When GPS was first being put into service, the US military was concerned about the possibility of enemy forces using the globally available GPS signals to guide their own weapon systems. Originally, the government thought the "coarse acquisition" (C/A) signal would give only about , but with improved receiver designs, the actual accuracy was . Starting in March 1990, to avoid providing such unexpected accuracy, the C/A signal transmitted on the L1 frequency () was deliberately degraded by offsetting its clock signal by a random amount, equivalent to about of distance. This technique, known as Selective Availability, or SA for short, seriously degraded the usefulness of the GPS signal for non-military users. More accurate guidance was possible for users of dual-frequency GPS receivers which also received the L2 frequency (), but the L2 transmission, intended for military use, was encrypted and was available only to authorized users with the decryption keys.
