GPS III sees the addition of four new signals, three new civilian signals and a military signal. One of the civilian signals and one of the military signals will be used to provide redundancy, making the signal difficult to interfere with either through intentional or accidental jamming of the signal.
Another handy little benefit: by using two different signals, you can directly measure any delay error that happens in the ionosphere as the signal makes its way towards Earth, and thus remove it from the calculations the receiver makes during triangulation. This will subtly, but significantly increase the accuracy of GPS receivers. Receivers that are capable of receiving two signals are known as dual frequency receivers.
The new redundant civilian signal is known as L2, compared to the original L1. The new military signal is known as M-code.
Another of the civilian signals will be used exclusively for aviation safety services, particularly the commercial aviation that occurs on a mass scale in today's globalized world. Radar technology is slowly being replaced by GPS technology in commercial aviation to provide more accurate and more reliable positioning, and these benefits may be guaranteed through use of a dedicated signal. This signal is appropriately referred to as safety of life, the formal name being L5.
The fourth, and final civilian signal is designed to allow for interoperability between GPS and other GNSS systems such as GLONASS, COMPASS and Galileo, as all of these systems plan on broadcasting this signal in addition to their own. This will allow for previously unknown coverage, a truly global network, with all of the GNSS constellations capable of working as one. It will operate on the same frequency as the original civilian signal, L1.