RAdio Detecting And Ranging: any of several systems or devices using transmitted and reflected radio waves for detecting a reflecting object, as an aircraft, and determining its direction, distance, height, or speed, or in storm detection, mapping, navigation, etc.

For the sake of those that would take this as free license to attempt construction of their own radar or microwave emitters that is probably not the best idea. The acronym as defined above stands for RAdio Detection And Ranging. Meaning that it essentially does the same thing as SONAR only out of the water. Simple enough. At sea level, a single pulse from an antenna can be expected to cover 1 nautical mile in 6.18us. (Microseconds.) For future reference 1 radar range mile is the space covered by the pulse in 12.36us. This constant can be used to compute the distance to target in nautical miles where delta (e.g. elapsed,) time from pulse transmission to reception is divided by 12.36us. This is primarily of use in distance measuring equipment (DME, as a practical example TACAN systems use this extensively for distance to antenna installation computation and display,) and navigational radars.

Typical Classifications of Radar Systems
There are several different forms of radar which all have a specific function tied to their design, type of pulse, operating frequency and use. Most civilian radar systems are of the navigational variety, as they are not intended for high-resolution scans of target areas or intended to feed precise information into targeting systems. Almost all ships capable of crossing large areas of open ocean are equipped with some form of simple navigational radar, Raytheon and Furuno being two common manufacturers of such systems. Fire control radars are slightly funny fish however. With most typical surface radars the emitter footprint resembles something like someone wandering into a room and lazily looking around. Not particularly desperate, but nonetheless looking around curiously. Military search radars are different in that the individual walking into the room is looking for someone specific, however still not making much of an effort to find them. The spectral footprint of a fire control radar on the other hand is the equivalent of a person running into a room with a bat while screaming and yelling. Types of radars can usually be classified by looking at their PRF, or pulse repetition frequency. PRF in essence is the number of pulses being sent out, obviously this is much higher in fire control applications as detailed target information is a must for reliable operation.

There are five primary types of radar system:
1. Search: Continuously scans a volume of space.
-Surface search radar: primarily used for navigation and the location of targets around the emitter on or very near the surface. Uses PPI and is primarily limited to LOS, (Line of Sight.) Radar emissions suffer from the same limitations that UHF/VHF radio waves do in that they will very rarely be of any use over the visible horizon.
-Air search radar: detects and determines range, relative bearing, distance, position, course, and speed of airborne targets. Air search radars can be broken down further into 2D and 3D. 2D (two dimensional,) radars are limited to determining two of three factors at a time due to the method in which the data is displayed to the system operator. Either height and range or bearing and range can be determined by two-dimensional radar-plotting systems. 3D (three dimensional,) systems on the other hand are capable of displaying height, range and bearing simultaneously and have far more complex presentation apparatus than their far more common cousins.

2. Tracking: Provides continuous tracking of targets in fire control radars. Characterized by narrow circular beams, very high PRF (Pulse Repetition Frequency,) and narrow PW (Pulse Width.) Tracking radars usually have three phases of operation being designation, acquisition and tracking. During the designation phase inputs from additional systems scanning a large volume of space may provide bearing and height information to the tracking radar to kick start it into its next phase. During the acquisition phase, the system will scan a very small volume of space for the target that it is attempting to find. This may either be manually or automatically controlled. The final stage of target tracking is done for one purpose only, to provide a weapons system with the necessary inputs to insure destruction of the intended target.

3. Missile Guidance Radar: Furnishes information used to guide missiles to a hostile target. Beam riding guidance radars follows a single beam to the illuminated target. (This would be analogous to someone following a laser beam across a dark and smoke filled room.) Homing radars follow either existing radiation (this is passive homing as with the Shrike missile and later HARM or High-speed Anti-Radiation Missile,) or reflected radiation already being pointed at a given target. Homing missiles following existing radar illumination are unique in that they require no emissions from the unit or person firing the missile. At one point research was conducted by the Raytheon corporation to determine if it would be feasible for a single E-2C Hawkeye, (Update 2 or later,) to provide targeting information to entire airborne squadrons of F-14 Tomcats. Since the AWG-9 (see JTEDS for breakdown,) fire control radar in the F-14's would never go active the adversary would not know that between 30 and 50 planes had launched 7-10 missiles until too late to employ effective countermeasures. A single E-2C flying three or four hundred miles behind the air wing would not be perceived as a threat and could loiter nearly off station in this case.

