Ing. Václav Navrátil, Ph.D.

Přednáška přibližuje aktuální stav systémů družicové navigace (GNSS). Dále stručně popisuje princip jejich funkce a různé metody určení polohy, i z toho vycházející limity jejich přesnosti a dostupnosti. Je diskutována zranitelnost GNSS vlivem rušení případně podvržením signálu (spoofing). Budou zmíněny i nové služby GNSS systémů, které přinášejí modernizované signály.

This paper proposes an approach of TDoA positioning in UWB networks, where user tags localize themselves by means of exploitation of the broadcasted synchronization messages of the anchor network. Such approach promises unlimited number of localized devices, moreover, the position is available directly at the user terminal. The key challenge of this method is to eliminate errors caused by tag clock drifts, which render the TDoA measurements useless when left uncorrected. Our method employs extended Kalman filtering (EKF) for the estimation of position and elimination of the drift-induced errors. It is shown that the system performance is similar to the more common TDoA method, where the tags transmit blinks received by the anchors. However, the anchors are still able to receive the blink messages and estimate position of those tags, since the synchronization messages are exploited. Therefore, it is possible to use both directions of the TDoA positioning concurrently; a limited number of tags is tracked by the infrastructure and all tags may compute their positions. The TDoA solutions have achieved RMS horizontal accuracy of 25.9 cm and 33.6 cm, respectively.

In the paper, a theoretical background of the unexpected third-order intermodulation power decrease at some levels of input power is discussed first. This anomaly can be partially explained by various physical phenomena, RF stress, e.g., as shown in the state-of-the-art review in the paper. Then a simulation of the IM3 dependency is performed, including a comparison between the computed and measured IP3 points. Finally the single- and double-band low-noise amplifiers designed by multi-objective optimization are measured from this point of view, and a frequency dependence of the IM3 anomalies is demonstrated. An improved optimization method is defined too.

The asymmetric double-sided two-way ranging (ADS–TWR) is a method of range estimation through precisely timestamped messages sent between two transceivers. In contrast to the symmetric double-sided two-way ranging (SDS–TWR), the ADS–TWR suppresses the errors induced by transceiver clock drifts even when the message reply delays are unequal. However, due to the nonlinearity of ADS–TWR estimator, its mean and variance cannot be evaluated in neither straightforward nor exact way. This article presents an analytical approach of evaluating an approximation of bias and variance of the range estimators and applies it on an exemplary set of parameters, which is typical for ultra-wide band (UWB) localization systems. Moreover, the obtained results are verified by a Monte–Carlo simulation. It will be proven that the ADS–TWR is safe to be used in most of the present applications and its precision is rather similar to SDS–TWR.

Improving performance of Doppler shift evaluation could be generally attempted by enhancement of hardware or software. The costly and inflexible nature of the former motivates us to gather as much as possible from the signal processing. The aim of our research has been to develop a real-time algorithm improving precision of Doppler shift evaluation for a standard, low-cost CW (continuous wave) Doppler radar. Emphasis has been put on close-to-zero Doppler shifts (i.e. low velocities), where standard algorithms usually do not provide accurate and reliable results. The proposed algorithm uses simple neural networks in order to provide the approximate estimate of the Doppler shift. According to their results, a proper processing method is chosen for the evaluation of the accurate Doppler shift value.

The Time Difference of Arrival is a popular method for UWB-based positioning, since it allows high position update rate even for multiple users. However, it requires the network infrastructure (anchors) to be synchronized, preferably with sub-nanosecond accuracy. Herein, an approach for synchronizing multiple anchors in a wireless, line-of-sight manner is described. This method is able to deal with UWB modules equipped with inexpensive drift-prone oscillators, as such impairments are estimated and compensated. By applying the proposed approach the influence of generally variable environment (e.g. temperature) on timing and positioning performance is reduced. Moreover, the presented algorithm is suitable for straightforward chaining of the line-of sight-segments in order to allow synchronization of distant anchors that cannot be synchronized with the master anchor directly.

