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This paper presents a solution for autonomous orbit determination and time synchronization of spacecraft in Halo orbits around Lagrange points using inter-satellite links. Lagrange points are stable positions in the gravitational field of two large bodies that allow for a sustained presence of a spacecraft in a specific region. However, a challenge in operating at these points is the lack of fixed landmarks for orbit determination. The proposed solution involves using inter-satellite links to perform range and range-rate measurements, allowing for accurate computation of the spacecraft's orbit parameters without the need for any facilities on Earth. Simulations using a fleet of three satellites in Near Rectilinear Halo Orbits around the Earth–Moon Lagrange point, proposed for the Lunar Gateway stations, were conducted to demonstrate the feasibility of the approach. The results show that inter-satellite links can provide reliable and accurate solutions for orbit determination with a DRMS error lower than one meter (90th percentile) and synchronization errors of around one nanosecond. This solution paves the way for a fully autonomous fleet of spacecraft that can be used for observation, telecommunication, and navigation missions. [ABSTRACT FROM AUTHOR]

The automatic dependent surveillance-broadcast system is a pillar of the future air traffic control system. It is a surveillance system in which air planes transmit their information (identity, position, velocity, etc.) in broadcast to any listener equipped with a receiver. Networks of ADS-B receivers have been installed all over the world, producing a clear traffic awareness for air traffic controllers. For historical reasons, this protocol does not have any authentication/encryption mechanism. The aim of this article is to improve the ADS-B system security by introducing a physical layer protocol modification for the introduction of an authentication scheme. The proposed evolution fully complies with the current standard and exploits the phase modulation to increase the number of bits that can be transmitted with a single packet of data. These additional bits can be used to carry authentication information, making the overall system more secure (and safe). The proposed protocol evolution is developed ensuring backward compatibility and respecting the ICAO and RTCA recommendations; it can be introduced and used together with the present standard in a mixed scenario, allowing a smooth transition from the present protocol to the new one. Protocol performances were evaluated by simulations and, moreover, its feasibility and compliance with recommendations were tested using a software-defined radio (as transmitter) and a laboratory ADS-B receiver. [ABSTRACT FROM AUTHOR]

Automatic dependent surveillance-broadcast (ADS-B) is an air traffic control system in which aircraft transmit their own information (identity, position, velocity etc.) to ground sensors for surveillance scope. The tracking of the different sensors’ clocks by the use of time difference of arrival of ADS-B messages is proposed to check the veracity of the position information contained in the ADS-B messages. The method allows detecting possible on-board anomalies or the malicious injection of fake messages (intrusion) without the use of the multilateration (or any other) location algorithm. It follows that it does not need the inversion of the location problem (usually strong nonlinear and ill-posed), and, contrary to the multilateration, it works also with less than four sensors. [ABSTRACT FROM AUTHOR]

Satellite navigation systems are more and more used in geomatics. This penetration has not always taken into account the relative safety and security aspects and the consequent threats to the humans. This work focuses on these aspects that are often overlooked in geomatics. After a brief introduction on the evolution of satellite navigation systems and on future trends, the related safety and security risks are analyzed and possible countermeasures (Integrity, Awareness, and Protection) are discussed. [ABSTRACT FROM AUTHOR]

The Automatic Dependent Surveillance-Broadcast (ADS-B) system is one of the major surveillance systems for air traffic management, providing identity, position, and status information of all cooperating aircraft. It is being used more and more frequently in modern air traffic management [1]-[4]. The ADS-B is a dependent and cooperative surveillance system used for air traffic control in which aircraft and surface vehicles periodically transmit their own information to any equipped listener for surveillance scope. The ADS-B system is one of the pillars of future air traffic systems and estimates suggest that about 80% of all commercial aircraft are now equipped with ADS-B transponders. [ABSTRACT FROM PUBLISHER]

Automatic Dependent Surveillance-Broadcast (ADS-B) is a surveillance system used in Air Traffic Control. With this system, the aircraft transmits their own information (identity, position, velocity, etc.) to any equipped listener for surveillance scope. The ADS-B is based on a very simple protocol and does not provide any kind of authentication and encryption, making it vulnerable to many types of cyber-attacks. In the paper, the use of the airplane/transmitter carrier phase is proposed as a feature to perform a classification of the aircraft and, therefore, distinguish legitimate messages from fake ones. The feature extraction process is described and a classification method is selected. Finally, a complete intruder detection algorithm is proposed and evaluated with real data. [ABSTRACT FROM AUTHOR]

