According to some embodiments, a method for use in a first spectrum access system (SAS) server of authorizing spectrum access comprises: receiving, from a second SAS, information about one or more priority access license (PAL) protection areas (PPAs) served by the second SAS; receiving a spectrum grant request on behalf of one of a first PPA or a first general authorized access (GAA) user; determining whether granting the spectrum grant request may interfere with a second PPA geographically proximate the one of the first PPA or the first GAA user, wherein the second PPA is served by the second SAS; and granting or denying the spectrum grant request based at least in part on the interference determination. The interference determination may comprise modeling the radio environment within the second PPA and surrounding areas based on the spectrum grant request to determine whether an aggregate interference level exceeds a threshold.