Long range sensors - Ultrasonic/IR/LiDAR/laser - for microcontroller
How large is an obstruction? I think your options are probably either "build a RADAR set" (difficult, pricey) or "modify a laser rangefinder to do what you want". There exists an open source laser rangefinder base unit: http://www.lightware.co.za/shop/en/ - that $150 sounds expensive but realistically you're not likely to do it for less unless you have a really great supply of scavenged electronics and optics.