Paolo Mulassano

GNSS Signal Acquisition in the Presence of Sign Transition

In this paper, the problem of acquiring the pilot and data channel of the next-generation systems (as Galileo, and GPS modernization) is addressed and analyzed, focusing the majority of the attention to the problem of managing the higher sign reversal transition rate due to the navigation message in the data channel and to the secondary code in the pilot channel. It must be considered that in the case of the Galileo E1 signals, each period of the spreading sequence may suffer of a sign change which can reduce the correlation gain. Moreover, the sign transition occurred on the block of samples being processed produces a correlation peak splitting along the Doppler axis of the search space matrix constructed during the acquisition phase and this may lead to a wrong Doppler estimate. Here, we propose an acquisition detection methodology in order to cope with the aforementioned impairments. The main idea behind the proposed algorithm derives by the fact that even though a sign transition occurs, the total useful signal energy remains unchanged, so it is possible to process the search space in order to recover in a nonambiguous way not only the presence of a satellite but also the correct spreading code delay phase and its Doppler shift. The effectiveness of the proposed method will be deeply assessed with a simulation campaign in terms of detection and false detection rate, which will be presented by means of the so-called receiver operative characteristic curves and compared with the state-of-the-art acquisition methodologies.

Performance analysis of MBOC, AltBOC and BOC modulations in terms of multipath effects on the carrier tracking loop within GNSS receivers

The objective of the work is the evaluation of the impact of new modulation strategies foreseen for the galileo signal in space (SIS) on the carrier tracking operation within the global navigation satellite systems (GNSS) receiver (namely frequency locked loop (FLL)/phase locked loop (PLL)) in presence of multipath. Although multipath has more relevant effects in term of measurement errors on the code tracking, its consequences on the carrier tracking can not be neglected in high accuracy applications that rely on the carrier phase measurement such as, for example, real time kinematics (RTK) survey. Particular attention is addressed to the multiplexed binary offset carrier (MBOC) signals and the alternate BOC (AltBOC) signals. The investigation is conducted considering a two ray model with the aim of producing an error diagram showing the multipath effect in the carrier tracking: the delay locked loop (DLL) PLL interaction is considered in a static - low dynamic environment. Closed loop simulations are conducted by means of specific software tools based on the GNSS fully software receiver developed by the NavSAS group. Performance verification is done for a noisy environment. Results show loops performance in a multipath affected environment and the effects of noise in the carrier tracking operations. The PLL performance evaluation under multipath conditions, considering as input MBOC and AltBOC modulated signals, is important to understand the behavior of the new modulations especially in case of carrier tracking based applications that are the more demanding in terms of final positioning performance (i.e. geodetic applications).

Performance of the Proposed Galileo CBOC Modulation in Heavy Multipath Environment

In the framework of the modernization plans of Global Navigation Satellite Systems (GNSS) a new Multiplexed Binary Offset Carrier (MBOC) modulation has been proposed as a possible Signal-In-Space (SIS) with improved performance with respect to the robustness against external sources of degradation. Discussion around such signal format are still on-going at the time of writing, anyway it is the most probable candidate signal that will be adopted by both GPS L1C and the Galileo E1 Open Service (OS) signals [(http://ec.europa.eu/dgs/energy_transport/galileo/documents/doc/2004_06_21_summit_2004_en.pdf, 2004)] as modernization of the present BOC(1,1). In the context of the integration between wireless communication and satellite navigation, one of the major problems is that usually the GNSS receiver has to work in a critical environment characterized by a heavy presence of multipath or interference sources. Some examples are urban canyons or light indoor spaces. These are also typical situations for the usage of personal communications systems. The new MBOC modulation have been studied with the specific goal to create a signal more robust with respect to multipath, bringing the high performance of Galileo in situations in which the present BOC(1,1) shows limitations. In this sense, future Location Based Services (LBS) provided on the basis of the GNSS OS will benefits from this intrinsic robustness of the signal. Throughout the paper a review of the MBOC structure will be presented, specifying that in the Galileo framework such a structure has been proposed in a version named Composite BOC (CBOC). In particular, the paper presents the simulation of the CBOC behavior in presence of multipath showing the extreme improvement with respect to other BOC solutions.

Performance assessment of low cost GPS receivers under civilian spoofing attacks

Services based on Global Navigation Satellite Systems (GNSS) are currently in use in many critical networked infrastructures and crucial applications. Within the described scenario, the importance for the GNSS community of investigating the effects of spoofing attacks, together with anti-spoofing techniques, appears essential. This paper describes the results of preliminary tests performed in lab on low cost GNSS receivers under spoofing attack. Such tests have been performed at the Istituto Superiore Mario Boella (ISMB) research institute and at the European Commission Joint Research Centre (EC-JRC) with the final goal of identifying effective countermeasures to increase receivers robustness.

