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.
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
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.
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.
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.)