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 Classification of Radar

“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

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.

Facet-shaped on Lockheed F-117. Adopted from [6]

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].

Heterogenic Magnetic Antenna (MA) on B-2 Spirit. Adopted form [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.


Passive radar has several systems or ways to detect targets, that are

  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]

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.

The principle of MLAT TDOA. Adopted from [13]

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.


[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/

[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.


[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.



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Muhammad Naufal Ar.

Sophomore student of Telecommunication engineering in Bandung Institute of Technology.