Advances in Bistatic Radar updates and extends bistatic and multistatic radar developments since the publication of Willis' Bistatic Radar in 1991. New and recently declassified military applications are documented, civil applications are detailed including commercial and scientific systems and leading radar engineers provide expertise to each of these applications. Advances in Bistatic Radar consists of two major sections: Bistatic/Multistatic Radar Systems and Bistatic Clutter and Signal Processing. Starting with a history update, the first section documents the early and now declassified military AN/FPS-23 Fluttar DEW-Line Gap-filler, and high frequency (HF) bistatic radars developed for missile attack warning. It then documents the recently developed passive bistatic and multistatic radars exploiting commercial broadcast transmitters for military and civilian air surveillance. Next, the section documents scientific bistatic radar systems for planetary exploration, which have exploited data link transmitters over the last forty years; ionospheric measurements, again exploiting commercial broadcast transmitters; and 3-D wind field measurements using a bistatic receiver hitchhiking off doppler weather radars. This last application has been commercialized. The second section starts by documenting the full, unclassified bistatic clutter scattering coefficient data base, along with the theory and analysis supporting its development. The section then details two major clutter-related developments, spotlight bistatic synthetic aperture radar (SAR), which can now generate high resolution images using bistatic autofocus and related techniques; and adaptive moving target indication (MTI), which allows cancellation of nonstationary clutter generated by moving (i.e. airborne) platforms through the use of bistatic space-time adaptive processing (STAP).
Other keywords: sonar; multistatic radar; comprehensive reference; bistatic radar
This an introduction to the book 'Advances in bistatic radar'. It provides the definition, applications, purpose, scope, and history of bistatic radars.
Radar was first patented and demonstrated in 1904 by the German engineer Christian Hülsmeyer. As is well documented, his telemobilskop used a special spark-gap transmitter operating on a 40-50-cm wavelength and a separate receiver that rang a bell when detecting ships up to 5 km from the receiver. The first fully documented demonstration of the telemobilskop was in Cologne, in May 1904, with both the transmitter and receiver located on a platform under the Rhine Bridge (a chain suspension bridge destroyed during World War II) in a monostatic configuration to detect ships on the Rhine River. Purists might argue that Hülsmeyer's telemobilskop was not a radar because it did not directly measure range, as in 'radio detection and ranging,' and technically they would be correct. However, his invention included the essential elements of a radar, all reduced to practice: antennas, transmitter, receiver (with adequate shielding), and indicator, in this case an audio alarm to signal when a target was sufficiently close in range to require attention. More sophisticated indicators would have to await the development of timing circuits and displays.
This chapter disccuses a brief history of the bistatic radar known as Fluttar, developed by MIT Lincoln Laboratory under the sponsorship of the U.S. Air Force. The experimental Fluttar system led to the AN/FPS-23, which was the first operational bistatic radar the United States deployed. It was used in the Distant Early Warning (DEW) Line in the late 1950s as a gap-filler radar to detect low-flying aircraft approaching North America from the north.
Radar systems designed to operate at high frequencies between 5- and 30-MHz are often classified as over-the-horizon (OTH) radars, because at such frequencies the signals can travel via ionospherically refracted paths, called sky waves. A second mode of HF (high frequency) radar operation is surface-wave propagation, where HF energy propagates along the Earth's curvature by diffraction. An HF bistatic radar might exploit a surface-wave mode on one path while using a sky-wave mode for the other path. With a few early exceptions, HF radars use separated transmitters and receivers to reduce the complexity and cost of the receiving array, as well as to establish receiver isolation from the direct path transmitted signal. When a HF radar uses sky-wave propagation for both paths, this separation is small compared to the target range; thus, the radar operates with single-site characteristics, for example, with small bistatic angles, and is usually called a monostatic (or near-monostatic) OTH radar. Nearly all HF radar literature has focused on these monostatic systems. Recently, however, considerable information on bistatic HF radars has become available, which describes their military applications, specifically for ballistic missile launch warning. The purpose of this chapter is to document this new information, prefaced by a review of relationships between HF and VHF/ UHF (very-high frequency/ultra-high frequency) bistatic radar operation.