Multifunction radars based on active phased arrays are well known and widely studied systems. The concepts of bistatic architecture, conformal array and digital beam forming (DBF) are combined in this paper to define a novel multifunction radar for point defense. The conical shape of the antenna overcomes the significant limitations in the azimuth coverage of 360° of fixed-faces phased arrays due to the beam scanning up to 45°. The usage of separate transmit/receive arrays and the DBF technique adds the operational flexibility and the possibility of multiple simultaneous functions, with an optimal time-energy resources exploitation. After a short description of its technological demonstrator, some significant design trade-off, and operating aspects of the proposed architecture, called d-Radar, are described, showing the main differences with respect to the classical, four faces, and phased-array multifunction radar architecture. It is described how the operating modes can be made more and more similar to a “staring” or “ubiquitous” radar permitting an instantaneous detection and location of short-range, low-elevation targets for sea and ground operations. Finally, some remarks about the resources management and scheduling are shown with the results from a case of study. [ABSTRACT FROM PUBLISHER]

The secondary surveillance radar (SSR), that is an evolution of the military identification friend-or-foe systems, is widely used by air traffic control service providers to localize and identify co-operating aircraft equipped with a standard transponder [1]. The ground SSR installation transmits interrogations, from a rotating, narrow azimuthal beam antenna. The airborne transponders, once they have received an interrogation, transmit at a 1,090-MHz carrier a reply signal containing the requested data, i.e., identity (mode A reply) or fight level (mode C reply). Azimuth and range of the aircraft are measured by the interrogator, based on the delay of the reply and on the antenna pointing angle. The current SSR standard is based on the use of selective interrogations and replies and is called mode S: to reduce the interferences, the mode S protocol is based on a message format that includes the unique address of the aircraft. The airborne segment of the SSR is composed of the transponder and a pair of antennas on top and on the bottom of the fuselage. As the aircraft antennas are omnidirectional, many ground stations can receive the replies. This allowed the development of multilateration (MLAT) systems for the aircraft localization based on 1,090-MHz signals [2]. A typical MLAT system is composed of a distributed network of 1,090-MHz sensors, an interconnecting facility, and a central processor for the fusion of the sensor data. The data fusion relies on the estimation of the signals arrival time at the different stations and on hyperbolic localization. To identify the emitters, the processing is done with mode A replies or mode S replies that contain the emitter identity. A transponder with mode S capability can also transmit a particular downlink format message, called squitter, containing the aircraft's unique address and other information. The squitter signals are not elicited by the SSR interrogation, but they are spontaneously emitted at pseudoperiodical intervals. They are the basis of the automatic dependent surveillance-broadcast (ADS-B) concept [3]: the ADS-B OUT function periodically transmits information (identity, position, state, etc.) about the aircraft, and the ADS-B IN function receives the messages from nearby traffic. On the airborne side, the ADS-B OUT broadcasts data with onboard equipment using the squitter signals. The airborne ADS-B IN equipment provides to the pilot the traffic scenario, receiving the messages from ground and nearby aircraft ADS-B OUT. Moreover, the ADS-B IN can receive other ground services: the traffic information services-broadcast (TIS-B) and the fight information services-broadcast (FIS-B) [3]. Airport vehicles can be equipped with an ADS-B OUT, a simplified, nonflyable device transmitting 1,090-MHz squitter signals containing the identity and the position. As a matter of fact, there are various users of the 1,090-MHz channel: 1) SSR transponder replies (modes A, C, S); 2) ADS-B OUT messages; 3) MLAT systems with interrogation capability; and 4) TIS-B. [ABSTRACT FROM PUBLISHER]

In this paper, an effective procedure to emplace standard and enhanced mode-S multilateration stations for airport surveillance is studied and developed. This procedure is based on meta-heuristic optimization techniques, such as genetic algorithm (GA), and is intended to obtain useful parameters for an optimal system configuration that provides acceptable performance levels. Furthermore, the procedure developed here is able to evaluate and improve previous system designs, as well as possible system enhancements. Additionally, the design strategies to be used along with the procedure proposed here are fully described. Parameters such as the number of stations, the system geometry, the kind of measurements to be used, and the system accuracy are obtained taking into account the basic requirements such as the Line of Sight, the probability of detection, and the accuracy levels. [ABSTRACT FROM PUBLISHER]

In this paper, the use of regularization methods to solve the location problem in multilateration systems, using Mode-S signals, is studied, evaluated, and developed. The Tikhonov method has been implemented as a first application to solve the classical system of hyperbolic equations in multilateration systems. Some simulations are obtained and the results are compared with those obtained by the well-established Taylor linearization and with the Cramér–Rao lower bound analysis. Significant improvements, for the accuracy, convergence, and the probability of location, are found for the application of the Tikhonov method. [ABSTRACT FROM PUBLISHER]

