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To achieve the highest level of accuracy, most survey-grade receivers use two GPS radio frequencies: L1 and L2. Currently, there is no fully functional civilian signal at L2, so these receivers leverage a military L2 signal using "codeless" techniques.
The ongoing GPS modernization program is adding a dedicated civil signal at L2 that supports high-accuracy positioning without the use of military signals. The GPS program is also adding a third civil signal at the L5 frequency that will enhance performance even further. Since 2020, the government no longer supports codeless access to military GPS signals.