4. Approach radar: Allows aircraft to approach an airfield in IFR, (instrument flight rules,) conditions. Also known as ILS, (Instrument Landing System,) Automatic Approach Control, and Automatic Landing System. Broken down into GCA (Ground Control Approach) and CCA (Carrier Control Approach.) CCA is obviously the more complex of the two systems given that it integrates ship movement into data passed to the aircraft. Static airfields that are not moving, (i.e. no pitch, roll and forward velocity,) have no use for this and simply transmit enough data to put the aircraft on an approach pattern to the end of the runway.

5. Airborne radar: Designed to fit specific weight and space requirements for use in aircraft. Airborne radars are unique in the sense that often they are required to handle large amounts of power safely and in a minimum of space. The earlier mentioned U.S. fielded E-2C Hawkeye is an excellent example of such a system in use. AWACS, (Airborne early Warning and Control System,) aircraft in general possess huge radar installations in terms of power output.

Waveforms and Transmission Types
Radar systems can then be broken down into transmission methods beyond their specific use. There are four primary types of transmission:

1. Pulse Modulation: Relatively short bursts of RF are released from the transmitter, (between .1us and 50us.) Pulse width varies directly with range to target, these systems cannot provide velocity data.

2. Frequency Modulation (FM): Continuously varied beam in a set pattern, (similar to commercial FM transmissions,) at set frequencies. The received reception pattern is used in sum and difference, (similar to Doppler,) computations to yield distance. Phases received out of turn can cause indicator ghosting and Doppler shift will cause inconsistencies in ranging. Mainly used in radar altimeters.

3. Continuous Wave (CW): Uses Doppler effect for generation of display. Transmitter operates at a single frequency and uses Doppler shift in returned waveform to compute velocity differential between the emitter and a given target. The frequency variation of the received waveform is directly proportional to relative velocity of the target object. (Relative velocity meaning the difference between emitter and target, despite the fact that one of the two may not be moving.) CW radars are most often used in fire control systems given that they very easily detect fast moving objects. They are however hindered by an inability to distinguish between two targets on the same bearing at the same velocity and difficulties detecting very slow or stationary targets.

4. Pulse-Doppler: Most versatile of all transmission types. Modifies pulse shaping and detection used in pulse modulated radars to use Doppler shift to detect velocity differences.

Several factors affect the performance of a radar system and their ability to detect targets and accurately present information from received pulses to their operators. Due to the inherent complexity involved in exceeding maximum published ranges as a result of atmospheric conditions, that topic will not be covered here completely. However, radar waves can under certain conditions enjoy the same bounce effect as HF radio waves and return from targets beyond the horizon. Maximum standard range, (barring unforeseen anomalies,) is a function of carrier frequency, peak output power, PRF (Pulse Repetition Frequency,) and receiver sensitivity. Discrimination of ambiguous returns also plays a role in maximum range, that is if an antenna were to rotate at such a rate as to receive echoes from other, earlier transmitted pulses. This is also referred to as 'Second Sweep Pulse.' Azimuth resolution comes into play when two targets are at the same range from the transmitter but at slightly different bearings. Range resolution on the other hand is the exact opposite, meaning that it is the ability of a radar to distinguish between two targets at the same bearing but different ranges. Slant range also affects resolution by damping effects caused by differences in attitude. If the transmitter is not located on a stable platform, (such as an aircraft,) ambiguous returns caused by reflections off of the surface of the earth can create difficulty for the operator of such a system. This also figures far more significantly in radar altimeters where antennae are usually mounted on one of the low points of the fuselage. A sudden shift in pitch or roll can be expected to introduce errors into the system of many thousands of feet if slant range effects are not taken into account automatically.

