How many Sat's req' to get a fix?

Wicksy · Occasional Visitor Joined: Apr 11, 2006 Posts: 6

How many satellites and what minimum strength does TTN 5 need to get a useable fix. I haven’t noticed any pattern between having a valid GPS signal and not.

lbendlin · Posted: Sun Apr 30, 2006 1:56 am Post subject:

you need a minimum of three sats for a 2D fix, and a minimum of 4 for a 3D fix (with altitude).

the signal strength has no meaning, it is different per receiver. Some can lock on with signals of 17dBm and less.
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mcwarre · Regular Visitor Joined: Mar 25, 2004 Posts: 157

I thought that there were 3 unknowns in 2d? i.e. lat / long & time. This leads to 3 simultaneous equations which needs 4 sats to solve it. 3D has another unknown and therefore needs 5 sats.

lbendlin · Posted: Mon May 01, 2006 2:27 pm Post subject:

Not exactly. You need to imagine the time bubble (sphere) traveling from each satellite. Your receiver is at one of the intersections of the spheres.

Two sats intersect with a circle, three sats intersect in two points (one of which is in outer space and can be ignored), starting with four sats the meeting piont is well defined.

The receiver doesn't really need to know the time, it just needs to receive the same time beacon from the sats, and from the time difference calculate their locations.
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timber · Posted: Mon May 01, 2006 4:25 pm Post subject:

Wicksy · Occasional Visitor Joined: Apr 11, 2006 Posts: 6

Thanks for your response. All the above apply to my GPS ( Fortuna GPS Smart BT) to give a fix with a minimum of three sats. But this does not seem good enough for for TomTom Nav 5 (On XDA Mini S) which requires a longer wait for more sats with stronger signals before kicking into life.
Any ideas why this is so?

lbendlin · Posted: Mon May 01, 2006 11:08 pm Post subject:

The Fortuna GPSmart is a generally weak device, and requires undisturbed sky view to get sufficient Signal to noise ratio. I think your three sats are not locked on stably enough for TomTom.

Either get an external antenna (not even sure if the Fortuna supports that) or consider getting a SiRF III based device. A difference like day and night.
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