Two-way Antenna Research Articles

An Approach to the Assessment of Detectability of Subsurface Targets in Polar Ice From Satellite Radar Sounders

A satellite mission onboard a radar sounder for the observation of the earth’s polar regions can greatly support the monitoring of the cryosphere and climate change analyses. Several studies are in progress proposing the design and demonstrating the performance of such an earth-orbiting radar sounder (EORS). However, one critical aspect of the cryospheric targets that are often ignored and simplified in these studies is the complex geoelectrical nature of the polar ice. In this article, we present a performance assessment of the polar ice target detectability by focusing on their realistic representation. This is obtained by simulating the orbital radargrams corresponding to different regions of the polar cryosphere by leveraging the data available from airborne campaigns in Antarctica and Greenland. We propose novel performance metrics to analyze the detectability of the internal reflecting horizons (IRHs), the basal interface, and to analyze the nature of the basal interface. This performance assessment strategy can be applied to guide the design of the signal-to-noise ratio (SNR) budget at the surface, which can further support the selection of the main orbital instrument parameters, such as the transmitted power, the two-way antenna gain, and the processing gains.

A Study of Comb Beam Transmission on Phased Array Weather Radars

The comb beam transmission (TX) approach, which forms a power antenna radiation pattern with multiple mainlobes, was studied for application to phased array weather radars. Combining the use of comb TX and a digital beamforming receiver enables a weather radar to observe multiple directions simultaneously. Numerical simulations show that the two-way antenna radiation patterns formed by comb TXs have properties comparable to conventional weather radar observation: almost the same mainlobe widths are achieved, and with reference to the mainlobe peak, the maximum sidelobe level is less than −37 dB, and the sidelobe level reaches −60 dB at an angle of 13.77°. These properties are superior to the wide TXs that are normally utilized by phased array weather radars. A simulation that applied the comb TX approach to a current C-band weather radar showed that the volume scan time can be reduced from almost 500 s to 1 min.

Doppler Velocity Bias Mitigation Through Sidelobe Whitening for Multistatic Weather Radar

Multistatic radar is a promising option for the low-cost collection of multiple-Doppler weather observations. However, due to the use of low-directivity antennas at the receivers in these systems, they typically suffer from extremely high two-way sidelobe levels compared to monostatic radars. Doppler velocity estimation biases induced by sidelobe contamination have proved to be a significant obstacle to more widespread adoption of this technology. It has been noted in the existing literature that the technique of sidelobe whitening, first developed for use in monostatic systems, has the potential to mitigate this issue. However, the existing sidelobe whitening algorithm is not suitable for use in this application, as it is only capable of achieving whitening in the two-way antenna pattern, whereas an algorithm to be used in the multistatic case must be able to achieve this result in the transmit pattern alone. This article proposes an alternate sidelobe whitening technique based on the method of alternating projections which allows for effective whitening in the one-way antenna pattern. A multistatic weather radar time-series simulator is used in conjunction with numerical weather prediction data to demonstrate the effectiveness of this method in mitigating multiple-Doppler measurement biases.

Reducing Doppler noise with multi-station tracking: The Cassini test case

Doppler tracking of Solar System probes is used for spacecraft navigation, planetary geodesy, and tests of the theory of General Relativity. The spacecraft radial velocity is measured by observing the Doppler shift of a radio signal transmitted from an Earth station to the spacecraft and then re-transmitted back, while preserving phase coherence, to the same station (two-way link) or to a different station (three-way link). Specialized orbit determination software is then used to reconstruct the spacecraft trajectory and estimate planetary gravity field coefficients or relativistic parameters. The measurement noise is a crucial element for the accuracy of the final estimates, thus considerable effort has been devoted to improve the range rate accuracy by adopting higher frequency links to reduce the dispersive noise from interplanetary and ionospheric plasmas, and by calibrating the tropospheric path delays with microwave radiometers. While Ka-band radio links (32–34 GHz) allowed a successful suppression of plasma noise, reducing tropospheric noise and ground antenna mechanical noise has been more challenging. The Time-Delay Mechanical noise Cancellation (TDMC) technique is a promising method to reduce mechanical and tropospheric noises and to improve further the accuracy of Doppler measurements. The TDMC is a linear combination of simultaneous Doppler data from a main antenna providing the two-way link and a three-way antenna (generally smaller and stiffer). If the listen-only, three-way antenna is also located in a particularly dry site, the TDMC can considerably reduce both tropospheric and antenna mechanical noises, which are the leading disturbances in two-way Ka-band radio links. For an operational test of this method, we applied the TDMC to Doppler data at X-band (7.2–8.4 GHz) from the Cassini spacecraft acquired during the Saturn tour phase of the mission. Although X-band links are generally dominated by the highly-variable interplanetary plasma noise and are not suitable for the TDMC, we found that, when local noises are particularly large at the two-way antenna, this technique may still lead to up to a factor-of-three noise reduction (at 60-s integration time) with respect to the two-way link. The TDMC can maximize the data quality during unique events, mainly planetary or satellite flybys (such as those considered in the Europa Clipper and JUICE missions), where the scientific results could be severely hampered by adverse conditions at the tracking station.

