In late April of 2014, the Air Force inaugurated a new capability for GPS satellites with the broadcast of two new signals. GPS users wouldn't have noticed any difference because no current GPS recovers are set up to use these signals yet, but these civilian navigation (CNAV) signals will soon provide more accuracy and redundancy
The new signals are called L2C and L5. They join the standard L1 and L2 signals that GPS satellites have broadcast since the beginning (the L3 frequency transmits data on detected nuclear detonations in the atmosphere; the L4 frequency is not used).
The 1575.42 MHz L1 frequency broadcasts both the civilian signal and an encrypted precision military signal (exactly how these two data streams are modulated onto a single frequency is an obscure topic beyond our scope here).
Meanwhile, the 1227.60 MHz L2 signal also receives multiple modulation inputs and sends data for a second encrypted precision military signal.
Note that military receivers have two signals available. This is important for maximum accuracy because a important source of error is caused by GPS signals passing through the charged particles in the ionosphere. If you are receiving just one signal, you have no way of knowing how much error has been incurred. However, with two known frequencies you can compare them, determine the effect of the ionosphere and remove it from your calculations.
Now, the new L2C signal will provide a second civilian signal, allowing civilian GPS receivers to cancel out ionospheric error, making them more accurate.
The other new signal at 1176.45 MHz is L5. This signal will provide several advantages, including higher transmit power than the signals at the L1 and L2 frequencies, will provide a back up signal and it modern signal design will give it robust forward error correction and multiple message types. It is broadcast at a frequency in the aviation frequency band and will be used extensively by civilian airliners.
Right now the Air Force is testing the L2C and L5 frequencies, with initial operational capability set for December of 2014. Currently 13 of the 31 operational satellites can broadcast the L2C signal and six satellites can broadcast both L2C and L5. As the other six of the Block IIF satellites and the new Block IIIA spacecraft are launched into the the constellation, all GPS satellites will eventually have the capability to send both L2C and L5. Users GPS units will need to be designed to take advantage of the added accuracy and redundancy of the new signals.
The upshot of these improvements will be even more accuracy and reliability from GPS.