Improving the availability of the SAR/Galileo return link service via network coding

In this paper we investigate how to improve the availability (deliverability) of the Return Link Service (RLS) of the Search and Rescue service of Galileo (SAR/Galileo). More specifically, we focus on improving the probability that the emergency beacon receives the Return Link Message (RLM) sent in the I/NAV signal. It is wellknown that beacons in distress events may be in unpredictable and arbitrary position and particular surrounding conditions such as harsh weather (heavy rain) or suffer blocking (tilted ships, crashed aircraft, big waves, etc.). Hence, it is possible that some of the 4/8 SAR RLM frames which form the short/long RLM, sent by two satellites, are lost. In this paper we propose two solutions, which could be considered for the next generation of GALILEO, to improve the chances of obtaining the RLM even when the beacon loses some frames from both satellites. Our proposals take advantage of the spatial diversity inherent in the SAR/Galileo system. The first solution implies increasing and changing the bits of the I/NAV signal which signal the SAR RLM frames. The second solution also implies increasing and changing these bits and uses Network Coding (NC). Issues such as backward compatibility with the current system and complexity are also assessed in the paper.