Using GPS2 as primary altitude source

In another blog we discussed the drift of the baro due to atmospheric pressure so I would like to start using multiband GPS for altitude.

But I am a bit confused about the settings of the use of GPS1 and GPS2.

The problem I see here is this:

Two GPS units do not report the same altitude. I know this from experience and it can be quite significant. So if I start and ARM, my home altitude is set to zero, using for example GPS1.

If, during flight, GPS2 reports a better lock, it will switch to GPS2 and consequently use the altitude from GPS2 as well. Which can be very different from GPS1.

So actually I would like to set a primary GPS, and only allow switchover in case the primary fails.

I also would like to keep the Here GPS connected to GPS1 with the lights and arm button, which is very convenient and use a multiband GPS for positioning and altitude on GPS2.

There is the setting GPS_AUTO_SWITCH. The way I read it, is like this:

For example, if set to “3”, it would use GPS2 and switch to GPS1 if GPS2 fails, or GPS1 reports a better lock.

In the description it says:

“Automatic switchover to GPS reporting best lock”.

So by setting this parameter, you determine how to switch, not which one to use by default. It doesn’t say which GPS is default.

So my question is this:

I would like to use GPS2 as my primary GPS and use that for navigation and altitude. GPS1 should only be used as redundant in case GPS2 fails.

Is this possible?

Is there anybody who know how to do? @tridge maybe?