Gaussian Monopulse Research Articles

Development of a high-time/spatial resolution self-impedance probe for measurements in laboratory and space plasmas.

Plasma impedance probes are often used in laboratory experiments as well as in space to make measurements of important plasma parameters such as the electron density. Conventional impedance probe methods involve sweeping the frequency applied to the probe through a range containing the plasma frequency, which can take on the order of a second to complete. This acquisition time leads to very low spatial resolution when making measurements from sounding rockets in the ionosphere. A high-time resolution impedance probe is under development at the U.S. Naval Research Laboratory with the goal of increasing the spatial resolution of measurements in space. To achieve this, a short-time Gaussian monopulse with a center frequency of 40MHz and containing a full spectrum of frequencies is applied to an electrically short dipole antenna. Laboratory experiments were performed with the Gaussian monopulse triggered once every 10 µs and averaged over ten shots, equating to a spatial resolution of 13cm for a typical sounding rocket speed. This paper discusses the development of the new high-time/spatial resolution self-impedance probe and illustrates that the short-time pulse method yields results that match well with data taken using conventional methods. It is shown that plasma parameters such as the electron density, sheath frequency, and electron-neutral collision frequency can also be derived from the data. In addition, data from the high-time/spatial resolution impedance probe are shown to compare well with those from theoretical impedance models.

Design and Development of an Inexpensive Sub-Nanosecond Gaussian Pulse Transmitter

A design of a hybrid Gaussian pulse transmitter using a transistor-based pulse generator and two pulse-shaping networks, a shorted stub delay circuit and a 90° hybrid coupler, is presented. A transistor-based pulse generator drives the pulse-forming network to achieve a sub-nanosecond pulsewidth. The shorted stub delay line was modified to optimize the frequency response and phase linearity. The transistor-based pulse generator is an inexpensive way of producing a square pulse of sharp rise time; this design achieved 280-ps rise time. A Schottky diode was used to eliminate the negative part of the output pulse as well as to reduce the ringing. Multiple simulations and measurements were performed to study the circuit behavior. The measurement results demonstrated a pulsewidth as sharp as 153 ps with 0.83-V amplitude and a low ringing on the order of 10%. The Gaussian monopulse was obtained by feeding the Gaussian pulse to a broadband 90° hybrid, which acts as a differentiator in the time domain. The pulse has a width on the order of 100 ps. The transmitter circuit was completed by connecting the monopulse circuit with an antenna. The measured radiated pulse in the far field of the transmitter is a second derivative Gaussian pulse with 85 ps of pulsewidth.

Mathematical analysis of RF imaging techniques and signal processing using wavelets

Non-linear optics plays an important role in the imaging. In this paper using non-linearity, we have derived three different techniques for imaging. In the first technique, an algorithm is developed for the scattering of electromagnetic waves from the medium, which gives higher-order harmonics of the EM wave. Here, the linear and non-linear interactions of molecules with the applied electromagnetic waves play an important role in target detection. We assume the material is inhomogeneous and represented by its susceptibility tensor. Second technique for detection of non-linearity or higher-order harmonic is using anharmonic oscillator model where the perturbation due to non-linearity in the electron moment is derived and mapped to the second harmonic generation of electromagnetic waves. Third technique is applied using Gaussian monopulse where the non-linear interaction of wave with the matter makes the phase change of the wave. When wavelet transforms with dilations and translation are applied to these non-linear waveforms, we get the details of region of interest in terms of wavelet coefficients. The region of interest may be 1-dimensional, 2-dimensional or 3-dimensional. These methods can be used in biomedical applications and other areas where target is in the near-field range.

Mathematical analysis of RF imaging techniques and signal processing using wavelets

Non-linear optics plays an important role in the imaging. In this paper using non-linearity, we have derived three different techniques for imaging. In the first technique, an algorithm is developed for the scattering of electromagnetic waves from the medium, which gives higher-order harmonics of the EM wave. Here, the linear and non-linear interactions of molecules with the applied electromagnetic waves play an important role in target detection. We assume the material is inhomogeneous and represented by its susceptibility tensor. Second technique for detection of non-linearity or higher-order harmonic is using anharmonic oscillator model where the perturbation due to non-linearity in the electron moment is derived and mapped to the second harmonic generation of electromagnetic waves. Third technique is applied using Gaussian monopulse where the non-linear interaction of wave with the matter makes the phase change of the wave. When wavelet transforms with dilations and translation are applied to these non-linear waveforms, we get the details of region of interest in terms of wavelet coefficients. The region of interest may be 1-dimensional, 2-dimensional or 3-dimensional. These methods can be used in biomedical applications and other areas where target is in the near-field range.

