What must be entered to maintain synchronization between SINCGARS radios?

Maintaining synchronization between SINCGARS (Single Channel Ground and Airborne Radio System) radios is crucial for effective communication in tactical environments. The correct answer is the Julian date because it serves as a standardized way to identify the current date, typically expressed as the day of the year. This synchronization is essential as it ensures that the radios are configured to operate on the same temporal reference, which reduces the chances of communication errors by aligning the internal clocks of the devices.

When the Julian date is correctly entered into the radios, it helps the systems coordinate with each other for timing-related processes, such as encryption and secure communications. This synchronization is particularly important in military operations where timing can significantly impact the success of missions.

The other options, while relevant to the operation and functionality of radios, do not play a direct role in achieving synchronization. Battery status pertains to the power available for operation, frequency range relates to communication channels, and the serial number is for identification purposes, but none of these factors addresses the synchronization timing aspect that the Julian date provides.