Steering The Radar
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The use of radar systems for automotive applications, notably for collision-avoidance chores, has been in planning since the late 1950s. Because of the high frequencies employed for automotive radar systems, however, commercialization of automotive radar—even for luxury car models—wasn’t feasible until the 1990s with the support of millimeter-wave semiconductors. With automotive radar systems focused largely at 24 and 77 GHz, advances in microcontrollers and RF/microwave integrated circuits (ICs) and monolithic microwave integrated circuits (MMICs) during the 1990s made the design and fabrication of high-frequency radar modules more practical for inclusion in commercial vehicles. Early automotive radar applications based on surround sensing technologies included parking-assistance systems, collision-warning, and adaptive-cruise-control (ACC) systems.
In 1992, commercial carrier Greyhound Bus took a lead in the use of traffic radar, installing forward-looking K-band (24-GHz) and side-looking X-band (10.525-GHz) radar systems with frequency-modulated-continuous-wave (FMCW) signals on about 1500 of its buses. The forward-looking radar systems employed phased-array antennas and about 0.5-mW transmit power for a detection range of about 350 ft. From 1992 to 1993, the firm reported a reduction of about 20% in accidents for the buses, although also struggled with operational problems caused by interference with the automotive radar from traffic radar signals.
In 1999, Mercedes introduced the 77-GHz “Distronic” radar system into its S-class vehicles, with other high-end automobile manufacturers (including Audi, BMW, Cadillac, and Jaguar) to follow. This ACC system helps maintain safe distances between vehicles. Japanese car manufacturers such as Honda and Toyota also applied brake-assist systems for collision mitigation applications based on 77-GHz long-range radar electronics. As a general trend, automotive radar systems are moving from driver/passenger comfort functions to more functions that serve both comfort and safety.
A number of organizations exist to assist in minimizing interference among automotive radar signals. The European Telecommunications Standards Institute established the 76 to 77 GHz frequency band for European automotive radar use in the 1990s by means of the ETSI EN 301 091 standard. The band is now allocated for Intelligent Transport Services (ITS) in Europe, North America, and Japan. As dictated by the FCC and the North American Free Trade Agreement (NAFTA), ultrawideband (UWB) technology in the range of 22 to 29 GHz is allocated for the North American market at a maximum mean power density of -41.3 dBm/MHz.
The short-range automotive radar frequency allocation consortium (SARA) was founded in the early 2000s to promote technology and UWB regulation for 24-GHz automotive radar in Europe. A range of 21.65 to 26.65 GHz was selected for UWB short-range automotive applications, with frequencies from 77 to 81 GHz for UWB short-range automotive use later added.
In addition to SRR sensors, several companies currently offer long-range-radar (LRR) sensors at 77 GHz. The ARS 300 radar sensor from A.D.C. Automotive Distance Control Systems GmbH uses FMCW signals to measure distance and velocity for collision-avoidance applications. The 77-GHz LRR3 long-range radar sensor from Bosch is based on silicon-germanium (SiGe) semiconductor technology and features a rugged mechanical design to withstand high vibration levels—most notably, when used in commercial trucks.