Communications Satellite Corp., one of the country's fast-growing advanced telecommunications firms, yesterday reported a profit decline for 1980 due to start-up costs of its new Satellite Business Systems venture and the U.S. Navy's decreased use of the Marisat communications system.

Net income for the year dropped 5 percent to $38.3 million ($4.79 a share) from $40.2 million ($5.02). Revenues increased for $299.7 million from $255.8 million.

Satellite Business Systems is supposed to begin providing communications satellite services for U.S. domestic customers this spring.

Net income for the fourth quarter was $9.7 million ($1.22), up from $7.5 million (94 cents). Total operating revenues were $82.0 million, up from $69.5 million.

Cerberonics Inc. of Falls Church reported earnings for its second quarter ended Dec. 27 of $93,379 (20 cents a share), up from $71,052 (11 cents) a year earlier. Revenue rose to $2.4 million from $2.1 million.

First-half earnings were $175,195 (37 cents), up from $160,125 (25 cents) and revenues were $4.5 million, up from $4.4 million.

Easco Corp. of Baltimore reported net income decreased from $8.2 million ($2.21 fully diluted) in 1979 to $3.7 million (97 cents) last year as revenues rose from $357.3 million to $351.7 million. Net income for the fourth quarter rose from $1.2 million (31 cents) to $1.7 million (50 cents). Revenues were $103.1 million, up from $97.6 million.