A couple of nights ago I watched a fascinating documentary on the "US Secret Service - On the Front Line" (aka Protecting the President).
Nothing is left to chance. Encrypted radios, backup plans, weapons, counter snipers, assault teams and so on
On a whim I googled "United States Secret Service Radios" and these two documents came up immediately
Re: old radio system
https://www.oig.dhs.gov/assets/Mgmt/201 ... -Jan16.pdf
Re: new radio system
https://www.dhs.gov/publication/dhsusss ... er-ip-roip