Conceptualizing IFF for Dismounted Soldier

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1 Conceptualizing IFF for Dismounted Soldier Meenakshi Durga, Sushil Kumar Singh, Pramendra Kumar Verma, Lalit Suthar and Ashok Kumar Defence Electronics Applications Laboratory, Dehradun Abstract The paper deals with the conceptual design of Identification of Friend-or-Foe (IFF) System for Dismounted Soldiers using radio-laser transceivers i.e. laser transmitter mounted on gun and laser detectors & UHF transceiver mounted on the helmet. The gun assembly initiates the query towards the potential target soldier using a low divergence laser beam signal. The helmet assembly on the target soldier detects the laser beam, generates and transmits response in UHF band. This response is received and decoded by the helmet assembly of the interrogating soldier and the gunner is informed whether the target is friendly or unknown via audio/led display. Keywords-IFF, Dismounted Soldier, Laser, UHF, Transceiver I. INTRODUCTION Potential target identification is a serious long-term problem. Identification of Friend or Foe (IFF) systems have become important, as speed and ferocity with which conflicts are being addressed in the current urban warfare scenario with similar human anatomy. Friendly killing known as fratricide is one of the most devastating consequences of a conflict. IFF systems are more important for dismounted soldier who may be moving covertly through an unknown combat zone at night with limited visibility [1]. The rate of fratricide has increased manifold, as positive visual identification is difficult under degraded natural and man-made conditions, with adversaries having same/similar/civil uniforms and having almost same physical structure. Equipment for target identification is thus an essential element of soldier s kit like any other safety kit. The proposed system will provide real-time identification information directly to the gunner s sight through a cooperative question-and-answer approach. Laser based Interrogation system will be mounted on gun and all helmets of the forces will have embedded laser detection system. Miniaturized radio frequency transmitter and receiver will be mounted on the helmet. The interrogation process is proposed to be completed within a maximum time of one second and prime power will be supplied through battery. II. SYSTEM ARCHITECTURE AND CONFIGURATION IFF system for the dismounted soldier comprises of two main units Gun assembly and Helmet assembly. Figure 1 shows the block level architecture of IFF system. The gun assembly includes button/switch for initiating interrogation, laser transmitter for sending query to a target, bluetooth transceiver and antenna for communication with helmet, LEDs for friend/unknown indication and battery. The helmet assembly includes laser receiver, main control unit, UHF transceiver and antenna, bluetooth transceiver and antenna for communication with gun, and battery. Laser receiver consists of laser detector array and current to voltage converter. Laser detector array consists of GaAs APD arranged for omni-directional reception in azimuth and to cover 360 degrees field of view and the current to voltage converter converts the output current of APDs into the required voltage level which can be processed by control unit. The wavelength of laser transmitter proposed is1550 nm due to eye safety, and low attenuation due to the presence of atmospheric window [2]. The 1550nmwavelength is also a standard for Fiber Optics communication. Therefore laser transmitters and receivers are well developed and COTS available [2]. The disadvantages for this wavelength are reduced sensitivity and strict alignment requirements [3]. But this is not an issue as IFF systems for dismounted soldiers are for short range and the power density at 1550 nm band nearly 50 times that at 780 nm and is still safe for the human eye [4]. The modulation used for data transmission for laser interrogation is OOK. The factors considered for choice of optical detector are operating wavelength and each element size, as array of detectors will be mounted on helmet. GaAs based APDs are proposed for low noise characteristics. The UHF transmission and reception is done using single transceiver IC. The modulation scheme chosen for UHF transmission/reception is GFSK. The Antenna for UHF transceiver is PIFA for its low profile. The control unit selected is an 8-bit microcontroller based on 8051 core having UART, SPI and in-built ADC. The system proposed is compact and light in weight as it will be carried by the dismounted soldier. The proposed time for continuous operation is of the order of 12 hours in active mode

