San Diego County - Imperial County Regional Communications System (RCS)
From The RadioReference Wiki
On 8/24/11, a change was made removing 863.0000 from the North Zone and adding 861.5000 to the North Zone. I've been monitoring the North Zone for a week (8/24-9/1) using UniTrunker. There has been no activity on 861.5000, and there still remains plenty of activity on 863.0000.