The following is excerpted from an early report from Dalmo Victor, the pioneer developer of digital (reprogrammable) radar warning receivers. The report has received wide circulation because it very concisely describes a digital RWR to those who now understand the principles of radar described in the previous sections of this tutorial.
To completely identify an electromagnetic signal received at a remote point as originating from a particular radar system at a particular location, the following seven parameters must be measured:
1. Frequency
2. Pulsewidth
3. PRF patterns
4. Missile guidance
5. Scan pattern
6. Power density (transmitter power and beamwidth)
7. Angle-of-arrival.
These "fingerprints" identify and locate the system which generated them. Since the parameters of all radar systems are fairly well known, a digital radar warning receiver identifies a particular radar by storing its known fingerprints in digital form in a memory bank for comparison with incoming signals. It is the function of the signal processor, then, to receive unknown signals, digitize them into a word format and present them to the identification words stored in its memory for matching. When a match is found to exist, the processor display circuits present an alphanumeric visual warning of the location and threat status.
To use this stored data, the incoming signal must be processed to generate a computer word which contains the following identification data:
A receiver can be designed to determine each fingerprint to the accuracy to which they are known, but any operational self-protection receiver must be designed to minimize equipment size and complexity and maximize aircrew simplicity. The frequency measuring device can be simplified by identifying only the band in which the radar exists. Likewise, since radar can be classified as threat/no threat by PW -- threat radars have short pulsewidths -- the pulsewidth measurements can be simplified to Long/Short. Then, if an emitter of a precisely measured PRF is located within a frequency band and has an acceptable pulsewidth, it is assumed to be at the discrete frequency/pulsewidth which corresponds to that PRF. So term (1) becomes:
This simplification allows a great deal of reduction in hardware, but it causes some ambiguous identifications since radars in the same Freq/PW bands can have overlapping PRF.
Ambiguities can be resolved for missile fire control radars by correlating the missile guidance signal (the "uplink") with the fire control radar with which it is known to be used. Now the word becomes:
Note that if a SAM signal can be resolved, it is resolved prior to, or at the time that, it becomes a real threat. That is, when missile guidance is activated, the system is ready for firing or the missile is in flight and the warning receiver has resolved its ambiguity to a true alert.
Since threat (fire control) radars must have some sort of scan method for auto-tracking, transmitted scan patterns (or the absence of them) can sometimes be used to help resolve ambiguities.
Scan resolution can be particularly useful in identifying AAA radars. The word now becomes:
Many emitters use staggered or jittered pulse trains or a combination of the two to optimize their own capabilities. Thus, these PRF patterns can sometimes be used to resolve ambiguous identifications:
The term (5) above is the processed identity word that is applied to the digital memory for threat matching. This format should not be interpreted as the order in which the various fingerprints are actually processed. Instead, most of them are transmitted and processed simultaneously; but if they are not, each parameter can be added to the basic word (1) as they are applied to the transmitter.
It is important to note that (5) will not always unambiguously identify a threat since the radar windows imposed by nature and state-of-the-art cause many threats to have closely related fingerprint sets. When (5) results in an ambiguity, the RWR uses additional receivers to provide additional "resolving power" on the parametric data of (5).
To determine the location of the radar, azimuth and range must be added to (5). Azimuth can be obtained from DF antennae while approximate range can be determined from the received power level. Thus, (5) becomes:
where the first 5 terms are used for Identification and the last 2 terms for Location.
The characteristics of the weapons associated with radar systems are also well known. To enable the warning receiver to display its data in the most meaningful manner, the Power information can be weighted by known weapon lethality to obtain a range display which is a function of range and relative danger. (6) becomes:
where the first 5 terms again are used for Identification and the last 3 terms for Relative Location.
With term (7) the identification is complete and the appropriate symbol is displayed on the Azimuth Indicator. At the same time, the PRF is "stretched" and applied to the aircraft ICS as an audio tone or alert as directed by the aircrew.
