Assisted GPS | EPFL Graph Search

vignette|Composants impliqués lors de la mise à jour de l'almanach GPS à l'aide d'A-GPS et d'un réseau GSM L’Assisted GPS, ou A-GPS, est une technique pour améliorer le positionnement par satellites. L’A-GPS a été introduit pour améliorer la réactivité du GPS, d’où son nom de « GPS assisté ». Le système GPS repose sur une table d'éphémérides des satellites (voir l'article Global Positioning System pour plus de précision) afin de calculer la position géographique en latitude et longitude ainsi qu'en altitude d'un récepteur GPS (trois satellites minimum, le calcul du temps nécessitant l'utilisation d'un quatrième satellite). Cette table, préalable nécessaire à l'obtention d'une position, est transmise via les signaux des satellites mais souffre dans ce cas de deux défauts majeurs :

elle est lente à télécharger car le débit de données par le signal du satellite est seulement de , et si le signal du satellite est perdu durant le téléchargement il faut recommencer à partir du début ;

Doppler-based positioning using LEO augmented satellite constellation - Simulations of perspectives

Alice Andreetti

The Global Navigation Satellite System (GNSS) refers to a constellation of satellites emitting signals from space, used to provide Position, Navigation and Timing (PNT) services to the receivers on Earth. Nowadays, the GNSS is exploited in a wide range of applications among which ground mapping, transportation, machine control, timing, surveying, defense, or aerial photogrammetry [1]. However, as we become more dependent on GNSS technology, we also become more vulnerable to its limitations [2]. The GNSS operates with satellites orbiting in Medium Earth Orbits (MEO, approximately 20200 [km] in altitude). On one hand, this great distance allows to have a large footprint of the satellite signal on Earth, but on the other hand, it requires the signal to cross a wide path in the atmosphere before reaching the receiver. This results in a decrease of signal strength, which is not an issue in open-sky condition, but becomes a limitation in deep attenuation environments, with little or no service in urban canyon, in dense cities or indoors [3]. In this contribution, a possible strategy to cope with this issue is proposed and evaluated through a simulation process. It implies the use of Low Earth Orbits (LEO) satellites as a support for the GNSS PNT service. Since nowadays no broadband big LEO constellation broadcasting navigation messages exist, in this project various LEO constellation setups have been simulated and tested. Because LEOs are much closer to Earth (200 to 1500 [km]), they move faster in the sky, passing overhead in minutes instead of hours like MEOs. This gives rise to the possibility to exploit the Doppler effect to accurately locate the user. Therefore, three different PNT algorithms, employing the Kalman filter, are implemented and the evaluation of the localization quality is made at four distinct locations on the globe. The results show that, for most of the cases, compared to the standard GPS pseudorange only PNT, the addition of LEO pseudoranges already improve the localization quality. The performance of the PNT algorithm increases even more with the exploitation of both pseudoranges and pseudorangerates (Doppler) measurements. Nevertheless, the choice of constellation parameters such as satellites altitude, total number of satellites, orbit inclination or the frequency of the emitted signal, has a great impact on the magnitude of the improvement in the localization quality. The factor that has the most significant influence seems to be the signal frequency and for that reason it must be carefully chosen: the higher the better. Moreover, it has been demonstrated that at high elevation cut-off angles (denied environments), the addition of LEO constellation is a valid option and allows us to obtain an accurate position even when the GNSS only can not provide a solution. Finally, the LEO only system seems to be a promising alternative to cope with GNSS jamming or spoofing. However, the optimal LEO parameters providing an accurate solution for all the location on the globe has not yet been identified. In conclusion, the result of this effort is the awareness that, once the optimal parameters for the LEO system have been found, it is a valid option to augment the GNSS, or even as a standalone backup, especially when the Doppler-based positioning is applied. Nevertheless, we are still far from that end and much more research needs to be done.

2020

Turning Point - Trajectory Analysis for Skiers

Jan Skaloud

As with a growing number of athletes, competitive skiers are looking to GNSS technology to help evaluate and improve their performance. Positions logged during practice runs or races can be transformed into 3-D models of the exact trajectories skied to measure velocity, accelerations, and other performance variables. However, obstructions along a ski run, such as terrain and trees, can block satellite signals - suggesting a need to add low-cost inertial sensors to support continuous, accuracte and affordable positioning.

2007

Doppler-based positioning using LEO augmented satellite constellation - Simulations of perspectives

Alice Andreetti

2020