Aviation In Canada

Thursday, August 11, 2005

ATC Radar (Light History - Analog)

Don't be put off by the title if you hate history. This is background, more so than anything else.

The term Radar isn't specifically meant for something that rotates and plots targets. It's meant as RADio Detection And Ranging. Strictly speaking, it's the use of radio waves to determine if something's out there, and if so, how far away it is. Naturally in our business, if you find something, you'd like to know where it is to make the information more useful. So now, we add the rotating antenna and measure its azimuth (which direction it's pointing) at any given moment so we know where a return (in the case of ATC radar, a reflection of radiation or a transponder reply) comes from. The antenna emits very short pulses of radiation at rapid intervals, and where the antenna is when the return comes in is known. The rate of pulses given out over a 360° rotation of the antenna is called the Pulse Repitition Frequency (PRF) and the more pulses given out in a single rotation, the more accurate the radar can be in terms of azimuth. You'll notice that long range radars tend to rotate more slowly than short range (compare airway surveillance and ASDE, the very short range, airport oriented radars), and this aids their ability to be accurate at long ranges. The more rapidly a radar rotates, the more quickly a radar image is updated, thereby allowing for a closer-to-real-time plotting of movements. The radar's intended usage is considered at the time of design, and the best compromise of update rate and range required is determined before the system is built.

Another issue for radar is the nature of the display. There are several methods of plotting information on a display. Perhaps you'll recall the old home video game called "Vectrex", or the old-style arcade games of "Battlezone" or the oringial "Asteroids". These were plotted in a vector-based method. Straight lines, single color, each line being comprised of a start and end point (even a dot on the screen had two points, they just happened to be the same). Anyone remembering these will recall how crisp the lines were regardless of direction. Your computer monitor in front of you is drawn in the method called "raster", which simply means the image is drawn on the screen a line at a time, starting from the top left and drawing one line lower on each pass. The downside of this display is the "pixel" effect, where a diagonal line is drawn as a series of blocks, making it look jagged when compared to a true horizontal or vertical line.

The old radars used by ATC were drawn differently again. Though I don't remember a name for it, they were plotted from the center out as returns were gathered. Each pulse of the antenna was drawn on the screen as it was sent, so the azimuth on the screen was tracked in time with the antenna. This is called an analog radar display, since there is no digital conversion of any kind taking place. If the radar received something, it would plot it on the display on the current azimuth at the range determined. There was no computer processing to determine if a return was an actual airplane or background clutter, or weather. If it "saw" something, it showed it to you. There was a mess of switches and knobs that were used to vary any of a number of "filters" (affecting the polarization of the signals, reducing the antenna gain, and so on) that would allow ATC to peer through heavy weather systems to see airplanes and so forth, but it was all controlled by the controller, not a computer.

The big downside of the analog systems is that since there was no digital data returned by the antenna, the range that the data could be transmitted was very limited. This resulted in Terminal Control Units being located at the airport, away from the parent Area Control Centers. I doubt you'll find a terminal controller who worked in the old days hating this fact. The airport was a good place to be for a TCU, and offered a few advantages, too. They could hear their departures, their backup communications were often simple, and they also worked in rooms very near the tower staff, so close working relationships were formed with the individuals on both sides. But not all TCU's are 24-hour operations and when the TCU closed for the night, the parent ACC didn't have access to the data from that radar antenna in many cases, leading to inefficiencies for those flying through the night time.

More tomorrow.