Radar Basic PrinciplesThe following figure shows the operating principle of a primary radar set. The radar antenna illuminates the target with a microwave signal, which is then reflected and picked up by a receiving device. The electrical signal picked up by the receiving antenna is called echo or return. The radar signal is generated by a powerful transmitter and received by a highly sensitivereceiver.

TRANSMITTING PATHTRANSMITTER DUPLEXER ANTENNA ELECTROMAGNETIC WAVE AIM

RECEIVING PATHAIM ECHO SIGNAL ANTENNA DUPLEXER RECEIVER DISPLAY

Figure 1: Block diagram of a primary radar(interactive picture)All targets produce a diffuse reflection i.e. it is reflected in a wide number of directions. The reflected signal is also called scattering.Backscatteris the term given to reflections in the opposite direction to the incident rays.Radar signals can be displayed on the traditional plan position indicator (PPI) or other more advanced radar display systems. A PPI has a rotating vector with the radar at the origin, which indicates the pointing direction of the antenna and hence the bearing of targets.TransmitterThe radar transmitter produces the short duration high-power rf pulses of energy that are into space by the antenna.DuplexerThe duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna need be used. This switching is necessary because the high-power pulses of the transmitter would destroy the receiver if energy were allowed to enter the receiver.ReceiverThe receivers amplify and demodulate the received RF-signals. The receiver provides video signals on the output.Radar AntennaThe Antenna transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception.IndicatorThe indicator should present to the observer a continuous, easily understandable, graphic picture of the relative position of radar targets.The radar screen (in this case a PPI-scope) displays the produced from the echo signals bright blibs. The longer the pulses were delayed by the runtime, the further away from the center of this radar scope they are displayed. The direction of the deflection on this screen is that in which the antenna is currently pointing.

Chapter: Secondary Radar Technology

Figure 1:Large Vertical Aperture AntennaRadar was born in the due to the pressure of war. The need to detect hostile aircraft led to a vast investment in intellect and money to developRADAR. Classical Radar (now called Primary Radar) by definition is a non co-operative technology, that is it needs no co-operation from the Target being detected. Why do we need a different system then?As well as seeing hostile aircraft it soon became apparent that Radar was agood toolto see friendly aircraft and hence control and direct them. If the friendly aircraft is fitted with atransponder(transmitting responder), then it sends a strong signal back as an echo. An active also encoded response signal which is returned to the radar set then is generated in the transponder. This proved very useful for the military in seeing their own aircraft clearly. In this response can be contained much more information, as a primary radar unit is able to acquire (E.g. anAltitude, an identification code or also any technical problems on board such as a radiocontact loss ...).The objectives of this chapter of the homepage Radar Basics are to indicate the principles of the operation of Secondary Surveillance Radar (SSR). Firstly, the functionalblock diagramof the SSR (Mode A/C) system will be described, including both the Up Link formats and the Reply Messages. Secondly, the main aspects of the forthcoming Mode S system will be described.

The following Air Traffic Control (ATC) surveillance, approach and landing radars are commonly used in AirTraffic Management(ATM): en-routeradar systems, Air Surveillance Radar(ASR) systems, Precision Approach Radar(PAR) systems, surface movement radars, and specialweather radars.

SRE-M7, a typically en-route radar made by the GermanDASAcompanyEn-Route RadarsEn-route radar systems operate inL-Bandusually. These radar sets initially detect and determine the position, course, and speed of air targets in a relatively large area up to 250nm.

The Air Surveillance Radar ASR-12Air Surveillance Radar (ASR)Airport Surveillance Radar (ASR) is an approach control radar used to detect and display an aircraft's position in the terminal area. These radar sets operate usually inE-Band, and are capable of reliably detecting and tracking aircraft at altitudes below 25,000 feet (7,620 meters) and within 40 to 60nautical miles (75 to 110km) of their airport.

PrecisionApproach Radar PAR-80 made byITTPrecision Approach Radar (PAR)The ground-controlled approach is a control mode in which an aircraft is able to land in bad weather. The pilot is guided by ground control using precision approach radar. The guidance information is obtained by the radar operator and passed to the aircraft by either voice radio or a computer link to the aircraft.

Surface Movement Radar ASDESurface Movement Radar (SMR)TheSurface Movement Radar(SMR) scans the airport surface to locate the positions of aircraft and ground vehicles and displays them for air traffic controllers in bad weather. Surface movement radars operate inJ- to X- Bandand use an extremely short pulse-width to provide an acceptablerange-resolution.

Microburst radar MBRSpecially weather-radar applicationsWeather radaris very important for the air traffic management. There are weather-radars specially designed for the air traffic safety.

Basic Principle of Operation

transmitted energybackscatter

Figure 1: Radar principle: The measuring of a round trip time of a microwave pulseThebasic principleof operation of primary radar is simple to understand. However, the theory can be quite complex. An understanding of the theory is essential in order to be able to specify and operate primary radar systems correctly. The implementation and operation of primary radars systems involve a wide range of disciplines such as building works, heavy mechanical and electrical engineering, high power microwave engineering, and advanced high speed signal and data processing techniques. Some laws of nature have a greater importance here.Radar measurement of range, or distance, is made possible because of the properties of radiated electromagnetic energy.1. Reflection of electromagnetic wavesTheelectromagnetic wavesare reflected if they meet an electrically leading surface. If these reflected waves are received again at the place of their origin, then that means an obstacle is in the propagation direction.2. Electromagnetic energy travels through air at aconstant speed, at approximately the speed of light, 300,000 kilometers per second or 186,000 statute miles per second or 162,000 nautical miles per second.This constant speed allows the determination of thedistancebetween the reflecting objects (airplanes, ships or cars) and the radar site by measuring the running time of the transmitted pulses.3. This energy normally travels through space in astraight line,and will vary only slightly because of atmospheric and weather conditions. By using of special radar antennas this energy can be focused into a desired direction. Thus the direction (inazimuthandelevationof the reflecting objects can be measured.These principles can basically be implemented in a radar system, and allow the determination of the distance, the direction and the height of the reflecting object.(The effects atmosphere and weather have on the transmitted energy will be discussed later; however, for this discussion on determining range and direction, these effects will be temporarily ignored.)

