This concept paper aims to investigate the requirements for correlation. Correlation is the process of associating surveillance data with the flight plan and data derived from the flight plan for the flight concerned, of radar reports with tracks in an air traffic control (ATC) radar data processing system (RDPS). The main purpose of correlation as a part of the ATC is to guarantee a safe distance between aircraft, to avoid collisions between aircraft, to maintain information useful toward the safe and efficient conduct of flights, and to notify the appropriate organizations in case of an emergency.
The correlation of radar reports with tracks is
necessary in order for the RDPS to establish the location of an aircraft
visible to the radar equipment by using Secondary Surveillance Radar
(SSR). SSR provides data on the
position, identification and altitude of an aircraft, permitting individual
flights to be tracked in three-dimensional space. Aircraft send discreet transponder codes, allowing the radar and the
Flight Data Processing System (FDPS) in the ground station to automatically correlate the discreet code with the aircraft
call sign. The
RDPS uses the radar reports to correlate with the established tracks in order
to keep track of aircraft. The RDPS also uses
correlation to determine whether the aircraft can still be reliably
tracked. Correlation is currently
based on the assignment of a discrete Mode A code to a single flight in
accordance with the Originating Region Code Assignment Method (ORCAM). The
principle of ORCAM is to specify the codes that may be assigned or retained
within defined regions, thus minimizing both the duplication of a code in a
region and the frequency with which the code is changed during a flight. This
allows for the unambiguous correlation of surveillance data with a flight
plan. If an aircraft is transmitting its assigned Mode A
Code and a flight plan exists in the FDPS associated with that particular Mode
A Code, then a correlated track containing the aircraft flight identification
will be sent to the controller's radar display. If, after correlation, radar contact
with an aircraft is lost, then the aircraft identification is displayed in the
“lost label” tabular area of the controller's radar display.
Sometimes a small or disabled aircraft will not have an SSR signal. When such an aircraft is within range of the radar, Primary Surveillance Radar (PSR) must be used. PSR works by sending out radio waves and detecting reflected waves, which indicates an object. When reflected radio waves are detected, they are taken note of by the correlation system. When enough detections occur, they are correlated and a track is established, which means that the correlation system believes that the reflected radio waves are from an aircraft.
The other systems related to correlation are track update generation, and alerts and warnings generation. When correlation is successful, the track is updated with the incoming radar report; when correlation fails, a new track is generated if information provided in the radar report is sufficient. In the case of an emergency, the correlation of an SCC code should result in alerts and warnings generation.
Sometimes miscorrelation, or track swap, may occur. The basis upon which correlation is established is a match between the SSR code in the track with the one assigned to the radar report. However, due to errors in the code allocation or shortage of available SSR codes, even a discrete code in a track can not be considered unique, and therefore a further check is made to ensure that the track position is in conformance with the radar report.
It is desired that the system be capable of maintaining
multiple radar reports provided that they can be uniquely identified by SSR
code, and other information if necessary.
The system correlates a received incoming report with existing tracks
and radar reports in order to determine whether it matches an existing track or
if a new track is required to be generated. The system provides a one-to-one mapping
between existing tracks and aircraft within range using the unique SSR code.
The system would permit the creation of an incomplete radar report upon entry
of minimum flight data (SSR code).
Upon entry of sufficient information into a Minimum Flight Plan, the trajectory
and track is calculated by the track generation system. If the report contains an SSR Code
different than that in the RDPS no code is assigned in the radar report, and
SSR Code Assignment is performed accordingly. The system allows manual assignment of
an SSR code, regardless of whether it has already been assigned to another
radar report. Furthermore, the
system will be capable of automatic radar report and track correlation for
system tracks containing a discrete code.
Correlation between a track and radar report is achieved if there is a
match in SSR codes between the track and radar report, and if the track is in
conformance, within predefined limits, with the trajectory. Correlation will be attempted for an
uncorrelated radar report upon assignment or update of a discrete code in the
radar report. Correlation will also
be attempted for an uncorrelated radar report that has failed the correlation
position check upon update of the trajectory. Furthermore, correlation will be
attempted for an uncorrelated track upon change of a discrete code in the
track. Correlation will be
attempted for an uncorrelated track that fails the correlation position check
upon update of the track position or on a periodic basis. Once correlation is established between
an SSR code and a track, the correlation process continues until it is broken
by a manual decorrelation request, a track cancellation or a transition of the
radar report.
The benefits of the correlation system include guarantees of a safe distance between aircraft, avoidance of collisions between aircraft, maintenance of information useful for the safe and efficient conduct of flights, and the ability to notify the appropriate organizations in case of an emergency.
When
a special condition code (SCC), such as 7700/Emergency code is correlated,
communication to the ATC facility is immediately established. Once an SCC has been detected for a
track, the correlation of a track with radar reports continues to remain in the
SCC state until the radar report no longer indicates SCC.
In situations where no established
radar report exists for an SSR-aircraft during correlation, a radar report is
created which may contain only the SSR code. Subsequent input may be made to complete
the radar report.
For an unassumed track, the
system should generate a warning if the track SSR Code does not match correlate
with the existing track.
For an assumed track, the
system should generate a warning if the track SSR Code does not match the SSR
code in the radar report or a special purpose code.
In the case where the
correlation of the discrete code in the radar report does not match the ones in
the RDPS, a further check is made to ensure that the track position is in
conformance with the radar report trajectory. The PSR code can be used for the
purpose of correlation when SSR code does not match the existing track. Also, other criteria such as range,
azimuth and altitude information shall be compared to existing tracks. If a match exists, then the SSR code for
the established track is assigned to the radar report. Otherwise, SSR Code Assignment shall be
performed. The RDPS shall then
derive new a track from the radar report.
In the situation where two
identical SSR codes are correlated, a further check is made to ensure that the
existing track position matches one of the 2 SSR codes. The PSR code can be used for the purpose
of correlation when SSR code does not match the existing track. Also, other criteria such as range,
azimuth and altitude information shall be compared to existing tracks. If a match exists, then the one matching
the existing track is updated and the one not matching is warned to change the
SSR code.
Once established, correlation
is maintained regardless of an SSR code change, or loss, in the track until the
SSR code be released upon transition of the radar report to which it assigned
to, or upon assignment of a new code to that radar report.
Correlation
between a track and radar report shall be achieved if there is a match in the
SSR codes between the track and radar report, and the track is in conformance,
within pre-defined limits, to the trajectory.
The function of correlation is
to augment the surveillance picture with information from the radar report and
to enable the monitoring and updating of the radar report with actual
progress. Tracks should be
established as early as possible, so correlation is ideally attempted upon
assignment or update to any of the items in an uncorrelated track or radar
report that is a factor in establishing the correlation such as track SSR code
or track position.
http://www.canairradio.com/acc.html
http://imansolas.freeservers.com/ATC/ATC_Tools.html
http://www.eurocontrol.be/eatmp/egd/ATM_System-02.htm
http://bathursted.ccnb.nb.ca/vatcan/training/sqcodes.html
http://www.faa.gov/atpubs/aim/index.htm
http://www.faa.gov/atpubs/aim/Chap1/aim0102.html#1-2-1
http://home.columbus.rr.com/lusch/rtudslide00.html
http://atcsl.tripod.com/radar_theory.htm
http://www.airsport-corp.com/modec.htm