ATC Radar (Primary Surveillance Radar)
I was thinking about radar the other day (considering the weather abilities of our stuff), and I began to wonder what about radar would be interesting to talk about anyway. I honestly can't remember what I knew about radar before I got into ATC, so I thought I'd take a couple posts and write a little about it. I'm not getting into true radar theory partly because I don't know everything about it and partly because I feel some background is good but the truly technical issues are better left to technicians to describe and deal with than for us. Today's post will deal mostly with one type, and I'll continue tomorrow.
The two classes of radar that ATC use are called Primary Surveillance Radar (PSR) and Secondary Surveillance Radar (SSR). Primary radar has nothing to do with its importance, but rather it is the name of the class that deals with reflected radiation. No equipment is necessary on the airplane for this radar to work, and indeed no equipment on the airplane will help this radar in its ability to see an airplane. This is strictly radio waves emitted from the radar antenna and the reflected radiation being picked up by the antenna.
The primary radar antenna is a parabola and is normally encased in a big dome (or "golf ball" on top of some staging. In the old days, and at some locations still, the antenna is not encased and may be recognized by its size. It's a big net of metal, often a frame work rather than a solid sheet, that's bent on both the horizontal and vertical axis so as to focus radiation to a point in the same fashion as a satellite dish focuses signals from a larger area on a smaller area, increasing the gain of the antenna. A larger aircraft will reflect a larger amount of radiation and will be visible to the controller at a greater distance, generally speaking, than a smaller one. The materials an aircraft is made of will make a difference in reflected radiation as well, since metal will reflect more radio waves than some composites will. So generally speaking, the further the aircraft is from a radar antenna, and the smaller the aircraft is, the less radiation is returned to the antenna, so the less likely it will be that the radar can "see" the airplane.
In the case of PSR, the time between the emission of the radio signal and the time that some reflected radiation is received is directly related to the distance between the antenna and the object. Of course, since this is the round trip time, the distance accounted for is twice the actual distance, so the radar has to account for this fact. The azimuth of the antenna is measured continuously, so the direction of the reflected radiation is known as well, thereby allowing the display of a radar hit, or "target", in an appropriate place.
As mentioned earlier, no equipment on the aircraft is read through this method, and therefore altitude is not something can be determined by the simplest of primary radar antennas. The only way altitude can be figured out is to add another antenna that would "sweep" the sky vertically, such as with a "quad radar", those used for precision approach guidance. In this way, a controller can see the vertical axis on one section of the screen as well as the horizontal axis, and both would plot distance. But the average radar antenna used for airport and airway surveillance is incapable of determining an altitude for a given primary radar return.
The antenna doesn't tilt upward, but rather the radiation emitted is more like a triangle if seen from the side, looking from the horizon upward. This leads to a blind spot directly above the antenna, similar to the "cone of ambiguity" over a VOR or NDB. If an aircraft flies very close to the geographical location of an antenna at high altitude, its straight path would seem to curve as it approached the antenna and eventually resume its normal path once it got far enough on the other side. This is due to slant range, similar to DME. It may be nearly right over the antenna, and therefore nearly zero miles from the location, but it's 5 miles up and that distance is also measured simply by the way radar functions. The radar screen would have the target plotted at 5NM from it, and the speed would also appear to shift dramatically as it passed directly over (passing a huge azimuth range in a short time due to the actual horizontal distance being short).
More will follow in the days to come on other types of radar and the changes technology has brought to us.
The two classes of radar that ATC use are called Primary Surveillance Radar (PSR) and Secondary Surveillance Radar (SSR). Primary radar has nothing to do with its importance, but rather it is the name of the class that deals with reflected radiation. No equipment is necessary on the airplane for this radar to work, and indeed no equipment on the airplane will help this radar in its ability to see an airplane. This is strictly radio waves emitted from the radar antenna and the reflected radiation being picked up by the antenna.
The primary radar antenna is a parabola and is normally encased in a big dome (or "golf ball" on top of some staging. In the old days, and at some locations still, the antenna is not encased and may be recognized by its size. It's a big net of metal, often a frame work rather than a solid sheet, that's bent on both the horizontal and vertical axis so as to focus radiation to a point in the same fashion as a satellite dish focuses signals from a larger area on a smaller area, increasing the gain of the antenna. A larger aircraft will reflect a larger amount of radiation and will be visible to the controller at a greater distance, generally speaking, than a smaller one. The materials an aircraft is made of will make a difference in reflected radiation as well, since metal will reflect more radio waves than some composites will. So generally speaking, the further the aircraft is from a radar antenna, and the smaller the aircraft is, the less radiation is returned to the antenna, so the less likely it will be that the radar can "see" the airplane.
In the case of PSR, the time between the emission of the radio signal and the time that some reflected radiation is received is directly related to the distance between the antenna and the object. Of course, since this is the round trip time, the distance accounted for is twice the actual distance, so the radar has to account for this fact. The azimuth of the antenna is measured continuously, so the direction of the reflected radiation is known as well, thereby allowing the display of a radar hit, or "target", in an appropriate place.
As mentioned earlier, no equipment on the aircraft is read through this method, and therefore altitude is not something can be determined by the simplest of primary radar antennas. The only way altitude can be figured out is to add another antenna that would "sweep" the sky vertically, such as with a "quad radar", those used for precision approach guidance. In this way, a controller can see the vertical axis on one section of the screen as well as the horizontal axis, and both would plot distance. But the average radar antenna used for airport and airway surveillance is incapable of determining an altitude for a given primary radar return.
The antenna doesn't tilt upward, but rather the radiation emitted is more like a triangle if seen from the side, looking from the horizon upward. This leads to a blind spot directly above the antenna, similar to the "cone of ambiguity" over a VOR or NDB. If an aircraft flies very close to the geographical location of an antenna at high altitude, its straight path would seem to curve as it approached the antenna and eventually resume its normal path once it got far enough on the other side. This is due to slant range, similar to DME. It may be nearly right over the antenna, and therefore nearly zero miles from the location, but it's 5 miles up and that distance is also measured simply by the way radar functions. The radar screen would have the target plotted at 5NM from it, and the speed would also appear to shift dramatically as it passed directly over (passing a huge azimuth range in a short time due to the actual horizontal distance being short).
More will follow in the days to come on other types of radar and the changes technology has brought to us.
posted by Michael at
08:00