Direction-determination

Figure 1:Direction-determination (bearing)The angular determination of the target is determined by the directivity of the antenna. Directivity, sometimes known as the directive gain, is the ability of the antenna to concentrate the transmitted energy in a particular direction. An antenna with high directivity is also called a directive antenna. By measuring the direction in which the antenna is pointing when the echo is received, both the azimuth and elevation angles from the radar to the object or target can be determined. The accuracy ofangular measurementis determined by the directivity, which is a function of the size of the antenna.Radar units usually work with very high frequencies. Reasons for this are: quasi-opticallypropagation of these waves. High resolution (the smaller the wavelength, the smaller the objects the radar is able to detect). Higher the frequency, smaller the antenna size at the same gain.The True Bearing (referenced to true north) of a radar target is the angle between true north and a line pointed directly at the target. This angle is measured in the horizontal plane and in a clockwise direction from true north.(The bearing angle to the radar target may also be measured in a clockwise direction from the centerline of your own ship or aircraft and is referred to as therelative bearing.)

Figure 2: Variation of echo signal strengthThe antennas of most radar systems are designed to radiate energy in a one-directional lobe or beam that can be moved in bearing simply by moving the antenna. As you can see in the Figure 2, the shape of the beam is such that the echo signal strength varies in amplitude as the antenna beam moves across the target. In actual practice, search radar antennas move continuously; the point of maximum echo, determined by the detection circuitry or visually by the operator, is when the beam points direct at the target. Weapons-control and guidance radar systems are positioned to the point of maximum signal return and maintained at that position either manually or by automatic tracking circuits.In order to have an exact determination of the bearing angle, a survey of the north direction is necessary. Therefore, older radar sets must expensively be surveyed either with a compass or with help of known trigonometrically points. More modern radar sets take on this task and with help of theGPS satellitesdetermine the northdirection independently.Transfer of Bearing InformationThe rapid and accurate transmission of the bearing information between the turntable with the mounted antenna and the scopes can be carried out for servo systemsand counting ofazimuth change pulses.Servo systemsare used in older radar antennas and missile launchers and works with help of devices like synchro torque transmitters and synchro torque receivers. In newer radar units we find a system ofAzimuth-Change-Pulses(ACP). In every rotation of the antenna a coder sends many pulses, these are then counted in the scopes.Newer radar units work completely without or with a partial mechanical motion. These radars employ electronic phase scanning in bearing and/or in elevation (phased-array-antenna).

Minimal Measuring Range

RminFigure 1: The Radars blind rangeMonostatic pulse radar sets use the same antenna for transmitting and receiving. During the transmitting time the radar cannot receive: the radar receiver is switched off using anelectronic switch, calledduplexer. Theminimalmeasuring rangeRmin(blind range) is the minimum distance which the target must have to be detect. Therein, it is necessary that the transmitting pulse leaves the antenna completely and the radar unit must switch on the receiver. Thetransmitting timeand the recovery timetrecoveryshould are as short as possible, if targets shall be detected in the local area.Rmin=c0( + trecovery)in [m]

2

Targets at a range equivalent to the pulse width from the radar are not detected. A typical value of 1s pulse width of a short range radar corresponds to a minimum range of about 150m, which is generally acceptable. However, radars with longer pulse width suffer a relatively large minimum range, notablypulse compressionradars, which can use pulse lengths of the order of tens or even hundreds of microseconds. Targets at ranges closer than this minimum are said to be eclipsed.

Pulse CompressionPulse compression is a generic term that is used to describe a waveshaping process that is produced as a propagating waveform is modified by the electricalnetwork propertiesof the transmission line. The pulse is frequency modulated, which provides a method to further resolve targets which may have overlapping returns. Pulse compression originated with the desire to amplify the transmitted impulse (peak) power by temporal compression. It is a method which combines the high energy of a long pulse width with thehigh resolution of a short pulse width. The pulse structure is shown in the figure1.

UinUoutFigure 1: separation of frequency modulated pulsesSince each part of the pulse has unique frequency, the returns can be completely separated.This modulation or coding can be either FM (frequency modulation) linear(chirp radar) or non-linear, time-frequency-coded waveform (e.g. Costas code)or PM (phase modulation).Now the receiver is able to separate targets with overlapping ofnoise. The received echo is processed in the receiver by the compression filter. The compression filter readjusts the relative phases of the frequency components so that a narrow or compressed pulse is again produced. The radar therefore obtains a better maximum range than it is expected because of the conventionalradar equation.

Figure 2: short pulse (blue) and a long pulse with intrapulsemodulation (green)The ability of the receiver to improve therange resolutionover that of the conventional system is called thepulse compression ratio(PCR). For example a pulse compression ratio of 50:1 means that thesystem range resolutionis reduced by 1/50 of the conventional system. The pulse compression ratio can be expressed as the ratio of the range resolution of an unmodulated pulse of lengthto that of the modulated pulse of the same length and bandwidthB.PCR =(c0 /2)= B (1)

(c0/ 2B)

This term is described asTime-Bandwidth-productof the modulated pulse and is equal to the Pulse Compression Gain,PCG, as the gain inSNRrelative to an unmodulated pulse. Alternatively, the factor of improvement is given the symbolPCR, which can be used as a number in the range resolution equation, which now achieves:Rres= c0 ( / 2) = PCR c0/2 B(2)

The compression ratio is equal to the number of sub pulses in the waveform, i.e., the number of elements in the code. The range resolution is therefore proportional to the time duration of one element of the code. The radar maximum range is increased by the fourth root of PCR.Theminimum rangeis not improved by the process. The full pulse width still applies to the transmission, which requires theduplexerto remained aligned to the transmitter throughout the pulse. Therefore Rminis unaffected.AdvantagesDisadvantages

lower pulse-powertherefore suitable forSolid-State-amplifierhigh wiring effort

higher maximum rangebadminimum range

good range resolutiontime-sidelobes

better jamming immunity

difficulter reconnaissance

Table 1: Advantages and disadvantages of the pulse compressionPulse compression with linear FM waveformAt this pulse compression method the transmitting pulse has a linear FM waveform. This has the advantage that the wiring still can relatively be kept simple. However, the linear frequency modulation has the disadvantage that jamming signals can be produced relatively easily by so-called Sweeper.The block diagram on the picture illustrates, in more detail, the principles of a pulse compression filter.

filters for frequency componentsdelay lines for the time durationsummary devicesUinUinUoutUouttime duration ofa frequency componentFigure 3: Block diagram (ananimationas explanation of the mode of operation)The compression filter are simply dispersive delay lines with a delay, which is a linear function of the frequency. The compression filter allows the end of the pulse to catch up to the beginning, and produces a narrower output pulse with a higher amplitude.As an example of an application of the pulse compression with linear FM waveform theRRP-117can be mentioned.Filters for linear FM pulse compression radars are now based on two main types. Digital processing (following of theA/D- conversion). Surface acoustic wave devices.

