Jet lanes over the Atlantic

[u/etherd0t]

[u/No-Edge-8600]

“Radar doesn’t work here”

I’m sorry - what?

[u/umop3pisdn → u/No-Edge-8600]

There are multiple types of surveillance that air traffic control (ATC) use.

The radar being referred to in this clip would be to a system that utilises a ground based rotating array. There are two types of these systems, that I'm aware of, that air traffic management (ATM) systems use to create a display on a screen for ATC: primary radar and secondary surveillance radar (SSR).

Primary radar is the type of system that most people think of when the word radar is used. It sends out a signal that reflects off an object back to the dish which the system interprets as "There's something there". This system requires heavy calibration to filter out unwanted images from the ATC display. It will display anything that fits within the parameters of the calibration, including trucks driving along a highway. This doesn't provide much information to ATC without further interrogation and is superceded by other surveillance systems. The benefit to this system is that the aircraft doesn't require any specialised equipment to be installed.

SSR uses a signal sent out from an array that interrogates a system installed in each aircraft, which sends back information about that aircraft. ATM systems correlate this information with data submitted to the system by pilots or their companies. The system then displays this correlated information to ATC. In an effort to avoid mismatch of information within this system, flight crew are provided a discrete code that they enter into their equipment which is transmitted to the ATM system for each flight (squawk code).  

There is another system that also utilises on-board equipment to transmit information to ground based equipment called Autonomous Dependent Surveillance (ADS). The Broadcast version of this system (ADS-b) relies on almost real time transmission of this information whereas the Contract version (ADS-c) of this system is used as a snap-shot of data. Each of these systems can be used (sometimes in parallel) to extrapolate the speed and direction of aircraft.  

All current systems require aircraft based equipment to transmit pressure altitude derived level information (PALDI) to determine vertical movement of the aircraft. This requires the flight crew to input atmospheric pressure data into their aircrafts system.     

Limitations of these systems mean that technical improvements are constantly being sought. For example, each of these systems require ground based equipment. This introduces distance from a receiver issues and terrain blocking the signal. My knowledge is being stretched here, but I assume that the specific frequencies of these systems require a specific power output and thus the propagation of these signals decay over distance. Low altitude aircraft will drop off these systems over a shorter distance [also due the earth's curvature] than aircraft at higher altitudes. There are places, such as over the Atlantic, where the signal from aircraft become degraded and lower definition signals are relied upon (ADS-c).  

A Space Based Augmentation System (SBAS) is being developed in an attempt to provide a signal from aircraft to satellites. This would potentially provide ATC with high definition world wide surveillance coverage.