F. Vejrazka

Interoperable GPS, GLONASS and Galileo software receiver

The new and modernized GNSS navigation systems will provide various navigation services and signals for civil user. The system operators work on basic interoperability agreements which simplify multi-system navigation receiver architecture and reduce its cost. The most current GNSS signals of GPS, GLONASS and Galileo can be processed by one or several classical E-L correlators with a look up table PRN generator of a length of 10,230 chips. Most signals are processed optimally, but for some signals, like Galileo E1b and E1c or GPS L1C, this signal processor does not utilize all signal components and properties. The proposed E-L correlator was implemented to the FPGA of the experimental software GNSS receiver and was tested on the Galileo E1b, E1c, and E5a signals. The target multi-system GNSS receiver architecture based on an ExpressCard peripheral card for the standard PC computer or notebook is described herein. Up to one hundred universal correlators and signal snapshot capture unit for DSP acquisition are programmed to the receiver FPGA.

Multi system navigation receiver

High demands on mobile user positioning cannot be satisfied by single navigation system. The paper proposes multi system solution which utilizes navigation and communication systems and signals. The experimental software receiver is described as development tool of such combined system. The proposed receiver consist of three GNSS RF front ends of bandwidth of 24 MHz tunable to any navigation frequency from 1 to 2 GHz and one DVB-T(H) tuner. The programmable receiver hardware is based on FPGA and PC workstation which communicate each other via Gigabit Ethernet. In the second part of this paper we are presenting methods of estimation of time of arrival of OFDM (Orthogonal Frequency Division Multiplex) signal. Described estimation algorithm is used in the DVB-T(H) (Digital Video Broadcasting - Terrestrial) receiver which can be used for navigation or for support of satellite navigation receivers. The last part of this paper describes our results of experiments with DVB-T(H) navigation. They were obtained in various environments (urban, suburban, etc.) including indoor environment.

Low complex interoperable GNSS signal processor and its performance

The recent development of the GNSS systems and international cooperation resulted in important technical problems of the GNSS systems which are an interoperability and compatibility. In the interoperable receivers the most expensive parts - front ends - can be shared for signals reception of different systems. The unification of the signal processor is also possible with some small performance deterioration but the hardware complexity reduction is considerable. The paper analyses applicability of a classical E-L correlator for processing of various GNSS signals and compare its performance with optimal method. The low complex interoperable processor of software receiver based on a FPGA for the GPS, Galileo and GLONASS systems is proposed. The results of testing on the Galileo E1 and E5 signals are presented. The last part of the paper proposes architecture of a low cost multi system GNSS receiver based on mass market components.

Galileo and the Other Satellite Navigation Systems

GPS is the best-known satellite navigation system in the world. Next to GPS, a Russian system GLONASS is operating, and a European system Galileo is intensively prepared. The first satellite of the Galileo system should be launched in 2009, and the whole system Galileo should start to regularly operate in 2011 or 2012. Both in the community of researchers and in the public, questions about the accuracy, compatibility and helpfulness of those systems are discussed. Those questions are not only of the technical nature but also of the political one. The paper tries to give explanation of the principles of the operation of those systems, and their mutual comparison. At the end of the paper a concept of satellite navigation receiver designed at the Czech Technical University in Prague is sketched.

GNSS receiver for GLONASS signal reception

This paper deals with the latest version of Experimental GNSS receiver built at the Czech Technical University and describes integration of GLONASS signal processing to the receiver. The new FPGA platform Virtex-D Pro by Xilinx is used and enables integration of whole digital signal processing of GNSS receiver into the single chip. The RE unit of the receiver is capable of processing all GLONASS frequency of the Li and L2 bands in two independent RE channels; each channel can process one band. The frequency selection of the appropriate satellite is accomplished in a digital correlator. The development flow of the GLONASS correlator is discussed herein. The complexity of the GLONASS correlator with complexity of GPS correlator is compared. The developed GLONASS correlator was tested in Simuelink tool during development. The next test was carried out using GLONASS simulator and real GLONASS satellite signal.

