A novel emergency system for low earth orbit satellites using Galileo GNSS

Abstract

Low Earth Orbit (LEO) satellites have a limited direct contact time with the stations of their ground segment. This fundamentally constraints a timeliness reaction of the mission control center in case of emergency situations onboard the LEO spacecraft. To enable such a rapid reaction to emergency situations onboard LEO satellites, it is proposed to use a Search and Rescue (SAR) beacon onboard that spacecraft to transmit an alert message via Galileo satellites which support SAR through the Cospas-Sarsat (C/S) system to the satellite mission control center. While SAR up to now is limited to terrestrial, maritime, and aviation user scenarios, this space user concept presents a novel emergency system which helps facilitating the valuable space assets which LEO satellites in many cases represent. However, such a space user system faces various technical, system, and business challenges as well as legal and regulatory issues. The frequency band assigned for the SAR system is limited to low power satellite emergency position-indicating radio beacons and is foreseen for earth-space transmissions only. The International Telecommunication Union (ITU) should agree on opening this band for space-space communication for space user distress beacons. The Distress Alerting Satellite System (DASS) and the SAR/Glonass system will also operate in this band and an agreement will be required for these as well. A visibility analysis is presented for LEO to Galileo satellites. Depending on the placement of the antenna of the distress beacon on the LEO spacecraft, between 6 and 21 Galileo satellites are visible. The space user beacon may cause interference to the current SAR system when it’s signals collide with those of Earth-bound users in time or overlap in frequency at the Galileo transponder. When they collide in time one of the signals might still be processed if one of the signal levels is significantly higher than the other. Upon sharing the same frequency, both signals could be lost in a worst case scenario. This overlap in frequency can be caused by Doppler shifts. Therefore, a Doppler analysis was performed and Doppler shifts of about ?11 kHz were identified. Next to frequency overlaps the traffic load in the adjacent channels can increase. Different methods to prevent these Doppler shifts were analyzed. To reduce system complexity and benefit from existing technology, the space user beacon could be similar to that of an Earth beacon. However, the repetition time could be increased and the frequency channel selected for the Doppler analysis is chosen such that the interference is minimal. A high level design of the SAR payload onboard the LEO satellite was performed and different protocol options were valuated.