2 and 6 weeks in standby mode. The prime source of power is rechargeable battery. Proposed system specifications are shown in Table I. System Specification Range Identification Time IFF Indication Interrogation Response 700 m 1 second (max) Audio signal, LED Laser (narrow beam) UHF (omni) Laser Source Laser Receiver FOV UHF Transmit /Receive pattern Battery Operation Weight Query - LASER-OOK Response - UHF-GFSK Eye safe Laser (1550 nm) Beam divergence 4mrad 40x360 degree (10 detectors on helmet/belt with 40x40 degree FOV) Omni directional 12 hours (continuous) 6 Weeks (Standby ) ~300 gm. (Gun assembly + Helmet assembly) TABLE I. PROPOSED SPECIFICATION FOR IFF SYSTEM FOR DISMOUNTED SOLDIER III. CONCEPT OF OPERATION The IFF system is a query-response based system. Figure 2(a) depicts the concept of operation. To avoid any synchronization issues between the interrogation and response, there is only one main control unit which is mounted on helmet. By pressing the interrogation button on Gun, the gun assembly communicates with the control unit on helmet to get the query data packet via bluetooth. The data packet received by bluetooth is transmitted through a low divergence laser beam signal to the potential target soldier detected on the cross-wire of the gunner s sight. Simultaneously the control unit on helmet prepares itself for receiving response and switches on the UHF receiver. Transponder System mounted on helmet of a friendly target soldier detects the laser beam using laser detectors. The output of laser receiver goes to the control unit which decodes the query and sends acknowledgement with its identity to the interrogating soldier through UHF transmitter. Simultaneously the target soldier is informed that he is under friendly interrogation. UHF response transmitted by the target soldier is received by the UHF receiver mounted on helmet of gunner and the corresponding information goes to the control unit where it is processed and an audio signal is generated which tells the gunner that it is a friendly target. Simultaneously a signal is transmitted through bluetooth to gun for status indication (friend-or-foe) on LED. All other targets are identified/processed as unknown, thus facilitating the gunner s firing decision at the point of engagement. The query signal being transmitted by a narrow beam laser is secure and data level encryption can be done before the packet is transmitted via Laser/UHF. A typical data packet format has been shown in Figure 2(b). The payload bits can contain the information of the frequency at which the UHF response is to be transmitted. The chosen frequency will be randomly selected to minimize the effect of jamming at UHF signals. IV. LINK BUDGET ANALYSIS A. Link Budget Analysis for Laser Channel Typical Link parameters for a system are given in Table II. Power P T required to be transmitted from source for a propagation distance L, is given by P T = P R + L DIV + L ATM + LM (1) Where P R (dbm) = Minimum signal at the input of laser receiver L DIV (db) = Beam divergence Loss L O (db) = Optical Loss L P (db)= Pointing Loss L ATM (db) = Atmospheric Loss LM (db) = Link Margin 1) Receiver Sensitivity For APDs, thermal noise usually dominates in practice even though shot noise is multiplied. For thermal noise limit, and minimum power PR required at the input of detector is given by the set of following equations [5]. PR = Q/Rd (q*f A *Q*Δf + σ T / M) (2)

3 σ T = NEP*(Δf) 1/2 (3) Where F A is the excess noise factor and Δf is the bandwidth of the detector and Q is given by the following equation [5] BER = ½ erfc (Q 2 /2) 1/2 (4) For BER of 1xe- 9, Q=6. The receiver sensitivity for the parameters in Table II is calculated to be-64.5 dbm. 2) Beam Divergence Loss For a source with divergence θ, Power P T and diameter D T, the power received (for non-diffraction limited spread) at a distance L is given by [3] P r = D R* PT/(D T + θ*l) (5) Where D R is the diameter of receiver. The divergence loss L DIV in db is L DIV = 10*log (P t /P r ) (6) = 20* log ( (D T + θ*l)/d R ) (7) LDIV calculated for DR= 200 um, DT = 84 um, θ = 4 mrad and L =0.7 Km is 82.9 db. 3) Atmospheric Loss The transmitted power Pt (Watts) at a distance, L, from the transmitter is given by Beer Lambert s Law[3]: Pt = Pt * exp( -γ(λ)* L) in Watts (8) Where Pt is the output power at transmitter (Watt) and γ(λ) is total attenuation/extinction coefficient per meter. The attenuation coefficient is the sum of absorption and scattering coefficients from aerosols and molecular constituents of atmosphere, therefore γ(λ) is given by : where γ(λ) = α m (λ) + α a (λ) + β m (λ) + β a (λ) (9) α m (λ) = molecular absorption α a (λ) = aerosol absorption β m (λ) = molecular scattering β a (λ) = aerosol scattering Atmospheric Attenuation in db can be written as L ATM = 10 log exp (γ(λ)* L) (10) = 10 * log 10 e * (γ(λ)* L) (11) = (γ(λ)* L) (12) Since for FSO systems, wavelengths are so chosen that they coincide with the absorption windows such that attenuation is mainly dominated by scattering (Mie-scattering) from aerosols i.e. γ(λ) ~ β a (λ) which is given by [3] β a (λ) = (3.91 / V ) * (λ/550) -δ (13) Where V is Visibility in Km and λ is in nm. Value of δis given by using Kruse model [3] as below: δ = 1.6 for V > 50 Km 1.3 for 6 Km < V < 50 Km (14) 0.585*V 1/3 for V < 6 Km For light fog visibility V= 0.77m [3], β a (λ) = 2.91 and total atmospheric loss L ATM in db = 4.343*β a (λ)*l =8.857dB. Parameter Data Rate Operating Wavelength (λ) Range (L) Beam Divergence (θ) Transmitter Aperture Dia (Dt) Receiver Aperture Dia (Dr) APD Bandwidth(KHz) Value 5 Kbps OOK APD 0.9 Visibility (V) 1550 nm 0.7 Km 4 mrad 84 um 200 um 25MHz 0.77 Km Noise Equivalent Power (NEP) 0.01 pw/ Hz 1/2 BER Required 1e -9 Link Margin (LM) TABLE II. 4) Link Margin 10 db PARAMETERS OF A TYPICAL FSO LINKS There are several parameters which degrade receiver sensitivity such as finite extinction ratio, jitter, dispersion etc. which have been not taken into consideration for calculating receiver sensitivity [5] for ease of calculation. Attenuation due to absorption by molecules in rain/fog and turbulence are very small compared to the attenuation caused by scattering of aerosols. Also pointing and optical losses etc. have not been taken into consideration. An additional margin of 10 db is included to cater to such losses. Detailed calculation can be done using [6, 7]. 5) LASER Transmitted Power For P R = -64.5dBm, the total power P T required to be transmitted from source is calculated as below P T = = dbm which is 5.34 watts. B. Link Budget Analysis for UHF Channel Typical Link parameters for a UHF system are given in Table III. Power P T required to be transmitted from UHF transmitter for a propagation distance L, is given by

