HF-OTH Skywave Radar for Missile Detection

TITLE In this paper missile detection capabilities of an HF-OTH skywave will be analysed and assessed. Specifically we focus on a challenging scenarios that is the detection of ballistic missiles in their boost phase for early activation of defence systems. Missile Radar Cross Section (RCS) is calculated during target flight taking into account of frequency and viewing angle. Detection capabilities are assessed in terms of peak power estimation and discrimination in the range-Doppler domain. 1.0 INTRODUCTION A very wide area surveillance capability is becoming a crucial requirement in order to deal with national and international safety issues such as clandestine immigration, naval and air traffic control, illegal actions surveillance (e.g.: oil spill, building abusiveness, chemical pollution, etc). Moreover, a system with the aforementioned features can be particularly suitable for large-scale environmental monitoring and common military user requirements, such as target detection, recognition and identification. Two approaches are currently in use in order to provide a practical solution for these kind of requirements: 1) A satellite constellation or a fleet of airplanes placed in permanent flight with onboard radar sensors, 2) A very dense radar network distributed over the national territory. Even though both solutions are practically feasible, they imply some disadvantages. Firstly, a satellite constellation does not allow a continuous surveillance over the whole area, although it usually ensures a short revisit time. Secondly, an airplane fleet is generally a very costly solution that can provide only a limited coverage. Finally, a network of radar sensors ensures a continuous surveillance with the drawback of a poor coverage capability. As a matter of fact the national costal radar network cannot provide an effective surveillance further than 100-200 km from the coastline. A different approach to this problem, that is efficient against the above mentioned downsides, makes use of a HF skywave (ionospheric reflection) Over The Horizon (OTH) radar [1]. Distances well beyond the horizon can be reached by exploiting the effect of ionosphere reflection, since in the HF band the e.m. waves are gradually bended through the ionosphere [2-3]. The ionosphere acts like an electromagnetic mirror at these frequencies. The signal is reflected back by the Earth to the radar system that can perform the detection operations. This type of radar achieves the remarkable advantage of a very wide and time continuous coverage that ranges from 600 km up to 3000 km. The cost-effectiveness of such a system is outstanding if compared with the aforementioned configurations. Moreover, the advent of 2D-array HF skywave radar [4] has offered some new capabilities in terms of detection and tracking performance as well as low probability of intercept (LPI). In this case, different radar functionalities can be imaged: 1) Theatre surveillance of ship or slowly moving surface targets; 2) Air traffic monitoring in regional areas; 3) Detection and tracking of missiles launched from a relative large local area; 4) Other. The goal of this paper is to assess the missile detection capabilities of a 2D-array skywave. The analysis will be conducted on ballistic missiles launched from 2000-3000 km far from the radar site. RTO-MP-SET-125 16 1 UNCLASSIFIED/UNLIMITED HF-OTH SKYWAVE RADAR FOR MISSILE DETECTION UNCLASSIFIED/UNLIMITED Peak power and missile Range-Doppler echoes map are calculated during the target flight in the boost phase to see whether detection is possible. 2.0 2D-ARRAY HF-OTH SKYWAVE RADAR SYSTEM A 2D-array HF radar is a very complex system because it is characterized by a set of features which are very unusual if compared to ordinary microwave radars: 1. Transmission frequency must be selected upon the ionosphere propagation behaviour in the wide band [3-30 MHz]. 2. Long radar coverage is allowed up to 4000 km corresponding to a zero antenna elevation angle. In practical applications it is convenient to limit the maximum distance to about 3000 km to avoid low antenna elevation angle as well as layer ionosphere internal multipath. 3. When the ionosphere e.m. incidence angle is greater than a critical value, the transmitted signal is not reflected and no returns occur. This phenomenon produces a blind area for distances less than about 400-600 km. 4. Ionosphere channel behaviour depends on date, sun activity and spatial coordinate. Therefore, ionosphere propagation is very changeable from night and day. 5. In the HF band, radar performances are heavily affected by background noise, which is mainly due to external noise [5]. More precisely, the external noise is composed by atmospheric noise, cosmic noise and man-made noise. Internal noise caused by thermal effect is almost neglectable. 