SAR unmanned air vehichles could fly though dense forests without using GPS

https://youtu.be/jfnOt_2IXdg

MIT's press release

• Fleets of drones could aid searches for lost hikers
• System allows drones to cooperatively explore terrain under thick forest canopies where GPS signals are unreliable

http://news.mit.edu/2018/fleets-drones-help-searches-lost-hikers-1102