Integrity Monitoring in Navigation Systems: Fault Detection and Exclusion RAIM Algorithm Implementation

The use of GPS is becoming increasingly popular for real-time navigation systems. To ensure that satellite failures are detected and excluded at the receiver is of high importance for the integrity of the satellite navigation system. The focus of this paper is to implement a fault detection and exclusion algorithm in a software GPS receiver in order to provide timely warnings to the user when it is not advisable to use the GPS system for navigation. The GPS system currently provides some basic integrity information to users via the navigation message, but it is not timely enough for safety-critical applications. RAIM is a means of providing integrity with the capability of detecting when a satellite failure or a measurement error has occurred. It is the simplest and most cost effective technique for integrity monitoring. After applying the iterative fault detection and the exclusion algorithm, a significant improvement in positioning accuracy is achieved.

The Downlink Adjacent Interference for Low Earth Orbiting (LEO) Search and Rescue Satellites

NOAA (National Oceanic and Atmospheric Administration) - LEO environmental satellites provide continuous coverage of Earth, supplying high-resolution global meteorological, oceanic and space observation data. In addition, these satellites are part of the international COSPAS – SARSAT program, which aides search and rescue teams worldwide. The USA segment, referred to as SARSAT (Search and Rescue Satellite Aided Tracking) system, is designed to provide distress alert and location data to assist on search and rescue operations. SARSAT locates distress beacons (406MHz) activated at distress locations. The system calculates a location of the distress event using Doppler processing techniques. Processed data is continuously retransmitted through the SARSAT downlink to Local User Terminals (LUT) when satellites are in view. The downlink adjacent interference is expected when two satellites operate in close proximity and share the same frequency. The downlinks of all SARSAT LEO satellites use the same 1544.5 MHz frequency. In cases where the satellites are within the main lobe of the local user terminal antenna, transmissions from adjacent satellites act as interference to one-another, effectively decreasing the signal-to-noise ratio of the desired downlink. This can result in missed distress beacon bursts or no stored solutions received at the LUT, consequently no data is provided about a distress location. Analysis on interference prediction, impacts on system operation and recommendations for mitigating interference periods where the duration may be significant, are presented in this paper.

Rain Attenuation Impact on Performance of Satellite Ground Stations for Low Earth Orbiting (LEO) Satellites in Europe

Low Earth Orbits (LEO) satellites are used for public communication and for scientific purposes. These satellites provide opportunities for investigations for which alternative techniques are either difficult or impossible to apply. Ground stations have to be established in order to communicate with such satellites. Usually these satellites communicate with ground stations at S-band. The communication quality depends on the performance of the satellite ground station, in addition to that of satellite. The performance of the satellite ground stations is expressed through Figure of Merit. The aim of this paper is to analyze the rain attenuation impact on the performance of the respective ground station. Rain attenuation depends on geographical location where the satellite ground station is implemented. In order to compare this effect on satellite ground station performance, some cities of Europe are considered. Finally, the rain attenuation impact on the satellite ground station Figure of Merit for the hypothetical satellite ground station installed in Prishtina is analyzed.

The Injection Velocity and Apogee Simulation for Transfer Elliptical Satellite Orbits

Basic resources for communication satellites are communication radio-frequencies and satellite orbits. An orbit is the trajectory followed by the satellite. The communication between the satellite and a ground station is established only when the satellite is consolidated in its own orbit and it is visible from the ground station. Different types of orbits are possible, each suitable for a specific application or mission. Most used orbits are circular, categorized as low, medium and geosynchronous (geostationary) orbits based on the attitude above the Earths surface. The launching process heading the satellite in geostationary orbit, by the first step places the satellite in a transfer orbit. The transfer orbit is elliptical in shape with low attitude at perigee, and the apogee of the geostationary orbit attitude. The apogee of the parking orbit depends on the injection velocity applied at perigee. Simulation approach of injection velocity at perigee to attain different apogees, considering an incremental step is presented in this paper.

Simulation and Measurements of VSWR for Microwave Communication Systems

Nowadays, microwave frequency systems, in many applications are used. Regardless of the application, all microwave communication systems are faced with transmission line matching problem, related to the load or impedance connected to them. The mismatching of microwave lines with the load connected to them generates reflected waves. Mismatching is identified by a parameter known as VSWR (Voltage Standing Wave Ratio). VSWR is a crucial parameter on determining the efficiency of microwave systems. In medical application VSWR gets a specific importance. The presence of reflected waves can lead to the wrong measurement information, consequently a wrong diagnostic result interpretation applied to a specific patient. For this reason, specifically in medical applications, it is important to minimize the reflected waves, or control the VSWR value with the high accuracy level. In this paper, the transmission line under different matching conditions is simulated and experimented. Through simulation and experimental measurements, the VSWR for each case of connected line with the respective load is calculated and measured. Further elements either with impact or not on the VSWR value are identified. Interpretation of simulation and experimental results allows to judge about improving the VSWR, and consequently increasing the microwave transmission systems efficiency.

The Range and Horizon Plane Simulation for Ground Stations of Low Earth Orbiting(LEO)Satellites

Communication via satellite begins when the satellite is positioned in the desired orbital position. Ground stations can communicate with LEO (Low Earth Orbiting) satellites only when the satellite is in their visibility region. The ground station’s ideal horizon plane is in fact the visibility region under 0o of elevation angle. Because of natural barriers or too high buildings in urban areas, practical (visible) horizon plane differs from the ideal one. The duration of the visibility and so the communication duration varies for each LEO satellite pass at the ground station, since LEO satellites move too fast over the Earth. The range between the ground station and the LEO satellite depends on maximal elevation of satellite’s path above the ground station. The dimension of the horizon plane depends on satellite’s orbital attitude. The range variations between the ground station and the satellite, and then ground station horizon plane simulation for low Earth orbiting satellites as a function of orbital attitude is presented. The range impact and horizon plane variations on communication duration between the ground station and LEO satellites are given.