A large fixed wing UAS remote pilot reported operating in the traffic pattern when they suffered a communication failure and no defined procedures were in place for a UAS suffering lost communications.

Narrative

During taxi; ZZZ Tower Controller asked us to fly a VFR pattern before we departed. We agreed; performed all checklists; took off; and were cleared for a right turnout to downwind; non-standard pattern altitude of 2;400 feet. We were also advised of traffic to follow; unmanned aircraft approaching base turn. We were told to 'extend your downwind; I'll call your base' from Tower; to which we read back instructions and continued on downwind. While passing mid-field; we heard the traffic in front of us call a base turn. Then we heard a chirp on the radios and then nothing. At a 2-mile perch point we called Tower with no response. At a 4-mile perch point we called Tower again and heard nothing. Shortly thereafter; our Visual Observer called us on his handheld radio saying 'Tower is trying to get a hold of you; they want you to turn base.' We turned base just inside of our approved airspace. At this point we determined we had a radio failure. As the Pilot In Command (PIC) I elected to break out of the specified pattern and proceed to our lost link orbit inside the Delta airspace; at which we would orbit and troubleshoot. I relayed this intent to our Visual Observer; who copied and reported he would relay to the Tower via his handheld radio. As we began our right turnout and headed to our emergency mission orbit; our technician found our radio issue and corrected it. As soon as our radios came back we heard multiple climb and descent instructions from Tower to multiple aircraft including our own. The first intelligible instruction we heard for us was to descend immediately to 1;500 feet. We elected to maintain our heading in our descent. After the descent; Tower moved other aircraft with respect to altitude and directed us to rejoin downwind for a full-stop landing in order to troubleshoot our radios. We informed Tower we had fixed our problem; requested a low approach to a right turnout for departure; and was cleared that. After the occurrence I contacted the Tower Controller to ask if I had incorrectly interpreted any procedures for radio failure in the specified pattern; (for example; wing rock up initial and break for a full stop). The Tower Controller checked the Letter of Agreement between [the] Airport and Company; and stated there were no defined procedures for NORDO UAS in the pattern. It is my opinion this was the primary contributing factor to this incident.