A-GPS is based on the use of a data connection (for example, a cellular network) to
provide predicted satellite information from an A-GPS server to the GNSS receiver. With
the help of this data, the receiver is usually able to achieve a positional fix faster than
using live data only. Although the term “A-GPS” is commonly used, the server-based data
can also refer to other constellations (for example, GLONASS predictions).
A "cold" start is when the GNSS receiver must receive all data before starting navigation,
which can take several minutes.
A "warm" start is when the GNSS has most of the data it needs in memory and will start
quickly, within a minute or less.
A “hot” start is when the receiver has all the data from the satellites (time, almanac,
ephemeris) and only needs to calculate the positional solution. The fix is usually acquired
in few seconds.
In other words, using A-GPS allows the device to start in a condition similar to “warm”
and “hot”, thus shortening the TTFF.
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