Improving Galileo OSNMA Time to First Authenticated Fix

Galileo is the first global navigation satellite system to authenticate its civilian signals through the open service navigation message authentication (OSNMA) protocol. However, OSNMA adds a delay in the time to obtain a first position and time fix, the so-called time to first authentication fix (TTFAF). Reducing the TTFAF as much as possible is crucial to integrate the technology seamlessly into existing products. In cases where the receiver already has cryptographic data available, the so-called hot start mode and focus of this article, currently available implementations achieve an average TTFAF of around 100 s in ideal environments. In this work, we explore TTFAF optimizations available to general OSNMA-capable receivers and to receivers with tighter time synchronization than that required by the OSNMA receiver guidelines. We dissect the TTFAF process, describe optimizations, and benchmark them in three distinct scenarios (open-sky, soft urban, and hard urban) using recorded real data. Moreover, we also evaluate these optimizations using a synthetic scenario from the official OSNMA test vectors. The first block of optimizations centers on extracting as much information as possible from broken subframes by processing them at the page level and combining redundant data from multiple satellites. The second block of optimizations aims to reconstruct missing navigation data through the intelligent use of fields in authentication tags that belong to the same subframe as the authentication key. Combining both optimization ideas improves the TTFAF substantially for all considered scenarios. We obtain an average TTFAF of 60.9 s for the test vectors and 68.8 s for the open-sky scenario, with a lowest TTFAF of 44.0 s in both cases. Similarly, the urban scenarios show a drastic reduction in the average TTFAF between the nonoptimized and optimized cases. These optimizations have been made available as part of the open-source OSNMAlib library on GitHub.