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It should be obvious to the reader at this point that the many radars in an air defense network have similar parameters. From Section 3 of this tutorial, signal processing techniques used by RWR systems cause additional parameter overlaps. Thus, ambiguous identifications will always exist in RWRs (i.e, the computer has used its best logical analysis to eliminate most radar types, but several candidates still exist). These are displayed on the RWR screen as alternating symbols and the aircrew must make the final identification. Such identifications are called ambiguities.
Cockpitology is the study of the integration of the aircrew with the mission, ECM gear, and RWR so as not to interfere with his combat role but yet let him translate ambiguities into self-protect ECM or maneuvers. It differs from present day man-machine interface studies in that it concentrates on defining very simple rules for interpreting ambiguities based on data available to the pilot only after his mission is scheduled and while it is being flown. Effective utilization of an RWR requires a thorough understanding of the cockpitology of that system.
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ASW: anti-submarine warfare
BEAM RIDER: weapon or missile which, through on-board electronics follows an electromagnetic or light beam
CHAFF: small metallic dipoles that resonate at the radar frequency and cause large radar back scatter
CLUTTER: the presence of reflections (echoes) from objects in the area of the target
COMMAND GUIDANCE: a data link whose purpose is the transmission of information from one system to another system.
CONTINUOUS WAVE (CW): continuous flow of electromagnetic energy (nonpulsed)
DATA LINK: the system by which information is transferred between two locations. Generally refers to RF or optical transmitting and receiving equipment
DATA RATE: the rate at which data can be converted, transmitted, received and reconstructed
DETECTION: acquisition of an electromagnetic signal with the same output characteristics as the original transmitted data
DOPPLER (EFFECT): continuous wave (CW) doppler radar modules are sensors which measure the shift in frequency created when an object (being "illuminated" by energy waves) moves. A transmitter emits energy at a specific frequency which, when reflected, can indicate both speed and direction of that target.
For instance: DFT = 2TF (v/c) cos A, where DFT is the doppler frequency shift, TF is the transmitter (or head) frequency, v is the velocity of the target, c is the speed of light and A is the angle between the perpendicular axis of the transmitter and the direction of the target. When objects move closer to the doppler source, they increase in shift (positive value) and when they move further away, they decrease in shift (negative values). Hence the expressions "up doppler or down doppler" as used so familiarly to signify movements of a target.
DUTY CYCLE: the percentage of time a device or system is activerelative to continuous operation
FREQUENCY AGILITY: the rapid and continual shifting of a transmitter's mean frequency, generally to avoid jamming
FREQUENCY BAND: a continuous and specific range of frequencies
FREQUENCY BANDPASS: the number of Hertz where maximum output is obtained between two limits usually defined and bounded by lower and upper half power (3 dB) points
FREQUENCY HOPPING: an anti-jamming technique used by a radar system. The carrier frequency of the pulsed transmissions are periodically or continuously shifted within limits on each successive pulse.
FREQUENCY MODULATION (FM): the modulation of a sine wave carrier so that its instantaneous frequency differs from that carrier by the amount proportional to the modulating wave
FREQUENCY SHIFT KEYING (FSK): frequency modulation in which the modulating wave shifts the output frequency between predetermined values and the output wave has no phase discontinuity
FREQUENCY SPECTRUM: the entire range of frequencies of electromagnetic radiation
GAIN: any increase in power when energy is transferred from one point to another
IFF: Identification-Friend or Foe. A major problem in modern warfare is the inability to discriminate between friendly and non-friendly forces. IFF equipment provides appropriate responses upon interrogation.
JAMMER: a device used to deprive, limit or degrade the use of a communications system. Radio frequency jammers include barrage, noise, discrete frequency repeater and deceptive equipments.
LEADING EDGE TRACKER: a tracking radar which obtains its data from the leading edge of the echo pulse from the target
MISSILE LAUNCH DETECTION: Certain military aircraft are equipped with specialized receivers for the purpose of detecting actual missile launch conditions.
MISS DISTANCE: the distance measured between the closest paths of a target and intercepter (i.e., aircraft and missile). One objective of Defensive ECM equipment is to increase the miss distance to a safe distance if detection and launch cannot be prevented.
