Linear system construction of multilateration based on error propagation estimation
 Yanjun Hu^{1, 3},
 Lei Zhang^{1, 3},
 Li Gao^{2}Email author,
 Xiaoping Ma^{3} and
 Enjie Ding^{1}
https://doi.org/10.1186/s1363801606576
© Hu et al. 2016
Received: 13 March 2016
Accepted: 14 June 2016
Published: 29 June 2016
Abstract
Iterative localization algorithms are critical part in the control of mobile autonomous robots because they feed fundamental position information to the robots. In a harsh unknown environment, the estimation of environmental noise is hardly obtained during the movement of the robots. It means that the stateoftheart methods, which increase localization accuracy using error management, are unsuitable. In this paper, we deduced an upper bound of the localization error without knowing the precise model of environment noise when the anchor nodes have position errors. Utilizing the minimum upper bound, we can construct an optimal localization linear system of iterative localization algorithms based on least square. An algorithm of generating localization linear system is proposed by using the minimum upper bound. The algorithm reduces the impact of the shortage of environmental information on localization error propagation. Our simulation results show that the algorithm is insensitive to noise and can improve the localization accuracy by constructing a proper localization linear system with a high probability.
Keywords
1 Introduction
Indoor iterative localization algorithm of autonomous robots is an active subject because of the environmental complexity. The coordinates of autonomous robots are the fundamental parameters of robot control [1]. Due to the absence of Global Positioning System signals in an indoorlike environment, the autonomous robots need an iterative localization algorithm to provide their position information. Laser, sonar, infrared, visual sensors, or some combinations of these methods are used to locate the robots [2]. However, those technologies may fail in some harsh environments, such as a firing building which is dusty, smoky, and dark.
Iterative localization based on Received Signal Strength (RSS) is a suitable option to provide the position information in a harsh scenario [3]. Indoor iterative localization based RSS is exploited in rangingbased techniques, which maps the distance by a measurement of RSS, see e.g., [4–6]. Since the measurement noise is inevitable in the practical localization system, algorithms are proposed to improve the localization accuracy. In those algorithms, the key component of decreasing localization error is to estimate the measurement noise. And, those algorithms work effectively if the noise is estimated precisely [7]. However, in a harsh scenario, the noise estimation is almost impossible to be achieved because of the insufficiency on measurement noise caused by the robots mobility [8]. A new strategy of improving localization accuracy is needed to solve the problem. Actually, localization accuracy is influenced by the construction of localization linear system (LLS) when least square is used to calculate the position of the unknown node. Therefore, we can decrease localization error by constructing proper LLS. In this paper, through the studying of upper bound of localization error of LLS, an upper bound of error propagation in the localization is proposed. And, an algorithm which can be used to improve the localization accuracy without environmental noise estimation, by utilizing the upper bound to generate an optimum LLS.
The paper is organized as following: In Section 2, related works are introduced. In Section 3, the key step of iterative localization is briefly described to introduce the symbols. In Section 4, after introducing the orthogonally invariant norms, it is given that the error upper bound of localization using anchors with localization error. And, an algorithm of improving localization accuracy is proposed by constructing optimum LLS which uses the minimum upper bound. In Section 5, the random and efficient of algorithm are verified by simulations.
2 Related work
The iterative localization algorithm is a distributed, infrastructurefree positioning algorithm to calculate nodes’ positions in the ad hoc networks [9, 10], which is a “spreading” process of node information. The process consists of three processes, which are node registry, neighbor selection, and update criterion [11].

Initiating nodes: Each node in the ad hoc network initiates its coordinate and the coordinate’s errors.

Selecting originanchors: Three or more nodes are selected as originanchors, whose coordinate errors are considered as zero. Then, a relative coordinate system is built by using those nodes.

Generating pseudoanchors: An unknown node selects at least three located nodes from the neighbor nodes set to calculate its position. After the node is located, this node is updated as a pseudoanchor.
In this perspective, an iterative localization algorithm is a process of transforming the nodes into the pseudoanchors.