Position estimate using radio signals from terrestrial sources

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. Range estimation using packet round trip delay (similar concept to the DME ranging used in aviation) is not possible because most of the systems do not have a stable response delay. However, some systems with synchronous transmitters (or base stations) can be used by the way of hyperbolic navigation concept. Luckily many of the systems with high transmitted power, such as TV broadcast or cellular data systems downlink have the precise transmission timing incorporated. This contribution describes utilizing the OFDM Signal of Opportunity for the positioning purposes. The proposed method is based on the peak search in an estimated radio communication channel model. The time differences of arrivals of signals from individual transmitter pairs can be measured in this way. At least two independent measurements must be used for the final two-dimensional position estimate by the iterative LevenbergMarquardt algorithm. With three independent measurements the 3D positioning is available. Because the measured differences do not have generally to share the common timescale (only the measurement pairs have to), combining of more independent systems together is possible by nature of our approach. The validity of the above mentioned theoretical concept is demonstrated and verified by means of an experiment on real captured DVB-T (OFDM modulated) signals. Moreover, an analysis of transmitter geometry influence on the positioning accuracy is also shown and the graphical output for the real scenario is provided. Considering these results, future goals and improvement possibilities are formulated.

Multi-objective optimization of a low-noise antenna amplifier for multi-constellation satellite-navigation receivers

Although the major parts of function blocks for the satellite navigation receivers are fully integrated in a CMOS chip in most cases, it is convenient to create an antenna preamplifier as a separate circuit based on a low-noise pHEMT. Such an RF front end can be strongly optimized to attain a trade-off between the noise figure and transducer power gain. Furthermore, as all the principal navigation systems (GPS, GLONASS, Galileo, and Compass) work in similar frequency band (roughly from 1.1 to 1.7 GHz), it is reasonable to create this low-noise preamplifier for all of them. In the paper, a sophisticated method of the amplifier design is suggested based on multi-objective optimization. First, an extraction of pHEMT model parameters was performed, including comparisons among several models. The extraction was carried out by our original three-step robust identification procedure based on a combination of meta-heuristic and direct optimization methods. Second, a substantial improvement of a standard method for the multi-objective optimization is outlined. Third, the equations of passive elements of the circuit (including transmission lines and T splitters) were carefully defined using frequency dispersion of their parameters as Q, ESR, etc. Fourth, an optimal selection of the amplifier operating point and essential passive elements was performed using the previously improved goal attainment method. Finally, the s-parameters and noise figure of the proposed preamplifier were measured, and the third-order intermodulation products were also checked.

A new criterion for stability assessment of the microwave pHEMT-based low-noise amplifiers

At present, the foremost low-noise amplifiers are based on usage of the pseudomorphic high-electron-mobility transistors (pHEMTs). However, in many cases, the amplifiers are often constructed at the limits of absolute stability or even in the region of potential instability because only such solutions give requested circuit properties as a trade-off between the transducer power gain and noise figure. The stability properties are mostly checked by the classical criteria such as the Rollett conditions, μ-factor etc. In the paper, a novel additional very efficient criterion is proposed, finding the most critical couple of poles and evaluating the ratio of imaginary and real part. The efficiency of the method is demonstrated on a low-noise antenna preamplifier for a multi-constellation satellite-navigation receiver based on an ATF-54143 pHEMT.

A new assessment of pHEMT models by comparing relative errors of drain current and its derivatives up to the third order

At present, there are relatively more precise pHEMT models available for computer-aided design, and they are frequently compared to each other. However, such comparisons are mostly based on absolute errors of drain-current equations and their derivatives. In the paper, a novel method is suggested based on relative root-mean-square errors of both drain current and its derivatives up to the third order. Moreover, the relative errors are then relativized to the best model in each category to further clarify obtained accuracies of the drain current and its derivatives. Furthermore, one our older and two newly suggested models are also included in the comparison with the traditionally accurate Ahmed, TOM2 and Materka ones. The assessment is performed using measured characteristics of a 110 GHz pHEMT. Finally, a usability of the models of the higher-order derivatives is illustrated using an IP3 computation/measurement of a multi-constellation receiver for a satellite navigation with ATF-54143.

Comparing the steady-state procedures based on epsilon-algorithm and sensitivity analysis

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 ϵ-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 ϵ-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.