file: faa_radars.wpd Oct 8, 2004 jbh

Tracking and Search Radar Characteristics

Primary Radar (detect reflection from targets)

Secondary Radar (detect signal from interrogated transponders on aircraft).

FPS: Fixed (ground) installation Pulse Search

Transponders are also sometimes referred to as "beacons", and another name for secondary radar is "ATC Radar Beacon System" or ATCRBS (pronounced "at-crabs").

Today, virtually all ATC radar installations are equipped with both primary and secondary radar capability. However, the FAA is threatening to decomission primary radar from its enroute facilities. If they did this, centers would no longer be able to "see" any aircraft that were not transponder-equipped or whose transponder failed. Center controllers and pilot groups like AOPA are opposed to this, and it remains to be seen if anything comes of it.

Mode A transponder "squawks" aircraft I.D.

Mode C: transponder reports aircraft barometric altitude

There's a new type of transponder called a Mode "S" transponder that is required for TCAS and may support datalink. A few years ago, the FAA tried to mandate that the fleet retire their Mode A/C transponders and upgrade to Mode S, but this move was strongly resisted by AOPA and other general aviation groups. At present, the rules to force the transition to Mode S have been put on indefinite hold, and aircraft that aren't required to be TCAS-equipped (which includes most of our owner-flown airplanes) continue to use the older and far less expensive Mode C units.

The FAA is now talking about a future ATC system based not on radar surveillance, as is used today, but on "automatic dependent surveillance" in which aircraft continually transmit their GPS position to ground stations which keep track of them and tell controllers where they are. In view of this, it's possible that Mode S may become obsolete before we need to worry about it.

Radar ground stations

If you take a close look at an ATC radar ground station, you'll see that they actually consist of three separate antennas. The biggest and most obvious one is the primary radar antenna, which looks like a parabolic dish that goes round and round (10 times a minute for approach control radar, 6 times a minute for long-range center radar). This antenna transmits powerful pulses and then listens for echoes. It is used to detect aircraft "skin paint" and also can detect weather to some degree.

The second ground station antenna, called the directional antenna, is used to send interrogations to airborne transponders and to receive replies from those transponders, providing secondary radar capability. It is a bar-shaped affair that is usually perched atop the primary radar antenna and rotates along with it. It's called "directional" because, like the primary radar antenna, it is designed to beam the interrogations and to receive the replies only from the direction it is pointed.

However, the directional antenna is less than perfectly directional. To design a perfectly directional antenna, you'd have to make it infinitely large...not too practical if you need to rotate it 10 times a minute. Real-world directional antennas have weaker "side lobes" in addition to the "main lobe". The side lobes are too weak to be a problem for distant aircraft, but for aircraft close to the antenna site they are a big problem. Unless something was done about them, the side lobes would cause a close-in aircraft to show up as three or four different targets on the controller's screen, causing no end of confusion.

Side lobe supression

That's where the third antenna comes in. It's called the omnidirectional antenna because it radiates equally in all directions. Every time the directional antenna sends out an interrogation (which consists of a pair of pulses known as P1 and P3), the omnidirectional antenna sends out its own pulse (called P2). The signal from the omnidirectional antenna is designed to be much weaker than the main lobe of the directional antenna, but stronger than its side lobes.

When the transponder receives an interrogation, it compares the strength of the three pulses it receives. If the P2 pulse is weaker than the P1 and P3 pulses, then the transponder knows it's hearing the main lobe from the directional antenna, and it replies to the interrogation. On the other hand, if P2 is stronger than P1 and P3, then the transponder knows it's hearing a side lobe and so it doesn't respond. Pretty clever, huh?

Interrogations and replies

The ground station transmits its interrogation pulses on an "uplink" frequency of 1030 Megahertz, and the transponder replies on a "downlink" frequency of 1090 Megahertz. Effective range of secondary radar is usally about 110 miles, although this can vary considerably from one site to another.

The ground station may send a Mode A interrogation to request the transponder code or a Mode C interrogation to request the aircraft altitude. The only difference between Mode A and Mode C interrogations is the time interval between the P1 and P3 pulses. Ground stations normally alternate the two types of interrogations, and send them quickly enough that the transponder receives several of each on each sweep of the radar antenna. (Modes B and D are used by the military and in other countries, but are of no concern to most of us.)

By the way, the transponder's output power (typically about 200 watts), frequency (1090 MHz), and "side lobe supression" logic are among the things that your radio shop tests when you bring in your aircraft for its biennial transponder certification check. These biennial tests are mandated by FAR 91.172, and are required for all aircraft, not just those that fly IFR.

Squawk 5678?

There's a new type of transponder called a Mode "S" transponder that is required for TCAS and may support datalink. A few years ago, the FAA tried to mandate that the fleet retire their Mode A/C transponders and upgrade to Mode S, but this move was strongly resisted by AOPA and other general aviation groups. At present, the rules to force the transition to Mode S have been put on indefinite hold, and aircraft that aren't required to be TCAS-equipped (which includes most of our owner-flown airplanes) continue to use the older and far less expensive Mode C units.

Mode S transponders also broadcast or squitter their address, a 24-bit identification unique to each individual aircraft. The address is assigned by the country of registry and is usually derived from the aircraft registration number.

The TDR-94D is classified as a Level 3A Mode S transponder according to the International Civil Aviation Organization standards and recommended practices. As a data link terminal, it receives short (Comm A) and long (Comm C) uplink messages and transmits short downlink messages (Comm B). Separate messages may be linked in chains to transmit large amounts of information.

AOS-260

Mode-S / CD-2 Digitizer

The Mode S is a secondary surveillance and communication system which supports Air Traffic Control (ATC) and could support other data link services, such as weather information, directly to the cockpit. Each Mode S transponder equipped aircraft is assigned a unique address code. Using this unique code, interrogations can be directed to a particular aircraft and replies can be unambiguously identified. Mode S limits its interrogations to specific targets, and proper timing of interrogations permits replies from closely spaced aircraft to be received without mutual interference. Mode S also provides monopulse detection for improved azimuth accuracy and includes RMM capabilities. The Mode S also interrogates and receives aircraft position and altitude information from Air Traffic Control Radar Beacon System (ATCRBS) transponder equipped aircraft. As a back-up, the Mode S has the capability to operate as an Air Traffic Control Beacon Interrogator (ATCBI). As of the end of 1998, there are approximately 140 Mode-S sensors commisioned in the backup mode (IBI) with around 135 of those fully commissioned in Mode-S.

The FAA is now talking about a future ATC system based not on radar surveillance, as is used today, but on "automatic dependent surveillance" in which aircraft continually transmit their GPS position to ground stations which keep track of them and tell controllers where they are. In view of this, it's possible that Mode S may become obsolete before we need to worry about it.