Chirp Signals and Noisy Waveforms for Solid-State Surveillance Radars

Since the advent of “pulse compression” radar, the “chirp” signal (Linear Frequency Modulation, LFM) has been one of the most widely used radar waveforms. It is well known that, by changing its modulation into a Non-Linear Frequency Modulation (NLFM), better performance in terms of Peak-to-Sidelobes Ratio (PSLR) can be achieved to mitigate the masking effect of nearby targets and to increase the useful dynamic range. Adding an appropriate amplitude modulation, as occurs in Hybrid-NLFM (HNLFM), the PSLR can reach very low values (e.g., PSLR < −60 dB), comparable to the two-way antenna sidelobes in azimuth. On the other hand, modern solid-state power amplifier technology, using low-power modules, requires them to be combined at the Radio Frequency (RF) stage in order to achieve the desired transmitted power. Noise Radar Technology (NRT) represents a valid alternative to deterministic waveforms. It makes use of pseudo-random waveforms—realizations of a noise process. The higher its time-bandwidth (or BT) product, the higher the (statistical) PSLR. With practical BT values, the achievable PSLR using pure random noise is generally not sufficient. Therefore, the generated pseudorandom waveforms can be “tailored” (TPW: Tailored Pseudorandom Waveforms) at will through suitable algorithms in order to achieve the desired sidelobe level, even only in a limited range interval, as shown in this work. Moreover, the needed high BT, i.e., the higher time duration T having fixed the bandwidth B, matches well with the low power solid-state amplifiers of Noise Radar. Focusing the interest on (civil) surveillance radar applications, such as ATC (Air Traffic Control) and marine radar, this paper proposes a general review of the two classes of waveforms, i.e., HNLFM and TPW.

Adaptive Beamspace Processing for Phased-Array Weather Radars

The next generation of weather radars, which may also support other missions, is likely to be based on phased arrays that will utilize simultaneous receive beams to achieve the required update times. Some of the disadvantages of using simultaneous receive beams, such as higher two-way antenna radiation pattern sidelobes, can be mitigated by using adaptive beamforming. A majority of the existing adaptive beamforming algorithms are designed for point targets, and direct application to distributed scatterers (e.g., hydrometeors) can lead to significantly biased estimates of key radar variables. This paper presents an adaptive beamspace processing algorithm specifically designed for weather-surveillance radar applications. Through both simulated and real data, it is shown that the proposed adaptive beamspace processing algorithm can produce accurate and calibrated estimates of radar variables while also automatically rejecting interference signals.

Employment of New Techniques for Characterizing Indian MST Radar Phased Array

ABSTRACTScientific investigations of dynamical processes in the earth's atmosphere are being performed over the past two decades using a 53 MHz mesosphere–stratosphere–troposphere (MST) radar, which is located at Gadanki, India. This radar consists of a phased antenna array of 1024 Yagi-Uda antennas. Antenna phase calibration is an important aspect for accurate measurements of atmospheric parameters by phased array MST/ST radars. In this paper, we describe two newly employed techniques to calibrate the large phased array antenna system of the Indian MST radar and the results obtained by these techniques. A “total power radiometer” technique has been employed for the first time to calibrate the antenna pattern in the receive mode. Radio source Virgo-A has been used as source in the far field for this purpose. The new technique has enabled us to characterize for the first time the first side lobe level of the antenna pattern and also the beam pointing accuracy in the E-plane. In the receive mode, antenna pattern is characterized with a 3 dB full beam width of 2.8o, first side lobe level of −18 dB, beam pointing accuracy is better than 0.12o. While the beam width and beam pointing accuracy are found to be close to the design values, the first side lobe level is 2 dB higher than that expected. To characterize the two-way antenna beam, active radar experiments are conducted and beam pointing accuracy has been estimated to be ∼0.1o using radar echoes from the moon; again for the first time. Improvement of signal-to-noise-ratio is observed with the newly developed antenna phase calibration method that used receiver quadrature detector. The agreement in comparison of wind vector derived from radar observations and Global Positioning System (GPS)-radiosonde measurements shows the performance status of the array antenna with the applied phase calibration.

Experimental validation of coherent multiple‐input multiple‐output radar antenna patterns

This study specifies the antenna radiation patterns of a linear array radar operating in a phased array configuration and in two colocated multiple-input multiple-output (MIMO) configurations. The MIMO-1 configuration uses orthogonal transmission on all elements, while the MIMO-2 configuration uses orthogonal transmission on the two end elements. The theoretical two-way antenna radiation patterns are derived for all three configurations. Field experiments with an eight-element, X-band linear array radar and trihedral targets are then described. The experimental mainbeam patterns are shown to match those of the theoretical patterns. MIMO-1 is shown to have the same two-way radiation pattern as that of the phased array radar configuration. Compared with MIMO-1 and phased array, MIMO-2 has enhanced angle estimation accuracy, lower gain, and higher sidelobes while only requiring two orthogonal waveforms on transmit.

Asymmetric physical layer design for high-speed wireless digital communications

High data rates and portability are conflicting requirements in the design of reliable indoor wireless data communication systems. Asymmetric system design addresses this problem by exploiting the major differences in the availability of resources (i.e., energy, space) that exist between base stations and portable units. Such an approach seeks to concentrate most of the signal processing tasks involved in the operation of the two-way wireless link at the base station. This paper presents and discusses the implications of a set of techniques for asymmetric physical layer system design based on the use of channel precoding for forward transmission. System performance is assessed via simulations using a realistic time-varying channel model. Simplified two-way antenna diversity implemented exclusively at the base station and the key issue of automatic gain control at the receive side of the precoded link are also addressed and evaluated. Simulation results confirm that the adoption of these techniques enables reliable digital communication at a data rate of 20 Mbit/s on both links while significantly reducing the power consumption of the portable unit.