The Defense Advanced Research Projects Agency announced Monday that it will test an unmanned aircraft tracking system throughout the week in San Diego and National City.

The test period, which started Monday and runs through Friday, will focus on a potential drone tracking system for large cities, which present tracking challenges due to tall buildings.

The test will involve sensors placed on two tethered aerostat balloons roughly 500 feet above ground level and optical sensors on hovering drones and building tops.

“While the defense research agency’s focus is on protecting U.S. troops from drone attacks in urban settings overseas, the system under development could ultimately help protect U.S. metropolitan areas from potential drone-enabled terrorist threats,” the agency said in a statement.

Read more: San Diego Union Tribune