Originally Posted by
Volume
Is there anybody un this forum who can shed some light on how modern radar equipment works? On the old fashoned CRT design you could simply adjust brightness to get rid of all the noise on the screen (birds etc.). How is sensitivity adjusted today? How is information filtered (i.e. are single returns filtered automatically by the software, because they are obviously noise, and are only repetitatve returns moving within a certain reasonable speed range shown)?
What exactly is recorded? Raw data? Displayed Data? Analyzed, filtered Data? Can we (the civilians...) run special analysis software on that data to scan through it for interesting details?
Normally with modern systems the display shown to the controllers is totally synthetic and is made up of information from different radars and from other sources such as Automatic Dependent Surveillance (both Broadcast at half Herz and sometimes Contract every 10 minutes). The radar information is normally 'digitized' at the remote radar sites, this process can remove some of the primary radar noise. Similarly the secondary radar normally will have up to 20 'hits' on the aircraft even out at 150 miles less than 10 and the response may be discarded. The digitized signals and other position reports from other sensors are then fed to the control center where a 'multi-sensor tracker' changes them into 'tracks' these are built as soon as the aircraft transponders switch on on the ground and are continually updated with sensor positions as they are received. Filters are then used to ensure that there is no jitter due to slight inconsistencies in the reported positions. It is these tracks that are sent to the controller display processors.
The tracks are correlated with the known flight data of the aircraft so that the controller can access flight data by clicking on the position symbol, but also the flight data can be used by the tracker to 'expect' when aircraft will turn. If the aircraft is tracked and late the surveillance processing can update the flight data processing so that times remain correct. The system can also alert when aircraft deviate from cleared levels etc.
What the controller sees is entirely synthetic and the controller has no control over gain etc. It is not normal for an ATC controller to use primary radar only indeed many will switch it off to reduce clutter. However, once an aircraft is being tracked the primary response is 'correlated' with the other track data, so that if the aircraft transponders cease for some reason the labeled display will continue with just the primary response if there is one. If all surveillance sensor data for a track is lost the systems
may coast the position symbol for a period along the projected ground track sometimes modified by flight plan track data. This is actually quite common in oceanic areas where position reports may be only every 10 minutes on ADS=C. (Coasting can be seen on systems like Flight Aware.)
Data is recorded at almost every point in the process with greatly differing accessibility. The recordings at the radar heads will be mainly for maintenance purposes not normally for investigation. Once information comes into a center it is recorded again mainly for maintenance and fault finding. All data delivered to controllers for their use is recorded and stored in accordance with ICAO rules which is normally 28 days or more. Most centers will have almost immediate playback capabilities for incident resolution.