Obviously, the localization accuracy is influenced by the last two steps. The method of selecting originanchors is studied, such as choosing the nodes with maximum density factor [9]. Consider the executing time, the process of pseudoanchor generation will be executed more frequently than the process of originanchor selection. Therefore, improving the localization accuracy in the step of generating pseudoanchors will significantly decrease the localization errors of all pseudoanchors. To meet the requirement of improving localization accuracy, physical methods and cyber methods can be used.
In particular, physical methods are based on the idea that the less measurement error is the less localization error is. It improves localization accuracy by using more sensitive sensors. For example, passive broadband harmonic nonlinear transmissionline tags were used to measure the distance of two nodes [12, 13] or the distance was estimated by using the channel state information [14].
Meanwhile, under the constrains of the measurement accuracy limitation in the physical methods, cyber method is to design algorithms for finding an optimal position estimation of the unknown node. Multilateration based on LeastSquares (MLS) [15] is one widely used cyber method. Also, the localization error of MLS was studied to improve the localization accuracy [16]. Cramer Rao Bound is used to calculate the localization error bound [17–19], in which probability density function of noise is needed to calculate Fisher information matrix. And, localization accuracy was characterized by using a noise covariance bound when anchor nodes have location uncertainty [20]. To calculate localization error accumulating during an iterative process, the mean of localization error was given in the literature [11]. All those literatures assumed that the probability density function or covariance of noise is known. However, according to the description above, the assumption is not always satisfied in a harsh scenario. The peculiarity of localization in an anonymous environment is noticed [21]. But, the literature is focused on converting the RSS into the distance when little information on the radio propagation model is provided. It is still not studied that how to improve localization accuracy in the scenario of the insufficiency on measurement noise caused by the robots mobility. Xu et al. [22, 23] proposed a crowdsourcingbased framework for processing mobile information and have been proved to be a high accuracy and efficiency.
3 Iterative localization based on least squares
To introduce the notions and symbols used in the following contents, here, we briefly describe the process of MLS.
Let x = (x, y) represents the coordinate of an unknown node located based on an anchor nodes set {x _{ i } = (x _{ i }, y _{ i })i = 1, ⋯, n} where n is the cardinality of the anchor nodes set, and ∥ ∥ _{2} is the Euclidean norm. Localization based least squares performs as following:
It is obvious that there are n constraints in a localization system.
The subscript i of A _{ i } and b _{ i } emphasizes that the A and b are generated in the case of choosing ith anchor node as BAN.
The estimation coordinate of pseudoanchor always deviates its physical coordinate since the measurement error exists. The literatures introduced in Section 2 have studied the methods of improving localization accuracy based on measurement error estimation. Unfortunately, as it is discussed, those algorithms are disabled because of the insufficiency on measurement noise in a harsh scenario. It is noted that the localization is influenced by choosing BAN. Therefore, generating proper LLS is a way to obtain the optimal localization accuracy instead of measurement error estimation.
The following section discusses the upper bound of the localization error propagation. We use the boundary to guide the construction of LLS which can be used in distribution infrastructurefree localization algorithm.
4 Localization error upper boundary of anchors with errors
One character of iterative localization is that the localization error propagates. We deduce an upper bound of the localization error propagation based on orthogonally invariant norms. In addition, an algorithm is proposed by using the upper bound as a LLS measurement.
4.1 Orthogonally invariant norms
The orthogonally invariant norms are used to conduct the upper bound of the localization error propagation. The notion and its characters are introduced as follows.
 (1)
Φ(x) > 0, x ≠ 0,
 (2)
Φ(α x) = αΦ(x), ∀ α ∈ ℝ,
 (3)
Φ(x + y) ⩽ Φ(x) + Φ(y),
 (4)
\( \varPhi \left({\varepsilon}_1{x}_{i_1},\dots, {\varepsilon}_n{x}_{i_n}\right) = \varPhi \left(\boldsymbol{x}\right), \)
where α is a scalar, ε _{ i } = ± 1 for all i, and i _{1}, ⋯, i _{ n } is a permutation of 1, 2, ⋯, n.
