What does a successful "SYNC" indication signify after power-up on an Inmarsat-C system?

The successful "SYNC" indication after power-up on an Inmarsat-C system signifies that sufficient signal strength on the NCS (Network Control Station) common channel is being received. This is a critical first step in the operation of the system, as the "SYNC" status indicates that the Inmarsat-C terminal is effectively receiving signals from the NCS, which is necessary for further communications to occur.

Receiving a "SYNC" signal shows that the terminal has access to the network, which means it can potentially establish a connection for data transmission and reception. This is fundamental before any additional functions, such as logging into the network or sending and receiving traffic, can take place, as a good signal is required for the system to properly synchronize with the network's operations.

The other choices reflect activities or statuses that cannot occur until after the "SYNC" status is achieved. For instance, logging in to perform specific commands requires the initial synchronization, hence making option A contingent upon this indication. Similarly, options C and D pertain to actions that can only be taken after the system successfully confirms synchronization and locks onto the NCS signal.