I spent some time experimenting with SRF-08 sonar for outdoors robot navigation usage. While this sonar is said to have range of 6m I haven’t gotten any measurements beyond ~1m even indoors.
Outdoors it’s of course even more unreliable and there the usable range at the 20cm height seems to be about 30cm, which depends a bit on height of the grass and evenness of the terrain. Since the speed of this robot is about 5 km/h I have about 200 ms from the beginning of the measurement cycle to actually hitting the obstacle. This might be just about enough if I turn up the ping frequency to 10ms range and avoid doing anything else in the main loop.
Another option is of course to cap the robot speed to something slower but that wouldn’t be much fun.
I wonder if MaxSonar WR range would offer better range in this scenario, especially the models with narrower beam width. For some reason I couldn’t find any actual reviews of its outdoor performance and 100$ seems a bit much just for finding out.