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Docs GPS & Navigation GPS Warm vs Cold Start

GPS Warm vs Cold Start

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Cold, warm and hot GPS starts explained, how long each takes on a NEO-6M, and how the beacon's sleep behavior affects them.

GPS Warm vs Cold Start

A GPS receiver needs three pieces of data to lock: the satellite almanac, the ephemeris for each satellite, and the current time and position. How much of this it remembers determines the time to first fix (TTFF).

The three start types

StartRetainsTypical TTFF (NEO-6M)
ColdNothing26 - 30 s outdoors
WarmAlmanac, time, position2 - 5 s
HotEphemeris tooUnder 1 s

A receiver that has been off for days does a cold start. One that slept for a few hours usually still has a valid ephemeris and does a hot start.

What the beacon does

The beacon has a firmware setting for GPS power during sleep:

  • Keep powered: the module retains everything and does a hot or warm start on every wake. Cost: a few milliamps continuously.
  • Power cut: the module cold-starts after every deep sleep cycle. Cost: tens of seconds of no-fix time after each wake, plus more battery during the search.

The right choice depends on the mission: continuous beaconing with position matters most (keep powered), versus maximum battery life with occasional position updates (power cut).

Time source matters

The NEO-6M can be told the time and rough position over UART, which turns a cold start into a warm one. The firmware does this after the first fix of a session, so subsequent power cycles acquire faster. This is one reason the first fix of a day is the slowest.

The 30-second rule

When you power the beacon after days of storage, allow at least a minute of open-sky time before judging the GPS. The first fix is always the worst.