Static and Moving Target Imaging Using Harmonic Radar
Nonlinear radar exploits the difference in frequency between radar waves that illuminate and are reflected from electromagnetically nonlinear targets. Harmonic radar is a special type of nonlinear radar that transmits one or multiple frequencies and listens for frequencies at or near their harmonics. Nonlinear radar differs from traditional linear radar by offering high clutter rejection and is particularly suited to the detection of devices containing metals and semiconductors. Examples
... rs. Examples include tags for tracking insects, tags worn by humans for avoiding collisions with vehicles, or for monitoring vital signs. Such tags contain a radio-frequency (RF) nonlinearity, often a Schottky diode, connected to a suitable antenna. Targets with inherent nonlinearities, such as metal contacts, semiconductors, transmission lines, antennas, filters, and ferroelectrics, also respond to nonlinear radar. In this paper, the successful exploitation of harmonic radar for moving target imaging and synthetic aperture imaging of targets, while suppressing clutter signals from linear targets, are presented. Our results demonstrate some unique advantages of harmonic radar over its traditional linear counterpart. Circuit elements exhibit nonlinear properties either by design or by consequence. By design, P-N junctions, such as diodes, are inherently nonlinear. The nonlinear effects of diodes are especially important in the field of RF for their use in mixers, which are used to up-and down-convert signals from one frequency to another. The operation of mixing two signals together to create a new frequency is a nonlinear operation. Upconverting signals to higher frequencies allows for higher bandwidths, thus information can be transmitted faster  . By consequence, many RF and microwave circuit elements exhibit unintended nonlinear properties. The best example of this phenomenon is the amplifier. Amplifiers are generally expected to operate linearly, and boost the input signal without creating extraneous frequencies at the output. In practice, building an ideal linear amplifier is not possible. As a result of an amplifier's inherent nonlinear character, extra frequency content will be generated and possibly distort the desired signal. The unintended frequency content generated by the nonlinear properties of the amplifier can interfere with other radar and communication systems    as well as affect the sensitivity of the receiver. Much research has been done to linearize amplifiers [6-10]. Many nonlinear effects occur which are subtler and do not manifest themselves as often. Among these, the most prevalent is PIM, which is observed when high power signals interact with components that are weakly nonlinear. Weakly nonlinear components do not exhibit measurable nonlinear distortion under normal conditions. An early example of PIM in a RF system is the Luxembourg effect or cross-modulation, which was first observed in 1933 when a high-power radio transmitter caused interference to other radio stations by modifying the conductance of the ionosphere [11, 12] . A more recent example of PIM causing unintended nonlinearities occurs in mobile phone towers, which communicate with multiple users simultaneously. PIM distortion in mobile phone towers is generated by the antennas and connectors      . Lesser known observations of nonlinearities come from electrothermal coupling. As components heat and cool, they begin to exhibit nonlinear properties, which are generally very weak  . Starting in the 1970s, research began focusing on detecting, measuring, and exploiting nonlinear circuit elements at standoff distances. Most naturally occurring objects do not exhibit significant nonlinear characteristics [19, 20] . This permits detection of small nonlinear objects in the presence of larger linear clutter. Theoretical and experimental data have shown that different nonlinear junctions can be discriminated against at standoff distances           . These systems traditionally operate at a single frequency and detect nonlinear devices connected to an antenna. A harmonic radar (a specialized form of nonlinear radar) transmits a frequency f t and processes returns at a harmonic frequency n f t , where n = 2, 3, 4, ..... Usually, the second harmonic, i.e., 2 f t , is used since it generally provides the strongest response from these types of targets. This radar approach effectively separates the harmonic target response from the clutter band, thereby making harmonic radar attractive for several applications. The automobile industry has been investigating harmonic tags as early as the 1970s for avoiding collisions with other automobiles and humans carrying nonlinear tags at X-/Ku-Bands [31-33]. Such a system has also been used by biologists and entomologists interested in tracking insects at 917/1834 MHz [34,     . Since most systems traditionally used a single frequency waveform and a single transmit and receive antenna, only detection of nonlinear targets was performed. No ranging, tracking, imaging or identification of nonlinear targets was possible. For two-dimensional localization of nonlinear targets, a wide bandwidth and a large aperture are needed to achieve good down range and cross range resolutions, respectively. To address the shortcomings mentioned above, we have constructed a wideband harmonic radar operating transmitting over the 800-1000 MHz range and receiving over the fundamental frequency range as well as the second harmonic (1600-2000 MHz) frequency range  . In this paper, we present the basic principles of harmonic radar, and provide experimental evidence of its ability to suppress linear clutter, perform detection and indication of slow moving nonlinear targets, and perform high resolution synthetic aperture imaging. While our work on moving target and synthetic aperture Electronics 2017, 6, 30 3 of 20 radar imaging has been published in conference proceedings, this paper combines the two imaging modalities and presents a unified treatment of the two topics.