PA-28 Cherokee/Archer/Dakota/Pillan/Warrior
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 5
Communication Breakdown; Confusion; Situational Awareness
ATC Issue All Types; Conflict Ground Conflict; Critical; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic; Flight Crew Executed Go Around / Missed Approach
Human Factors; Airport; Staffing; Procedure
I was working local 1; 2; and 3 combined after dark due to staffing shortage. A solo pilot licensed; student cross country unknown that she was solo at the time; called ready for departure from run-up position holding short of the runway. I replied to Aircraft X to 'standby'; which is agreed to be used with aircraft when they call ready. I then turned to the runway to observe a sequence between two other aircraft and missed Aircraft X's read back of 'line up and wait.' Meanwhile Aircraft Y had turned final for the runway. Approximately 15 seconds later I turned to observe Aircraft Y approaching 1/2 mile final. This is when I noticed Aircraft X appeared to be moving. It was hard to tell at first because she did not have her Nav lights on and if I remember correctly just her strobes which also caught my attention. I asked her if she was moving onto the runway and she replied that she was lining up to wait. At this point Aircraft Y went around and I instructed them to offset east of the runway. Aircraft Y was 1/2 mile final when they began their go around procedure. I believe this to be partly my error. I'm not sure how someone can be told 'standby' and hear 'line up and wait.' But I suppose this is a high traffic-training environment and nothing surprises me anymore. I was working my 3rd hour of Local 1 at the time of the event and was geared down to the traffic. I should have caught the incorrect read back but was multitasking with the other controller in the tower cab and was searching for my parallel runway traffic. I think the automatic responses I am used to hearing may have created a bit of complacency as I was becoming more tired throughout the shift. I had no clue how I didn't hear her read back. I think better staffing to create a little more of a rhythm and variety in the daily rotation so one doesn't work the same position all day would help. I think if all the [local] student solo pilots; regardless of whether or not they already have their license; use the call sign of 'green' this would identify a mistake-prone low-hour pilot. It would alert us to the greater likelihood of incorrect actions/read backs.
Tower Controller reported a runway incursion after instructing an aircraft to standby.