As you alluded to, the Garmin Astro system for dogs is used by many SAR K9 handlers to locate their dog in the field. For humans, some teams use the Garmin Rino unit which can display the location of other searchers. Having played with those units, though, I've found them limited in range when showing locations. My team has started utilizing a few smartphone apps that broadcast your real-time location in the field. The disadvantage, however, is that you must be within the range of a mobile network to use the service.