Of new radar technologies and their future applications
Air Cmde Trilok Chand (Retd)
The last two decades saw the greatest strides in the development of Information and Communication Technologies (ICT) especially for commercial use. Radar and related technological development pace, on the other hand, remained slow mainly due to their highly specialsed applications and higher cost involved in hardware and infrastructure associated with these systems. Of late, situation has changed dramatically, as many ICT applications have found their way into radar systems.
Advancements in nano technology and semiconductors have also contributed to miniaturisation and diversified utilities of the radar. Most radar applications are of dual use in nature. Any radar designed for surveillance, whether, navigation and landing use can also be used for early warning, battlespace weather forecast, mission planning and recovery of military aircraft. Exclusive military applications such as tracking, fire control, identification of Friend and Foe (IFF) are also revolutionised equally by the newer radar technologies.
Basic radar range equation in its new avatars continues to inspire new design and innovations of both commercial and military radars. Important new radar technologies such as Multiple Inputs, Multiple Output (MIMO) systems, Digital Beam Forming (DBF) techniques, Active Electronically Steered Array (AESA) radar, millimeter wave radar, Passive Coherent Location Radar (PCLR) Systems, semiconductor Power Amplifiers (PA), Intelligent signal coding and radar Digital Signal Processing (DSP) have inspired many modern radar designs in recent times.
Multiple Inputs, Multiple Output Radar
MIMO radar technology has evolved from communication systems where it has been used for improving the coverage area and signal quality. MIMO radars simultaneously radiate uncorrelated signals, with orthogonal polarisation. This improves the coverage and the received signal quality. The decorrelation of each transmit signal is important for picking up small targets at long distances. A decorrelation of about 70 decibels can be possibly achieved through proper modulation. Newer generation of Synthetic Aperture Radar (SAR) system makes use of multiple elevation and azimuth receiver channels combined with digital beam forming (DBF) capability. This allows for the synthesis of multiple digital receiver beams for improved signal discernment and reduced noise figure.
Digital Beam Forming Technique
The Digital Beam Forming (DBF) is achieved by transmitting and receiving multiple independent weighted beams formed by an array of antenna elements. The received signals of each antenna element are down converted from analogue to digital form and stored in a memory. From the memory an arbitrary number of beams can be digitally processed simultaneously. The major advantage is that the large beam coverage can be simultaneously processed to form multiple beams. Thus, DBF could be used for achieving higher angular resolution and wide coverage without mechanical moving parts in modern radars.
Active Electronically Steered Array Radar
AESA radar technology employs new generation of Trans-Receive (TR) modules which are highly capable Software Defined Radios (SDR) that can also be used for radio communication with very high data rates. AESA radar is exceedingly being used for the upgrade and replacement of erstwhile radar technology. The AESA design uses modular concept and thus enhances reliability. A failure in the critical TR module will not make the whole radar unserviceable and the system could be restored by replacing the module in very little time.
Millimeter Wave Radar
Millimeter Wave Radar technology is another new development which has the ability to penetrate dust and fog and enables the radar to accurately detect and identify vehicles and road objects. For achieving high precision and better resolution capabilities, many applications have found frequency range above 20 GHz more useful and convenient. At this millimeter wave frequencies there are several bands that could be designated with up to 4 GHz of available bandwidth.
Radiation at millimeter wave frequencies tends to suffer higher atmospheric losses but is more directional than at lower microwave frequencies. The millimeter wave radars are smaller in size, have reduced noise and better resolution due to broad bandwidths. The characteristics of millimeter wave radar are finding use in detection and surveillance of Unmanned Aerial Systems (UAS) and even for medical monitoring. Multi channel radar designed for perimeter surveillance and scanning surveillance radar systems using Frequency Modulated Continuous Wave (FMCW) principals, with 1 GHz bandwidth at millimetric wavelength, have been used to achieve very high range resolution. Also, millimetric band radar has been used in remote heart rate monitors.
The reduced susceptibility of millimeter wave radar to light conditions, weather and clutter provides surveillance advantages over visual spectrum and IR camera technology as well. Reportedly, security and safety management technology for concealed threat detection has been developed that can reliably detect threats at a standoff distance of about 35 metres. Another technological radar innovation has eliminated the necessity of bulky radars altogether.
Passive Coherent Location Radar System
Passive coherent location system (PCLR) is an application of bistatic radars. These radars under development make use of public broadcast stations (FM radio stations, cellular phone base stations and digital audio broadcast) as target illuminator also called transmitters of opportunity and are inherently very difficult to detect and to jam them all.
PCLRs are highly cost effective compared to active radar systems with comparative performance as available transmitters are used at no cost. Individual receivers can be easily relocated for better performance. These radars are likely to find applications in broad area surveillance around airports and other important places complementing the existing radars which use own transmitters with conventional power amplifiers.
Semiconductor Power Amplifiers
Conventional Power Amplifiers (PA) system designs have undergone dramatic changes. The use of Gallium Nitride (GaN) power transistors in the PAs has modified the design. Power handling capacity of these new generation semiconductor devices is quite comparable to the erstwhile Travelling Wave Tubes (TWTs). The main reason is that GaN PA has power handling capacity, reliability and bandwidths far exceeding other solid-state technologies. They have overcome the reliability, size and maintenance issues generally associated with the TWTs. GaN PAs are very rugged and offer a very high thermal stability.
GaN technology is also used for Low Noise Amplifiers (LNA) with similarly rugged characteristics as the GaN PAs, providing sensitive receiver technology that is not easily jammed, damaged or affected by the limited dynamic range due to a low input voltage threshold. It also offers improved detection ranges considered unachievable due to noise associated with the signal. Techniques such as intelligent signal coding and newer generation digital signal processing have also contributed to the better design features of modern radars.
Intelligent Signal Coding
Like communication systems, signal coding schemes such as OFDM (Orthogonal Frequency Division Multiplexing), DSSS (Direct Sequence Spread Spectrum) and CDMA (Code Division Multiple Access), are finding applications in the new radars. Employment of these schemes results in an increase in compression gain by 50 to 70 decibels and reduction in transmitter power requirements. Variations of OFDM carrier modulations such as QAM (Quadrature Amplitude Modulation) and PSK (Phase Shift Keying) could also be employed. For military applications the detectability of these radar signals for localisation and countermeasures becomes much more difficult.
Radar Digital Signal Processing
The echo signal input to the receiver varies in amplitude, range and doppler. The information contents are separated using discriminators or demodulator devices which is in the form of amplitude, range and doppler. Range and doppler can be determined by simple Fourier transformations. It is possible to handle a large number of targets by using (Digital Signal Processing) DSP in the radar. The overall efficiency of the radar using DSP generally increases by up to 10 decibels.
The innovative system technologies of the radar are likely to provide many new functions and applications, which could replace most of the existing system concepts. The future radars are likely to be smaller in size and low on cost but high on information contents. Most of the technologies maturing in the ICT domain are likely to be integrated further in to the future radars. Automobile radar for safe and autonomous driving are meanwhile being produced in large numbers. The fastest growing market for radar applications is likely to be in the automotive systems.