Passive Radar Applications for Detecting Stealth Aircraft

The Story of Radar

The fundamentals of Radar is invented by Hertz in 1886 who demonstrated the concept reflectivity of EM wave. In 1900, Tesla also explained the concept of speed calculation and detection an object using the EM wave. In 1903 and 1904, Hülsmeyer did an experiment for detecting ships when at low visibility using the principle of radio wave reflectivity. The idea from this Germany’s engineer was reappointed by Marconi in 1922. In same year, corporate research laboratory for United States of America, U.S. Naval Research Laboratory (NRL) demonstrated the radar for detecting ships and also accidentally, the radar could detect the aircraft. Then, the development of NRL’s radar was used as a patent which was named Continuous Wave Radar (CW)[3]. In 1930, there were so many countries developed the radar for military purpose, for instance, British, France, Germany, Russia, Italy and Japan. At that time, radar was still working at HF (high frequency) and VHF (very high frequency)[3].

The Classification of Radar

Based on the principle of work, radar can be divided as Active Radar and Passive Radar. In a nutshell, active radar emits a signal to detect the object, then the echo will be received back by the antenna receiver on the radar. While, passive radar doesn’t emit any signal to detect the object, but this radar exploits any signals that is emitted by the target and objects around the radar[4].

Stealth Aircraft

Nowadays, the fifth-generation of Jet Fighter has been widely used by countries as one of their ‘troops’ to protect and defend their country. This generation jet fighter must have all the stealth aspect, even when they bring weapons and low-probability-of-intercept radar (LPIR) based on Whitford, Design for Air Combat[5]. Radar Cross Section (RCS) is the ability of an object to reflect the radar signal back to the transmitter source. The smaller the RCR of an object, it will be harder detected by the radar. As a comparison, an insect have average RCS of 0.001 meter squared and a bird have average RCS of 0.01 meter squared, while the fifth-gen jet fighter such as F35 have RCS of 0.005 meter squared. It means that the fifth-gen jet fighter is very difficult to detect by the conventional active radar.

Facet-shaped on Lockheed F-117. Adopted from [6]
Heterogenic Magnetic Antenna (MA) on B-2 Spirit. Adopted form [9]

Application

Passive Radar has several advantages than Conventional Active Radar, therefore in military field, passive radar is more suitable for detecting stealth aircraft. First, passive radar doesn’t emit any signals to detect targets, passive radar usually use the principle of bistatic and multistatic radar, so that this radar can capture the scattering signal (burst) wider than monostatic radar. Second, passive radar is difficult to locate because it is not emit any signal and we can say that it is indestructible using the anti-radiation weapons, such as the AGM-88 missile which can detect and search the source of signal emission from the radar then destroy it. In addition, passive radar has a wider frequency band than active radar, started from VHF (50 MHz) to K-Band (18 GHz), it means that passive radar is more possible to detect stealth aircraft because in low frequency band the Radar Absorbing Material (RAM) is not working effectively[4].

  1. Passive Emitted Tracking (PET), the concept of this system is to identify the emissions of RF signal from target transmitter such as communication signal, IFF (identical friend or foe) or target navigation radio system, then tracking the target[10]. This system will work effectively when the target emit a signal. Furthermore, we can do signal analysis, such as knowing the frequency, bandwidth to the modulation from the target. By using multistatic and signal processing, then we will get an intersection line that will provide the information on the coordinates of the target. Unfortunately, this radar system cannot work effectively when the target is not emitting a signal at all.
  2. Passive Coherent Location (PCL). How about if the target do not emit any signal? In this case, we can use signal around us or third party signal, usually a signal from broadcast transmitter that operates on a low frequency band (HF and VHF) for instance, signal form FM radio, analog TV, Digital TV, GSM, Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB) and GPS satellite. The principle work of PCL as follows, a third party signal is captured by the receiver and an echo signal from the target as well captured by the receiver, then based on these two signal and by using bistatic range and cross-correlation information, the Time Difference of Arrival (TDOA) can be obtained, so that it can be obtained the direction of the target based on the ellipsoidal intersection [11]. The accuracy of the position of the target is greatly influenced by the third party signal’s bandwidth and beamwidth of the receiver antenna [11]. The PCL is very suitable for use in area that emits the third party signal, such as in urban areas and airports
The Principle of PCL TDOA. Adopted from [11]
The principle of MLAT TDOA. Adopted from [13]

References

[1] M. Skolnik, Radar handbook, Third edit., vol. 28, no. 01. McGraw-Hill Education, 1990.

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Muhammad Naufal Ar.

Muhammad Naufal Ar.

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Sophomore student of Telecommunication engineering in Bandung Institute of Technology.