Uouttside lobe of antenna(angularly)aimtime (range) side lobes

Figure 4: View of the time side lobes at an oscilloscope (upper figure) and at B-scopeTime-Side-LobesThe output of the compression filter consists of the compressed pulse accompanied by responses at other times (i.e., at other ranges), called time or range sidelobes. The figure shows a view of the compressed pulse of a chirp radar at an oscilloscope and at a ppi-scope sector.Amplitude weighting of the output signals may be used to reduce the time sidelobes to an acceptable level. Weighting on reception only results a filter mismatch and some loss of signal to noise ratio.The sidelobe levels are an important parameter when specifying a pulse compression radar. The application of weighting functions can reduce time sidelobes to the order of 30db's.Pulse compression with non-linear FM waveformThe non-linear FM waveform has several distinct advantages. The non-linear FM waveform requires no amplitude weighting for time-sidelobe suppression since the FM modulation of the waveform is designed to provide the desired amplitude spectrum, i.e., low sidelobe levels of the compressed pulse can be achieved without using amplitude weighting.Matched-filter reception and low sidelobes become compatible in this design. Thus the loss in signal-to-noise ratio associated with weighting by the usual mismatching techniques is eliminated.A symmetrical waveform has a frequency that increases (or decreases) with time during the first half of the pulse and decreases (or increases) during the last half of the pulse. A non symmetrical waveform is obtained by using one half of a symmetrical waveform.The disadvantages of the non-linear FM waveform are Greater system complexity The necessity for a separate FM modulation design for each type of pulse to achieve the required sidelobe level.

Figure 6: A non-symmetrical waveform (Output of theWaveform-Generatorpulse widthlinear FMnon-linearsymetrically

Figure 5: symetrically waveform

pulse width

Figure 7: non-symetrically waveform

Phase-Coded Pulse Compression

Figure 8: diagram of a phase-coded pulse compressionPhase-coded waveforms differ from FM waveforms in that the long pulse is sub-divided into a number of shorter sub pulses. Generally, each sub pulse corresponds with a range bin. The sub pulses are of equal time duration; each is transmitted with a particular phase. The phase of each sub-pulse is selected in accordance with a phase code. The most widely used type of phase coding is binary coding.The binary code consists of a sequence of either +1 and -1. The phase of the transmitted signal alternates between 0 and 180 in accordance with the sequence of elements, in the phase code, as shown on the figure. Since the transmitted frequency is usually not a multiple of the reciprocal of the sub pulsewidth, the coded signal is generally discontinuous at the phase-reversal points.Length ofcode nCode elementsPeak-sideloberatio, dB

2+--6.0

3++--9.5

4++-+,+++--12.0

5+++-+-14.0

7+++--+--16.9

11+++---++--+--20.8

13+++++--++-+-+-22.3

Table: Barker Codes

Elevation Angle

N (0)ESWRhorizon

Figure 1: Definition of elevation angleAltitude- or height-finding search radars use a very narrow beam in the vertical plane. The beam is mechanically or electronically scanned in elevation to pinpoint targets. Height-finding radar systems that also determine bearing must have a narrow beam in the horizontal plane in addition to the one in the vertical plane.The elevation angle is the angle between the horizontal plane and theline of sight, measured in the vertical plane. The Greek letter Epsilon () describes the elevation angle. The elevation angle is positive above the horizon (0elevation angle), but negative below the horizon.

Range Resolution

Figure 1: range resolutionThe target resolution of a radar is its ability to distinguish between targets that are very close in either range or bearing. Weapons-control radar, which requires great precision, should be able to distinguish between targets that are only yards apart. Search radar is usually less precise and only distinguishes between targets that are hundreds of yards or even miles apart. Resolution is usually divided into two categories; range resolution and bearing resolution.Range resolution is the ability of a radar system to distinguish between two or more targets on the same bearing but at different ranges. The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver and indicator. Pulse width is the primary factor in range resolution. A well-designed radar system, with all other factors at maximum efficiency, should be able to distinguish targets separated by one-half the pulse width time. Therefore, the theoretical range resolution cell of a radar system can be calculated from the following equation:Src0 (1)

2

Figure 2: Animation: One target includes two aimsThe following figures show the range resolution for a pulse with of one microsecond. If the spacing between two aircrafts is to small, then the radar see only one target as shown in Figure 2.And now the other example when the spacing is large enough:

Figure 3: Animation: two aims and two targetsRadar using Intrapulse-Modulation

Figure 4: Range resolution as a function of transmitters bandwidthIn apulse compression system, the range-resolution of the radar is given by the length of the pulse at the output-jack of the pulse compressing stage. The ability to compress the pulse depends on thebandwidthof the transmitted pulse (BWtx) not by itspulse width. As a matter of course the receiver needs at least the same bandwidth to process the full spectrum of the echo signals.Src0(2)

2 BWtx

This allows very high resolution (and a small radar range resolution cell) to be obtained with long pulses, thus with a higheraverage power. Figure4 shows the variation of slant range resolution with bandwidth. An 1.5m resolution will be achieved with a -3dB bandwidth of 100MHz theoretically.

