  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.
