What does Field E display when no other information is available?

When no other information is available, Field E displays ground speed. Ground speed is a critical parameter that indicates how fast the aircraft is moving over the ground, which is particularly important for air traffic control. Understanding ground speed aids in maintaining safe separations between aircraft and in managing traffic flow effectively. In the absence of more detailed data, ground speed serves as a reliable indicator of an aircraft's performance and helps controllers make informed decisions regarding navigation and timing. This focus on ground speed is essential for ensuring safety and efficiency in airspace management.