Simulation on change of generic satellite radar cross section via artificially created plasma sprays

Recent advancements in antisatellite missile technologies have proven the effectiveness of such attacks, and the vulnerability of satellites in such exercises inspires a new paradigm in RF Stealth techniques suitable for satellites. In this paper we examine the possibility of using artificially created plasma sprays on the surface of the satellite’s main body to alter its radar cross section (RCS). First, we briefly review past research related to RF Stealth using plasma. Next, we discuss the physics between electromagnetic waves and plasma, and the RCS number game in RF Stealth design. A comparison of RCS in a generic satellite and a more complicated model is made to illustrate the effect of the RCS number game, and its meaning for a simulation model. We also run a comparison between finite-difference-time-domain (FDTD) and multilevel fast multipole method (MLFMM) codes, and find the RCS results are very close. We then compare the RCS of the generic satellite and the plasma-covered satellite. The incident radar wave is a differentiated Gaussian monopulse, with 3 dB bandwidth between 1.2 GHz and 4 GHz, and we simulate three kinds of plasma density, with a characteristic plasma frequency ωP = 0.1, 1, and 10 GHz. The electron-neutral collision frequency νen is set at 0.01 GHz. We found the RCS of plasma-covered satellite is not necessarily smaller than the originally satellite. When ωP is 0.1 GHz, the plasma spray behaves like a dielectric, and there is minor reduction in the RCS. When ωP is 1 GHz, the X–Y cut RCS increases. When ωP is 10 GHz, the plasma behaves more like a metal to the radar wave, and stronger RCS dependency to frequency appears. Therefore, to use plasma as an RCS adjustment tool requires careful fine-tuning of plasma density and shape, in order to achieve the so-called plasma stealth effect.

Spectral and Spatial Properties of Antennas for Transmitting and Receiving UWB Signals

The spectral and spatial properties of antennas for transmitting and receiving ultrawideband (UWB) pulses and the corresponding filter response designs for receiving such pulses over severe multipath are studied in this paper. Based on the transmit and receive transfer functions of a UWB antenna, we analyze the UWB link behavior. A metric that is based on the receiver output signal-to-noise ratio (SNR) in multipath is used as a criterion to design an appropriate receive filter response. A lower bound on this metric allows for a tractable maximization. The solution is a linear combination of incoming pulses from all directions, which are weighted by their relative intensity. A simpler best candidate solution for the receive filter response is shown to provide a performance close to that of the linear combination solution. These results are given for a diamond planar antenna and a Gaussian monopulse at the transmitter side. For this case, the impact of direction angle on the receiver output SNR is also shown.

Narrowband Interference Suppression in Time-Hopping Impulse Radio Ultra-Wideband Communications

Ultra-wideband (UWB) technology has been considered an innovative solution for future short-range high-speed wireless communications. Interference suppression is important for the UWB devices to operate over spectrum occupied by narrowband systems. In this paper, the use of a notch filter in time-hopping impulse radio (TH-IR) for UWB communication is considered, where a Gaussian monopulse is employed with pulse position modulation. Lognormal channel fading is assumed, and a complete analytical framework is provided for the performance evaluation of using a transversal-type notch filter to reject narrowband interference (NBI). A closed-form expression of bit-error probability is derived, and the numerical results show that the use of a notch filter can improve the system performance significantly. Furthermore, a performance comparison between TH-IR and multicarrier code-division multiple-access (MC-CDMA) UWB systems is made under the conditions of the same transmit power, the same data rate, and the same bandwidth. It is shown that in the presence of NBI, the TH-IR system and MC-CDMA system achieve similar performance when both use a notch filter