Operational Use of ADS-B In Non Radar Airspace Generic Design Safety Case
6.0 Potential Mitigators and Safety Requirements Safety The safety requirements derived from the hazard identification process requirements are detailed at Appendix 2. These will be expanded as work continues derived from within SASP in deriving ADS-B safety assessments. hazards identified This Safety Case will provide evidence and arguments to demonstrate
that the safety requirements can be met by States implementing ADS-B systems.
Operational Use of ADS-B In Non Radar Airspace
Generic Design Safety Case
APPENDIX 1 – ADS B COMPARISON TO RADAR WORKING PAPER APPENDIX 2 – HAZARD LOG APPENDIX 3 – AMENDMENT PROPOSAL FOR PANS-ATM Doc 4444
APPENDIX 1
ADS B COMPARISON TO RADAR WORKING PAPER
Appendix 1 is subject to change as more information is
obtained, catalogued and assessed.
1. COMPARATIVE ASSESSMENT BETWEEN RADAR AND ADS-B
Principle Characteristics. The key pieces of data provided by an ATC Radar to a Controller and their origin for both Radar and ES-ADS-B is summarised in the table below:
(Shaded areas indicate that ADS-B and Radar are the same)
DATA RADAR (SSR) ADS-B
Position Radar itself measures range
Down-linked from aircraft and azimuth
Altitude Down-linked from aircraft
Down-linked from aircraft (Mode C)
Identity Down-linked from aircraft
Down-linked from aircraft (Mode 3/A and use of
(Unique 24-bit address and Special Purpose Ident)
flight identity) Velocity Vector
Computed from successive
Down-linked from aircraft position determinations
(more responsive) Emergency Alerting
Down-linked from aircraft
Down-linked from aircraft (Reserved Mode A codes)
(Contained in status field)
COMPARATIVE ELEMENT Position.
RADAR
Radar measures an aircraft’s position in range and bearing. Range is measured by accurate measurement of elapsed time from transmission of an interrogation to the time of reception of the reply from the aircraft. The azimuth (bearing) of the aircraft is determined by the direction the very narrow beam-width antenna is facing. In modern monopulse radar the azimuth within the beam is also determined to further increase bearing measurement accuracy. Radars measure accurately the range but azimuth is measured in angle and hence at long range has lower linear accuracy than range.
Air Traffic Controllers assess and utilise the presented radar positional data in the context of the complete traffic management picture. ATC is not based on the presentation of a single positional data report (measurement). Rather, the history of the track together with the most recent data is used to predict where the aircraft will be in the future. Controllers also use other information such as knowledge about aircraft intent and the clearances issued to modify their perception of the data presented. No tool is perfect, and controllers use radar as a significant tool, but take account of its strengths and limitations.
Radars measure position in range and azimuth. The range noise errors are 0.125 Nm (1∑ ) and the noise errors in azimuth are 0.08 degrees (1 ∑). Since GPS (ADS-B) errors are expressed with respect to a 95% confidence, this paper will use 2.00∑ (95% assuming Gaussian distribution of errors) - namely a 0.16 degree error. In addition to
ADS-B
In an ADS-B system, positional data is determined by the aircraft’s navigation system and broadcast to the ground station. Hence the accuracy of the positional data is a property of the aircraft’s navigation system which is further explored below. The positional accuracy of ADS-B is determined by the navigation system in the aircraft. For high-end aircraft this is typically FMS/IRS/GPS. These navigation systems have a knowledge of the accuracy of the aircraft position report and this is passed as figure of merit to the ground system which can exclude reports of insufficient accuracy. Similarly GPS based navigation systems can determine figure of merit based on satellite geometry.
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