RWR/ESM and Passive Geolocation

Brief:

It is undeniable that the impact of RWR-ESM on the design of modern aircraft is enormous. It has been the centre of many heated debates over the years, with many believing that modern RWR/ESM will make powerful radars and low radar cross section obsolete, as aircrafts will be instantly targeted and shot down the moment they turn on their radars. On the other hand, others believe that RWR/ESM are not much better than a warning system. The truth is far more complex than that, however.

To clear out some of the common misconceptions about RWR/ESM and passive geolocation, this article will explain how they work.

Radar warning receiver (RWR)

RWR are systems designed to detect radio wave emissions of radar systems. Their original purpose was to warn pilots when a radio signal, that might be a threat, is detected. However, depending on the complexity of technologies involved, while some RWR systems (such as Mig-29’s SPO-15) can only do simple tasks such as detecting the presence of energy in a specific radar band and giving general direction, modern RWR systems (such as ALR-94 , ASQ-239 , Spectra) are capable of classifying the source of the radar by the signal’s strength, phase and waveform type, and even capable of pinpointing the exact location of a ground threat. Complex radar warning receivers are also known as electronic support measures (ESM) systems.

In general, an RWR will analyse the following characteristics of the pulse that it detects:

1. Radio Frequency (RF measured in GHz)
2. Amplitude (power measured in kW)
3. Direction of Arrival (DOA) – also called Angle of Arrival (AOA)
4. Time of Arrival (TOA)
5. Pulse Repetition Frequency (PRF measured in pulses per second )
6. PRI type
7. Pulse Width (PW)
8. Scan type and rate
9. Lobe (beam width)

More sophisticated ESM systems can measure additional parameters, such as PRI modulation characteristics, inter and intra-pulse Frequency Modulation (FM), missile guidance characteristics (e.g., the pattern of pulse spacing within a pulse group), and Continuous Wave (CW) signals.

full-17362-77339-rcs1_mig_21

PASSIVE DIRECTION FINDING AND EMITTER LOCATION

Direction Finding techniques

Lacking the transmitting element of a radar, RWR and ESM systems have to use various different alternative methods to determine the direction of the signal they receive. Each technique has its own advantages and drawbacks that will be explained below:

Rotating Directional Antenna

Rotating Directional Antenna

A rotating directional antenna is used, which has a polar gain pattern that varies as a function of the angle from the antenna foresight (The antenna polar diagram). In its simplest form, it consists of a loop (with or without a monopole), an array of elements such as Yagi, or a reflective dish with a small horn. By rotating the antenna structure past a signal of interest, the DOA of the signal can be determined from the time history of the signal amplitude relative to the antenna orientation (or pointing angle). The orientation at which the greatest signal strength is received is the DOA. The shape of the sensor antenna characteristic is known, so two or more intercepts of the signal of interest within the main beam are sufficient to determine the orientation that would place the emitter at the antenna foresight. It is possible to achieve very good DOA measurement accuracy if antennas having narrow beams are used. But the physical size of the antenna required will limit this technique to higher radar frequencies, where it is possible to have high gain directional antennas that are not too large. Typical DF accuracies are of the order of 1/10th of the beam width, but the technique is not instantaneous and therefore is associated with a time to intercept which itself is a function of the antenna rotation rate.

Watson – Watt –

Watson - Watt -

This technique was developed by Sir Robert Watson-Watt, known for his radar technology developments. The basic principle makes use of three linearly aligned antennas. The centre antenna is the sense antenna and the outer two are spaced approximately a quarter wavelength apart. The sum and difference of the signals from the outer antennas are normalized by the sense antenna and they result in the creation of a cardioid (or heart-shaped) pattern with respect to the angle of signal intercept. By rotating the antenna, the signal of interest can be moved into the null of the beam pattern thus identify its DOA. In order to rotate the cardioid pattern over a range of azimuth angles without physically having to move the antenna, a number of symmetrical pairs of outer antennas are often incorporated, which can then be electronically switched in order to rotate the pattern null and thus measure the signal DOA. This technique is widely used in communication DF systems, offering DOA accuracies of the order of 2 to 3 degrees RMS.

