The correct option is "I and II only." GAGAN, which stands for GPS-Aided Geo Augmented Navigation, is indeed designed to enhance the accuracy and integrity of GPS signals, particularly for aviation navigation, ensuring safe and efficient air traffic operations. It also facilitates more efficient air traffic management by providing precise positioning information, thus improving the capacity and safety of airspace use. However, statement III is incorrect because, while GAGAN primarily benefits aviation, the technology can also be extended to enhance navigation for other transportation modes, underscoring its broader applicability beyond aviation alone. This understanding aligns with the comprehensive scope of technological advancements in navigation systems covered in UPSC-level sources.