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I've seen different callsigns for en-route air traffic control, including Center, Radar, and Control (and Radio?). I suspect the only difference is the geographic location - Center in the U.S. (e.g. Salt Lake Center) and Radar and Control in Europe (e.g. Langen Radar, London Control).
Are there any other differences besides location? (is the location-assumption correct?) If not, why is it not standardized then?
The Center suffix is indeed used in the US for the en route call signs under radar control. There is also New York Radio for the *non-radar* service over the Atlantic, for example.
Another example is the Director suffix that is not used in the US, but is used elsewhere for the final approach controller.
In this case we need a referee, enter ICAO Annex 10 Volume II Communication Procedures:
5.2.1.7.1.2 The unit or service shall be identified in accordance with the table below except that the name of the location or the unit/service may be omitted provided satisfactory communication has been established.
Unit/service available Call sign suffix Unit/service available Call sign suffix area control centre CONTROL precision approach radar PRECISION approach control APPROACH direction-finding station HOMER approach control radar arrivals ARRIVAL flight information service INFORMATION approach control radar departures DEPARTURE clearance delivery DELIVERY aerodrome control TOWER apron control APRON surface movement control GROUND company dispatch DISPATCH radar (in general) RADAR aeronautical station RADIO
Above are the standards, but member states are free to deviate. Since you mentioned UK and US examples, below is from UK AIP listing its differences:
GEN 1.7 DIFFERENCES FROM ICAO STANDARDS, RECOMMENDED PRACTICES AND PROCEDURES [...] 5.2.1.7.1.2
- Approach control radar arrivals = DIRECTOR/ARRIVAL (when approved).
- Precision approach radar = TALKDOWN.
- HOMER (not used in UK).
- Ground movement planning = DELIVERY.
Similarly from US AIP:
GEN 3.4 [...] 4.4.5.1 Pilots, when calling a ground station, should begin with the name of the facility being called followed by the type of the facility being called, as indicated in the following examples.
Facility Call Sign Airport UNICOM Shannon UNICOM FAA Flight Service Station Chicago Radio Airport Traffic Control Tower Augusta Tower Clearance Delivery Position (IFR) Dallas Clearance Delivery Ground Control Position in Tower Miami Ground Radar or Nonradar Approach Control Position Oklahoma City Approach Radar Departure Control Position St. Louis Departure FAA Air Route Traffic Control Center Washington Center
Also note:
GEN 1.7 [...]
The U.S. does not use the term "area control service" to indicate controlled flight in controlled areas.
The U.S. equivalent facility for an Area Control Centre (ACC) is an Air Route Traffic Control Center (ARTCC).
Related: Where to find the callsigns of the en route controllers?
Actually, it is standardized, but not in the way you assume.
The suffix ("radio", "information", "radar", etc.) specifies the type of services offered by the station or, in some cases ("ground", "approach", etc.), the type of traffic or phase of flight handled by the station.
For example, a station named Springfield Radio won't offer radar or ATIS service; if it did, the suffix would be different ("Radar" and "Info", respectively, in this case). Just by a quick glance at the call sign, or overhearing it on the radio, the pilot can know what to expect from them.
Consequently, if Approach hands the aircraft off to Control during approach and landing, the pilot knows that something is probably amiss and can ask for confirmation; but if Approach hands them off to Tower or Tower hands them off to Ground, they know that all is probably well.
There are a number of different "ATC" authorities:
Responsible for the movement areas on the physical airport. This includes both aircraft and airport service vehicles. Aircraft refer to this as "Ground".
Gives an aircraft IFR (or sometimes VFR) clearance for flight plans.
Manages arriving and departing aircraft, usually takes over responsibility for aircraft as they approach a runway (take-off clearance) or as they approach the airport (hand-over from approach control).
Manages the airspace around the airport for departing flights providing separation and routing. Sometimes combined with Approach Control.
This is the first authority you talk to that is actually located at the airport. Manages spacing and arrivals and usually assigns runways. Also called "Terminal Control" and may be combined with departure control.
This is the authority you talk to when you are going between airports. Usually a center controls a large area of airspace and can manage aircraft in those areas. Here are a list of FAA Centers, called "Air Route Traffic Control" or ARTCC for short. On the radio pilots usually use the term "Center".
This is a service provided to pilots but is not an air traffic control authority. Pilots may open/close flight plans or get en-route weather information on these frequencies. They are usually called "Radio", for example, "Stevens Point Radio, N12345 on 131.2"...
