The Parallax sonar sensor was on sale a while back at Radio Shack, where I got it for a good price. There are good online tutorials for this. I was able to get this sensor working with my Teensyduino 3.2, and printing out ranges and testing.
The range of this sensor is less than 50 feet. I have gotten good results for 20 to 25 foot range. This sensor is needed to determine when to separate the payload from the descent chute. I added some code to the sample program to light up leds to indicate when we are too close to the ground and need to fire the squib to decouple from the descent chute.