In SQL Server ATN2 function returns arctangent of two numbers. In PostgreSQL you can use ATAN2 function.
SQL Server:
-- Get arctangent SELECT ATN2(35.17, 129.44); # 0.265303934222674
PostgreSQL:
-- Get arctangent SELECT ATAN2(35.17, 129.44); # 0.2653039342226741
For more information, see SQL Server to PostgreSQL Migration.