navigation. Chansarkar [1999] utilized RBF networks for GPS positioning and navigation. Forrest et al., [2000] suggested an inertial navigation data fusion system employing an unsupervised neural network as the data integrator to estimate INS errors. Ojeda and Borenstein [2002] proposed a fuzzy logic rule-based position estimation algorithm for mobile robots as one of the prototypes of marsian rovers.

As for INS/GPS integration, Chiang and El-Sheimy[2002] and Chiang et al., [2003] first suggested an INS/ GPS integration architecture utilizing Multi-Layer Feed-Forward Neural Networks (MFNNs) for fusing data from DGPS and either navigation grade IMUs or tactical grade IMUs. Chiang [2003] proposed an MFNN based INS/GPS architecture for integrating IMUs with Single Point Positioning (SPP). Chiang [2004] proposed an optimal GPS/ MEMS integration architecture for land vehicle navigation utilizing neural network. Chiang and El- Sheimy [2004] proposed the idea
of developing the conceptual intelligent navigator that used ANNs approach for next generation land vehicular navigation and positioning application. In addition, El-Sheimy and Abdel-Hamid [2004] suggested a model derived from Adaptive Neuro- Fuzzy Interference System (ANFIS) to bridge GPS outages in MEMSINS/ GPS land vehicle navigation.

The common characteristic of those research works is to reduce the impact of those limiting factors mentioned above. According to the results and conclusions given by Mohamed [1999], Frykman [2003], Ojeda and Borenstein [2002], Shin and El-Sheimy [2004], Chiang [2004] and El-Sheimy and Abdel- Hamid [2004], a comparison between different INS/GPS integration schemes is given in Table 1.

As indicated in Table 1, the AIAs are able to provide more advantages for implementing alternative INS/ GPS integration schemes. Due to the limited scope of this article, only ANN based INS/GPS integration scheme developed by the authors will be discussed as an example to demonstrate the benefits of incorporating of incorporating AIAs as the core component for alternative INS/GPS integration schemes.

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