Flight Plans (FPLs) are filed in ICAO standard format. Part of the FPL is the route. Each national ANSP must, within its own specific flight plan processing system (FPPS), convert the FPL route into a route expressed in terms of its own national airspace computer model.
The UK airspace model is built using the same architecture as the US one. It is part of the National Airspace System (NAS), which the UK obtained from the US FAA in the 70s. The UK NAS airspace architecture is essentially the same today as it was in the 70s, though of course the model itself has changed to reflect the changing airspace over time.
A key part of the NAS software is a sub-programme called Route Conversion and Posting. This converts the ICAO FPL route into the internal format of the FPPS. It then determines which sectors (ATCOs) need to be provided with the FPL information (the “posting” part). NATS does not publish details of this sub-programme, but the FAA does. It is documented in a volume called NAS-MD-312. These details have naturally diverged over time, but the essentials remain the same.
To quote NAS-MD-312 “The 3-dimensional volumes of airspace that comprise an [airspace model] are described by points and lines with a specified altitude range for each. These volumes of airspace are fix posting areas (FPAs). Geographic points are described in source information in terms of latitude and longitude in units of degrees, minutes, and seconds and are converted to conventional X and Y coordinates, in units of one-eighth mile, and stored in that form. A boundary line is described by its geographic end points. Since each line segment has a specified altitude range adapted to it, a series of connected lines is used to describe a 3-dimensional volume of airspace. An FPA is the fundamental unit of airspace within the [airspace model]. Other volumes of airspace within a centre, such as sector or approach control areas, are described in terms of FPAs that comprise them. A fix posting area is a volume of airspace identified by a series of connected line segments that form a polygon when viewed in the horizontal and vertical plane, with each boundary line having a specified altitude range. The polygon may be convex or concave, permitting a variety of geometric shapes.”
This process of route conversion is obviously a very complex exercise. I understand at least one major UK NAS outage in the past was caused by errors in this process. Someone had managed to input an FPL route that passed NAS route validation (described in NAS-MD-311 Message Entry and Checking) but “did not compute” when route conversion was attempted. Of course, all possible errors should be trapped, but…
A flavour of the complexity of route conversion can be had here:
https://www.tc.faa.gov/its/worldpac/...nas-md-312.pdf
I am not saying such a problem was the cause of the outage yesterday – though it could be. It is just some background to the way NATS processes flight plans.