What must all centers do during non-radar operations for aircraft clearance?

During non-radar operations, it is essential for air traffic control centers to provide clearances to aircraft using established navigation aids or defined airways. This is primarily because non-radar operations do not allow for continuous positional tracking of aircraft, making it crucial to rely on fixed points and routes for safety and to ensure efficient traffic flow.

Clearing aircraft via radials of NAVAIDs or airways ensures that pilots have a defined route to follow, which helps maintain separation from other aircraft and avoids conflicts. It also supports pilots in navigating based on instruments and VFR references, which is particularly important in non-radar environments.

In contrast, using flight levels only may not adequately address horizontal navigation needs. Direct GPS routing could complicate air traffic control without established routes and might lead to airspace conflicts. Issuing visual flight rules, while useful under certain conditions, is not a standard practice during non-radar operations and does not necessarily provide the structured route clearances required. Thus, utilizing radials of NAVAIDs or airways is the most reliable and standardized method during such operations.