Basic System Components
All radar systems have similar basic parts. Whether they are divided as they are here is primarily caused by engineering needs such as space requirements, convenience, and part interoperability. The decision has been made here to divide a basic radar system into six parts for the purposes of clarity. Note that each unit in the complete system controls a different facet of the total operation of the radar. Part of the problem with locating all of the parts of a radar in the same space comes from the amount of power being radiated by the antenna. In multiple megawatt systems, (such as the Aegis SPY-1A phased array system fielded by the United States Navy,) there is enough radiated power coming from side lobes to stop unshielded digital watches. Obviously locating sensitive receiver circuits directly next to such a transmitter would be impractical. In lower power systems, (such as radar altimeters,) the entire assembly can fit into spaces half the size of an ordinary cigar box. Newer radar altimeters in service today actually integrate all necessary components, (including indicator and excluding antenna,) into a package smaller than a hand held radar gun used by law enforcement.

1. Synchronizer: Controls radar operation by coordinating timing and produces trigger pulses coupled to the transmitter which cause it to fire.

2. Transmitter: Generates RF pulses at regulated intervals governed by the synchronizer which are then passed through a waveguide to the antenna/duplexer for transmission.

3. Antenna/Duplexer: Routes the pulse from the transmitter and then radiates energy into free space in a highly directional beam. The antenna also picks up the returned pulse and routes this energy to the receiver for processing and display. Switching between reception and transmission is referred to as recovery time which essentially governs the minimum range of a radar. Minimum range can be computed by adding pulse width to recovery time and then multiplying it by 164 yards.

4. Indicator: Presents information to system operator. This can come in several forms, including large presentation boards, traditional circular scopes and velocity differential information. Indicators are usually tailored to present the information the system was designed to present. This may include navigational information, target data and position.

5. Power supply: Supplies the necessary operational voltages to system components.

6. Receiver: (May be included as a part of the synchronizer.) Handles received RF passed to it from the antenna and passes this information onto an external signal data converter for display or packages it locally and then passes it to an indicator. This is dependant on the complexity of the information presented to the operators. More simple systems will not have video generation units, analog or digital display driver circuits or large display apparatus incorporating range data.

Since conventional circuits deal with frequencies at power far below the RF spectra used in radar systems, coaxial cable is not an effective way to move energy through a radar system. Specifically transmitted pulses must be moved through something called a waveguide. The microwave spectrum runs from 1GHz to 100GHz, with lower and upper harmonics found at the usual ranges. (Note: For those of you at home with 900MHz cordless and PHS, PCS, CDMA and other multi-mode cellular phones, those are microwaves right there next to your ear. In essence, you are putting your head into a very low-power microwave oven every time you decide to answer the phone. There is insufficient data either way to the answer of the obvious question being asked here, which is the amount of harm posed to the users of such devices. Ultimately this is left to personal discretion.) Most normal circuits rely on current and voltage as primary characteristics for determining what type of wire or cable should be used and where. However, microwave circuits deal in electromagnetic fields that are far more demanding in terms of resources required to move them from place to place. For starters, unshielded magnetrons and resonant cavities at full power generate magnetic fields with sufficient force to induce current in nearby unshielded lines. This will lead to static, transient electromechanical and computer malfunction caused by spurious inputs and possible damage to nearby equipment.

The waveguide itself is composed of an infinite number of one-quarter wavelength shorted stubs. One needs look no further than the face of a microwave oven for an example of a shorting grid composed of the same quarter wavelength stubs. The screen set into the glass on the door, (glass is RF transparent,) of the oven is actually a shorting grid designed to stop RF energy from escaping the inside of the oven. In the same way, waveguides are designed to prevent RF from escaping by essentially using the physics of microwave transmission against the waveform. Energy traveling through the waveguide, (unless properly coupled,) arriving at a break in the line will arc over and ground out against the sides of the line or the object in question. Dimensions of the waveguide are a function of two quantities, (known as the 'A' and 'B,') which are derived from the frequency and power handling required by the system. 'A' determines the range of frequencies that the line can handle and is .7 wavelengths long. The 'B' dimension is one half of 'A' or .35 wavelengths long and determines the power handling capabilities of the waveguide. Due to their close machining tolerances, (waveguides are hollow,) size, expense, (usually the insides are coated with Gold,) and support requirements waveguides are used in as limited quantities as possible. The medium inside the waveguide must be kept free of any humidity to prevent a capacitive effect that causes the energy to actually arc over inside the line and increase SWR beyond tolerances of the transmitter. (SWR: Standing Wave Ratio. Relationship between power transmitted and power reflected by conducting medium.) Extremely high SWR caused by such a problem will quickly kill a transmitter either due to overheat or component failure. Most U.S. military airborne radars overcome the problem of water entry by using self-purging positive pressure systems filled with a Freon and Nitrous Oxide (FeNO2) mix. (70% Freon/30% Nitrous Oxide at varying PSI. Actual internal pressure is determined by system requirements.) This in itself is impractical. As problematic leaks invariably develop, deployed units can quickly exhaust their supply of gas during troubleshooting and risk partial mission capability or non-mission capability status as a result.