Radars AccuracyAccuracy is the degree of conformance between the estimated or measured position and/or the velocity of a platform at a given time and its true position or velocity.Radio navigationperformance accuracy is usually presented as a statistical measure of system error and is specified as:1. Predictable: The accuracy of a position in relation to the geographic or geodetic co-ordinates of the earth.2. Repeatable: The accuracy in which a user can return to a position whose co-ordinates have been measured at a previous time with the samenavigation system.3. Relative: The accuracy which a user can determine one position relative to another (by neglegting all possible errors).Some results of radar units are indicated in the following table as example:radar unitaccuracyin bearingaccuracyin rangeaccuracyin height

BORA550< 0.3< 20 m

LANZA< 0.14< 50 m340 m 1150 feet (at 100 NM)

GM 400< 0,3< 50 m600 m 2000 feet (at 100 NM)

RRP117< 0,18< 463 m1000 m 3000 feet (at 100 NM)

MSSR-2000< 0.049< 44.4 m

STAR-2000< 0.16< 60 m

Variant< 0.25< 25 m

Table 1: Examples

En Route Separation StandardSpezified Azimuth Accuracy:Sliding Window ATCRBS (and ARSR)Monopulse ATCRBS/Mode SDistance from Radar (NM)Figure 1: Dependence of the accuracy of the range(Source: MIT Lincoln Laboratory)The stated value of required accuracy represents the uncertainty of the reported value with respect to the true value and indicates the interval in which the true value lies with a stated probability. The recommended probability level is 95 per cent, which corresponds to 2 standard deviations of the mean for a normal (Gaussian) distribution of the variable. The assumption that all known correction are taken into account implies that the errors in the reported values will have a mean value (or bias) close to zero.Any residual bias should be small compared with the stated accuracy requirement. The true value is that value which, under operational conditions, characterizes perfectly the variable to be measured/observed over the representative time, area and/or volume interval required, taking into account siting and exposure.Accuracy should not be confused withresolution.

Pulse Repetition Frequency (PRF)

Pulse width Pulse Repetition Time (PRT)

Figure 1: Radar pulse relationshipsThe Pulse Repetition Frequency (PRF) of the radar system is the number of pulses that are transmitted per second.Radar systems radiate each pulse at the carrier frequency during transmit time (or Pulse Width PW), wait for returning echoes during listening or rest time, and then radiate the next pulse, as shown in the figure. The time between the beginning of one pulse and the start of the next pulse is called pulse-repetition time (prt) and is equal to the reciprocal of prf as follows:PRT =1(1)

PRF

The radar system pulse repetition frequency determines its ability to unambiguously measure target range and range rate in a single coherent processing interval as well as determining the inherent clutter rejection capabilities of the radar system. In order to obtain an unambiguous measurement of target range, the interval between radar pulses must be greater than the time required for a single pulse to propagate to a target at a given range and back. The maximum unambiguous range is then given byRunamb.=c0=c0 PRT(2)

2 PRF2

wherec0is the velocity of electromagnetic propagation.Peak- and Average Power

Duty Cycleaverage powerpulse powerpulse widthpulse repetition timeFigure 1: Duty cycle, peak- and average powerThe energy content of a continuous-wave radar transmission may be easily figured because the transmitter operates continuously. However, pulsed radar transmitters are switched on and off to provide range timing information with each pulse. The amount of energy in this waveform is important becausemaximum rangeis directly related to transmitter output power. The more energy the radar system transmits, the greater the target detection range will be. The energy content of the pulse is equal to thepeak(maximum)power levelof the pulse multiplied by the pulse width. However, meters used to measure power in a radar system do so over a period of time that is longer than the pulse width. For this reason, pulse-repetition time is included in the power calculations for transmitters. Power measured over such a period of time is referred to asaverage power.P=Paverage=Pulse Width ()= Duty Cycle(1)

PiPpeakPRT ()

Peak power must be calculated more often than average power. This is because most measurement instruments measure average power directly. Transposing the upper equation gives us a common way for calculating peak power/average power.Since the storage of the energy in themodulator, the power supply must make plant for the transmitter available a little more than the average power only.Duty cycleThe product of pulse width (pw) and pulse-repetition frequency (prf) in the above formula is called theduty cycleof a radar system. Duty cycle is the fraction of time that a system is in an active state. In particular, it is used in the following contexts: Duty cycle is the proportion of time during which a component, device, or system is operated. Suppose a transmitter operates for 1 microsecond, and is shut off for 99 microseconds, then is run for 1 microsecond again, and so on. The transmitter runs for one out of 100 microseconds, or 1/100 of the time, and its duty cycle is therefore 1/100, or 1 percent. The duty cycle is used to calculate both the peak power and average power of a radar system.

Dwell Time and Hits per ScanMost processes in pulsed radar are time dependent. Thus, some terms have been established to describe this time-dependence.

Figure 1: the target on the screen is a result of hitsDwell TimeThe time that an antenna beam spends on a target is called dwell timeTD. The dwell time of a 2Dsearch radar depends predominantly on the antennas horizontally beam widthAZand the turnspeednof the antenna (rotations per minute).The dwell time can be calculated using the following equation:TD=AZ 60; in [seconds](1)

360 n

Hits per ScanThe value of hits per scanmsays how many echo signalsper single targetduring every antenna swing are received. The hit number stands e.g. for a search radar with a rotating antenna for the number of the received echo pulses of a single target per antenna turn. The dwell timeTDand thepulse repetition timePRTdetermine the value of hits per scan.m =TD=AZ 60(2)

PRT360 n PRT

So that radar equipment can evaluate the target informations with sufficient precision, hit numbers are between 1 and 20 as necessary, which depends on the radar set operating mode.

Time-dependences in RadarRadar parameters such as antenna rotations per minute,dwell time,maximum unambiguous range, pulse repetition frequency (PRF), maximum number ofhits per targetare strongly interdependent. Finally also all other radar characteristics such as range and azimuth resolution, blind speed etc could be derived from this basic timing considerations. A classic radar (i.e. radar, not usingmonopulse technology) operating as an ATC-Radar needs a data renewal time of less than 5seconds. This requirement limits the receiving time and the maximum unambiguous range as following:

antenna revolution timedwell timehits per scanpulse periodmaximum possiblereceiving time