Amplitude Comparison

Amplitude Comparison

This technique involves the measurement of the relative amplitudes of a signal intercepted by the gain patterns of antennas that are oriented at different angles with respect to the target. Two or more antennas may be used, and the DOA for the signal of interest is calculated from the ratio of the instantaneous amplitudes measured in each antenna beam. This technique is sometimes known as monopulse DF, since the DOA can in theory be calculated from a single pulse from a radar, thus making it suitable for the purpose of rapid self-protection threat warning. (Often a hexagonal or octagonal array is mounted around a mast.)

Frequency Difference of Arrival (FDOA) or Doppler Difference of Arrival

This technique is based on the change in Doppler frequency over time as an emitter moves along a path relative to the sensor. It can also be used where the sensor platform is moving and the signal of interest stationary. In its most basic form, a single platform observes the received frequency. If the emitter moves in a straight line at constant speed, then the rate of change of frequency with time will reach a maximum negative value at the point of closest approach. Moreover, the received frequency at this point is precisely the frequency transmitted by the emitter. In principle, from knowledge of the actual differential Doppler as a function of time, it is possible to deduce the path of the emitter. This technique requires a very accurate and stable reference frequency source and a relatively fast moving emitter or sensor in order to make accurate DoA & PF measurements. This technique can be applied more easily to airborne sensors which may move in a more controlled fashion along pre-determined flight paths relative to the target scenario of interest.However for stationary sensor and target the technique does not work, and for totally ground based scenarios the technique becomes relatively inaccurate

In the case of sensor and target being stationary a further FDOA variant technique can be used which relies on a frequency difference induced into the measurement between signals received in two antennas, one of which is rotating with respect to the other. As one antenna rotates with respect to another a varying Doppler frequency shift is induced between the pair of received signal paths due to the relative path length changing between the moving antenna and the target emitter.The circular motion of the moving antenna results in a sinusoidal Doppler shift relative to the frequency of the signal received in the static antenna. The DOA of the signal of interest is the angle of the moving antenna at which the Doppler shift transitions through zero from positive to negative.In practical systems, the mechanical problems associated with a rotated antenna are overcome by replacing the moving element by a circular array of antenna elements that are switched sequentially into the second receiver path, comparison of the signal phase at adjacent antennas enables the calculation of relative frequency difference. The FDOA technique is widely used for navigation purposes in radio DF systems on civilian ships and aircraft. It offers a typical accuracy of 2 or more degrees RMS.

Phase Interferometer 

Phase Interferometer

This is a modern DF technique commonly used in ESM system such as Spectra , Falcon Edge , ALR-94..etc , it utilizes the measurement of signal phase difference between signals received along a linear or circular array of antennas. It is another instantaneous DF measurement technique offering monopulse capability, as was the case in the amplitude comparison method. In the case of a linear array a number of antenna elements (typically 4 or 5) are precisely spaced but at non-uniform intervals along a linear axis to provide phase ambiguity resolution across a sector coverage of up to 90 degrees. Each antenna element requires a separate phase matched or equalised receiver path in order to compare instantaneous phase differences between signal paths across the full azimuth sector. DF is obtained by instantaneously comparing signal phases between all antennas in the array. The direction of arrival information is derived directly from the phase differences among the elements and their known geometrical layout. The basic underlying principle is that the phase differences depend on the different times at which a signal arrives at different antenna elements, and this is a function of the direction of arrival. Interferometers are used when high accuracy DF measurement is important and it is possible to achieve high accuracy of the order (on the order of 0.1 to 1º).Interferometer DF accuracy is determined by the widest baseline pair. Typical cavity-backed spirals, track to 6 electrical degrees, and associated receivers track to 9º, resulting in an rms total of 11º. At a typical 16 dB signal to noise ratio, therms phase noise is approximately 9 electrical degrees. For these errors and an emitter angle of 45º, a spacing of 25 wavelength  is required for 0.1º rms accuracy while a spacing of 2.5 wavelength  is needed for 1º accuracy. For high accuracy, interferometer spacings of many feet are required. In airborne applications, this usually involves mounting interferometer antennas in the aircraft’s wingtips.A disadvantage of this method is the large number of receiver channels required to achieve full 360º coverage

Short Baseline TDOA

Sometimes called Differential Time Of Arrival (DTOA) this technique requires extremely accurate time delay measurements related to the leading edge or rise-time of a pulsed signal. This family of systems uses the time difference between a signal arriving at one antenna relative to another antenna as its measure of path difference, whereas an interferometer achieves the same by measuring phase difference. In one single sensor implementation a central antenna element becomes the reference point for comparison of time-of-arrival (ToA) of the signal, with a circular array of antennas equidistant from the reference. Depending on whether the same signal is received earlier, at the same time as, or later than the reference antenna, the relative DOA of the signal can be calculated, the earliest arriving signal at any instant being associated with the direct path to the signal of interest. They use much longer baselines (15-20metres) than interferometers (1 metre) in order to achieve an accuracy of about 1 degree.  Short baseline TDOA technique is not practical on CW or Communications signals.

