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I've recently traveled over Europe with a GPS-enabled mobile phone.

In Italy, it took around 10 minutes for the mobile phone to connect to a satellite, while in Latvia it took just 10 seconds.

How would you explain that?

(I know - it may depend on the weather, but it was sunny both in Latvia and in Italy.)

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2 Answers 2

up vote 11 down vote accepted

The difference sounds like a warm start vs a cold start.

With a warm start, the GPS already knows Ephemeris data for at least 3 of the satellites above it. It can therefore kick straight off with solving the time signal equations, and can get a fix quickly. (That Ephemeris data could either come from having recently used the GPS, eg previous 4-6 hours, or could come from the network with assisted GPS)

With a cold start, the GPS doesn't know the precise location (Ephemeris data) for the satellites above it. Instead, it needs to wait until it gets a full set from the satellites above it, which means it needs between 10 and 40 seconds of error free data from them (depending where in their data cycle they all are). Once it has that, it can solve the equations and knows your location. If you're moving, or surrounded by lots of buildings, or anything else triggering interference, it can take several tries for the GPS to get an error free set of Ephemeris data from enough different satellites, which can easily mean 5-10 minutes for a lock.

More informations here and here.

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In addition to Gagravarr's answer, some phones will use A-GPS, where they use surrounding cell-towers or wifi to give themselves a headstart in locating their position.

Certainly the position of satellites, mountains, cloud cover, nearby interference from powerlines, solar flares - there are a variety of different conditions that may affect it.

In Kazakhstan today, with clear sky I had 11 satellites in sight, and still couldn't get a fix. Go figure :)

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