With a predawn "call to stations," test conductor Andy Brown yesterday started the countdown for space shuttle Columbia's first satellite-carrying commercial flight, set for a fiery liftoff Thursday.

The count began on schedule at 3 a.m. when Brown summoned more than 50 technicians and engineers to their consoles in the launch control center three miles from the steel and concrete pad where Columbia is perched.

The four astronauts who will man the shuttle on its first operational mission were at Johnson Space Center in Houston, concluding months of training. They are Vance Brand, the commander; pilot Robert Overmyer, and mission specialists William Lenoir and Joseph Allen.

The early part of the count proceeded smoothly as Columbia's electrical power was switched on and the 60-foot-long cargo bay doors were closed, sealing inside the two 7,200-pound communications satellites that are the shuttle's first commercial payloads.

Satellite Business Systems of McLean, Va., and Telesat of Canada are the first in a long line of customers who will pay to have their payloads carried into orbit by the shuttle. They each have paid the National Aeronautics and Space Administration $10 million for hauling up the two $30 million satellites.