but if encoders ever used GPS altitudes presumably a mechanism would need to be found for ensuring every GPS was set indentically. In other words there would need to be a common standard of IFR approved GPS encoders - quite possibly one would imagine, cheaper, lighter and more reliable than existing technology.
Are solar flares a worry? We have had a period of weak solar activity but history suggests that will not be the case for ever. However it also leaves me wondering if the GPS signal can be interupted by solar activity can mode c / s transmissions and even primary radar be compromised.
Solar flares will disrupt GPS in 2011 - tech - 29 September 2006 - New Scientist