Two localization algorithms for multilateration systems are derived and analyzed. Instead of the classical time difference of arrival (TDOA), a direct use of the time of arrival (TOA) is made. The algorithms work for arbitrary spatial dimensions and overdetermined systems. These derivations are tested in a real-case implementation with simulated data (in particular, the multilateration (MLAT) system installed on the Malpensa Airport in Milan was considered for the MLAT simulation and its possible extension to wide area multilateration (WAM) system was considered for WAM trials). The results are also compared with the present-day algorithms performance, mostly based on TDOA. [ABSTRACT FROM PUBLISHER]

The article discusses an interpretation of the behavior of the apostle John and his dialog with Jesus at the Last Supper as depicted in the Gospel of John. It focuses on the unique situation of John as the beloved disciple and as the only one to be told the name of Jesus' traitor, Judas. According to the author this establishes an intimate bond between Jesus and John by which John comes to understand Jesus' love for Judas. The author reflects on the meaning of compassionate love and discipleship.

The use of wireless signals for the purposes of localization enables a host of applications relating to the determination and verification of the positions of network participants ranging from radar to satellite navigation. Consequently, this has been a longstanding interest of theoretical and practical research in mobile networks and many solutions have been proposed in the scientific literature. However, it is hard to assess the performance of these in the real world and, more importantly, to compare their advantages and disadvantages in a controlled scientific manner. With this work, we attempt to improve the current state of art methodology in localization research and to place it on a solid scientific grounding for future investigations. Concretely, we developed LocaRDS, an open reference data set of real-world crowdsourced flight data featuring more than 222 million measurements from over 50 million transmissions recorded by 323 sensors. We demonstrate how we can verify the quality of LocaRDS measurements so that it can be used to test, analyze and directly compare different localization methods. Finally, we provide an example implementation for the aircraft localization problem and a discussion of possible metrics for use with LocaRDS. [ABSTRACT FROM AUTHOR]

Automatic Dependent Surveillance-Broadcast is an Air Traffic Control system in which aircraft transmit their own information (identity, position, velocity, etc.) to ground sensors for surveillance purposes. This system has many advantages compared to the classical surveillance radars: easy and low-cost implementation, high accuracy of data, and low renewal time, but also limitations: dependency on the Global Navigation Satellite System, a simple unencrypted and unauthenticated protocol. For these reasons, the system is exposed to attacks like jamming/spoofing of the on-board GNSS receiver or false ADS-B messages' injection. After a mathematical model derivation of different types of attacks, we propose the use of a crowd sensor network capable of estimating the Time Difference Of Arrival of the ADS-B messages together with a two-step Kalman filter to detect these attacks (on-board GNSS/ADS-B tampering, false ADS-B message injection, GNSS Spoofing/Jamming). Tests with real data and simulations showed that the algorithm can detect all these attacks with a very high probability of detection and low probability of false alarm. [ABSTRACT FROM AUTHOR]

Radioastronomy and radiointerferometry operations below 20–30 MHz are severely hampered on the Earth surface by a number of reasons. The main two are the existence of the ionosphere acting as a rejecting filter for frequencies lower than about 20 MHz, and the presence in that spectral region of a strong electromagnetic pollution due to man-made activities (communications, broadcasting and radio frequency heating). Relevant advantages may be reached transferring those scientific investigations on the Moon and mostly on its “far side”. The paper presents these advantages and outlines some possible solutions. Three components are presented; the first one refers to the antenna and the antennae array to be realised on the hidden lunar surface. The second one deals with the receiver-transmitter to be deployed at each antenna with the needed synchronisation system since the phase must be preserved. The last one suggests some possible links needed to connect the lunar network to a data-collecting network, or to another interferometric station, space borne or on the Earth. The paper reflects the results of a thesis discussed at Politecnico di Torino. [ABSTRACT FROM AUTHOR]

Nowadays, aircraft safety is based on different systems and four of them share the same data-link protocol: Secondary Surveillance Radar, Automatic Dependent Surveillance System, Traffic Collision Avoidance System, and Traffic Information System use the Mode S protocol to send and receive information. This protocol does not provide any kind of authentication, making some of these applications vulnerable to cyberattacks. In this paper, an intrusion detection mechanism based on transmitter Radio Frequency (RF) fingerprinting is proposed to distinguish between legitimate messages and fake ones. The proposed transmitter signature is described and an intrusion detection algorithm is developed and evaluated in case of different intrusion configurations, also with the use of real recorded data. The results show that it is possible to detect the presence of fake messages with a high probability of detection and very low probability of false alarm. [ABSTRACT FROM AUTHOR]