Altitude data integrity
Tracker systems
There are large number of different radar tracking and display systems in use around the world. The major challenges in tracking are as follows:
a. minimising tracker anomalies such as the mismatching of target reports to the incorrect track
b. providing smooth track outputs
c. calculating accurate and responsive velocity vectors
d. extrapolation of track position to time of display
e. detection of manoeuvres
The displays for radar and ADS-B can be considered equivalent. Normally the same displays will be used..
The characteristics of the input data provided to any tracker is by far the largest determinant of tracker performance. High quality input data provides high quality result. These characteristics are further examined in this
equivalence of <0.25 Nm(@ 95%) to a <0.5 Nm
b. The integrity of each and (@ 10-7 level of certainty).
every message is protected by a strong cyclic redundancy check
when flags any message corruptions with high certainty.
The integrity of the received mode C altitude data
is not assured. The data is passed on the data link without error detection capabilities nor is there any check for aircraft encoder/altimetry error.
section:
Input data accuracy
Track swaps Radar often relies on downlinked 4 digit octal code that is subject to corruption in transmission. Code validation errors are not rare. Performance worse than ADS-B Velocity vectors Determined by real time computation and smoothing of noise radar position measurement
obtained every 3 to 12 seconds (depending on the radar). Performance worse than ADS-B Extrapolation error System radar tracks are typically updated between
4 seconds and 12 seconds.
Manoeuvre detection Positional updates between 4 seconds and 12 seconds
Reflections and duplicates Reception of the radar signal from reflecting objects and multipath can create position errors and “tracker duplicates” because the delay via the object can create position errors. Multipath can cause 2 tracks to appear instead of one.
Resolution Loss of resolution when two aircraft are nearby ADS-B data is more accurate than radar hence tracking is smoother and more responsive.
Downlinks a secure (protected by CRC) airframe unique code which makes the risk of mistracking significantly more remote than radar.
Determined by airborne avionics able to have precise and temporal measurement of velocity and heading. This data is down-linked inside a CRC protection. New values calculated and down-linked every second
Typically new positional data is presented to an ADS-B tracker every second. Therefore in any extrapolation required, for example to the time of display, is less for ADS-B because the extrapolation period is shorter. Extrapolation errors are also less when the velocity vector is more accurate (as it is for ADS-B compared to radar)
With a positional update of every 1 second, the commencement of aircraft manoeuvres can be detected earlier than radars which provide updates every 4 seconds.
Positional data is calculated by the aircraft. If the signal arrives at the ground station via different path, it has no impact on positional error nor on determination whether the target is the same as another.
The ADS-B tracker does not need to resolve
Slant Range Correction.
Emergency Navigation Service
can cause two aircraft to appear as one and cause significant disruption to the positional information from each aircraft. These present significant challenges to any radar tracker.
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