You are provided with the abstract of a scientific paper and the abstracts of the papers it references. Your task is to write a Related Work section for the main paper. This section should concisely summarize and integrate the key points from the cited abstracts, reflecting how they relate to the main paper's contributions and context. In your summary, use numerical citation markers like [1], [2], etc., to refer to the corresponding papers.

Main Abstract:
In wireless networks, radio-map based locating techniques are commonly used to cope the complex fading feature of radio signal, in which a radio-map is built by calibrating received signal strength (RSS) signatures at training locations in the offline phase. However, in severe hostile environments, such as in ship cabins where severe shadowing, blocking and multi-path fading effects are posed by ubiquitous met allic architecture, even radio-map cannot capture the dynamics of RSS. In this paper, we introduced multiple feature radio-map location method for severely noisy environments. We proposed to add low variance signature into radio map. Since the low variance signatures are generally expensive to obtain, we focus on the scenario when the low variance signatures are sparse. We studied efficient construction of multi-feature radio-map in offline phase, and proposed feasible region narrowing down and particle based algorithm for online tracking. Simulation results show the remarkably performance improvement in terms of positioning accuracy and robustness against RSS noises than the traditional radio-map method.

Cited Abstract(s):
[1]: This paper proposes the autonomous ultrasonic indoor tracking system (AUITS), an ultrasound based system for locating and tracking mobile objects inside a building. Ultrasound shows promise to be exploited for a practical indoor location system due to its high accuracy ranging, low cost, safety, and imperceptibility. However, conventional ultrasonic location systems pose such challenges as high installation cost and manual calibration. The key idea of AUITS is to use only one autonomous device, positioning on one device (POD), to not only process signal acquisition but also conduct position computation. Structural topology is designed to make POD easily deployed and easily calibrated. In addition, a structural localization algorithm is proposed to provide an effective and affordable algorithm for POD to calculate the object's position. We describe the of AUITS and evaluate its performance both experimentally and with simulation. The results show that the coverage area of a POD can reach 65 m2 and the positioning error is less than 15 cm with over 90 probability.

[2]: With proliferation of ubiquitous computing, digital access is facing an increasing risk since unauthorized client located at any place may intrude a local server. Location Based Access Control (LBAC) is a promising solution that tries to protect the client's access within some user-defined secure zones. Although a lot of prior work has focused on LBAC, most of them suffer from coarse judgment resolution problem or considerable manual setting-up efforts. This paper proposes LOCK, a highly accurate, easy-to-use LBAC system, which uses autonomous ultrasound positioning devices and an access control engine to precisely characterize the secure zones and accurately judge the online access authority. Particularly, the ultrasound positioning device provides relative 3D coordinate of the mobile clients. Measurement-Free Calibration (MFC) is proposed to easily calibrate these positioning devices to transform their relative positioning results into an absolute coordinate system. In this coordinate system, secure zones are characterized by a Coherent Secure Zone Fitting (CSZF) method to compensate the disparity between manually measured secure zone and the secure zone seen by the positioning devices. Furthermore, a Round-Trip Judgment (RTJ) algorithm is designed to fast online determine the geographical relationship between the client's position and such secure zones. A prototype of LOCK system was implemented by defining a meeting table as secure zone to control the client's access to a FTP server. Experiment results show that the system can be easily set up and can control the client's access with centimeter level judgment resolution.

[3]: The problem of position estimation from time difference of arrival (TDOA) measurements occurs in a range of applications from wireless communication networks to electronic warfare positioning. Correlation analysis of the transmitted signal to two receivers gives rise to one hyperbolic function. With more than two receivers, we can compute more hyperbolic functions, which ideally intersect in one unique point. With TDOA measurement uncertainty, we face a non-linear estimation problem. We suggest and compare a Monte Carlo based method for positioning and a gradient search algorithm using a nonlinear least squares framework. The former has the feature of being easily extended to a dynamic framework where a motion model of the transmitter is included. A small simulation study is presented.
A more accurate approach is to utilize the speed difference of signal propagation to measure distances from transmitters to receivers, so as to conduct indoor locating more accurately [1] [3] . Ultrasound and acoustic signals are the generally exploited low speed signals. In the case of measuring time of arrival, the transmitter broadcasts low-speed signal (ultrasound or acoustic) and RF signal simultaneously. The receiver receives the RF signal to synchronize timer with the transmitter and then measures the traveling time of the low-speed signal to estimate distance from the transmitter. Only when a receiver @math is within the communication range of the low speed signal (denoted by @math , and generally small) of the transmitter @math , can a distance @math be measured. When a set of distances, which is denoted by @math are obtained, least square estimation or multilateration is applied for position calculation. TOA-based positioning can provide centimeter level positioning accuracy [2] . But because the short transmission range of the low speed signals (ultrasound and acoustic), and the requirement of more than three non-collinear distances for location estimation, TOA-based positioning requires dense deployment of TOA beacons, which poses high cost to the positioning system.