In the ultra-wide band (UWB) localization networks the time difference of arrival (TDoA) is often used. The TDoA method is advantageous in comparison with two-way ranging approach, however, requires the UWB infrastructure of the network to be synchronized to a sub-nanosecond level. The wireless synchronization is in favor due to practical reason, nonetheless its application is constrained by the need of line-of-sight between the synchronized nodes. This paper focuses on an experimental evaluation of a Kalman-filter-based chained synchronization algorithm, which allows synchronization of a distant, directly unreachable UWB nodes through multiple nodes and the respective line-of-sight path segments.

This paper presents results from experiments that utilize GPS-like terrestrial positioning signals in a passive radar system to track aircraft over a metropolitan area. Unlike communication signals, these positioning signals offer unique properties that are advantageous for passive radar. Exploitation of these properties provides an opportunity to reduce hardware complexity and to enable passive radar systems with lower cost. This paper provides a detailed description of the signal processing in the context of low-cost hardware.

This study addresses the problem of short UWB (ultra‐wide band) outages in a UWB/IMU (inertial measurement unit) integrated navigation system. UWB outages may occur due to non‐line‐of‐sight between the UWB radios. To mitigate the errors during short UWB outages, we consider the special dynamics of the platform by adaptively applying constraints in the navigation filter. The application presented here is the navigation problem of the unexploded ordnances (UXOs) mapping platform. A UXO platform moves straight or performs turns or stops; these are the three dynamic states with pre‐defined constraints. Two neural networks are examined to determine the current dynamic state based on IMU data. Tests on open‐sky and canopied areas were carried out for the performance assessment. The networks were trained on the open‐sky dataset, and then tested on the canopied area. The detected improvement of the proposed solution is 10–30% (10–15 cm) for 5–10 s long outages. Copyright © 2018 Institute of Navigation

Passive radar uses illuminators of opportunity instead of a dedicated radar transmitter. A number of already transmitted signals can be used, however, their nature is mostly continuous and their utilization is a complicated task. One of the key limiting factors of passive radar is the strong direct signal interference (DSI), even though many physical and signal processing countermeasures have already been developed. In the case of using terrestrial signals for airborne target detection, elimination of a certain elevation angle of arrival is a possible solution. As the real-world environment is nonstationary, adaptive methods are expected to give better results. The paper shows and compares two space-time adaptive vertical antenna array processing methods for DSI mitigation, applied in terrestrial navigation signal based radar. In addition, both methods are verified by processing signals captured under real-world conditions. Both methods offer more than 10 dB DSI suppression.

Fast and stable processing optimized for given simulation problem is essential for any modern simulator. It is characteristic for electronic circuit analysis that complexity of simulation is given by circuit size and used device models. Implementation of electronic device models in program SPICE uses traditional functional paradigm allowing fast computation but further modification of model can be questionable. In this article, we propose modification of standard procedure inserting functional computation layer into the process. It allows on-the-fly modification of standard models and own functional definition during circuit definition without a loss in computational performance. It also gives a possibility of functional chaining mechanism and improves mapping performance of circuit variables to device models.

Acquiring geospatial data in GNSS compromised environments remains a problem in mapping and positioning in general. Urban canyons, heavily vegetated areas, indoor environments represent different levels of GNSS signal availability from weak to no signal reception. Even outdoors, with multiple GNSS systems, with an ever-increasing number of satellites, there are many situations with limited or no access to GNSS signals. Independent navigation sensors, such as IMU can provide high-data rate information but their initial accuracy degrades quickly, as the measurement data drift over time unless positioning fixes are provided from another source. At The Ohio State University's Satellite Positioning and Inertial Navigation (SPIN) Laboratory, as one feasible solution, UltraWideband (UWB) radio units are used to aid positioning and navigating in GNSS compromised environments, including indoor and outdoor scenarios. Here we report about experiences obtained with georeferencing a pushcart based sensor system under canopied areas. The positioning system is based on UWB and IMU sensor integration, and provides sensor platform orientation for an electromagnetic inference (EMI) sensor. Performance evaluation results are provided for various test scenarios, confirming acceptable results for applications where high accuracy is not required.

