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docs: eg25-control: elaborate on Almanac v.s. Ephemeris

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Colin 2024-06-24 00:25:54 +00:00
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pkgs/additional/eg25-control/eg25-control
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@ -8,6 +8,15 @@
#
# this script downloads assisted GPS (AGPS) data via the system's default gateway (i.e. WiFi)
# and shares that with the modem. this quickens the process of acquiring a GPS fix.
# AGPS data includes an almanac and "ephemeris" [Wikipedia-GPS_signals#Navigation_message]
# - Almanac: valid for 2 weeks; status & low-res orbital info for *all* satellites.
# - used to know which GPS signals to *search* for.
# - every sat broadcasts the global almanac over 12.5min cycles.
# - Ephemeris: valid for 4 hours; precise orbital info for each satellite.
# - takes 30s to learn this info w/o AGPS data.
# - every sat broadcasts *its own* ephemeris over 30s cycles.
#
# [Wikipedia-GPS_signals#Navigation_message]: https://en.wikipedia.org/wiki/GPS_signals#Navigation_message
#
# the script may also configure other parts of the modem as `eg25-manager` does.
# these options are less tested: see `--help` for more.