Emitters locating- passive ranging techniques:

Methods such as phase interferometer, short baseline TDOA can give very good angular information of emitters however unless target is within visual range, weapons delivery  often cannot be done without information about distance to target.Thus, many algorithms and techniques were invented to determine range to target from direction information.

Single ship  triangulation

single ship triangulation

Sensor platform is moving parallel to the signal of interest.Everytime platform moving  a certain distance, information about bearing relative to target are stored in computer memory. When the number of required line of bearing is reached, distance to target is measured using trigonometry function

Advantages:

  • Require single platform only

Disadvantages:

  • Not enough accuracy for BVR targeting
  • Does not work again forward target
  • Target have to stay stationary
  • Range measurement is not instantaneous

 

Multi-ship triangulation:

Radiotriangulation

Range measurement is accomplished by  determining the intersection of two or more lines of bearing (LOB) from the emitter to multiple assets that have detected it.

Advantages:

  • Quick measurement
  •  Effective again both air and ground emitters
  • Highly accurate ( enough for weapon deployment )

Disadvantages:

  • Require at least 2 platforms
  • Require asset to share information through datalink. Thus, they cannot stay silent and can be detected by enemy

Azimuth, elevation method:

Aimuth-elevation

Locate target by intersecting bearing line with vertical elevation line, distance to target is found through trigonometry function

Advantages:

  • Only require single aircraft
  • Instantaneous target geolocation
  • Highly accurate (enough for weapon deployment)

Disadvantages:

  • Does not work again airborne emitters
  • Accuracy reduce at low altitude
  • Accuracy reduce at extreme distance

Long baseline time difference of arrival:

Time different of arrival

The time difference of arrival (TDOA), or precision emitter location system (PELS) method measures the difference in time of arrival of a single pulse at three spatially remote platform.

Advantages:

  • Highly accurate , accuracy improve as distance increase
  • Effective again both ground and airborne emitters
  • Rapid ,near instantaneous target geolocation
  • Very effective again omi directional datalink

Disadvantages:

  • Require minimum of 3 platform located at a significant distance from each others
  • Require platfrom to share information with each others ( datalinks ) , thus they cannot stay passive
  • Require high performance proccessor
  • 3 platform need to receive and analyse exactly same pulse , thus TDOA is ineffective again  high gain radar ( thin beam width )

Kinematic ranging:

Kinematic ranging

kinematic ranging 2

Sensor aircraft performing zig zag maneuver ( minimum of two 90 degrees turn left and right )to create changing in bearing with airborne emitter   , the distance to target then being estimated overtime using a Kalman filter based tracking algorithm

Advantages:

  • Require single aircraft only
  • Work again both ground and airborne emitters
  • Can be accurate in specific condition

Disadvantages:

  • Slow target geolocation compares to others methods
  • Unless initial guesstimate of target heading and speed is very close to real value ( 0.5-1% errors) , the result isnot accurate enough for BVR targeting
  • Require sensor platform to perform maneuvers that will expose it’s beam aspect radar cross section which often has very high value compare to head on aspect
  • Require the emitters to move at constant speed, keeping constant heading and altitude otherwise estimation of range is not possible
  • Require emitters to  constantly illuminating while sensor platform perform maneuver

Conclusion :

RWR – ESM main advantages

  • Very long detection range independence of target radar cross section
  • Wide coverage, normally 360 degrees all around
  • Completely passive thus cannot be detected by adversary’s RWR or ESM

RWR – ESM  main disadvantages

  •  Require enemy to transmit for detection
  •  Passive ranging   is time consuming  and  require complex techniques that is often ineffective again airborne target.
  • Inaccurate compare to radar and infrared search and track.

while having many advantages over radar , RWR/ESM also have many drawbacks that prevent them from being the main sensor on fighters aircraft

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