Two conditions must be met for a waveguide to conduct: First, the E-field must be perpendicular to the direction of travel. Second, the H-field must exist in closed loops parallel to the conduit and perpendicular to the E-field. Coupling is achieved through one of three different means:

1. Probe Coupling: Sends signal or energy through a hole in the termination at the end of the transmission line parallel to the 'B' dimension and one-quarter wavelength from the shorted end. Similar to poking one's head through a hole in a wall and then yelling down the hallway.

2. Loop Coupling: Injects energy into a waveguide where it oscillates, thereby inducing an H-field. Inserted where H-field will be the strongest. Equates roughly to looking for a the acoustic sweet spot in a room and then placing a speaker there.

3. Aperture/Slot Coupling: Used where loop coupling is desired. The slot must be of proper size to minimize reflected waves coming back toward the transmitter or waveform generator. Take head, find hole in wall. Make sure hole is big enough that you are not going to blow out your own ears while yelling. Use megaphone.

4. Directional Coupling: Process where a device is used to couple RF to either external equipment, an antenna, or another circuit. This is for sampling purposes only.

5. Bi-Directional Coupling: Allows for sampling or injection of a signal in situations similar to those listed above.

Waveguides are shaped with the same rigid tolerances as mentioned above. One of several methods can be employed to direct a waveguide through or around a given obstruction. Another of the drawbacks of waveguide based systems manifests itself with this requirement, turning or twisting the line too much over a short enough length will render it unusable. All turns or changes in direction of the transmission medium must be made at about two wavelengths, anything less than this will cause internal arcing to occur. Gradual E bending is achieved by slowly curving or arcing the waveguide over the aforementioned two wavelength distance. Twist bending accomplishes the same directional change only by rotating the waveguide about the long axis. 45-degree turns can also be used where gently curving the waveguide is impractical for either internal structural or external installation requirements, this is also the maximum angle a waveguide can be bent or turned.

Antennae and Radiators
Antennae as a general rule fall into one of two categories, either omni-directional or directional. Omni-directional antennae radiate energy in all directions, where as directional antennae radiate in lobes with the vast majority of the energy being focused in a single way. Directivity of an antenna refers to the degree of sharpness that the beam takes. The more focused the beam means that less energy is required to run the system at a maximum range. Power gain is directly proportional to directivity and is the ration of power at any given point along the main lobe compared to that same distance in a dipole antenna. There are several primary types of antenna and a virtually infinite number of variations along a common theme. Only the four major types of microwave antennae are discussed here.

1. Parabolic: Beam is radiated from focal point at center of antenna. Parabolic antennae are between 10 and 20 wavelengths wide.

2. Truncated: Subdivided into horizontal and vertical truncation. Selected slice, (either horizontally or vertically,) of a parabolic antenna used to generate a fan shaped beam. Most ATC, (Air Traffic Control,) TACAN and rotating search radars use horizontally truncated antennae.

3. Cosecant Squared: Designed to produce a uniform beam, distributing energy in primarily in one general direction while limiting transmission in others. Commonly used in surface search radars from aircraft.

4. Array: Number of individually emitting antennae suitably spaced apart from each other. Radiating elements can be nothing more complex than feed horns. Arrays can either be mechanically or electrically steered. Linear arrays, (a number of elements arranged in one dimension in straight lines,) and planar (or phased) arrays (elements evenly placed around a plane,) are the most common types of array antennae. In most planar array systems, the beam is electrically steered by manipulating the main lobes of other elements to push the beam in one direction or another. The disadvantage in this lies in that the more steering that occurs the more damage is done to the beam itself. This is not particularly a factor with most modern phased array systems however.

