Air Traffic Advisory Service is a service provided within advisory airspace to ensure separation, insofar as practical, between aircraft which are operating on IFR flight plans.
The objective of the air traffic advisory service is to make information on collision hazards more effective than it would be in the mere provision of flight information service. It may be provided to aircraft conducting Instrument Flight Rules (IFR) flights in advisory airspace or on advisory routes (Class F airspace).
As a rule, advisory service is only implemented where the air traffic services are inadequate for the provision of air traffic control, and the limited advice on collision hazards otherwise provided by flight information service will not meet the requirement. Where advisory service is implemented, this is usually considered as a temporary measure only until such time as it can be replaced by air traffic control service.
Advisory service does not afford the degree of safety and cannot assume the same responsibilities as air traffic control service in respect of collision avoidance, since information regarding the disposition of traffic in the area may be incomplete. Advisory service does not deliver “clearances” but only “advisory information”. (ICAO Doc 4444: PANS-ATM, Chapter 9, Section 9.1.4)
Radar Advisory Service
Where available, radar may be used to assist in providing services to aircraft flying in Class F and Class G airspace.
ICAO Doc 4444: Procedures for Air Navigation Services - Air Traffic Management (PANS-ATM)