Originally envisioned as an uplink experiment with high-powered transmitters on Earth providing illumination, planetary bistatic radar needed a spacecraft receiver either in orbit or flying nearby to sample and record echo signals reflected from the target. The radio data could then be returned in the spacecraft science telemetry stream along with camera images, infrared spectra, magnetic held measurements, and other data acquired at or around the same time. Unfortunately, the added costs associated with uplink spacecraft radio instrumentation and the scheduling conflicts introduced by adding another investigation with unique attitude requirements (pointing the spacecraft antenna toward the planet) have largely relegated bistatic radar to add-on or piggyback status - an experiment of opportunity, which more readily could be conducted in a downlink configuration using existing hardware.
As outlined in Chapter 1, passive bistatic radars (PBRs) are a subset of bistatic radars that exploit nonradar transmitters of opportunity as their sources of radar illumination. They are often designed for military or civil air surveillance. When more than one transmitter is simultaneously exploited, the configuration becomes multistatic. In this case, measurements from transmit-receive pairs with overlapping coverage can be combined to locate a target usually via multilateration, for example, by determining the intersection of isorange contours generated by each pair. One transmitter operating with multiple receivers is also multistatic and can be used for multilateration. This chapter documents PBRs that have been proposed, designed, tested, and evaluated for military and civil air surveillance, starting with a PBR review in Section 6.2. It then assesses their utility in Section 6.3, specifically when compared to the entrenched, 70-year benchmark: monostatic radars. In subsequent sections, it documents the theory and available data to predict and assess the performance of a PBR, and then does so for a generic set of PBRs.
Scientific investigation of plasma turbulence in the lower ionosphere relies heavily upon VHF and UHF radar remote sensing. The neutral atmosphere is too thin to loft balloons above 40 km, and too thick for durable satellites below about 200 km. The entire in situ sounding rocket data set for the lower ionosphere probably does not exceed 2-h duration. This chapter is especially concerned with the technique of 'coherent scatter radar' which refers to the use of moderately sensitive VHF and UHF systems that detect scatter of radio waves from large amplitude fluctuations in the plasma density. This is in contrast to the much more sensitive 'incoherent scatter radar' technique, which refers to VHF and UHF systems that detect the scatter of radio waves from the thermal fluctuations in the plasma medium. The underlying radar waveforms and receiver systems can be quite similar, as both targets are volume scatterers with similar spatial and temporal scales. Both radar systems coherently detect the scatter of radio waves from a large volume of incoherent scatterers. From the point of view of the radar performance, the main difference is the level of sensitivity required; the scattering cross section of thermal scatter is 40-70 dB smaller than that of turbulent fluctuations. This chapter describes the Manastash ridge radar, which is a passive, bistatic VHF (100 MHz) radar performing a 'coherent scatter' function.
This chapter describes a commercial bistatic receiver, for convenience called a bistatic network receiver (BNR). Two or more BNRs hitchhiking off a monostatic doppler weather radar can retrieve full, three-dimensional vector wind fields.
Bistatic clutter is part of the ground environment that affects the performance of a bistatic radar system. In continuous wave (CW), moving target indication, and pulse Doppler radars, bistatic clutter residue. The phenomenology of terrain and sea clutter must be characterized, which is the purpose of this chapter. The radar return of transmitter energy scattered by terrain and sea (called surface clutter) from a specific area (called the clutter cell area) is a function of the bistatic angle. The bistatic angle is the angle subtended at the clutter cell by the lines joining it to the transmitter and receiver that are separated in space. The clutter return before signal processing can be orders of magnitude larger than returns from man-made targets such as aircraft or vehicles in the clutter cell.
The goal of a synthetic aperture radar is to observe a scene over a series of spatial sampling points and coherently combine those samples to synthesize a much larger aperture with finer azimuth resolution. The scene is persistently illuminated by the real beam of the antenna, and each spatial sampling point provides a range-resolved profile. Phase changes between profiles due to Doppler are exploited in Synthetic Aperture Radar processing to reconstruct a two (2-D) or three-dimensional (3-D) representation of the sampled scene.
A mechanism to suppress Doppler-spread ground clutter reflections is essential to the effective operation of an aerospace bistatic moving target indication (MTI) radar. Bistatic space-time adaptive processing (STAP) techniques combine spatial and temporal radar samples in a finite impulse response (FIR) filter to cancel stationary clutter returns while providing maximal gain on target. This strategy holds the potential to maximize output signal-to-interference-plus-noise ratio (SINR), thereby increasing the probability of detection for a fixed false alarm rate. This chapter describes adaptive bistatic clutter cancellation methods based on STAP.