Date: 201410
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: Amateur/Home Built/Experimental
Operator: Personal
Operating under: Part 91
Flight phase: Final Approach
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: Amateur/Home Built/Experimental
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Situational Awareness; Confusion
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types
Detector: Person Flight Crew; Person Air Traffic Control
Result: Flight Crew Executed Go Around / Missed Approach
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: [Two yellow experimentals] were inbound from the north or northeast.  Aircraft X was instructed to fly to right base and I can't recall if I instructed Aircraft Y for a right base or right traffic.  I observed who I thought was Aircraft X turning base to final in front of a Cessna on a practice approach; and instructed Aircraft X to turn in towards the numbers.  Being surprised that the base traffic was not flying a tight base as instructed; I immediately had the Cessna abandon the approach to the southeast.  There was some haze in the area that made it more difficult than usually to spot aircraft as well.  I was under the impression that Aircraft X was on final; but I believe he was actually still a few miles to the north; while Aircraft Y was on final. As this situation evolved; each time Aircraft X transmitted; I do not remember being able to understand anything other than the pilot's callsign.  In hindsight; I think he was trying to bring to my attention that I had the two aircraft confused; and that he was actually NOT on final or the first aircraft in the sequence.  I was attempting to get Aircraft X to fly a tight pattern to sequence him in front of the Cessna - but since I had the two aircraft confused; he actually turned base to final in front of Aircraft Y instead.  I attempted to mitigate the situation by telling Aircraft Y to fly east (no pilot response); and finally Aircraft X said he was in front of Aircraft Y flying over the runway - and asked if I still wanted him to go around?  (At this point I knew I needed one or both experimentals to go-around to protect runway separation).  I observed Aircraft X was on the go and I instructed him to fly runway heading; while Aircraft Y crossed the threshold behind him and safely landed.  From the go-around; I sequenced Aircraft X back into right closed traffic for an uneventful landing.As the aircraft proceeded inbound; I may have mixed up the targets with other aircraft operating outside the airspace to the north.  Adding some confusion was that there was also an unidentified airspace violator southbound between 070-075 at the time.  Some additional factors that added complexity were that I just changed runway configuration to accommodate practice approaches; and I was working from a different position in the cab than I usually would.  Finally; the runway designators recently changed; making transmissions more challenging as we grow accustomed to using the new numbers. I feel a number of different factors came into play to cause the event.  For whatever reason; I was convinced that I initially identified which aircraft was which; and then issued control instructions on that basis. I think some of the complexity factors identified above; along with a rush of inbound traffic; came together with the pilot/controller communications problem I encountered to make it difficult to detect that I confused the aircraft.  I believe I overlooked the clues that I had misidentified the aircraft while I was focused on resolving a potentially hazardous conflict between base traffic and a practice approach.   And finally; both aircraft have a similar appearance and perform similarly; which made it hard to tell them apart.  Bottom line: be ready for the unexpected and constantly be on the lookout for clues that aircraft mis-identification may have occurred.  In hindsight; it may have been more worthwhile to confirm the position of one of the other aircraft that was not experiencing radio problems - and this is another 'tool' I will take away and keep in mind for similar scenarios in the future.




Synopsis: Local Controller described a go-around event caused by his/her misidentification of two like aircraft in the pattern; radio communications adding to the confusion.