Sony VGP-BGU1 - Manual (Page 16)

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VGP-BGU1_2-691-136-12(1)
Charging the built-in battery
To shorten the time to position, almanac data or other required data can be
stored in the built-in memory. When you use the unit for the first time, or
have not used the unit for a long time, the memory data may be deleted due
to the battery discharging. In this case, charge the built-in battery using the
AC adapter.
About almanac data
To shorten the time to positioning, the unit memorizes the satellites orbit
information (almanac data) into the built-in memory.
Note
When you use the unit after a long period of time, the unit needs to
recognize the almanac data from the satellites again. When you turn on the
unit under the location where the reception condition is good, the unit
receives the almanac data from GPS satellites automatically.