PCAS Coverage
The second in a series of in-depth articles taking a look at the technology behind PCAS and today's advanced collision avoidance systems. This article demonstrates the amount of collision avoidance "coverage" in the U.S.

  

The Domino Effect

One of the most common questions I get asked when demonstrating PCAS is, "What about coverage?" It's a good question. First, it's important to note that PCAS (and every other collision avoidance system) receives transponder replies. Transponders in your area are interrogated by an active source, such as ground RADAR or TCAS-equipped airliners. Once interrogated, transponders emit the Mode A (squawk) and Mode C (altitude) codes that PCAS receives. (See "How Does PCAS Work?" for more on this). Think of it as a Domino Effect that starts with an active interrogation.

But what is the source of the interrogation?

PCAS has the unique ability to work in two interrogation envrionments. Ground RADAR, referred to as SSR or Secondary Surveillance RADAR) is perhaps the most obvious source and is prevalent throughout the US. The map below shows RADAR sites around the US. These include enroute, military, and terminal RADARs.

tcas-radar1
Ground RADAR Sites (ARSR-3, ARSR-4, ASR-9, and ASR-D)

Military RADAR

Dedicated military RADAR sites have an approx. average interrogation range of 200 NM (diameter). The following illustration shows the approximate coverage.

tcas-radar2
Military RADAR Coverage: 200 NM approx. avg. range

Terminal RADAR

Terminal RADAR sites typically exist around major airports and can extend to approx. 120 NM (diameter). The next illustration shows this interrogation source combined with military RADAR.

tcas-radar3
Terminal RADAR Coverage: 120 NM approx. avg. range (+ Military RADAR)

Enroute RADAR

The third class of RADAR interrogation source is enroute RADAR, which has a much greater range than terminal, approx. 400 NM (diameter). If we add this source to our map, we get the following:

tcas-radar4
Terminal RADAR Coverage: 120 NM approx. avg. range (+ Enroute and Military RADAR)

TCAS Interrogations

In addition to ground-based SSR interrogations, TCAS (Traffic and Collision Avoidance Systems), found in corporate aircraft and every airliner, also presents a viable interrogation envorinment. TCAS interrogation range varies from 40 NM to a maximum of 100 NM (radius). TCAS interrogations are often more informative than ground-based interrogations for a variety of reasons. For example, in mountainous areas, an airborne interrogation platform is not limited by mountains, unlike Ground RADAR, which is line-of-sight. This means that an aircraft in a valley and in the shadow of ATC will still be interrogated by any airliner within 40 to 100 miles, and PCAS should be able to receive the reply.

When we overlay the average number of TCAS-equipped airliners and their approximate interrogation ranges on our map, the total interrogation coverage area looks like this:

tcas-radar5
Average Interrogation "Coverage" Area: Military, Terminal, Enroute RADAR and TCAS Interrogation

Conclusion

As you can see, it is very difficult to find an area where interrogations are not occuring. However, please note the following:

  • The map does not include other active systems, such as Skywatch® or other GA-market systems
  • Coverage may vary or be more limited than illustrated in more mountainous areas
  • It is still possible for PCAS, or any other passive system, to miss aircraft due to any one of many reasons
  • Ranges illustrated are approximate or averaged. TCAS ranges are computed from a snapshot of an average weekday system workload.

Video Demonstration

To help better explain PCAS coverage, we've put the above information in the form of a video.

2.36MB .MOV (QuickTime required, available here)