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Why was the Transit satellite system not suitable for aircraft navigation?

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GPS satellite — GPS U.S. Navstar Global Positioning System (GPS) satellite in orbit over Earth, shown in an artist's conception.

The Transit satellite system was not suitable for aircraft navigation primarily due to its reliance on the Doppler shift to determine position.1

To calculate a location, the Transit system required a stable velocity during the navigation interval. Because aircraft move at high speeds and are subject to sudden, unexpected changes in velocity, these fluctuations modified the Doppler shift trace and introduced significant positioning errors. Specifically, an uncertainty of just two knots in a user's velocity could result in a position error of approximately one-half nautical mile. While this level of inaccuracy was acceptable for slow-moving ships at sea, it rendered the system unreliable and unsafe for the high-speed, dynamic requirements of aviation.1

In contrast, the Global Positioning System (GPS) overcame these limitations by utilizing signal travel time measurements from multiple satellites controlled by atomic clocks, allowing for precise, real-time positioning suitable for aircraft and other fast-moving vehicles.1