The other day, THIS arrived! HC-SR04 Ultrasonic Sensors

Ordered 5 of these a while back.  Bingo.  16.4' range, ultrasonic distance.  I need one more I think, for above SARA, it's fine.  One in each direction for now, N-S-E-W,  one pointing down.  Mixed with altitude from GPS and Barometric Pressure Sensor.  Good enough for now.  She's getting there...