Practical RF Handbook: Ian Hickman, 1999 (Newnes Publishing)
The ARRL Antenna Handbook, 17th Edition: Edited by R. Dean Straw, 1994 (American Radio Relay League Publishing)
AVA(A1) Avionics Technician School Part 3/4 Class Notes/Materials: Yurei, 1993/1994.
Navy Education Electronics Training Series (NEETS) Modules 1-27: CNET, various, (Government Printing Office)
Basic Electronics Theory, Fourth Edition: Delton T. Horn, 1994 (Tab Books)

Radar 101

As a backseater who has actually "flown low to beat the radar", I think that I can offer a few contributions to the understanding of radar. Aviators (we're not all pilots dammit!) usually use the phrase "fly low and avoid the radar", although the semantics are neither here nor there. While radar (RAdio Detection And Ranging) functions on the principle of reflected radio waves, there are a few steps that can be taken to detect low flying aircraft.

Most radar systems operate on the principle of pulse modulation, which is to say that RF at a given frequency is released in bursts at determined lengths, known as pulse width, giving range and azimuth (if the radar is a rotating sort) information. The process is simple, the antenna is pointed in a certain direction when the pulse goes out (azimuth) and the time it takes to get back is calculated based on the speed of light (162,000 NM/sec or 186,000 mi/sec).

RADAR ANTENNA transmit >>> >>> >>> TARGET

RADAR ANTENNA receive <<< <<< <<<

A somewhat different variation is the CW (continuous wave) radar, which broadcasts a steady stream of RF energy, giving only doppler speed and azimuth. The difficulty with this is that range cannot be estimated due to the fact that range is measured from the return time of individual pulses. If the RF is always going out, there is never a chance to measure the time that it is gone. The nice thing is that it does give the ability to determine relative speed to the operator. This speed is determined by the equation:

(Fh-Fl)/2 (3000/Fo) = speed in knots

Fh = highest returning frequency
Fl = lowest returning frequency
Fo = frequency emmitted by the radar
(this can also be used for acoustic applications)

PW RADAR ANTENNA transmit >>> >>> >>> TARGET
PW RADAR ANTENNA receive  <<< <<< <<< 

Advanced radars can infer doppler shift (frequency change due to relative velocity, like the sound of a car shifting from high to low when it passes you) for a single pulse and really advanced radars can perform NCTR (Non-Compliant Target Recognition) using a phenomenon known as JEM (it's on yahoo news so don't freak out CIA). While most details remain classified, JEM involves unique analog signal return traits to identify specific aircraft.

Stealth aircraft do no rely on terrain masking, the process of trying to mix with the confused reflections of stuff behind them. Instead, stealth is reliant on design features to produce a low target cross section, i.e. the amount of RF that will reflect off the aircraft from a given angle. Primarily, faceted, or angled, surfaces bounce the RF energy in a different angle from the radar receiver antenna. This is why stealth aircraft try to avoid having any 90 degree surfaces. As you will remember from geometry class, if something goes into a right angle, it will reflect out in the direction it came from. Try it yourself: throw a tennis ball in a square corner and marvel as it whacks you in the forehead. Add to all that radar absorbant materials and you've got yourself a hard to detect aircraft (and a sore head if you tried the geometry experiment).

A last consideration to take into mind is that the height of the radar antenna above the ground affects the distance it will be able to see it's target. To figure that out, the radar horizon is determined by the equation:

1.23 (square root of antenna in feet) = distance in nautical miles.

For a target above the surface of the Earth, the equation is:

1.23 (square root of antenna + square root of target) = distance in nautical miles.

With the above information, you can easily build an IADS (Integrated Air Defense System) for an emerging third world nation, but don't expect to shoot down any cruise missiles or stealth fighters anytime soon. That takes far more money, equipment, and classified documents than you will find on E2. Good hunting.

How does radar pertain to civil aviation? There has been no more significant development in the maintenance of safe skies since radio, in my humble opinion. 'See and be seen' worked for a while, but commercial aviation has long since passed the point past where it ceased to be practical. Much of the preceding writeups are relevant but I'd like to add some bits and bobs of my own.

Primary Radar

This is how we started out. Your basic Radio Detection And Ranging. An antenna that transmits measured pulses and listens for echoes from objects in their path. There are Continuous Wave and Pulse-Doppler radars that eschew or combine facets of pulse-based radar, but the civil radar is generally of the pulse type.

