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Conditions
Modes
Time Position Almanac Ephemeris
Cold Start None None None None
Warm Start Yes Yes Yes None
Hot Start Yes Yes Yes Yes
Table 4.1 Conditions for Start-Up modes.
For the cold-start mode, the module assigns all the available SVs to 16 channels in a
defaulted order. As a satellite is acquired, GPS time, associated ephemeris and
almanac data, which will take 12.5 minutes to download the data for all the available
satellites, are being downloaded and decoded, and the module’s status is then
transferred to tracking start. Once number of tracking satellites with valid
ephemeredes are greater than and equal to 3, the module’s position is calculated and
output, i.e., the module starts to navigate.
For the warm-start mode, based on the available time (from RTC), position, and
almanac data, the channels (up to 12) are assigned with observable satellites and the
rest of them are assigned to unobservable satellites. As the observable satellites are
acquired, time and almanac data are updated (if needed) and the corresponding
ephemeredes are downloaded and decoded. As soon as the module are tracking at
least three GPS satellites, the position is calculated and updated, and the module is in
the navigation mode.
For the hot-start mode, based on the available time, position, almanac, and ephemeris
data, the channels (up to 12) are assigned with observable satellites and the rest of
them are assigned to unobservable satellites. The module enters the navigation mode
almost instantly after power on. The time and position will be updated if needed as the
satellites are acquired. But the almanac and ephemeris data will not be updated since
they are already the “newest” information.
NOTE: To implement the warm and hot starts, a backed-up battery is needed to run
the RTC. The updated position, ephemeris, and almanac can be retrieved from BBR or
Flash memory.
4.3 Aiding - AGPS
The module can implement Assisted GPS (AGPS) function, which will accept
external input information, such as time, position, almanac, and ephemeris. This will
improve the performance of the module on Time To First Fix (TTFF). How much this