4 P T = P R + L P + L T +L R - G T G R + LM (15) Where P R (dbm) = Minimum signal at the input of Receiver L P (db) = Path Loss L T (db) = Feeder Losses in transmit chain L R (db)= Feeder Losses in receiver chain G T (db) = Transmit Antenna gain G R (db)= Receiver Antenna gain LM(dB) = Link Margin Parameter Data Rate Operating Frequency (f) Range (L) Transmitter Antenna Gain ( GT) Receiver Antenna Gain ( GR) Feeder Losses in Transmit Chain ( LT) Feeder Losses in Transmit Chain(LR) Receiver sensitivity (@ BER =1e-6) Link Margin (LM) TABLE III. Value 5 Kbps FSK 470 MHz 0.7 Km -13 dbi -13 dbi 1 db 1 db -106dBm 10 db PARAMETERS OF A UHF LINK The authors thank Director, DEAL for granting permission to publish this paper. Authors also thank Group Director Sh. K Siva Kumar for his valuable suggestions and guidance. REFERENCES [1] Gennadii Ivtsenkov, Alexandre Mantsvetov, Evgeny Berik, Combined IR-RF Combat Identification Friend-or-Foe (IFF) system for the Dismounted Soldier, U.S. Patent 2009/ A1, Feb. 19,2009. [2] Scott Bloom, Eric Korevaar, John Schuster, Understanding the performance of free-space optics, Journal of Optical Networking, vol. 2, No.6, June [3] Ghassemlooy Z. and Popoola W. O. (2010). Terrestrial Free-Space Optical Communications, Mobile and Wireless Communications Network Layer and Circuit Level Design, Salma Ait Fares and Fumiyuki Adachi (Ed.), ISBN: , InTech, Available from: [4] Akbulut, A., Efe, M., Ceylan, A.M., Ari, F., Telatar, Z., Ýlk, H.G. and Tugac, S., An Experimental Hybrid FSO/RF Communication System, Proc. IASTED International Conference on Communication Systems and Networks, , Benalmadena, [5] Govind P. Agrawal. (2002, Fiber Optic Communication System (3 rd edition) [On-line] Available: Fiber_Optic_Communication_Systems.html?id=yGQ4n1-r2eQC& redir_esc=y [June 19, 2012] [6] Srinivasan R, Dr. Sridharan D, The climate effects on line of sight (LOS) in FSO communication, IEEE International Conference on Computational Intelligence and Computing Research, [7] Al Naboulsi M., Sizun H, de Fornel F, Propagation of optical and infrared waves in the atmosphere, Proceedings XXVIIIth General Assembly, International Union of Radio and Science (URSI), October [8] Free Space Path Loss, Internet : Aug 29,2012 Where λ is the wavelength. Path loss L P in db can be written as L P = *log(fL) (16) For f = 470 MHz, L=0.7 Km, path loss calculated is 82.7 db. With link margin of 10 db, the total power P T required to be transmitted for the parameters given in Table 2 is given below P T = (-13) -(-13) + 10 = dbm V. CONCLLUSION The purpose of this paper is to propose a secure & automatic IFF system for dismounted soldiers having reduced weight and size. The system architecture and configuration for the proposed system has been elaborated. The concept of operation and its sequence has been explained. Selection and calculation of critical design parameters have been done for both Laser and UHF link and the transmitted power required in each case has been estimated. ACKNOWLEDGMENT

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