Aviation In Canada

Sunday, October 09, 2005

ATC and Non-Radar Separation

A reader asked about how ATC uses GPS to determine separation in a non-radar environment. Before we go there, we should take a look at the basics of non-radar separation. This is actually quite a lengthy topic to do in detail, so we'll just skim the surface.

The Canadian Aviation Regulations, or CARs, contain the separation standards used by ATC. There are a number of other procedures that are contained within an "in-house" document, but the foundations are all in the federal regulations.

One of the many concepts that ATC uses in a non-radar environment involves what's known as protected airspace. The ATC clearance issued to each aircraft defines the airspace that ATC will ensure is clear of other traffic for that aircraft to operate in. This contains a route of flight, an altitude, and a time frame, largely determined by speed. As such, ATC can separate aircraft by assigning different altitudes (1,000 feet for most airspaces as we all know), by assigning airspaces that don't overlap, or by ensuring that there is enough spacing between aircraft on the same or crossing tracks at the same altitude. At all times, ATC will monitor the positions of his aircraft to ensure that separation exists. This monitoring, in a non-radar environment, is accomplished through position reports by pilots over fixes and NAVAIDs and estimating the times of arrival for subsequent fixes along the route of flight. If it is determined that separation may be compromised, ATC will take action to ensure that another form of separation exists before he loses the current one he is working with.

As an example, two aircraft are operating on tracks that cross over a NAVAID. One is flying from north to south, the other from east to west. ATC will first check their altitudes. If they're separated by the appropriate minimum, then he has to check no further. If they are at the same altitude, he'll look at the times the aircraft are estimated to arrive over the NAVAID. ATC has conditions to meet, including the speeds of the aircraft and who is in front, to determine if there is enough spacing between the aircraft. If there is more than the minimum time between aircraft at the NAVAID, then longitudinal separation exists, even though the tracks converge, and nothing more needs to happen but monitoring to ensure they continue as expected. If it is determined that the minimum longitudinal spacing does not exist, ATC must ensure that he provides another form of separation before the aircraft are too close and separation is lost. In this case, ATC will likely force one of the aircraft to change altitude, and instruct the aircraft to reach the new altitude by a certain time or a certain distance from the NAVAID, and that time or distance would equate to the lateral separation minimum. ATC could also delay one of the aircraft from reaching the NAVAID on his estimated time of arrival there, and perhaps "build" more longitudinal separation at the NAVAID. The delay could consist of a speed restriction, or a short holding pattern far enough away from the NAVAID to ensure that lateral separation exists between the airway for one aircraft and the holding airspace assigned to the other.

Tomorrow we'll look at some more to do with this topic.