Passive Radar Applications for Detecting Stealth Aircraft
Radar, the acronym for Radio Detection and Ranging System, is a system for detecting and tracking to an object by using the principle of electromagnetic wave. sketchily, an electromagnetic wave is emitted by antenna on the radar, when the EM wave hits an object, there will be an energy that is absorbed and also reflected into all direction, including back to the radar’s antenna[1]. The reflected energy that is received by radar’s antenna is called echo. The time between emitting and receiving the echo will give us the distance from a target to the radar. Basically, conductive material will produce better echo, such as metal as the main body of the aircraft, while non-conductive material, such as wood, will produce a weaker echo, relatively[2].
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].
“Seeing without being seen”
is a cool term or the other name of passive radar. They actually work like that.
Based on the receiver radar location, radar can be divided as Monostatic Radar and Bistatic or Multistatic Radar. Monostatic radar has the same transmitter and receiver, in other words the echo must be received by the transmitter radar. Bistatic and Multistatic Radar, as their name, they have more than one receiver which is located at several points, so that to process the signals, we need more complex algorithm, instead of monostatic 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.
There are several ways to increase the “stealth” rate of stealth aircraft[6]. First, the airframe of the aircraft. Usually, the airframe of the stealth aircraft has a flat surface and sharp corners (facet-shaped), these aim to scatter the radar signal in various directions, so that the echo will not be captured by the radar in the monostatic case. However, behind it’s ability that is difficult to detect by conventional radar, stealth aircraft has to sacrifice aerodynamic aspect due to it’s facet-shaped geometric profile[5]. That’s why, there must be proper optimization in order to create an aircraft that has good aerodynamics as well as a small RCS.
Second, by using the Active Cancelation. It works by making the cancellation signal at the same time when the radar signal hits the target to clear up the reflexive signal (echo). This cancellation signal has the same amplitude and frequency but has a different phase by 180 degree with the echo, resulting a destructive interference[7][8], therefore the receiver radar will not read any signal. There are so many methods for emitting the cancellation signal, one of them by using the heterogenic magnetic antenna (MA) on the upper side of the airframe[9].
Third, by using the Radar Absorbing Material (RAM). Radar Absorbing Material is a material coated on the surface of the airframe which aims to absorb energy from EM wave and converting it into heat energy[6]. Unfortunately, Radar Absorbing Material is not effective on a low frequency band, such as HF and VHF[4], other than that the maintenance cost of Radar Absorbing Material is quite bit expensive and increase the load of the whole aircraft.
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].
Passive radar has several systems or ways to detect targets, that are
- 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.
- 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
3. Multilateration (MLAT), has just developed when the synchronization technology emerged [12]. This system should have at least four receiver station for getting the target’s burst signal. Similar to PCL, MLAT also uses TDOA to predict the position of the target. Two burst signals is obtained from two receivers will be carry out process of cross-correlation, from this process can be obtained TDOA [10]. In contrast to PCL, MLAT utilizes hyperboloides intersection on each receiver to predict the target position, so that the target can be represented in a three dimensional plane.
If these 3 systems is integrated on a passive radar system, each system will cover drawbacks with each other. A passive radar like this one is being developed by ITB Balitbang Kemhan.
References
[1] M. Skolnik, Radar handbook, Third edit., vol. 28, no. 01. McGraw-Hill Education, 1990.
[2] P. Folger, “Chapter 1 — Basic Radar Principles and General Characteristics,” pp. 1–34, 2014.
[3] P. D. Mark A. Richards, “Introduction to Radar Systems and Signal Processing,” 2nd ed., New York: McGraw-Hill Education, 2014.
[4] D. Oikonomou, P. Nomikos, G. Limnaios, and K. C. Zikidis, “Passive Radars and their use in the Modern Battlefield,” J. Comput. Model., vol. 9, no. 2, pp. 1792–8850, 2019.
[5] F. Sunjaya and R. Oktovianus, “Optimasi Aerodinamika-Radar Cross Section ( RCS ) pada Sayap Cropped Delta dengan Metode Design of Experiments ( DOE ) dan Multi Objective Genetic Algorithm ( MOGA ) Aerodynamic — Radar Cross Section Optimization of Cropped Delta Wing with ANSYS Design of,” J. Inov. Pertahanan dan Keamanan, vol. 01, no. 1, pp. 38–48, 2018.
[6] K. Ramya, “Radar absorbing material (RAM),” Appl. Mech. Mater., vol. 390, pp. 450–453, 2013, doi: 10.4028/www.scientific.net/AMM.390.450.
[7] I. A. Osman, M. Osman, A. Abdelrasoul, and J. Alzebaidi, “Active Cancellation Algorithm for Radar Cross Section Reduction,” Int. J. Educ. Res., vol. 1, no. 1, pp. 19–24, 2013.
[8] B. Yang, W. Yang, W. Xiao, and G. Chao, “Active Cancellation Stealth Technology Analysis and Verification in Experiment,” Procedia Comput. Sci., vol. 147, pp. 109–115, 2019, doi: 10.1016/j.procs.2019.01.199.
[9] A. I. Semenikhin and A. I. Chernokolpakov, “Active cancellation of the RCS of a large aircraft using of stealth 4-port magnetic antenna with heterogenic ferrite core,” Proc. 2019 IEEE Conf. Russ. Young Res. Electr. Electron. Eng. ElConRus 2019, pp. 882–884, 2019, doi: 10.1109/EIConRus.2019.8656844.
[10] H. Nabila, J. Suryana, and A. Izzuddin, “A 3D multilateration using RF burst,” Proc. — 2020 6th Int. Conf. Wirel. Telemat. ICWT 2020, pp. 0–3, 2020, doi: 10.1109/ICWT50448.2020.9243630.
[11] H. Kuschel, D. Cristallini, and K. E. Olsen, “Tutorial: Passive radar tutorial,” IEEE Aerosp. Electron. Syst. Mag., vol. 34, no. 2, pp. 2–19, 2019, doi: 10.1109/MAES.2018.160146.
[12] RONNIE BEASON, “UNIVERSITY OF OKLAHOMA GRADUATE COLLEGE NOVEL PHENOMENA IN CONFINED ELECTRONIC SYSTEMS A DISSERTATION in partial fulfillment of the requirements for the Degree of DOCTOR OF PHILOSOPHY By BAHMAN ROOSTAEI,” 2007.
[13] Z. Xu, D. He, Y. Tang, and J. Li, “A MLAT algorithm based on target pressure altitude,” 2015 IEEE Int. Conf. Mechatronics Autom. ICMA 2015, pp. 1800–1804, 2015, doi: 10.1109/ICMA.2015.7237759.