Comparison between Galileo CBOC Candidates and BOC(1,1) in Terms of Detection Performance

Many scientific activities within the navigation field have been focused on the analysis of innovative modulations for both GPS L1C and Galileo E1 OS, after the 2004 agreement between United States and European Commission on the development of GPS and Galileo. The joint effort by scientists of both parties has been focused on the multiplexed binary offset carrier (MBOC) which is defined on the basis of its spectrum, and in this sense different time waveforms can be selected as possible modulation candidates. The goal of this paper is to present the detection performance of the composite BOC implementation of an MBOC signal in terms of detection and false alarm probabilities. A comparison among the CBOC and BOC(1,1) modulations is also presented to show how the CBOC solution, designed to have excellent tracking performance and multipath rejection capabilities, does not limit the acquisition process.

Efficient Software Defined Radio Implementations of GNSS Receivers

The advent of the Galileo system in Europe and the modernization plans for global positioning system (GPS) are giving a strong impulse to development of global navigation satellite systems (GNSS). The availability of a variety of new signals is boosting the growth of new services and applications. The next generation of receivers will have to deal with signals in different bandwidths with different modulation schemes. In such a context the reconfigurability offered by software defined radio (SDR) technologies, employing programmable hardware like field programmable gate arrays (FPGAs) and digital signal processors (DSPs), can play an important role. The paper presents a case-study of an efficient hardware/software partitioning for the baseband processor and discusses in detail how to meet the real-time constraints. Finally some results concerning the implementation of the proposed architecture on a prototyping board are given.

Galileo AltBOC signal multiresolution acquisition strategy

Herein, some critical aspects related to the acquisition of the future Galileo signals are discussed. In particular, the Alternative Binary Offset Carrier (AItBOC) modulation that will be used by the Galileo satellites to broadcast navigation signals on the E5 band is considered, addressing acquisition issues only partially analyzed in previous works. The implementation of an acquisition section for the AltBOC signals is not straightforward, since several different receiver architectures can be used and remarkable differences are required with respect to the conventional signal processing used in GPS receivers. The main problems that must be handled (risk of false lock, resolution of the search space, and computational burden) are outlined in the following. An innovative technique, called multiresolution acquisition and tailored to the AltBOC signals, is then proposed as an effective solution to previous problems. As demonstrated by means of simulations, this novel strategy can be successfully used in coherent dual-band AltBOC receiver architectures with a feasible implementation complexity and it leads to remarkable advantages in terms of acquisition time.

Support infrastructures based on high altitude platforms for navigation satellite systems

In this article the use of an augmentation system based on high altitude platforms (HAPs) for supporting global navigation satellite systems is discussed. In fact, HAP-based systems are being studied and designed for several communication applications, but they can also be considered added value infrastructure if integrated with navigation systems, providing aiding services based on terrestrial stations or geostationary satellites. The article investigates the results of a feasibility study for the design of a stratospheric pseudo-satellite to provide additional ranging signal to improve the accuracy and availability of the overall navigation system, and an effective integrity service, strengthening the reliability of the positioning procedures. The work here presented discusses issues that have arisen in the design, and provides simulation performance of the proposed architecture.

Acquisition systems for GNSS signals with the same code and bit rates

In this paper we address the problem of the acquisition of GNSS signals for the next-generation systems (as Galileo, and GPS modernization), which will use signals with code rate equal to the bit rate. In this case there is a potential sign transition in each segment of the signal processed in the acquisition schemes. The search space could result greatly impaired, especially with the methods based on FFT. These methods are very efficient, but a bit transition leads to a modification of the shape of the Cross Ambiguity Function evaluated in the search space. Here we propose a method which mitigates this impairment, which fits the requirements of the new GNSS signals. We will show that the bit transition does not destroy the information on the presence of the satellite, but introduces an error in the estimation of the Doppler frequency and code delay. In fact the effect of the sign transition on the correlation peak is a splitting of the peak into two asymmetric lobes along the correct delay bin, so leading to a potential null value at the correct cell. However the integral of these lobes exhibits an invariance property, which can be exploited to recover the bit transition position, from which the acquisition process becomes feasible. In the paper we will provide the mathematical background of a method based on this invariance property and we will show some simulation results which prove the validity of the method.

NAV/COM Hybrid Architecture for Innovative Location Based Payment Systems

Identification and authentication procedures are really important aspects when talking about payments. Nowadays, credit and debit cards are the target for thieves since they can be easily cloned. One of the main fraud sources is represented by the Point Of Sale, the well-known POS. The paper describes an innovative payment system studied, developed and tested within the GAL-PMI Project, an initiative funded by the Industry Division of the Piedmont Region. The system provides users with a user-friendly application to perform payments by means of personal mobile devices like PDA or smartphone, using Global Navigation Satellite Systems as the baseline to increase security aspects. The paper gives an overview of this payment system architecture, going through the technlogical infrastructure that is based on navigation and communication capabilities. Finally, the paper presents a real test case, which has been set-up to demonstrate how security aspects can be improved within identification and authentication processes during payments at gas station.