MODULATION: the variation of amplitude, frequency or phase of an electromagnetic wave by impressing another wave on it
NOISE: any unwanted electrical or mechanical disturbance which modifies the desired performance
NOISE FIGURE (N.F.): the ratio of the total noise at the output to the noise at the input of a device. Generally attributable to the thermal noise of the signal source.
PASSIVE: an inert component which may control, but does not create or amplify information for the purpose of jamming
PHASE COHERENT: the continuous wave which has no discontinuity
PHASE LOCKED LOOP (P.L.L.): circuit in which the local oscillator is synchronized in frequency and phase with the received signal
PSEUDO CW: method of pulse transmission which can be received and integrated in a CW receiver as a normal CW signal
PULSE CODING: a technique which includes a variety of methods to change the transmitted waveform and then decode upon reception
PULSE MODULATION: the modulation of a carrier by a series of pulses generally for the purpose of transmitting data
PULSE POSITION MODULATION: the conversion of analog information to variation of pulse positions versus time
PRF: pulse repetition frequency. It is the frequency at which a pulse of certain width and amplitude is repeated.
REFLECTIVITY: the measure of a surface or a device to reflect impinging energy
SCINTILLATION: random and usually small fluctuations of a received field about its mean value, also called target glint or wander
ACQUISITION MODE: that mode of operation wherein the TTR scans its narrow beam over an angular segment to initially acquire the target so that the track mode can be used
ACQUISITION RADAR: those radars in an air defense network used to locate a target within an angular segment, generally 5-10 degrees
AIRBORNE FIRE CONTROL (AI) RADARS: those radars in an air defense network carried onboard interceptor aircraft. Their small size and short range characterize them as high frequency, low power, relatively large beam (3-degree) systems.
AIPD - AIRBORNE INTERCEPTOR PULSE DOPPLER: an airborne fire control radar which uses pulse-doppler techniques
ANTI-AIRCRAFT ARTILLERY (AAA) RADARS: those radars in an air defense network used to direct gunfire
BOMBING/NAGIVATION RADAR: used to guide an aircraft to a specific point and compute an optimum weapon release cue to hit the point
CONTINUOUS WAVE ILLUMINATOR (CWI): that radar in an air defense network which is used to illuminate a target with high-power CW so that missiles may home on the reflected CW energy
CONTRAST TRACKER: a target tracking system which uses light as an illuminating signal and generally presents continuous visual display (i.e., TV)
EARLY WARNING RADARS (EW): those radars in an air defense network used to initially detect the presence of an aircraft or strike force. These are large beam (10-20 degree) long-range (200 or more nautical mile) systems.
ECHO: in radar, that portion of energy reflected from the target to the receiver
FIRE CONTROL RADAR: specialized radar systems used to locate and track airborne and ground targets to determine optimum weapons firing point and which control the firing and sometimes guidance of weapons
GROUND CONTROLLED INTERCEPT (GCI) RADARS: those radars in an air defense network used to vector interceptor aircraft to a target area
HEIGHT FINDER: a radar system for determining the range and altitude in space of a potential target
JITTER PRF: a radar pulse chain whose PRI varies randomly from pulse-to-pulse; however, a set of PRIs such that unambiguous range is maintained must be used.
LOBE ON RECEIVER ONLY: methods of obtaining angle data by scanning with the receive antenna only
MAXIMUM UNAMBIGUOUS RANGE: the range beyond which targets appear as second-time-around echoes
MISSILE GUIDANCE RADAR (MGR): the name given to all radars in an air defense network which transmit guidance signals for missiles, whether or not the MGR tracks the missile
MISSILE TRACKING RADAR (MTR): that radar in an air defense network used to track the missile so that comparison of missile location with TTR data will generate guidance error correction signals which are transmitted by the MTR
MTI (MOVING TARGET INDICATOR): a pulse radar which observes the unambiguous range condition while utilizing Doppler effects (not Doppler) for ambiguous frequency resolution
PASSIVE TRACKING: target tracking without illumination from the TTR transmitter. This can be performed optically or by tracking aircraft radiation (normal or jamming).
RADAR: an acronym for Radio Detection and Ranging. It is used to detect a distant target, determine and display its relative direction (azimuth) and determine and display its relative distance (range).
