The transaction is valued at roughly $580 million including the assumption of Essex's debt.
The deal should close in the first quarter.
The Essex board approved the transaction unanimously, and shareholders who own around 11% of the company's stock are already planning to vote in favor of the acquisition.Northrop Grumman expects the purchase to be neutral to next year's earnings and to add to profits starting in 2008. Essex provides signal-processing services and products, along with advanced optoelectronic imaging for U.S. government intelligence and defense customers. Its shares were rising 17.8% to $23.53.