6. We must deal with heavy propagation losses due to the very long travelling distances as well as strong absorption losses mainly due to the D layer of the ionosphere. The whole loss contribution can be up to 100-150 dB. 7. The apparently simple propagation mechanism hides the complexity of the ionosphere structure. This implies a challenging target localization that could be achieved by a smart system calibration combined with a three dimensional reconstruction of the signal path through the ionosphere. 8. OTH radar system functionalities are strongly dependent on the ionosphere and on the environment noise level that means geographically dependent performances. Accordingly the radar siting represents one of the key choices. 9. The principle of operation for an HF OTH skywave radar shows a spatial resolution cell that is range dependent. 10. The antenna system requirements are particularly demandingly. It is remarkable that the radiating system should operate on a very wide frequency range (HF band) and a 2D-array requires an area of a few square km to be installed. 11. High values of peak power are necessary in such systems to deal with strong losses. This requirement makes the antenna siting more constrained in order to comply with the national laws on e.m. radiation limits. 12. HF radar cross section (RCS) of targets is regulated by different mechanism than in microwave regions. Targets lie in the Rayleigh and Mie region reporting a wide range of values. It is essential a simulative approach that can provide a predicted RCS variability as a function of the operating frequency and of the aspect angles that are unusual for ordinary radar systems. Therefore, it is evident that an HF radar must be an adaptive system, where transmitted waveforms, antenna beamwidth and gain, as well as signal processing, must be tuned according to the external environment. A 2D array skywave radar allows this level of flexibility jointly with the capability of controlling the beam pattern in elevation. A large number of single antennas in a planar configuration permits a narrow beam forming, and as a consequence a significant performance improvement. That is, higher signal to noise ratio, low probability of intercept, multi hop rays avoidance and ionosphere propagation stability. The main functionalities of the radar are reported in the diagram block of Fig. 1. A distributed architecture is considered with centralized processing and control. The core of the system is the Radar Management and Control (RMC) block whose task is to manage all the radar sub-systems functionalities. 16 2 RTO-MP-SET-125 UNCLASSIFIED/UNLIMITED HF-OTH SKYWAVE RADAR FOR MISSILE DETECTION UNCLASSIFIED/UNLIMITED RADAR CONSOLLE RADAR MANAGEMENT AND CONTROL (RMC) Frequency Selection in TX TX/RX PHASED ANTENNA ARRAY Signal Processing (Detection) Data Processing (Tracking) Synthetic Representation Range-Doppler Representation Na Na Na Control and synchronism signals Data Fig.1 – 2D array HF radar architecture The other main elements are: 1) Frequency selection block, that decides what frequencies must be transmitted according to the ionosphere behaviour, spectral occupancy and noise level. 2) Phased array antenna, whose elements must be suitably feed in amplitude and phase in order to form different beams and scan all the radar coverage area. 3) Transceivers are integrated into each single antenna of the array. Because of low operating frequency (HF band) fully digital architecture can be conjectured. Use of Direct Digital Synthesizer (DDS) for waveform generation and Digital Down Conversion (DDC) for digital received signal pre-processing make the fully digitization affordable. 4) The detector must take into account of the space-time variability of external scenarios and channel. Space Time Adaptive Processing (STAP) with reduced rank seems to be a promising technique for target detection. 5) Tracking is a complicated task and it requires dedicated resources of the system. The additional information provided by a 2D array might support an operationally useful altitude estimation capability. Moreover a specific micro-multipath technique could be used in order to improve target altitude measurement accuracy. 6) A number of visualizations is available for the user: range-Doppler and frequency-angle domain and synthetic representation that allows a deep insight on the point or area of interest. 3.0 RCS OF MISSILES In order to assess the detection performance of a 2D-array HF-OTH radar system, RCS of missile during flight time must be computed in accordance to transmitted frequencies, ray-paths and target trajectory. RTO-MP-SET-125 16 3 UNCLASSIFIED/UNLIMITED HF-OTH SKYWAVE RADAR FOR MISSILE DETECTION UNCLASSIFIED/UNLIMITED 3.1 Missile trajectory By referring to Fig.2 and denoting the missile polar coordinates as ( ) ( ) ( ) , B B r t t φ , the missile trajectory is governed by the following parametric equations: ( ) ( ) ( ) ( ) ( ) 2 2 2 2