RADAR CROSS SECTION: the equivalent area intercepted by a radiated signal and, if scattered uniformly in all directions, produces an echo at the radar receiver equal to that of the target. Typical radar cross sections of aircraft vary from one to over 1000 square meters. Ships may exceed 105 square meters.
RANGE GATE TRACKER: radar system which tracks a target in range by measuring the elapsed time from the transmitted pulse to the echo return
RANGE RESOLUTION: the ability of radar to discriminate two targets closely located in range
RANGE TRACKING: pulse radar measure the time difference between radar pulse transmission and echo reception. The range gate is positioned at a range where the target is expected. The receiver is blanked off except during the period where the range gate is positioned. Range tracking may occur at the leading edge of the return pulse or between ON and OFF gages.
SCAN: the cyclic movement of the beam as a radar examines an angular segment or point
STAGGER: PRF - a-radar pulse chain composed of two or more pulse trains of identical PRF
TARGET TRACKING RADAR (TTR): that radar in an air defense network used to track the target, thereby obtaining azimuth, elevation and range information from the target and the rates of change of these coordinates so that the radar can keep itself boresighted with the moving target
TRACKING: the continuous monitoring of range, velocity and position of a target in space from a reference position
TWS - TRACK WHILE SCAN: radar system using computer techniques to track targets in range, velocity and position without interfering with the acquisition scan rate
TWSRO: Track-While-Scan Radar where the tracking data are obtained by scanning the receiving antenna only. The transmitting or illuminating signal is non-moving and is used to illuminate a fixed section of space.
ABSORPTION: dissipation of energy of electromagnetic waves, sound and light waves into other forms of energy as a result of interaction with matter. Absorption characteristics of specific materials are used as blankets, coatings or structural and surface materials for aircraft to reduce effective radar cross sections.
ADAPTIVE JAMMING: the adjustment of the jamming signal as a result of observation of its effects on the offensive system
BARRAGE JAMMING: an attempt to "outshout" the opposing communications equipment by providing continuous or high duty cycle power within the desired frequency band. Barrage jamming may consist of broadband noise, narrow band noise, discrete frequencies or repeater radiations.
BEACONING EFFECT: Once a jammer is activated, the presence of a penetrating aircraft or other equipment is known. Since the jamming transmitter power output is generally greater than the normal return echo of a radar system prior to burnthough, the intruder's presence may be known prior to the time of radar detection. This is referred to as "beaconing effect".
BURNTHROUGH: the range at which the reflected echo power from the surface of the target exceeds the jammer power measured at the receiver or in the presence of ECCM; the point at which, by adjustment, the target can be distinguished from the jam signal.
CHIRP: a repetitive and continuous change of carrier frequency of a pulse-modulated wave. Generally for the purpose of coding or pulse compression.
CHAFF: ribbon-like pieces of metallic materials which are dispensed by aircraft to mask or screen other aircraft or to cause a tracking radar to break lock. The foil materials are generally cut into small pieces for which the size is dependent upon the radar interrogation frequency.
DECEPTIVE JAMMING: deception jamming uses a repeater, VCO or frequency memory to provide a precise return which has been changed in time or frequency in order to interfere with normal missile or projectile intercept calculations
DECOY: a device used to improve aircraft surivability by delaying or denying acquisiton of the real target. Decoys may be equipped with passive or active devices to enhance decoy credibility as a target. As detection and tracking systems improve, decoy credibility will need to improve by providing nearly exact radar reflections of the aircraft. Optical devices used to enhance radar tracking will cause decoy producers to design for aircraft shape similarity. For infrared tracking systems, flares are used as the decoy.
DEFENSE SUPPRESSION: a term applied to weapons systems which are intended to eliminate enemy detection, acquisition, or tracking equipments
ELECTRONIC RECONNAISSANCE: specific reconnaissance directed toward the collection of electromagnetic radiations. Examples include:
COMINT - Communications intelligence
ELINT - Electronic Intelligence
OPINT - Optical Intelligence
RINT - Radiated Intelligence
SIGINT - Signal Intelligence
EXPENDABLE JAMMER: a nonrecoverable jammer. Early expendables were limited to chaff and flare deployments; however, various radiating jamming systems have also been developed.