 (1)
‖A ^{T}‖ = ‖A‖, ∀ A ∈ ℂ^{ m × n }
 (2)
‖x‖ = ‖x‖_{2}, ∀ x ∈ ℂ^{ n }
 (3)
‖AB‖ ⩽ ‖A‖_{2}‖B‖, ∀ A ∈ ℂ^{ m × n }, ∀ B ∈ ℂ^{ n × l }
 (4)
‖AB‖ ⩽ ‖A‖‖B‖_{2}, ∀ A ∈ ℂ^{ m × n }, ∀ B ∈ ℂ^{ n × l }
 (5)
‖A − B‖ ≤ ‖A‖ + ‖B‖, ∀ A, B ∈ ℂ^{ m × n }
 (6)
‖ A ‖ − ‖ B ‖  ⩽ ‖ A − B ‖
4.2 Upper boundary of localization error of LLSRSS iterative algorithm
4.2.1 Upper boundary of localization of error LLS
We propose a lemma which describes the upper bound of localization error for a LLS. The lemma issues an abstract but a useful formula for calculating the upper bound of the error propagation.
where Â ^{†} _{ i } is MoorePenrose pseudoinverse of matrix Â _{ i }.
where \( \beta ={\left{\left\Vert {\widehat{\boldsymbol{b}}}_i\right\Vert}_2/{\left\Vert \varDelta {\widehat{\boldsymbol{b}}}_i\right\Vert}_21\right}^{1} \).
where \( \kappa =\left\Vert {\widehat{A}}_i^{\dagger}\right\Vert \left\Vert {\widehat{A}}_i\right\Vert \). Finally, combining (17) and (19), the conclusion is obtained.
4.2.2 Error upper boundary of LLSRSS
The Theorem 1 gives a universal upper bound of the measurement error. Since RSS is widely used as the measurement data, we propose a concrete numeral upper bound of the measurement error of a LLSbased RSS (LLSRSS). The upper bound will be fundamental of algorithm which can construct the optimum LLS in the next subsection.
To calculate the upper bound of Lemma 1, we need to calculate k, α, and β. The k and α are calculable, because all components are only related with known coordinate of anchors (originanchors, pseudoanchors, or combination of them). However, the measurement data noise \( \varDelta {\widehat{\boldsymbol{b}}}_i \) is random and unmeasurable. It makes instantaneous value of β that cannot be calculated. The value can be obtained is the mean of β which is upper boundary. As an extension of Theorem 1, the mean of error upper bound of LLSRSS is given by Lemma 2.
where \( {\left\Vert E\left(\varDelta {\widehat{\boldsymbol{b}}}_{i,2}\right)\right\Vert}_2=\left\Vert E\left(\varDelta {\widehat{d}}_i\right)E\left(\varDelta {\widehat{d}}_k\right)\right\Vert \) since the random variables are independent and identically distributed.
Use Theorem 1, Combine (25), (29) and norm definition, the (20) and (21) are obtained.
4.3 Optimum algorithm of constructing LLS
Theorem 2 gives the numerical result of localization accuracy influenced by node information and measurement data. The theorem can handle the situation that the positions of the anchor nodes can exist error. Although the error upper bound is calculated in statistical significance, ∆b _{i}/b _{i} appears with a high probability, which is tested in experiments.
Remark: The environment parameter σ/η is needed to calculate the E _{ i } in (19). But, a estimated value can be used. It will not significantly affect the result. It means that the algorithm could be fully “blind” based on an assumption value. Of course, any knowledge of parameter can improve the algorithm performance. This conclusion is discussed in Section 5.2.
5 Simulation and discussion
 (i)
We use three originanchor nodes, whose coordinates are (0, 0), (50, 0), and (25, 50), respectively. The nodes are numbered by their orders. The fourth node is a pseudoanchor whose coordinate is (25, 25). The fifth node is a pseudoanchor.
 (ii)
The radio propagation model uses distancedependent path loss with lognormal fading with Gaussian noise N (0, 1.5).
 (iii)
The position of the unknown node is calculated as \( {\widehat{\boldsymbol{x}}}_m={\left({\widehat{A}}_m^{\mathrm{T}}{\widehat{A}}_m\right)}^{1}{\widehat{A}}_m^{\mathrm{T}}{\boldsymbol{b}}_m \).
