How many satellite's are needed for a GPSDO to work correctly

[davebb]

How many satellite's are needed for a GPSDO to work correctly
Thanks Dave 2E0DMB

[Ian.M]

3 satellites are required for a 2D GPS fix (which assumes zero altitude with respect to the WGS84 geoid).  4 satellites are required for a 3D fix.   Without a known position, the receiver cannot correct for the effects of Doppler shift (due to the satellites' 14,000 km/hour orbital velocity) so cannot establish an accurate local timebase.   All receivers will have a valid reference clock output and/or 1pps output when they have established a 3D fix, but the status of those outputs with a lower grade fix, or even a manually entered position and altitude would be dependent on the receiver firmware. 

[dom0]

Yes, that's what the survey-in feature is for. The GPS receiver in survey mode tries to very accurately determine the antenna position over a few hours (at least) or days and then locks that position in.

I think this kind of GPS receiver can maintain an accurate clock after the survey with just one satellite, but I'm not certain. On the other hand I also question whether this is important - if the GPS weather is so poor with the current antenna setup that you can't get enough sats for a reliable 3D fix then clearly another antenna setup is needed for reliable GPS operation.

[davebb]

Hi thanks for the info
Thanks Dave

[jpb]

Ublox timing GPS modules have a single satellite mode so long as the survey-in has happened. In fact, I think this mode is more consistent at least whilst it can still see the same satellite.

e.g. see the highlights page for the Lea6T:

https://www.u-blox.com/en/product/neolea-6t