EFFECTIVE RADIATED POWER (ERP): input power to antenna; time gain of antenna expressed in watts
ELECTRONIC WARFARE: a general term used to describe the use of communications systems in warfare. As such, EW includes the Electronic Order of Battle (EOB), reconnaissance, intentional interference, intrusion or intelligence collection.
ELECTRONIC COUNTERMEASURES (ECM): the intentional use of electronics equipment for the purpose of interference, or confusion in order to obtain a tactical advantage in support of a larger operation. The intention of ECM is to deny or degrade the enemy's use of his communications systems (including radar) in order to gain a time or position advantage. Two basic types exist: passive measures and active measures.
ELECTRONIC COUNTER-COUNTERMEASURES (ECCM): the design or redesign of equipments to make a communication system or equipment techniques less vulnerable to a known or projected ECM equipments
ELECTRONIC ORDER OF BATTLE (EOB): the establishment, in a tactical situation, of the systems (friendly and unfriendly) and determination of their probable use. This effort may include the use of reconnaissance, passive and active ECM systems.
FALSE TARGET: radiated bundle of electromagnetic energy which is displaced in time from the real target echo which creates a response in the receiver where no reflecting surface exists
FALSE TARGET GENERATOR: device for generating electromagnetic energy of the correct frequency of the receiver which is displaced in time from the reflected energy of the target
FREQUENCY MEMORY: a device useful in storing coherent RF energy in a circulating loop for delayed transmission
HOME-ON-JAM (HOJ): Certain missile guidance and fire control systems are equipped with this feature. Should the system or systems operator detect jamming or a jamming intent, the operational mode may be changed to track or home on the center of jammer-radiated power.
IFM (INSTANTANEOUS FREQUENCY MEASUREMENT): a specific receiver technique for single pulse frequency measurement
INTRUSION: (1) the entry of a non-friendly aircraft or system into friendly air space. (2) the intentional interference in a communication system whereby the intruder attempts to confuse, delay, or cause error by the selective introduction of additional data.
INVERSE CON SCAN: One method of confusing a radar operator or fire control radar system is to provide erroneous target bearings. This is accomplished by first sensing the radar antenna or antenna dipole scan rate and then modulating repeater amplifier gain so that the weapons system will fire at some bearing other than the true target bearing.
JAMMING: the intentional interference between two communications systems whereby the one system attempts to degrade or make the second system useless
JAM-TO-SIGNAL (J/S): the ratio of jammer power to the reflected power from the surface of the target as seen at the receiver
LOOK-THROUGH: During the process of jamming, the operator may interrupt his jamming efforts to determine the enemy's operation mode and to determine if his jamming efforts are effective. If not, he may select a new jamming mode.
MODE: In EW this generally refers to the particular operational characteristics of a device or system.
NOISE JAMMING: brute force jamming by selective jamming equipment. The intent is to induce sufficient noise power into receiving equipment so that the true information cannot be detected.
SET-ON-JAMMING: technique for measuring the threat frequency and adjusting a sine wave generator within the jammer to retransmit the threat frequency
SWEPT SPOT JAMMING: a jamming technique in which an oscillator is swept over a specific range of frequencies in the band of interest in order to be assured of exciting a receiver tuned to any frequency in that band
RADAR WARNING RECEIVER (RWR): device for monitoring the direction and type of potentially hostile systems relative to the observing platform
RANGE GATE JAMMING: a deceptive jamming technique used against pulse tracking radars. By varying the delay of the return signal, the range gate is pulled off the true position of the target.
RECONNAISSANCE: the collection of data especially related to opposing forces. Such information includes, but is not limited to, troop position and movement, weapons emplacement and quality, communications data, and communicating systems operational parameters.
REPEATER: a system which receives, memorizes, and re-transmits the emissions of energy radiation
REPEATER JAMMING: a system which modifies the re-transmission of potential hostile radars for the purpose of denying accurate positional data
STAND-OFF-JAMMING (SOJ): systems which provide jamming information but remain outside of the range of defensive weapons
TRACK BREAKER: jammer system which will cause a tracking radar to be removed from the true echo of the target
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