Will users of dual-channel GPS receivers need to purchase new units to take advantage of GLONASS and GALILEO ?
If you mean those that currently use dual-channel codeless & semi-codeless type receivers some will have to totally replace other might be able to upgrade the internal boards etc? There are currently GPS/GLONASS receivers available and some manufacturers claim GALILEO "ready" but GALILEO hasn't even been finialized as yet so who knows?
Low end civil receivers will have to be replaced to use the new civil freq's
Come Dec 31 2020 then all current dual-channel codeless and semi-codless receivers will be obsolete and require replacing.
Will GPS vertical error be reduced to about the same level as horizonatal error, so GPS (by itself) will deliver reliable elevation data ?
No, but vertical accuracy will be better but still not as good as horizontal accuracy.
Exactly to what magnitude will GPS accuracy and signal acquisition/retention improve with the new Navstar channels ? And with the other two systems ? And with both combined ? In short, what sort of GPS accuracy and performance can one expect after 2013 ? I cannot find any information about this, even estimates.
Civil dual-freq should be able to provide around 1.5 metres 95%. If one includes the other systems then accuracy probably won't improve much but coverage and availability will which might push the 95% out to 99++ % ?