Primary Radar—or Primary Surveillance Radar (PSR), to use the full title—produces the basic radar picture that an air traffic controller sees, though in today's world it's a frighteningly basic picture. If you've ever seen a photo or video of ATC radar, it might seem a confusing picture, with clusters of numbers huddling around dots swarming around the screen. Trust me: it's a lot more frightening without those numbers. And that's the information that primary radar gives you. A bunch of blips.

It's a basic system of echo-location that radar uses to determine the position of stuff nearby. But a couple of factors determine certain of its capabilities. As pointed out above there are several components in a radar system, one of which is the transmitter. This transmits pulses of R/F energy for a specific interval. This interval is called the 'pulse length'; for instance, five microseconds. The time interval between pulses is called the 'pulse recurrence interval'.

The reason these things affect the detection capabilities of the radar is that it generally cannot transmit and receive at the same time. It transmits for a time, then listens for echoes. Because pulse echoes are so much fainter than the pulses themselves, the listening equipment greatly amplifies them before they are processed. If this processing equipment were subjected to a full-power pulse it would fry, so it is isolated from the radar receiver while the pulse is being transmitted.

So, while a radar system is transmitting a pulse, it cannot "hear" anything. Let's use our earlier figure of five microseconds for the pulse length. Radio waves propagate at the speed of light (186,000 miles/sec), and in five microseconds would travel just under a mile before the antenna stops transmitting. If the pulse hits anything less than half a mile away, the echo will reach the antenna before it has finished transmitting. Pulse length, therefore, affects the minimum range of primary radar systems. The longer it is, the larger the 'blind' area surrounding the antenna.

What of the pulse recurrence interval (PRI)? This affects maximum range, for the simple reason that after transmitting the pulse, you have to allow some time for the echoes to reach you before you start transmitting again. If you have already started transmitting pulse two when the echoes from pulse one arrive, you won't hear them. A short PRI will give you relatively fast updates on short-range echoes, because the system doesn't 'wait' very long between pulses, but you won't be able to see anything reliably past a certain range. Conversely, long PRIs allow the radar to see further, at the expense of refresh times, which will be particularly long for stuff far away. As always, it's a trade-off.

Presumably there was a time when PSR was everything? Yes, indeed. To initially identify aircraft—which would all, of course, appear as identical blips—controllers had to instruct them either to make a turn (which would subsequently be observed on the controller's radar screen), or to report their position in relation to a specific geographical location (which would be rendered in the appropriate position on the screen). Once identified, controllers had nothing more than their working memory informing them which blip was which aircraft. This still happens on the odd occasion, when the more up-to-date tools fail, and it usually ain't pretty.

Secondary Radar

Forgive my slight regurgitation of other writeups in this section.

Secondary Radar—Secondary Surveillance Radar (SSR)—is an 'active' system, where PSR is purely passive. SSR is a two-part system, using equipment on the ground and installed on aircraft. The part on the aircraft is called the 'transponder' and it's from here that most numbers you'd see on a controller's screen originate.

Like PSR, SSR transmits a measured pulse and then listens. But rather than echoes, it receives replies. When the transponder on an aircraft detects an SSR pulse, or 'interrogation', it transmits a 'reply'. The fact that this is a transmission rather than an echo means that secondary radar has at least double the range of PSR; increasing the PRF of a primary radar system does increase range to a point, but beyond that it becomes difficult to pick out echoes from background noise.

The data that SSR replies may contain depends on the capabilities of the transponder sending it. There are several categories of data, called 'modes':

Mode A

Mode A, or 'mode alpha,' is the most basic kind of information available from a transponder. A pilot-selectable code can be set in the cockpit as an identity code of sorts for the aircraft. The radar system correlates the code with the relevant primary radar echo, and the code appears next to it on the radar display. The code is four digits long, each of which is represented by a 3-bit binary code, giving a range of 0-7 for each digit.

There are plenty of simply hilarious controller jokes involving setting an '8' in a transponder code.

