In order to obtain high accuracy GPS positioning (better than 2 meters) receiver must know local ionosphere corrections. This requires two GPS signals at a minimum, the difference between both is used to calculate ionospheric corrections which is then applied to either one.
However since GPS doesn't offer two usable civilian signals, a technique known as semi-codeless was devised, that calculates ionosphere corrections by using the two military encrypted signals L1P(Y)+L2P(Y), then applies the iono corrections to the civilian L1 C/A signal, however the military was never to fond of that usage, since it limits what changes they can do with the military signals. Specially changes in power levels.
Since September 26, 2005 GPS satellites capable of broadcasting the L2C (2nd civilian signal) and since May 2010 GPS satellites capable of broadcasting the L5 (3rd civilian signal, usable for aviation) have been launched, however those signals are still not fully usable.
Today those signals were enabled in a pre operational format meaning:
1 — All messages required for full L2C and L5 utilization are broadcast
2 — L5 signals are broadcast with an alert flag (not usable), L2C is broadcast without an alert flag
3 — L2C/L5 almanac and ephemeris will be updated about twice a week, while regular L1 C/A updates happen typically twice a day, so L2C and L5 signals will be less accurate in this phase
However this means there is no technical excuse for GPS equipment manufacturers to finalize their L2C and L5 offerings, since they now have a complete signal to test against, and right after GPS satellites have received an upload they should have similar accuracy as L1 C/A.
This should continue for the next few years, until the new GPS control segment, OCX comes online, OCX block 1 is needed for full L2C capabilities and OCX block 2 is needed for full L5 capabilities.
Notice that it will take at least another 12 GPS launches for L2C to reach a state known as FOC (full operational capability), meaning that are enough satellites with L2C capability for L2C to be usable for standalone positioning and it will take another 19 GPS launches for L5 to reach FOC as well.