Figure 1: Time-dependences in RadarSince the radar processing in this surveillance radar is still in real time (with relatively low, but constant delay), the data renewal time depends on the antenna revolution time. To direct in the same azimuth angle after 5seconds, so that the radar can measure the co-ordinates again, the antenna must turn with 12revolutions per minute at least.The dwell time, the time that an antenna beam spends on a target, depends predominantly on the antennas horizontallybeam widthand the turn speed of the antenna. If we assume, that a well designed parabolic antenna got a beam width of 1.6degrees, the full circle of 360 degrees is divided by360/1.6 = 225different directions. 5seconds divided by the number of 225 gives a dwell time of5 s / 225 = 22.22milliseconds.These radar sets need a given number of hits per scan. This is necessary, to integrate the signals (seepulse integration) of different pulse periods for a better distinction of wanted signals from unwanted noise, as well as to measure a correct angular direction. Assumed a necessary number of 20 hits per scan, the maximum pulse period take a time of 1millisecond therefore. Assuming a receiving time less than 1 millisecond, the maximum unambiguous range of the ATC-radar is smaller than 150kilometers. If the radar uses astaggeredpulse repetition frequency to avoid blind speeds in radarsignal processing, then the smallest period gives the base to the range calculation. So we must calculate with a period of about 0.8ms instead of 1ms. The maximum unambiguous range of this given ATC-radar is 120kilometers or 65nautical miles therefore.So we can see, that the time scheduling of radar is very important. Most of parameters are fixed and the maximum range of given radar set is time dependent predeterminated. Additional measuring of an elevation angle is not possible often. To promise a higher range, demands fundamental changes in the radar signal processing as like asmonopulse technologyand/ordigital beamforming. Even small changes in the needed number of hits per scan (as a possible alternative to increase the receiving time to achieve a better unambiguous range) have negative influence on the radarsprobability of detection.

Functional Block Diagram of Secondary Radar

TransponderReceiverDecoderTransmitterCoderInterrogatorCoderTransmitterDecoderReceiverPPI-screenSynchronizer ofPrimary RadarInterrogation Path(Uplink)Replay Path(Downlink)Figure 1: Block diagram of a secondary radarIntheinterrogator on the ground:The secondary radar set needs a synchronous impulse of the (analogous) primary radar set to the synchronization of the indication. The chosen mode is encoded in theCoder. (By the different modes different questions can be defined to the airplane.) Thetransmittermodulates these impulses with the RF frequency. Because another frequency than on the replay path is used on the interrogation path, an expensive duplexer can be renounced. Theantennais usually mounted on the antenna of the primary radar set and turns synchronously to the deflection on the monitor therefore.In the aircrafts transponder:Areceiving antennaand atransponderare in the airplane. Thereceiveramplifies and demodulates the interrogation impulses. Thedecoderdecodes the question according to the desired information and induces the coder to prepare the suitable answer. Thecoderencodes the answer. Thetransmitteramplifies the replay impulses and modulates these with the RF reply-frequency.Again in the interrogator on the ground: The receiver amplifies and demodulates the replay impulses.Jamming or interfering signalsare filtered out as well as possible at this. From the informations Mode and Code thedecoderdecodes the answer. The monitor of the primary radar represents the additional interrogator information. Perhaps additional numbers must be shown on an extra display.

Uplink-FormatsThe SSR interrogation format (sometimes called uplink format) inMark Xstandard is very simple, consisting of two pulses (P1 and P3) of 0.8s width which are separated by a certain time - this determines themodeof interrogation. The table shows the time spacing of the different military and civil modes and indicates their use.ModeDistance betweenP1 - P3modedescription

militarycivil

13 (0.2) sMilitary IdentificationMilitary mode1 is used to support 32 military identification codes (although 4096 mode1 codes could also be used). Normally, the 32 codes could be used to indicate role / mission / type. However, this modeitself is not in common use in a normal peacetime environment.

25 (0.2) sMilitary IdentificationMilitary mode2 provides 4096 ID codes for military use (as for modeA). Normally used to identify an individual aircraft airframe.

3A8 (0.2) sCivil / Military IdentificationProvides 4096 ID codes for civil / military use. The commonly used mode

B17 (0.2) snot used

C21 (0.2) sCivil, PressureAltitudeExtractionmodeC is used to extract the pressure altitude modeC value (or true altitude if below the transition altitude).

D25 (0.2) snot (never) used

Table 1: uplink-formats in Mark X StandardMilitary mode1 is usually used to indicate role, mission or type of aircraft (hence several aircraft may give the same mode1 reply value). Mode2 is usually used to indicate an individual aircraft airframe (which is a number set in the aircraft, usually before it takes off).Military mode3 and civil modeA are the same interrogation mode (hence often referred to as3/A). It requests an identity used for air traffic control purposes. Since this identity is only 12bits (constrained by the down link reply format - see later), there are only 4096possible values. Values of mode3/A codes to use in various regions are allocated by air traffic control authorities. The identity code value is set (as 4 octal digits) by the pilot, as directed by air traffic control instructions. The value may sometimes be changed during flight.The other essential information required by air traffic control is obtained from the modeC interrogation, requesting the aircraft flight level. This is derived from the aircraft pressure altimeter or theradar altimeter.Civil modeB and D, although originally defined, have never been used for civil ATC purposes. Hence, the present civil SSR system is usually referred to as SSR modeA/C. Not all aircraft transponders are able to reply to all modes of interrogation. Military aircraft transponders will reply to modes1,2,3/A and many also have modeC capability. Civil transponders will not recognise Modes 1 and 2, but must recognise mode3/A. Most will also have modeC capability.The ground interrogator will change the interrogation modes made in a regular way this is usually referred to as the interlace pattern. Usually civil SSR interrogators alternate modeA and C each interrogation i.e. an AC interlace. Military interrogators may include mode1 or 2 e.g. a 1AC2AC interlace. (Some military interrogators may interchange mode1 and2 each scan.)Some military IFF systems (IFFMkXII) also include mode4. However, mode4 uses very different formats. In particular, the uplink format consists of a sequence of many pulses that contain encryption data so that only aircraft carrying the correct decipher key can be recognised.The P2 pulse, shown darkgreen colored in the pictures of the mode-table, is used for side lobe suppression purposes, as will be described later.

The Reply MessageTheSSR down link formatconsists of a number of pulses, nominally 0.45s (0.1s). F1and F2are always present and separated by 20.3s (0.1s) they are often referred to as a bracket or framing pair. Other pulse positions within this framing pair are spaced by 1.45s and are used to convey the required reply information in answer to the specific interrogation (e.gModeA identity or Mode C flight level values). The pulses are identified to give the bits of an octal code (ABCD). The X pulse at the centre of the reply is not used. The three blank positions may not be occupied by pulses, otherwise some decoders may reject the entire answer as interference.Note that the reply information itself does not contain any information to indicate which mode it is a reply to. The interrogator will assume that the replies received are in answer to it latest mode of interrogation.

