NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2018 Sep 29, 18:37 -0400
" The paucity of precise data from prior solar cycles may limit your ability to predict future levels of activity."
It's impossible to predict when a strong solar flare or CME large enough to disrupt GNSS might occur. They can even strike near the minimum of the roughly 11-year solar cycle. Most outbursts are not severe enough to prevent GNSS being used but simply reduce positioning accuracy with the vertical coordinate (as is always the case) being a few times worse than the horizontal coordinates. Potentially more impactful are strong solar radio storms, which drown out the GPS signals. NOAA reported on such a storm in December 2006 here: https://web.archive.org/web/20071213212715/http://www.noaanews.noaa.gov/stories2007/s2831.htm (from the "Wayback Machine" as the original item is missing or got moved on the NOAA site).
Much more impactful would be a repeat of the 1859 Carrington Event: https://science.nasa.gov/science-news/science-at-nasa/2008/06may_carringtonflare
-- Richard Langley
