What critical action must a satellite perform immediately after spacecraft separation during the 'checkout phase'?

Deploying solar arrays to begin generating power.

Once the satellite is gently pushed away from the spent upper stage—the moment of spacecraft separation—its own internal mission operations begin immediately. The launch vehicle's job is done, but the satellite's life is just starting. The very first priority for mission success is securing independent power generation, as all internal systems must be brought online. This necessitates the immediate deployment of its solar arrays. Ground controllers closely monitor this 'checkout phase' to confirm that the satellite successfully survived the violent ascent environment without damage before proceeding to maneuvers like deploying antennas or using the station-keeping engine for final positioning.