A93A0223-Loss of Separation Between Lufthansa German Airlines Airbus and American Airlines-Goose Bay, Labrador -06 October 1993

A93A0223 - Loss of Seperation
Goose Bay, Labrador - 06 October 1993

Safety Action Taken

Dissemination of Fact Finding Board Findings

During the investigation, it was learned that Transport Canada (TC) limits the dissemination of the findings from Fact Finding Boards in order to protect controller confidentiality. This practice reduces the opportunities for controllers to learn from the experiences of others. An Aviation Safety Information Letter was sent to TC pointing out that, considering the commonality of procedures and equipment at ATS facilities across Canada, a potential safety deficiency exists--that is, other occurrences could be waiting to happen for reasons already identified by a Fact Finding Board.

Safety Action Required

SSR Transponder Replies - Synchronous Garble

It is understood that the Goose Bay radar has been modified and now operates like other RAMP secondary surveillance radar (SSR) systems. Thus, the potential for the Goose Bay radar to disregard a transponder reply has been reduced; as with all SSRs, though, it has not been eliminated.

When two aircraft are approximately the same slant range distance from an SSR, the aircraft transponder replies to SSR interrogation can be synchronous. In such situations, the SSR digitizer attempts to ungarble the replies; however, any information that remains garbled will be disregarded. This results in a loss of radar target data from the radar display.

The probability of synchronous garble occurring depends upon the capability of an SSR to identify the specific block of airspace from which a transponder reply was emitted. These specific blocks of airspace (defined by azimuth, height, and distance) are referred to as extractor bins. State-of-the-art SSR technology has reduced bin size, thus reducing the chance of transponder replies being received simultaneously from the same bin; however, further significant reductions to bin size are not likely due to physical constraints. In addition, modern flight management systems facilitate precise track navigation, thereby increasing the potential for no lateral displacement from the centre line for two aircraft following a common track. This eliminates differences in azimuth from the extractor bin equation, making it more difficult for the SSR to determine the location of the transponder reply. Thus, synchronous garble will continue to exist with the type of SSRs presently in use in Canada's air navigation system.

Synchronous garble can be avoided by ensuring that individual aircraft transponder replies are either unique to individual aircraft by using Mode Select SSR or are transmitted by data link. TC's current plans for surveillance systems do not include Mode Select but do include data link. However, the use of data link for ATS will not be widely available before 1998, at the earliest.

As previously mentioned, some controllers at the Moncton ACC had developed a procedure to preclude the possibility of not all radar targets being properly displayed when conditions conducive to synchronous garble existed. For example, aircraft were being deviated slightly from cleared routes to create lateral displacement between the aircraft involved before a clearance was given to climb one aircraft through an altitude assigned to another aircraft. While this ad hoc procedure appeared to be acceptable, there was no evidence that management had participated in the procedure's development or approval prior to this occurrence. (The Board understands that, since the occurrence, local ATC management has approved such a procedure for use at Moncton ACC.)

The measures implemented at Moncton ACC should assist in mitigating losses of separation there due to synchronous garble. Although the Board has not investigated other occurrences involving synchronous garble, the Board is concerned that other SSRs are prone to target data degradation due to synchronous garble, with a continuing potential for risk of a collision. Therefore, the Board recommends that:

The Department of Transport implement procedures to reduce the possibility of radar target loss due to synchronous garble. A94-21

Transport Canada Response:

The loss of a Radar target due to synchronous garble is considered to be an extremely remote possibility with the Monopulse Secondary Radar (SSR) technology incorporated in the new RAMP radar system. All SSR target returns in Canada (including the Goose Bay Radar) are now secured by the monopulse method and processed by the RAMP Radar Data Processing System (RDPS).

The Goose Bay radar data being received and displayed in the Moncton Area Control Centre on the date of the loss of separation was downgraded due to the interference restrictions imposed while transitioning to the new RAMP Radar Data Processing System. This transition was completed on January 24, 1994, and the precision of the radar data now being received and processed is the most accurate possible in a any radar system.

The Air Traffic Control Manual of Operations provides direction to controllers to follow in the event of the loss of radar data. These procedures would be applicable to any situation where radar data is lost, including synchronous garble. The retention of all flight data manually on the flight data strip is a redundancy feature which is in place to ensure a safe operation can be maintained, using non-radar separation techniques, in the event of any loss of radar data or radar failure.

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