Radar has been a significant RF/microwave technology since the days of World War II. During that time, radar (a shortening of “radio detection and ranging”) proved an invaluable military tool for locating threats and targets and providing advanced warnings of an adversary’s position and direction. The basic operation of a radar system involves transmitting a high-frequency signal (usually a pulsed signal) towards the location of an expected target and receiving signals reflected from said target. By performing signal processing on these radar returns, information can be extracted regarding the target, its position, and its speed.
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Military uses were once the only applications for radar technology, but times have changed. Radar technology is now finding uses in many commercial, industrial, medical, weather, and especially automotive systems. These new and growing application areas are keeping radar designers—from integrated-circuit (IC) to system-level engineers—busy in search of high-performance, cost-effective solutions from RF through millimeter-wave frequencies.
Military systems still represent the most plentiful source of radar applications, with military radar systems found on land, at sea, and in the air (and in lesser numbers, in space-borne systems). Radar systems have been used in military applications for ground surveillance, missile control, fire control, air traffic control (ATC), moving target indication (MTI), weapons location, and vehicle search.
As land-based radar systems were being developed in support of American troops during World War II, the U.S. Naval Research Labs (NRL) developed radar systems for maritime applications, including onboard submarines. For such uses, a submarine would draw close to the water surface level, enabling a radar antenna to rise above the surface of the sea water to transmit signals in search of enemy aircraft.
Modern ground-based radar systems are transportable by personnel as well as by vehicles, with some systems—such as the AN/PPS-5A/B ground surveillance radar system—in service for a number of decades. Older military radar systems, whether of the ground-, maritime-, or avionic-based variety, are continuously upgraded as newer technologies become available.
With the AN/PPS-5A/B system, for example, systems based on magnetron tube power sources and weighing 125 lb. have largely been replaced by systems using solid-state transmit amplifiers and weighing only 70 lb., with a slight tradeoff in transmit power. This is considered a man-portable radar system that has been packed in waterproof enclosures for dropping into locations with infantry via parachute.
The AN/PPS-5A/B is fairly representative of a ground-based surveillance radar, operating over a fairly narrow bandwidth in the frequency range from 8.8 to 9.0 GHz with a pulse repetition frequency (PRF) of 4 kpulses/s. The system transmits pulses with 1 kW peak power and achieves ranges of about 6 km for detecting personnel and 10 km for detecting vehicles. The system is built for U.S. military customers by a number of different suppliers, including Eaton Corp., Telephonics Corp., and the Thales Group.
In the air, Lockheed Martin has long been an innovative developer of reliable military radar systems for surveillance. The company’s Tactical Reconnaissance and Counter-Concealment (TRACER) radar system (Fig. 1) provides effective long-term surveillance of suspect operations by means of synthetic-aperture-radar (SAR) technology. The basic principle of SAR is to use data from multiple radar returns to form the equivalent image that would be produced by a single large aperture antenna. The time delay information from returned radar signals is also converted to spatial dimensional information to produce additional details about a target.
TRACER is a dual-band (UHF and VHF) radar system capable of detecting targets through foiliage, rainfall, and even dust storms, providing real-time tactical ground imagery from the air. The use of the lower-frequency, longer-wavelength UHF and VHF signals compared to higher-frequency signals in many radar systems enables detection through dense foiliage.
The radar signals work with the company’s foliage penetration (FOPEN) technology to detect vehicles, buildings, and large metallic objects. TRACER features a portable ground station that works with the airborne electronics to collect and process data and develop precise ground images. The TRACER system is designed for use at low through high altitudes, either from manned or unmanned aircraft.
Military radar systems also are increasingly integrated into other weapons systems for guidance. One of the long-time suppliers of defense-based radar systems, Raytheon Co., has developed its Small Diameter Bomb II (SDB II) system for the Air Force and Navy to improve missile efficiency under all weather conditions, even when visibility is limited. The firm is currently involved in integrating the radar system onto F-35 Joint Strike Fighter aircraft, F/A-18E/F Super Hornet, and F-15E Strike Eagle aircraft.
The SDB II missile seeker system actually combines several different technologies, with a millimeter-wave radar to detect and track targets through adverse weather, an infrared (IR) imaging system to provide enhanced target discrimination, and a semi-active laser system that allows the SDB II system to track an airborne or ground-based laser designator for identification by allied troops. The radar/IR/laser weapons system can fly more than 45 miles to find a fixed or moving target, providing a great deal of flexibility to an airborne military team.
Raytheon’s AN/SPY-6 system is a next-generation air and missile defense radar (AMDR) system that incorporates multiple-frequency radar subsystems at S- and X-band frequencies. To be installed on DDG 51 Navy guided-missile destroyers beginning in 2016 (Fig. 2), the system packs receivers and transmitters together in a compact radar modular assemblies (RMAs) measuring just 2 × 2 × 2 ft. The RMAs are stacked together to form a complete system within the spacing requirements of each naval ship. The AN/SPY-6 AMDR is claimed to provide many times the range and sensitivity of existing naval shipboard radar systems, employing adaptive digital beamforming and advanced digital signal processing (DSP) to achieve the improvements in performance.
