Aviation In Canada

Wednesday, August 10, 2005

ATC Radar (Secondary Surveillance Radar)

We looked at Primary Radar (aka "Skin Paint") yesterday. Now we'll take a look at SSR, or Secondary Surveillance Radar.

SSR is really the mainstay of the ATC system these days, and for many reasons. First off, SSR works by having a radio transmitter that "replies" to "interrogations" it receives, and we pretty much all know these as transmitter-responders, or Transponders. The radar antenna sweeps the sky in much the same way as the PSR antenna does, but instead of emitting a powerful signal and looking for reflected radiation, the radar sends out a small signal on 1030MHz and listens for any transponders to reply to it on 1090MHz. The transponder replies in any of several "modes" which include different information. The SSR reply is received by the radar antenna and the distance is calculated along with the azimuth in the same method described yesterday to determine the position of the aircraft.

This method has several benefits. First, the range is increased since there is no need to detect reflected radiation. Instead, a simple radio signal broadcast from the aircraft is stronger than PSR returns, so the range of SSR is increased 2-3 times over PSR. Next, with the "IDENT" feature, a special code is sent to change the appearance of the target on the radar screen, thereby increasing the controller's confidence of which target is which. Further to that aspect, each transponder responding on what is known as Mode A will transmit a four-digit code. The numbers are octal values (they range from 0-7, giving eight possibilities for each place), giving a total of 4,096 different codes as possibilities. This allows a controller to see what code has been selected by the pilot, making it easier to track a particular aircraft. Mode C is the next greatest advance, in that aircraft so equipped can transmit their pressure altitude (note this is not the altitude indicated on the altimeter) to the radar station, providing an instant readout of altitude to the controller.

All those benefits and then some. In terms of equipment required for the radar antenna, there are advantages as well. Instead of a huge, parabolic antenna, the SSR system needs only a bar, much like what can be seen on ships. This bar still rotates as the other system's antenna does, but it can be mounted on top of a PSR antenna, or by itself as a stand-alone unit. This allows areas that don't warrant a full PSR antenna to be covered by less complex and less powerful antennas in an SSR-only environment. NavCanada, Canada's ATS provider, uses several of these types of stations called ISSRs (Independant SSR) to provide radar coverage over a much greater area than ever before. Using less power (only a radio signal to be transmitted, not needing to be enough to reflect a useful "radar shine" off an airplane), these antennas cost less to operate, too.

More tomorrow...