Within minutes, the first two arrived, confirmed the attack and sent a message over the radio system calling for backup units.

Racing across town, Harry sent a priority call for backup units to join him.

According to projections, this would be a backup unit in case the others were filled.

What do I tell our backup units and the ambulance?

Store officials say the air-conditioning tower is a backup unit.

But department officials said last night that Company 293 was in fact supposed to be the backup unit.

Nor did he smile when the backup unit could also not be located.

The Syrians called in backup units to deal with the situation.

I'd just ordered Peabody to call it in, send for a couple of backup units.

"You're trying to delay me until more backup units arrive."