Orbite terrestre moyenne | EPFL Graph Search

L'orbite terrestre moyenne, communément appelée orbite circulaire intermédiaire ou MEO (Medium Earth Orbit en anglais), est une orbite autour de la Terre située entre et kilomètres d'altitude, soit au-dessus de l'orbite terrestre basse et en dessous de l'orbite géostationnaire. Cette orbite est utilisée pour placer des satellites de navigation tels ceux de Glonass (à une altitude de kilomètres), du GPS (à une altitude de kilomètres) et de Galileo (à une altitude de kilomètres). On y a lancé également Telstar 1, un satellite de communication. Les périodes orbitales des satellites situés dans l'orbite terrestre moyenne varient de 2 à 12 heures . Notes et références Notes Références Voir aussi Articles connexes *Orbite terrestre basse *Orbite terrestre haute Liens externes *Les orbites circulaires ou quasi circulaires

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

Standalone GPS L1 C/A Receiver for Lunar Missions

Paul David Blunt, Cyril Botteron, Vincenzo Capuano, Pierre-André Farine, Jérôme Leclère, Jia Tian, Yanguang Wang

Global Navigation Satellite Systems (GNSSs) were originally introduced to provide positioning and timing services for terrestrial Earth users. However, space users increasingly rely on GNSS for spacecraft navigation and other science applications at several different altitudes from the Earth surface, in Low Earth Orbit (LEO), Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO), and feasibility studies have proved that GNSS signals can even be tracked at Moon altitude. Despite this, space remains a challenging operational environment, particularly on the way from the Earth to the Moon, characterized by weaker signals with wider gain variability, larger dynamic ranges resulting in higher Doppler and Doppler rates and critically low satellite signal availability. Following our previous studies, this paper describes the proof of concept “WeakHEO” receiver; a GPS L1 C/A receiver we developed in our laboratory specifically for lunar missions. The paper also assesses the performance of the receiver in two representative portions of an Earth Moon Transfer Orbit (MTO). The receiver was connected to our GNSS Spirent simulator in order to collect real-time hardware-in-the-loop observations, and then processed by the navigation module. This demonstrates the feasibility, using current technology, of effectively exploiting GNSS signals for navigation in a MTO.

Mdpi Ag2016

Cross-band aided acquisition on HEO orbit

Cyril Botteron, Vincenzo Capuano, Pierre-André Farine, Jérôme Leclère, Jia Tian, Yanguang Wang, Wei Wang

Global positioning system (GPS) has been demonstrated to be a valid and efficient system for various space applications in low Earth orbit (LEO) and medium Earth orbit (MEO), such as for location determination and time synchronization. However, in highly elliptical and high Earth orbits (HEO), the number of existing space applications is much smaller because of the very weak power of the received signals and the bad geometric dilution of precision (GDOP). To counteract these problems, one method is to add an L5 processing module to the existing L1 C/A receiver module. Indeed, tracking the L5 signal brings several benefits: 1) a higher ranging precision; 2) a pilot channel allowing long integration times, which is helpful for both acquisition and tracking. But this also implies several drawbacks, especially in resource restricted conditions: 1) the length of the fast Fourier transforms (FFTs) and the amount of memory needed for the L5 acquisition module is huge; 2) the acquisition time is very long because of the secondary code and large frequency search space due to the long integration time. In this paper, a cross-band aided acquisition method for the L1&L5 signals is presented, which satisfy the requirements for extremely high sensitive environment without any further assistance. Compared to a traditional architecture that would acquire the L1 and L5 signals sequentially, this method acquires the L1 signal in the first place because its structure is simpler and there is no need to search the secondary code, then uses this information to reduce the code and frequency search space of the L5 signals. This brings huge benefits to the receiver: 1) the resources needed for the L5 acquisition are tremendously reduced; 2) the acquisition time for the L5 signal is considerably reduced. The implementation on an Altera Stratix III FPGA shows that about 7 % of the logic, 18 % of the multipliers and 47 % of the memory can be saved with the proposed method, while the acquisition time is reduced by 99 %.