The National Park Service, in a number of locations, has multicast radio systems for parks. Multicast systems are those that carry the transmissions of each repeater site on all the other repeater sites, each repeater being on separate frequencies. You are speaking of simulcast, not multicast. The difference being simulcast is multiple repeaters or remote bases transmitting on the same frequency at the same time. The parks with multicast I know of include Sequoia-Kings National Park, Joshua Tree NP and Grand Canyon NP. I believe Mt. Rainier might have such a system also. Back east I think Natchez Trace and Blue Ridge Parkway might now be multicast.
Why multicast? First, so that every unit in the field hears all the traffic on a particular net, say park net, law enforcement net, backcountry net, frontcountry net, etc., hears all the traffic on the net, even though they can't hear the individual repeater the traffic is on. Field units often need to hear every unit on the net as the traffic might be relevant to what they are doing in their location. Second, in this way units on one repeater, who can't hear other repeaters, don't transmit at the same time someone on another repeater is transmitting. In mountainous terrain over distances of 50 miles or greater, having a system of multiple independent repeaters on the same frequency results in dispatch having to advise a unit on one repeater standby, due to traffic on another repeater. Third, if dispatch has traffic meant to be heard by all, such as a BOLO (be on the lookout), they can transmit it once over all the repeaters at the same time, rather than transmit it one repeater at a time. If a person is an urban scanner listener or one in small jurisdictions such as the counties east of the Rockies or Mississippi River, multicasting may not be a familiar concept.
We can all hear a multicast broadcast every night. If we tune into all the AM radio stations we can in the evening we will hear, let's say, WLS in Chicago, KOA in Denver, KOB in Albuquerque and KFI in Los Angeles, we might hear a syndicated program being broadcasted at the same time. Each station, of course, is transmitting the same time, but this is not simulcasting, because each station is on a different frequency. Many people would call this simulcasting, but it is not because all the stations are on different frequencies, not the same frequency.
Let's return to the Santa Monica National Recreation Area radio system. The RR database shows 4 repeaters, Solstice, Laguna, Castro and a newly discovered repeater we don't know the location of on a frequency of 170.0250. The new repeater carries the same traffic, not just of Castro, but of Laguna and Solstice as well. If units can only hear Solstice in their location and can't hear Laguna directly, they will still hear the traffic a unit using Laguna is transmitting. There might also be units that can only hear Castro, able to hear traffic being transmitting on Solstice, Laguna and the new, unknown location, repeater on 170.025.
