ImpSAR

Impulse Synthetic Aperture Radar

What is ImpSAR?

Impulse radar operates in a so-called transient regime, where transmitted waveform is just a short (pico-second) pulse and has no carrier. Consequently, the waveform is extremely broadband. For example, if the radiated pulsewidth τ =100 ps, it follows that the signal bandwidth is of the order of several Gigahertz, implying extremely high (sub-inch) resolution in range. Like a conventional SAR, 2D imaging is done by integrating impulse radar in a moving platform and applying special signal processing algorithms to synthetize radar aperture. Thus, an ImpSAR is an impulse radar operating in SAR mode.

Eureka Aerospace has developed and successfully demonstrated ImpSAR operations in both the strip-map and spot modes. To achieve a 3D/Stereometric ImpSAR capability, Eureka Aerospace utilized an additional antenna along the vertical direction.

It also needs to be emphasized that, for an impulse waveform, the dispersion effects due to conductivity of the medium, such as soil, have little impact on the quality of the signal, as long as pulsewidth, τ satisfies the following condition:

Figure 1. Instrumented ImpSAR van used for field exercies.

where ε_o= 8.854×10^-12 farad/m is the permittivity of free space, ε_r is the relative dielectric constant, and σ is the conductivity of the soil. The implication is that, compared to the long pulses, which have the same bandwidth, e.g., chirped waveforms, short pulses (impulses) are more effective. When propagating through the medium, they behave like Brillouin precursors that penetrate through the medium essentially unperturbed. This indicates the advantage of using transient waveforms (impulses) instead of long modulated pulses. For example, assuming ε_r~ 5 and σ ~ 0.01, it follows that in order to have efficient impulse penetration into soil, we need τ ≪ 3.5 ns. The proposed system’s σ is approximately 100 ps, which is well suited for this application.

ImpSAR is compact, lightweight and consists of very few consituent elements: an Impulse radiating antenna (e.g. IRA or CEM), a fast rise-time pulser, a digitizer and ImpSAR processing code(s). Therefore, ImpSAR can be easily integrated into any ground-based or airborne (e.g. UAV) platform.

ImpSAR Antennas

Impulse Radiating Antennas (IRAs) – IRAs are extremely broadband antennas consist of a parabolic reflector fed by a pair of transmission lines driven by a 100 ps rise time pulser. For these types of IRAs, the lower cutoff bandwidth occurs at f_min ~ c/2D, where c=3×10⁸ is the speed of light, and D is the IRA dimension. For example, using a 23 cm IRA, we get f_min ~ 640 MHz. The IRA’s upper cutoff bandwidth, however, is inversely proportional to the rise time of the pulser and thus inversely proportional to the width of the impulse t, i.e. fmax ~ 0.35/t. Thus, using a 100 ps rise time pulse, we get fmax ~ 3.5 GHz, which yields the total signal bandwidth of approximately 3 GHz and thus reaching sub-inch resolution. It can be shown that IRA’s 3dB beamwidth is q₃ ~ 2ct/D. For example, for 100 ps wide pulses and a 23 cm IRA, we have q₃ = 0.26 rad corresponding to a 15 degree beam.

Circular Electric Monopulse (CEM) – CEM is a new planar, compact, broadband antenna that was designed by Eureka specifically for unmanned aerial system (UAS) applications, where it can be arrayed and conformed to the fuselage of an airborne platform. Each CEM is a flat, square-shaped and only 1.5 mm thick module. We designed and fabricated CEMs of two different sizes: 25×25 cm and 8.3×8.3 cm, best suited for the applications in the 300MHz - 3GHz and 800MHz - 9GHz range, respectively.

ImpSAR Applications

There are 4 applications of Impulse Synthetic Aperture Radar (ImpSAR)

High Resolution Through-the-Wall ImpSAR
Polychromatic ImpSAR
Stereometric (3D) ImpSAR
Through-the-Foliage and Underground ImpSAR

News & Recognition

Through-Wall Imaging Exercise at Empire Challenge 10

Eureka's through-wall radar imaging team participated in Empire Challenge 10, an annual joint and coalition intelligence, surveillance and reconnaissance (ISR) interoperability demonstration sponsored by the Undersecretary of Defense for Intelligence and organized by USJFCOM. This year's exercise took place in Ft. Huachuca, AZ. The ImpSAR...

Through-the-Wall Radar Imaging — Book Chapter

A chapter by Eureka's Dr. James Tatoian, "Impulse SAR and Its Application for Through-the-Wall Detection and Identification of People and Weapons," is featured in a new book, Through-the-Wall Radar Imaging (CRC Press, ed. Moeness G. Amin). Details and ordering information are available via CRC Press and Amazon .

Advances in Environmental Remote Sensing — Book Chapter

A chapter by Eureka's Dr. Giorgio Franceschetti and Dr. James Tatoian, "The Impulse Synthetic Aperture Radar," is in a new book, Advances in Environmental Remote Sensing: Sensors, Algorithms, and Applications (CRC Press, ed. Qihao Weng). Details and ordering information are available via CRC Press and Amazon .

ImpSAR

Impulse Synthetic Aperture Radar

What is ImpSAR?

Figure 1. Instrumented ImpSAR van used for field exercies.

Elements of ImpSAR Operations

Antennas

Digitizer and Pulser

Illustrative Example of Radiated Waveform and Associated Spectrum