Figure 1: SSR down link formatIn the case of Mode A, theoctal code (ABCD)is set by a control panel in the cockpit. In the case of modeC, the flight level is encoded in a special way (by a special form of Gray code known as Gillham code - this has the characteristic of only one bit changing for each change in flight level).TheSPI(SpecialPurposeIdentification) pulse is used by air traffic controllers to confirm the identity of certain aircraft. The controller will ask the pilot to squawk ident the pilot pressing a button on the control panel which adds the SPI pulse to SSR replies for a certain period (181s). The display system will then highlight aircraft with SPI. (The SPI pulse may have been appropriate to distinguish aircraft on older display systems before fully plot extracted displays became available). The out of frame position of the SPI pulse is somewhat strange and, as will be seen later, the SPI pulse position chosen introduces rather unfortunate complications for automatic decoding purposes. According to ICAO the SPI-pulse will be added to Mode A reply only.By international standards it is possible to assign defined questions and answers to certain standard situations:CodeModusMeaning

77003/A, BGeneralairemergency

76003/A, BLoss of radio

4frame1,2,3/A,BMilitaryemergencycall

75003/AHijacking

Table 1: examples of different CodesEach answer receives its meaning only in connection with the respective question. For example: 7700 in Mode 3/A: general air emergency 7700 in Mode C: 20,000 ft height

Side Lobe SuppressionIn secondary surveillance radar technology thesidelobes of the antennae affect particularly unfavourably. Transponders also can be interrogated over the side lobes and then answer about these, too and a response telegram can be received also over the side lobes. This circumstance results from thefundamentally bettertransmitting- and reception conditions for secondary radar units.Such answers cannot be assigned obviously on the radar screen. They rather appear as several targets in the same range but in different directions. In the extreme case an airplane can be interrogated permanently during a turn of the antenna. Such an reply then appears on the PPI-scope as a ring around.There are two principles ofSideLobeSuppression (SLS) Interrogation PathSideLobeSuppression(ISLS), and Reply PathSideLobeSuppression(RSLS)The techniques for ISLS are very similar to those for RSLS.A supplementary so called ImprovedInterrogation PathSideLobeSuppression(IISLS)method uses the techniques of ISLS to reduce the influence of false replays caused by reflection.

Fruit

Figure 1: SSR-video with a disabled defruiterAll secondary surveillance radar transmissions are worldwide on the two frequencies only: 1030 MHz (the uplink frequency) and 1090 MHz (the downlink frequency)This is necessary since an aircraft passes through several radar control areas on itsflight path.One describes all asynchronous interferences as a Fruit, which arises from replies, these were triggered not by the own interrogator.The term Fruit is an acronym. There are a few definitions for the acronym Fruit in common usage today, e.g:FalseRepliesUn-synchronisedInTime orFalseRepliesUnsynchronised toInterrogatorTransmission.Fruit should only happen when at least one of the targets involved is in the main beam of at least two interrogators. Similar tosynchronous garbling, Fruit is in fact asynchronous interference where replies overlapping in time at the receiver antenna may be lost. The key difference is that one or more of these replies is not expected and is intended for another user of the frequency.As the population and activity of transponders (and ground stations) increases in response to increases in traffic level, the levels of Fruit increase, causing increased loss of replies and false targets.DefruiterIf the SSR- antenna sweeps once over the targets position, the aircrafts transponder has to transmit about 10 till 30 replies. These replies are stored in the defruiter and compared with each other in the next PRT now.

delay linesubtraction circuitvideo and FRUITdelayed videovideo without FRUITone pulse periodFigure 2: Principle of a Defruiter (Criterion: needed 2 replies from 2 PRTs)Depending on size of the Fruit such a comparison can be carried out for two or more replies (PRTs). Problems appear at this method by different circumstances, though: The more Fruit, one should adjust the Defruiters criterion the more sharply, the more Fruit, however, the probability of a reply is the smaller, the smaller the probability of a reply, one should adjust the Defruiters criterion the more weakly.Only a compromise can consequently represent the solution of the problem, by choosing a criterion which as many as possible suppresses Fruit, but still has much use information passed sufficiently.

Garbling/DegarblingGarbling is a fundamental problem in the design of the classical SSR system and the situation is made worse byincreased traffic. Aircraft are often closely spaced in range and azimuth but at different heights. Replies from two aircraft will overlap if their range separation is within the equivalent of the 20.3s reply length. This is approximately 1.7Nm. The most serious garbling situations occur when the azimuth separation is very small such that replies from both aircraft are received from all interrogations across the beam. With advanced reply processing techniques and algorithms, it may sometimes be possible to extract all some or all of the replies from the received signal.

Figure 1: synchronous garblingAt this, in principle, one distinguishes two manners of the overlapping: Non-synchronous Garbling; Synchronous GarblingTwo replies overlaps in time such that its time grids are not congruent, so one speaks aboutNon-synchronous Garbling. Such answers can separated and one by one be decoded correctly!But if two or more replies overlaps in time such that its time grids are congruent, so one speaks aboutSynchronous Garbling.It cannot to state in the decoding any more, whether this a single impulse belongs to one or the other ones response telegrams. Through this it would come to the decoding of completely new and wrong replies and difficult from the original replies. These replies must therefore be disabled!F1C2F2SPI

Figure 2: C2-SPI phantom bracketWirings which reduce the effects of the Garbling are called Degarbling Wirings. Bracket detection is usually implemented by a digital delay line in which the presence of the F1-F2 bracket pair is detected by tapping points 20.3s apart with some additional tolerance. Unfortunately the position of the SPI-pulse isspaced20.3s after the C2pulse and if both pulses are present in a reply thenC2-SPI phantombracket will occur:But in this case this reply may be decoded and displayed! The airplane with which you have a radio link is of special interest. It would be a pity, if it disappears of the screen as long as the operator talks with the pilot.Replies which are Closly Spaced represent a further special case shown in the bottom one example in the following table.GarblingPulsesto display

non-synchronuousGarbling

synchronuousGarbling

C2-SPI phantombracket

Closly spaced

Table 1: kinds of garbling

Degarble WiringTo recognize and be able to process the described garbling cases, special degarble wirings are used.

delay line DL 1: 20,3 sDL 2: 20,3 sDL 3: 20,3 svideoto the decoderH=enableC2-SPI phantom bracketH=bracket detectL=garblingFigure 3: Principle of a degarble wiringThe needed delay lines with the taps of1,45s(pulse grid!) can be created also as a digital shift register.The complete process also can be carried out by a processor controlled wiring.Sequences of operation during a correct replay1. Bracket detect2. delay of the recognized brackets3. check whether there brackets overlaps in time4. if no overlapping, then the decoder is enabled.Animation