Some of you are probably thinking "that isn't many codes," and you're right. 84 isn't much, and it has proven quite limiting. As it is, each ATC unit has an agreed—usually small—range of codes that it can assign to aircraft, so that there is as little potential for confusion as possible. When the aircraft enters the airspace of another ATC unit, it will probably be instructed to change its Mode A code to one of theirs, or to the 'neutral' 7000 code which basically means "I'm doing my own thing, no-one is controlling me." That way other controllers, in areas of overlapping radar coverage, will be able to see who, if anyone, is controlling the aircraft adjacent to their sector.

Regular, scheduled flights frequently have the same Mode A code assigned to them every time, and that Mode A code follows them for the duration of the flight rather than changing every time the aircraft enters another sector. A system called 'code/callsign conversion' allows suitably-equipped radar displays to show the callsign of the aircraft in the place of its Mode A code. This is very handy.

There are also a number of special-purpose codes. Here's a few of them:

0033 - dropping parachutists in the next five minutes
0036 - helicopter making a power line inspection
7003 - Red Arrows making a transit or air display
7500 - hijack or other unlawful interference with flight
7600 - radio failure
7700 - general emergency (engine failure, etc). Usually, if an aircraft selects this code, its callsign flashes SOS! in a different colour.

Plenty of RAF bases have their own ranges of Mode A codes too, as do many UK police forces.

Mode A codes are colloquially referred to as 'squawk' codes, which a few of you may be familiar with. It's actually slightly inaccurate, as 'squawk' is a secondary feature of a transponder. A controller may ask a pilot to "squawk ident", in response to which the pilot should press the 'IDENT' button on their transponder, after which a flashing circle will appear around the relevant blip on the controller's display for a few seconds. It's a handy way of picking out an aircraft if you lose its identity in background clutter. It's also a handy cover if you forget what you were going to say:

Me:     American 393...
AAL393: American 393?
Me:     American 393, uh... squawk ident.
AAL393: Identing now, American 393.
Me:     *clears throat*

Yes, you give the "squawk ident" instruction to an aircraft you're controlling if you lose its identity somehow (you are no longer sure which blip represents it on the screen), and need to confirm it. You don't say it to an unknown aircraft, so I confess I rolled my eyes when that army dude at the beginning of Transformers told Blackout to "squawk ident" after he intruded their airspace.

Anyway, moving on...

Mode C

I've gone into this a bit in another writeup so I'll be brief here - the Mode C, or 'mode Charlie' data from a transponder gives information on the aircraft's level, and this again appears next to the relevant blip on the radar display. Not wanting to go into the peculiarities of altitudes, heights and flight levels, the Mode C data is always provided as a flight level. If the aircraft is operating on an altitude, this will automatically be recalculated at the controller's end so that it is correctly displayed on the radar screen.

Mode C data is accurate to within 200 feet of the aircraft's actual altitude, and this is factored into controllers' decision-making. If one aircraft is in level flight and another is climbing or descending through that level, they are not considered vertically separated until the climbing/descending aircraft has exceeded the minimum separation by 200ft and is still going in the correct direction. If both aircraft are climbing or descending the rule applies to both. The 200ft rule increases to 400ft if either aircraft is supersonic, regardless of whether they are in level flight or not.

Since SSR data is broadcast indiscriminately, it can be picked up by any suitably-equipped receiver in range. Mode C is an essential part of TCAS (Traffic Alert and Collision Avoidance System), and is used by that system to gauge what, if any, collision threat is posed to an aircraft by those around it. TCAS uses several small antennas on the fuselage of the aircraft to triangulate the source of each SSR signal and build up a picture of the sky around it.

Mode S

This is the relatively recent one. It provides a huge leap in information potential, but has yet to be completely integrated into ATC systems. It has all the capabilities of Mode A and Mode C, but adds a raft of further information about the aircraft, including a unique 24-bit code which is set by the manufacturer and kept by an aircraft for life, eventually replacing Mode A altogether in theory.

Information transmitted by Mode S transponders includes heading, climb/descent rate, roll angle, speed and most interestingly, the selected altitude and heading. This is what the pilot has dialed into the autopilot (typically the autopilot flies the aircraft other than takeoff and the final stages of landing, and the pilot just dials into the autopilot what they want the aircraft to do), and is awesome because pilots have a long history of acknowledging one instruction and dialing something completely different into the autopilot.

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