To distinguish the different LLS, which use different anchor set of originanchors or pseudoanchors, we add a superscript on Â _{ i } to declare the used anchor nodes. Therefore, \( {\widehat{A}}_1^{1\hbox{} 2\hbox{} 3} \) means the LLS uses three nodes whose numbers are 1, 2, and 3, and 1st node are chosen as BAN.
5.1 Evaluation indicator
The fundamental of algorithm is the mean of error upper bound. It is noted that the upper bound is given in statistical perspective. It is possible that the upper bound is minimum while the localization is not optimum. The evaluation indicator of algorithm should reflect that the algorithm can choose optimum LLS or not using upper bound. Therefore, the coordinate error is unsuitable to evaluate the algorithm performance.

SMC: Let initial value of SMC be 0. SMC increases 1, if and only if, the minimum of rough upper bound and the minimum of localization absolute error are both obtained when ith anchor is selected as a BAN.

LMC: It is similar to SMC. However, LMC increases 1 when localization absolute error is smallest or second smallest in the case of the upper bound is minimum with the same BAN.
5.2 Feasibility of the algorithm
Each value is the match count per 100 times simulations using SMC. The calculation of minimum upper bound uses σ/η = 0.65 in each time.
To calculate the upper bound by using Theorem 2, σ/η is needed to obtain c. Experiments are implemented to illustrate the probability of construction optimum LLS varies with σ/η. The fifth node is placed at (25, 6). σ/η = 0.65 is used in each time of calculation of minimum upper bound.
5.3 Randomness of the algorithm
Experiments are executed to test the randomness of the algorithm. The fifth node is placed at (25, 6), and the experiments are executed for 100 × 100 times.
5.4 Effectiveness of the algorithm
Additionally, Figs. 5 and 6 show that the curve of absolute errors gently changes when the LLS is the one with minimum upper boundary. It means that the algorithm is stable. The factor of upper bound, κ = ‖Â _{ m }‖_{2}‖Â _{ m }‖_{2}, is a condition number, which means that the condition number of LLS will be smaller when the upper bound is minimum. Therefore, the algorithm, which uses minimum upper bound, is insensitive to noise.
5.5 Performance evaluation
5.6 Computational complex
In some application of mobile autonomous robots, the energy consumption and computation ability are constrained. It requires the localization to be realized onchip. Computational complexity, which is defined as the number of operations performed by the algorithm [27], is always used to evaluate the possibility implemented on chip.
Cost functions
Title  Add  MUL 

‖Â _{ i }‖_{2}(i = 1, ⋯, n)  n(n − 1)  n ^{2} 
\( {\left\Vert \varDelta {\boldsymbol{x}}_i\right\Vert}_2^2\left(i=1,\cdots, n\right) \)  (n − 1)  n 
\( {\left\Vert {\boldsymbol{x}}_k{\boldsymbol{x}}_i\right\Vert}_2^2\left(i,k=1,\cdots, n;k\ne i\right) \)  n(n − 1)  (n − 1)^{2}(n − 2) 
\( {\widehat{d}}_i^2\left(i=1,\cdots, n\right) \)  n  n 
\( {\left({\widehat{d}}_i^2{\widehat{d}}_k^2\right)}^2\left(i,k=1,\cdots, n;k\ne i\right) \)  n − 1  (n − 1)^{2}(n − 2) 
Therefore, the time complexity of algorithm 1 is O(3n ^{3}). The algorithm is more complex than the leastsquares algorithm. It is reasonable because the algorithm trades computational complex off for localization accuracy. It is also noticed that n is the anchor node number, which is a small value in practice.
6 Conclusions
An upper bound of error propagation of iterative localization is derived, which can be used in the situation that the precise distribution of the environment noise is unknown. The minimum upper bound is adopted to evaluate the localization result of LLS with certain measurement data. With this method, an optimum algorithm of constructing LLS is proposed. Even when the environment noise is unknown or unpriced evaluated, the algorithm still can construct the proper LLS with highly probability, which means it can still obtain the best localization accuracy with high probability.
Declarations
Acknowledgements
This work is supported partly by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China under grant (no: 2013BAK06B05) and the National Natural Science Foundation of China under grant (no: 61303183).
Competing interests
The author declares that he has no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
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