Passive radars use transmissions from signals of opportunity in order to locate targets. The absence of a dedicated transmitter reduce the cost, complexity and detectability of the radar as compared to active radar systems. This paper investigates - for the first time - the use of signals of a metropolitan-wide terrestrial positioning system for passive radar. We present simple theoretical analysis predicts the efficacy of these terrestrial navigation signals for this application. Additionally, we demonstrate that aircraft can be observed in a passive radar system exploiting a terrestrial positioning transmission at 10 km bistatic range.

A platform equipped with electromagnetic inference (EMI) sensor allows us to map underground areas and searching for unexploded ordnances (UXO). This mapping requires the precise navigation of the platform. In this paper, we use UWB/IMU integration to determine position and attitude of a UXO platform. UWB outages may often occur due to non-line of sight between the UWB network nodes and the rover. To mitigate the errors during short UWB outages, we consider the special dynamics of the platform by adaptively applying constraints in the navigation filter. Typically, a UXO platform moves straight, performs turns or stops; these are the three main dynamic states. Each dynamic state has a set of constraint equations that describes the specific motion. A neural network determines the current dynamic state based on IMU data. Two types of neural networks are examined: (1) a feed-forward network that uses the mean and variance of the IMU data, and (2) a proposed convolution network that takes the raw IMU data as inputs to determine the current dynamic state. The networks are trained on a dataset that was acquired during good GNSS signal reception, and UWB/IMU, GNSS/IMU solutions, whoever, the UWB had some outages. On this dataset, we found that the adaptive constraints mitigate the error of these outage and the UWB/IMU integrated solution by 10% (3-4 cm) using the GNSS/IMU solution as ground truth. The investigation did not show any statistically significant performance difference between the two neural network types.

Due to operational limits and known vulnerabilities of commonly used global navigational satellite systems a demand for alternative positioning has risen in recent years. One of the possible ways is to make use of so called Signals of Opportunity - already transmitted terrestrial signals, which are not primarily dedicated for the positioning purpose. The paper presents our results in the field of utilizing Terrestrial Digital Video Broadcasting signal (DVB-T), including real-world scenario analysis and experimental position estimate based on captured signals.

The satellite navigation is supposed to be used in applications that need coordinates generally. However we can meet a plenty of satellite signal reception problems in a real environment, often called as difficult conditions. The difficult conditions hinder reliable positioning with required accuracy, often in applications that are important for saving or securing the safety of human lives (work of rescue teams, protection of people in large warehouses, safety of lone forest workers, etc.). The main reason for this is the weakness of the received satellite signals. In addition to that, the weak signals are also highly vulnerable by interference, spoofing or jamming, even with the low-power and often generated by low-cost devices. In spite of this, radio systems complementing and making the backup of the satellite positioning are searched. There are terrestrial radio systems using high-power signals with properties which are suitable for the positioning purposes. The most important property in conjunction with the sufficiently high power is a very sharp and possibly unambiguous correlation function. Besides the signals of systems used primarily for the navigation (such as eLORAN, e.g.) signals of some systems primarily dedicated to communication have the acceptable properties mentioned above. They are usually called Signals of Opportunity. As an example let us mention signals of the DVB-T, LTE, Wi-Fi, etc. In the field of indoor navigation signal strength fingerprinting is frequently used. However, for much larger open areas this approach is not the best one, because it requires a kind of a site survey to be done. In case of outdoor applications, the use of different principles has to be considered. The methods based on signal power and angles of arrival measurements have been found unsuitable because they may be misleading even in a lightly obstructed area. ...

Although many programs have built-in various methods for finding the steady state nowadays, their actual implementations are often quite unsatisfactory regarding algorithm efficiency and reliability. We improved and checked procedures built on both epsilon-algorithm and sensitivity analysis in time domain. First of all, it was clearly demonstrated that increasing demands on the overall numerical accuracy do not lead to an excessive number of necessary integration steps and therefore LU factorizations correspondingly. This feature is especially significant for the suggested procedure based on the epsilon-algorithm. Furthermore, the practical experiments confirmed that a proposed arrangement of the extrapolation method is greatly insensitive to its order, which is even more important because a program user is unable to estimate the appropriate order for complicated circuits well. The properties of the methods are demonstrated using rectifier, C-class amplifier, and LNA for which exceptional attention was given to checking the insensitivity of the extrapolation to its order.