On the commercial side of marine radar, Raymarine is a major supplier of ship-board radar systems for a wide range of sea vessels for commercial and consumer applications such as boating and fishing. The company has grown steadily through the years, launching its popular Pathfinder radar system in 1997, and acquiring Raytheon’s recreational marine division in 2001. The firm, which also supplies sonar systems and VHF radios, offers a variety of different radar radome and array antennas for different environments, applications, and radar transmit power levels (for increased range).
Into the Ground
In contrast to traditional radar systems in which EM waves propagate through the air to strike a target, ground-penetrating-radar (GPR) systems propagate through different media (usually rock and soil) before striking a target of interest. GPRs usually operate from about 300 to 3000 MHz, at relatively low transmit power levels. Different three-dimensional (3D) scattering patterns will be formed by different target shapes, such as dielectric spheres, in different soils.
The usual wave qualities must be studied in the radar returns—such as signal phase shifts, time delays, and signal attenuation—but the effects of the different propagation media must also be calculated. A forward-looking radar wave will exhibit different vector components when striking the ground, depending upon the composition of the ground (e.g., clay versus sandy soil).
GPRs were initially developed during the Vietnam War for the detection of enemy tunnels. Different types of GPR systems include time-domain-based impulse radar systems, which use short pulses and measure the propagation time to and from the target, and stepped-frequency or frequency-modulated-continuous-wave (FMCW) systems, where the magnitude and phase of each frequency signal is measured and analyzed. Depending upon whether GPR systems are operating with transverse electromagnetic (TE) or transverse magnetic (TM) polarization, radar-system performance can be improved by finding the optimum height for the radar antenna above the ground. For even the short distance that the EM waves propagate through the air, the difference in propagation characteristics between the air and the soil must be calculated, and the refraction point at which the radar waves enter the soil must be found.
For example, an innovator in GPR systems, BAE Systems, used a stepped-frequency approach in their GPR systems for tactical ground-based military applications. With 20 transmit/receive antenna pairs in a forward-looking, vehicular-mounted system, the GPR operated from 0.5 to 2.0 GHz in 5-MHz steps and was effective in locating mines through a wide range of soil types. Penetrade Corp. also is a supplier of GPR systems.
Driving the Future
While radar technology has long been used for tracking and mapping weather patterns, and weather-based applications represent a strong market area for the technology, perhaps the most promising opportunities for radar technology lie in traffic and the consumer automobiles that make up that traffic. Many leading electronics and systems firms, such as Infineon and ZF TRW, have developed millimeter-wave automotive collision-avoidance radar systems for operation at 77 GHz.
In addition, numerous semiconductor companies, including Freescale and Qorvo, are developing radar ICs for the transmit and receive functions at millimeter-wave frequencies. TRW, in fact, now offers automotive radar systems in three different frequency bands—24, 77, and 79 GHz—with the recent introduction of its AC1000 automotive radar system for use at 79 GHz.
The firm’s earliest automotive radar system model, the cost-effective model AC100 (Fig. 3), operates within the 24-GHz ISM band, across the 100-MHz bandwidth from 24.150 to 24.250 GHz. Using a planar patch antenna, it is capable of accurate readings at velocities as high as 250 km/hr. The company’s long-range AC3 automotive radar system operates at 77 GHz and is already in the third generation of the product line. The most-recent system, the AC1000 automotive radar, operates at 79 GHz; it supports front-facing collision-warning, side- and rear-facing radar detection, and adaptive cruise control functions for a wide range of driving scenarios (Fig. 4).
ZF TRW earlier this year launched a commercial vehicle system that fuses radar with camera technology. The system uses a common set of sensors to combine data from the radar system and multiple cameras for increased vehicular safety. According to Ken Kaiser, vice president of engineering for the ZF TRW Global Electronics Business, “Fusing the data from camera and radar every 30 to 40 ms helps to confirm when a situation warrants action from on-board systems such as rapid braking via the electronic stability control system for Automatic Emergency Braking.” The trend of integrating radar technology with other electronic systems is quite strong for commercial automotive applications, as in military radar systems, and will continue as radar technology is applied to achieve complete 360-deg. safety around a commercial vehicle.
Radar technology comes in many forms and packages, and this article has only scratched the surface of the different types available, including in continuous-wave (CW) form in frequency-modulated CW (FMCW) radar systems. With the help of high-frequency IC manufacturers, radar technology is reaching well into the millimeter-wave frequency range at prices affordable and competitive for automobile manufacturers, who will be able to include radar-based safety features on commercial automobiles for customers in virtually every price range.
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