Galileo is the first global navigation satellite system to authenticate their civilian signals through the Open Service Galileo Message Authentication (OSNMA) protocol. However, OSNMA delays 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 the current products. In the cases where the receiver already has cryptographic data available, the so-called hot start mode and focus of this article, the currently available implementations achieve an average TTFAF of around 100 seconds in ideal environments. In this work, we dissect the TTFAF process, propose two main optimizations to reduce the TTFAF, and benchmark them in three distinct scenarios (open-sky, soft urban, and hard urban) with recorded real data. Moreover, we evaluate the optimizations using the synthetic scenario from the official OSNMA test vectors. The first block of optimizations centers on extracting as much information as possible from broken sub-frames by processing them at page level and combining redundant data from multiple satellites. The second block of optimizations aims to reconstruct missed navigation data by using fields in the authentication tags belonging to the same sub-frame as the authentication key. Combining both optimizations improves the TTFAF substantially for all considered scenarios. We obtain an average TTFAF of 60.9 and 68.8 seconds for the test vectors and the open-sky scenario, respectively, with a best-case of 44.0 seconds in both. Likewise, the urban scenarios see a drastic reduction of the average TTFAF between the non-optimized and optimized cases, from 127.5 to 87.5 seconds in the soft urban scenario and from 266.1 to 146.1 seconds in the hard urban scenario. These optimizations are available as part of the open-source OSNMAlib library on GitHub.

翻译：伽利略是首个通过开放服务伽利略消息认证（OSNMA）协议对民用信号进行认证的全球导航卫星系统。然而，OSNMA延迟了获得首个位置和时间锁定（即首次认证定位时间，TTFAF）的过程。尽可能缩短TTFAF对于将该技术无缝集成到现有产品中至关重要。在接收器已具备加密数据的情况下（即本文关注的"热启动"模式），现有实现在理想环境下的平均TTFAF约为100秒。本研究剖析了TTFAF过程，提出了两项降低TTFAF的主要优化方案，并在三种不同场景（开阔天空、软城市和硬城市）中使用实测真实数据进行了基准测试。此外，我们还利用官方OSNMA测试向量中的合成场景评估了这些优化方法。第一组优化方法侧重于通过页面级处理受损子帧，并融合多颗卫星的冗余数据，从而从子帧中提取尽可能多的信息。第二组优化方法旨在利用与认证密钥属于同一子帧的认证标签中的字段，重构丢失的导航数据。结合这两项优化方案，所有测试场景的TTFAF均得到显著改善。测试向量和开阔天空场景的平均TTFAF分别达到60.9秒和68.8秒，两者最佳情况下均为44.0秒。同样，城市场景中未优化与优化案例的平均TTFAF大幅降低：软城市场景从127.5秒降至87.5秒，硬城市场景从266.1秒降至146.1秒。这些优化方法已作为开源OSNMAlib库的一部分发布于GitHub。