Sequence of operation during a garbling reconnaissance4. overlapping detect, then the decoder is not enabled.Animation

Sequences of operation during C2-SPI Phantoms4. the phantom-bracket from the pulse pair of C2and SPI is ignored!5. the decoder is enabled in this case.Animation

Mode S Uplink FormatsA conventional SSR interrogator may have a typical sequence of ModeA interrogation, followed by ModeC interrogation or other modes. This would be repeated continually at ahigh rateto ensure that a position/identity plot can be produced for all targets in line-of-sight range of the interrogator during each antenna revolution.The Mode S ground station produces a larger variety of interrogation types. These types can be roughly classified into two types: All-call interrogations Roll-call interrogationsAll-call interrogations obtain replays from all aircraft in the beam dwell, although, under certain circumstances, ModeS aircraft can be locked out to all interrogations so that they do not reply.Roll-call interrogations are selectively addressed to acquired ModeS equipped aircraft using the unique24-bitaddress assigned to each aircraft. Only the addressed aircraft produce replies.The first problem for the ModeS system is to find the addresses of aircraft that are in radar cover so that selective addressed interactions can be made with them. This is achieved by ModeS all-call interrogator witch are made periodically from the radar.pulse width of P4Mode S inter mode

0.8 sno replayWhen the short P4is used by ModeS radar, an aircraft fitted with a ModeS transponder will be detected solely by the ModeS formats - i.e. aircraft will be first detected by the ModeS formats all-call and subsequently followed by ModeS selective address interrogations.

1.6 sall-call replyAlthough the long p4may be used to obtain ModeS all-call replies, its use is expected to be rather limited, in particular because the interrogator identity (ii) code and associated all-call lock features of ModeS are not relevant for this interrogation.

no P4Mode A replyThis is the downwards compatible modus providing the ModeS transponder to reply on early interrogators.

no P4but full P2Mode S replyThe interrogator sends a P2with full amplitude as for the P1pulse. This activates a ModeS transponder to then look for a following P6pulse containing the ModeS specific information. An older ModeA/C transponder seems this as anISLS- condition and don't reply.

Table 1: Mode SUplinkFormats

Mode S Individual Interrogation

Figure 1: Mode S - short interrogation (56 bits - 16.25 s)The ModeS up link interrogation format starts with two pulses, P1and P2, which are solely for the purpose of suppressing existing ModeA/C only transponders so that they are not aware of the main ModeS information. The ModeS interrogation data contained in the P6data block is phase modulated. The first phase reversal is the timing point for the subsequent bits (chips) of information. The ModeS interrogation may be of short (56 bits) or long (112 bits) format.

Figure 2: Mode S - long interrogation (112 bits - 30.25 s)The ModeS side lobe suppression pulse P5is transmitted from the control beam like the P2ISLSin the ModeA/C system. If P5is of more power than P6it has the effect of overwhelming the sync phase reversal of P6so that the ModeS transponder cannot read the subsequent information.

Mode S - Differential Phase shift Keying (DPSK)

Figure: Mode S - differential phaseshift keying(DPSK)Mode S Uplink interrogations use into the P6pulseDifferentialPhaseShiftKeying (DPSK) to modulate the data in the uplink format. It is a type of phase modulation that conveys data by changing the phase of the carrier wave. All subsequent information in the P6pulse is coded as 180phase reversals of the carrier frequency. DPSK is a kind of phase shift keying which avoids the need for a coherent reference signal at the receiver. Each reversal must have a duration of 0.08s. Each received phase section has a duration of 0.25s and is known as a chip The DPSK decoder compares the phase between two consecutive chips and verify what the data must have been.InICAOAnnex 10 Volume 4 is the interrogation data format described as follows:The interrogation data block shall consist of the sequence of 56or 112data chips positioned after the data phase reversals within P6. A 180-degree carrier phase reversal preceding a chip shall characterize that chip as a binary ONE. The absence of a preceding phase reversal shall denote a binary ZERO.After the sync phase reversal all subsequent phase reversals indicate the 56or 112bit P6information. All subsequent timing is taken from the point of the first phase reversal. The series of chips starts 0.5s after the sync reversal. At the end of P6pulse there is a guard interval of 0.5s to ensure that distinct transmissions do not interfere with one another.

Figure 2: Block diagram of DPSK receiverWhether the interrogation is short or a long pulse, the total duration of the P6pulse is either 16.25s (56data chips) or 30.25s (112data chips). The P6begins with an initial phase reversal at the start of the P6pulse with a length of 1.25s. This is known as the sync phase reversal. To supress antenna sidelobes the pulse P5is transmitted by an omnidirectional antenna. This pulse overlays the sync phase reversal and the transponder cannot decode the interrogation.Figure 2 shows an evident option method of demodulation. At this DPSK decoder, the original sequence is recovered from the demodulated differentially encoded signal through a complementary process. The whole received signal is delayed for exact 0.25 microseconds. The origin and the delayed part will be compared. If the signals are in phase to each other, there is a lower output than if the phases (and the maximum amplitudes) have a contrary magnitude. From this output signal, the original serial bit pattern can be restored, which is indicated only by a low pass filter with the following threshold device.

Figure 3: Decoder wave analysis

Mode S Reply Encoding

8 s56 s (or 112 s) = 56 or 112 bitpreambledata blockclockFigure 1: Mode S - pulse position modulation (PPM)Mode S replies consists of a certain number of pulses at a 1s spacing. (The bit update rate allows a 1s per data bittransfer ratethat can be translated to a one megabit per second data rate.) The ModeS reply consists of two distinct parts:1. a preamble and2. a data block.Pulse position modulation is a form of signal modulation in which thedata informationis encoded in the time delay between pulses in a sequence of signal pulses.

