GNSS devices often take a long time to get the first fix after first power-on or long power loss.
This is cold start. It is slow not because the module computes slowly, but because the receiver lacks prior information.
Positioning needs:
rough time
rough position
visible satellites
satellite orbit data
good enough signal quality
The more information is missing, the longer time to first fix becomes.
Cold Start Has Few Search Hints
If the receiver does not know roughly where it is or which satellites are visible, it must search a larger range of frequency, time, and satellite IDs.
Poor antenna environment makes this slower. Indoor use, garages, metal enclosures, weak signals, and multipath all extend cold start.
Warm or hot start is different. The device keeps previous position, time, and satellite data, so the search range is much smaller.
Ephemeris and Almanac Affect First Fix
GNSS satellites broadcast navigation data. The receiver needs it to compute position.
Two useful categories are:
- almanac: coarse, covering many satellites, useful for knowing what may be visible
- ephemeris: precise data for specific satellites, needed for accurate solution
During cold start, if local ephemeris is unavailable, the receiver must receive it from satellites. Weak signal or interruptions restart or delay this process.
AGNSS Reduces Waiting
Assisted GNSS can provide time, rough position, almanac, or ephemeris through the network, reducing the need to download everything from satellites.
Cellular devices often use AGNSS to shorten time to first fix. The cost is dependency on network, server, and data channel.
Without network, or with expired assistance data, cold start remains slow.
Debugging
Record:
- Whether this is a true cold start or backup data is retained.
- Whether the antenna has sky view.
- Satellite count, signal strength, and continuous tracking.
- Whether AGNSS data is available.
- Whether RTC time is credible.
- Whether restart after first fix is much faster.
GNSS cold start is the process of rebuilding time, satellite, and orbit knowledge under weak signals. Better antenna, retained backup data, credible time, and AGNSS usually help more than repeated module resets.