A 12 m diameter radio telescope will be deployed to the Summit Station in Greenland to provide direct confirmation of a Super Massive Black Hole (SMBH) by observing its shadow image in the active galaxy M87. The telescope (Greenland Telescope: GLT) is to become one of the Very Long Baseline Interferometry (VLBI) stations at sub-millimeter (submm) regime, providing the longest baseline >9000 km to achieve an exceptional angular resolution of 20 μas at 350 GHz, which will enable us to resolve the shadow size of~40 μas. The triangle with the longest baselines formed by the GLT, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the Submillimeter Array (SMA) in Hawaii will play a key role for the M87 observations. We have been working on the image simulations based on realistic conditions for a better understanding of the possible observed images. In parallel, retrofitting of the telescope and the site developments are in progress. Based on 3 years of opacity monitoring at 225 GHz, our measurements indicate that the site is excellent for submm observations, comparable to the ALMA site. The GLT is also expected to make single-dish observations up to 1.5 THz.
The presence of an obstacle in detecting respiration vital signs using a radar system does not only exhibit attenuation and phase shift effects on the radar signal but also causes additional beat frequencies to appear beside those from the target. When a conventional FMCW radar system detects the peak of the received signal spectrum as a beat frequency, this will give an error of detection. The respiratory vital signs will then be undetectable. A method to overcome this obstacle problem is needed. Therefore the implementation of the FMCW radar system in detecting respiratory vital signs behind the wall can be realized. In this paper, a method to deal with the previously mentioned problem is proposed. The proposed method consists of a method for identifying the obstacle response. Then the results are used to eliminate the beat frequency that arises from the obstacle structure, and therefore the signal to clutter ratio (SCR) can be improved. Furthermore, the method is combined with the phase detection method, which is applied to extract the Doppler response related to the target respiratory vital signs. The weighting process by elaborating cross correlation is also employed for minimizing clutter from other objects reflection under debris. A laboratory experiment by developing an FMCW radar system with a 24 GHz frequency operation was performed in this research. The proposed detection method was elaborated in the post-processing of the radar system. The experiment result demonstrates that the proposed method is capable of eliminating the effect of the wall and successfully extracting the respiration vital sign pattern.
IntroductionBistatic radar systems have been studied and drawn increasing interests in military applications [1]. The system has the advantage of counter-stealth capability and the receiving antenna system is passive, and hence undetectable. In modern electronic counter measure (ECM) environments, if a scattering object is undetectable for the receiving antenna, it would need to suppress its reflected signal at the specular direction because that by the Snell's law of reflection, most of incident electromagnetic energy will reflect at the angle of reflection that is equal to the incident angle. Although some adaptive nulling algorithms have this performance to make the null of re-transmitted radiation pattern at the angle of reflection [2, 3], they may require complicated digital signal processing techniques, giving restrictions on high-frequency and high-speed applications. In this study, a novel approach is developed for a retro-and reflecto-nulling antenna array that this array has two nulls occurring at the incident and specular reflection directions. This approach is implemented by a passive circuit with the use of 90 o hybrids and 90 o phase shifters which are properly connected to transmitting and receiving antennas. Measurement results of a four-element retro-and reflecto-nulling antenna array are presented. DesignConsidering an N-element antenna array, if a uniform plane wave illuminates this array at an incident angle ș i measured from the array broadside, the received signal at the m-th antenna can be given as ( ) ( ) 1 sin 1 , i i j m kd j m m a Ae Ae q f ----= = (1) where m = 1, 2, …, N. k is the wave number of the incident wave and d is the antenna element spacing. If the antenna array retransmits signals with an output signal b m at the m-th antenna, its array factor is given as ( ) ( ) ( ) 1 sin 1 1 1 . o o N N j m kd j m o m m m m F be be q f q ----= = = = å åFor designing a retro-and reflecto-nulling antenna array, one has to arrange two nulls at ș o = ±ș i . The method to arrange two zeros in the array factor is given below. An N-element antenna array which N is a multiple of 4 can be considered as two sub-arrays with spacing of Nd/2. The array factor shown in (2) can then be written as ( ) ( ) ( ) ( ) ( ) 2 2 1 2 1 2 1 1
In this paper, retro-and reflecto-nulling antenna array presented in the last year APMC is extended to the case of odd number of array elements. It can yield two nulls at the incident angle and the reflection angle of the incident wave. With the use of 90 • hybrids and the proper phase shifters, in order to design a retro-and reflecto-nulling antenna arrays with odd number of elements, a folding method is developed and results of three-and five-element retro-and reflecto-nulling antenna arrays are presented.
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