527 bits24 bitsformatnumbersurv. & comm.controladdress(parity)Figure 2: Content of the short messages data blockPreambleEvery Mode S reply starts with a preamble with a length of 8 microseconds. The pattern of the preamble consists of four pulses with a length of 0.5microseconds per pulse. The interspaces (to the first pulse) are 1; 3.5 and 4.5microseconds.Data blockThe data block consists of either 56 or 112 bits with a length of either 56 or 112 microseconds. The short data block format is divided in a format identifier of 5 bits, a surveillance and control word of 27 bits and the 24 bits for the individual airplane code including aparity information.Downlink formatmessage formatContent

DF0Fig. 2Short Air to AirACAS

DF4Bild 2Surveillance (roll call) Altitude

DF5Bild 2Surveillance (roll call) IDENT Reply

DF11Fig. 2Mode S Only All-Call Reply (Acq. Squitter if II=0)

DF16Fig. 3Long Air to AirACAS

DF17Fig. 31090 Extended Squitter

DF19Military Extended Squitter

DF20DF21Fig. 3Comm. B Altitude, IDENT Reply

DF22Military use only

DF24Fig. 4Comm. D Extended Length Message (ELM)

Table 1: Mode S Downlink format numbersThe longer downlink formats using 112 bit length of data block can exhibit an additional message field of 56bits, or an extended length message field of 80bits. All messages content the airplanes identification number including a parity information in co-operation with the surveillance and communication control word.The format number defines 25 coding formats. Each Mode S downlink format has a particular purpose. The formats DF0, DF4, DF5, DF11, DF16, DF20, DF21 and DF24 are used in civil aviation at present. The format DF0 provides informations forACAS. The DF17 format is used for theADS-Bsystem.Replies with the DF0 format are responses toACASorTCASinterrogations. Downlink format 16s are transmissions which are used byACASorTCASunits to communicate between aircraft. The responses for ground based interrogations have the DF4 format. DF11 and DF17 are squittered by Mode S transponders at a nominal rate of 1 Hz.

527 bits56 bits24 bitsformatnumbersurv. & comm.controlmessage fieldaddresss(parity)Figure 3: Content of the long messages data block (communication reply)

2680 bits24 bitsformatnumbercomm.controlmessage fieldaddresss(parity)Figure 4: Content of an extended length message data block (communication reply)The downlink format DF24is the one and only format number beginning with two High-bits and contains an extended length message data block. The decoder need to examinate these two bits only for reading this format number. The amount of bits can be shorten in the format number block therefore, as shown in Figure4.Publisher:Christian Wolff

Downlink BroadcastThe downlink broadcast frame is resent at regular intervals by the aircraft for a specific period time, at a nominal rate of 1Hz. It includes the transmission ofExtended Squitter- the unsolicited downlink broadcast of positional reports. The system depends on other aircraft systems, like a barometric encoder andGPSequipment for the position data.The preamble of the ModeS downlink allows a synchronisation to a clock for decoding the Data Block which is coded with the Pulse-Posistion-Modulation (PPM). The Data format of an ADS-B message is the format number DF17. The content of the datafield DF is here the decimal number 17, in binary code10001b.

PreambleData Block5324 bits56 bits24 bitsDFCAAAADS-B dataPIclockFigure 1: Data Block of an ADS-B messageThe three following bits (called Capability, or CA) ist the number of sub-type of the ADSB- message. The length 3 bit gives eight different kinds of reports. The next 24 bits are the individual Aircraft (ICAO) Address. The next Data field of 56 bits is the carrier of the ADS-B report, depending on the content of the CA field.This report can include: aircraft type andaircraft ID altitude, encoded latitude, encoded longitude (both coarse), and airborne velocity.The last 24 bits are the error detection code (Parity Information, PI).Figure 2: preview of a flyer for a real example of a virtual radarSBS-1The squitter information can be received and shown as a Real Time Radar display on your PCscreen of Mode-S/ADS-B equipped aircraft the airspace immediately around your home. Such a receiver is described atwww.javiation.co.uk. A lot of users of this ADS-B receiver are associated withwww.virtual-radar.de.The traffic around Zurich is represented in internet atradar.zhaw.ch, based on a diploma thesis from the suiss school of engineering in Zurich. The shown radar informations are originated on ADS-B squitter reports.

Transponder

Figure 1: Control paneel of an older transponderSecondary radar depends on a transponder (short-forTransmitter-responder) in the airplane to respond to interrogations from the ground station to make the aircraft visible and to report additional information like the aircraft's altitude.Figure 1 shows an older one transponder. The replays code for Mode1 and 3/A can be choosen with the black hand wheels. The yellow painted edges mark this device as reference unit of arepair shop.Newer one transponders operate with two antennae and two receivers in diversity mode. One antenna is mounted on top and the other one at the bottom of the airplanes fuselage. Additional informations are derived from the onboard avionics navigation systems.

Altitude Reporting(basically enablingMode C)Enables TCAS (whenselected, TAs andRAs are providedTransponderFailureIndicatorEnterMode ACodeEnterFlight IDSelf Test(press)Traffic(Auto orManual)TCASRangeSource ofAltitude (notselected Alt)Select(Enter)SPIClearFigure 2: Possible view of a main display of a Mode-s transponderThe Transponder maintains avionics data in 256 different 56 bit wide Binary Data Store (BDS) Registers that can be loaded with information and read-out by the ground system. Each register contains the data payload of a particular ModeS reply or extended squitter. These BDS registers are also referred to asGroundInitiatedCommB(GICB)registers. They are specified in the ICAO document Manual on Mode S Specific Services (Doc 9688). Registers not updated within a fixed period are cleared by the transponder. Registers are identified by a two digit hex number for example BDS05h (in some publications written as BDS0,5) is the position squitter. Commonly used registers are shown in Table1.RegisterContent

BDS01hData Link Capability Report

BDS02hAircraft Identity

BDS03hACAS Resolution Advisory

BDS04hSelected Vertical Intent parameters (Bit2840: Barometric Pressure Setting)

BDS05hExtended Squitter Airborne Position

BDS06hExtended Squitter Surface Position

BDS07hExtended Squitter Status(transmitted only in reply to interrogation)

BDS08hExtended Squitter A/C Id & Category

BDS09hExtended Squitter Airborne Velocity

BDS0AhExtended Squitter Event Report

BDS61hExtended Squitter Emergency/Priority Status(transmitted once per second during an emergency)

BDS65hAircraft Operational Status

Tabelle 1: Content of some binary data store registers

downconverterA/D-convertermonitorsingle-chip-processorpoweramplifierwaveform-generatorkeyboardexternal sensorslocaloscillatorFigure 3: Functional Block Diagram of a modern transponderPublisher:Christian Wolff