Dual Deployment
Some rockets use dual deployment to limit how far the rocket drifts on its way back down after the flight. Dual deployment generally relies on two parachutes - a smaller drogue parachute which deploys at apogee and a larger main parachute which deploys at a lower altitude. The rocket falls quickly under the smaller drogue 'chute before the larger parachute deploys slowing the rocket for a soft, safe landing.
Usually, an electronic altimeter is used to sense apogee and fire an ejection charge to deploy the drogue 'chute. The altimeter then waits until it senses the rocket has reached a pre-set lower altitude, usually 300 to 1000 feet, before firing a second ejection charge to deploy the main 'chute.
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