Warmstart
Warmstartis performedwheneverthe receiver hasonlyaccess tovalid almanacdata, andhas notsignificantly
movedsincethelastvalidpositioncalculation.Thisistypicallythecase,ifthereceiverhasbeenshutoffformore
than 2 hours, but still has knowledge of last position, time and
almanac. This allows it to predict the current
visible SVs. However, since ephemeris data is not available or outdated, the receiver needs to wait for the
ephemerisbroadcasttocomplete.
Hotstart
Hotstart is performed whenever the receiver still has access to valid ephemeris data and precise time. This is
typically the
case if thereceiver has beenshutoff forless than 2hours and theRTC has beenrunning during
that time. Furthermore, during the previous session, the receiver must have been navigating (to allow it to
decodeandstoreephemerisdata).
In Hotstart, thereceivercanpredict thecurrently visible
SVs,andisthereforeable toquickly acquireand track
the signal. Because ephemeris is already known, there is no need to wait for the ephemeris broadcast to
complete.
Current Time
and last position
known?
Valid ephemeris
available?
Coldstart
Warmstart
Hotstart
No
Yes
Yes
No
startup
Figure 3: Decision Tree on Startup Mode
If external aidinginformation like ephemeris and/orpreciseposition and timeare providedtothereceiver, the
TimeToFirstFixcanbesignificantlyimproved.RefertoSection 4.5.4forfurtherinformation.
GPSModules-SystemIntegrationManual(SIM)(incl.ReferenceDesign) GPSFundamentals
GPS.G4-MS4-05007-A1
Page 13
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