• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.45 no.9
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• pp.784-793
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• 2017

A two-axis gimbal-type X-band antenna mounted on an observation satellite can efficiently transmit high-capacity image data to a ground station regardless of both the satellite position and the orbital motion. However, this X-band antenna induces unnecessary micro-jitter which can degrade the image quality of the high-resolution observation satellite. Therefore, to achieve the high-resolution image quality from the observation satellite, micro-jitters have been required to be isolated. In this study, to resolve aforementioned drawback, we proposed blade gear using a shape memory alloy (SMA) applied to azimuth stage of X-band antenna. To investigate the rotational basic characteristics of the proposed SMA blade gear, we performed rotational static loading test. Futhermore, to evaluate the cycle to failure of the gear, accelerated life test was conducted. The temperature test was conducted to confirm rotational basic characteristics at various temperature conditions. To verify the isolation performance for micro-jitter, we performed micro-jitter measurement test.

• Journal of Aerospace System Engineering
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• v.12 no.2
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• pp.46-58
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• 2018

A two-axis gimbal-type X-band antenna is widely used to transmit bulk image data from high-resolution observation satellites. However, undesirable microvibrations induced by driving the antenna should be attenuated, because they are a main cause of image-quality degradation of the observation satellite. In this study, a pseudoelastic memory alloy (SMA) gear was proposed to attenuate the microvibrations by driving the antenna in an azimuth angle. In addition, the proposed gear can overcome the limitations of the conventional titanium blade gear, which is not still enough and is vulnerable to plastic deformations under excessive torque. To investigate the basic characteristics of the proposed SMA mesh washer gear, a static load test was performed on the thickness of the SMA mesh washer and the rotation of the gear. Moreover, The microvibration measurement test demonstrated that the SMA mesh washer gear proposed in this study is effective for microvibration attenuation.

• Publications of The Korean Astronomical Society
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• v.11 no.1
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• pp.109-123
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• 1996

We have developed a 5GHz continuum receiver system. The receiver is a direct type receiver. In order to reduce the noise due to the fluctuation of the gain in the amplifiers, the system employs the Dicke switching method. We made the 5GHz low-noise amplifier and the bandpass filter. The low-noise amplifier gives ${\sim}35dB$ gain and has ${\sim}210K$ noise temperature. The bandpass filter has a passband between 4.3 and 5.4GHz. We also made switch driver, video amplifiers, phase detector, and integrator. Using a 1.8 meter offset parabolic antenna, we measured the efficiency of the system. Since the antenna does not have a driver to track objects, observations were performed with the antenna fixed. The measured noise temperature of the system is ${\sim}650K$. From the observation of the blank sky, noise level was measured. It was found that the systematic noise(${\sim}0.5K$: peak to peak value) is much larger than the thermal noise. The systematic noise is possibly related to the stability of the DC power supplied to the receiver system. Besides the noise of the system, it was found that the airplanes are the very serious noise sources. We measured the radio flux of the Sun using the developed system. The observed radio flux of the Sun is ${\sim}10^6Jy$, which is close to the known value of the quiet Sun. The test observation of the Sun shows that the angular beam size of the antenna is ${\sim}2.2^{\circ}$.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.38 no.1
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• pp.27-33
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• 2001

This article describes an observation that the aperture coupling mechanisms in the aperture coupled microstrip antenna can be divided into two categories, cavity and parasitic types, depending on the separation between the microstrip patch and the ground plane. The similar phenomenon was observed in the relatively simple gap coupled microstrip antenna. The specific characteristics between two coupling mechanisms is discussed.

• Journal of Astronomy and Space Sciences
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• v.23 no.3
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• pp.269-288
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• 2006

We revised the OTFTOOL which was developed in Five College Radio Astronomy Observatory (FCRAO) for the On-The-Fly (OTF) observation. Besides the improvement of data resampling function of conventional OTFTOOL, we added a new SELF referencing mode and data pre-reduction function. Since OTF observation data have a large redundancy, we can choose and use only good quality samples excluding bad samples. Sorting out the bad samples is based on the floating level, rms level, antenna trajectory, elevation, $T_{sys}$, and number of samples. And, spikes are also removed. Referencing method can be chosen between CLASSICAL mode in which the references are taken from the OFFs observation and ELLIPSOIDAL mode in which the references are taken from the inner source free region (this is named as SELF reference). Baseline is subtracted with the source free channel windows and the baseline order chosen by the user. Passing through these procedures, the raw OTF data will be an FITS datacube. The revised-OTFTOOL maximizes the advantages of OTF observation by sorting out the bad samples in the earliest stage. And the new self-referencing method, the ELLIPSOIDAL mode, is very powerful to reduce the data. Moreover since it is possible to see the datacube at once without moving them into other data reduction programs, it is very useful and convenient to check whether the data resampling works well or not. We expect that the revised-OTFTOOL can be applied to the facilities of the OTF observation like SRAO, NRAO, and FCRAO.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.7
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• pp.601-609
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• 2014

Large number of SAR(Synthetic Aperture Radar) satellites, one type of earth observation satellite, have been developed as they have the advantage of not being affected by surrounding environment during the earth image acquisition. In order to gain high image quality, SAR antenna should have large diameter. However, internal space of satellite launch vehicle is limited and this leads SAR antenna to be designed deployable so that it can be folded in launch vehicle and unfolded in space. In this research, values of various design factors of deployable satellite antenna were chosen considering satellite's target mission. Configuration of deployable satellite antenna was designed by applying the chosen values of design factors, and variation in deployable satellite antenna during satellite maneuver was observed through simulation.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.05a
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• pp.119-122
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• 2002

The vibration qualification test of satellite antenna is required to verify that there will be no structural damage due to the severe vibration caused by the launch of satellite. For the qualification test, reasonable test load condition needs to be introduced by dynamic analysis. The present work has been performed to provide an understanding how the qualification test load can be evaluated by the results of both normal mode and sine vibration analyses with notching technique for a composite Ka-band antenna structure.

• Korean Journal of Remote Sensing
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• v.34 no.4
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• pp.639-647
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• 2018

This paper presents TerraSAR-X and KOMPSAT-5 sigma nought variation according to season and antenna observation configuration in Mongolia. Two types of landcover including bare surface and cropland were examined. The seasonal variation of sigma nought in cropland was about 7 dB and particularly a significant sigma nought reduction occurred after harvest. On the contrary, the Mongolia bare surface provides a consistent sigma nought values for several years with an annual variation less than 2.5 dB of standard deviation. However, the bare soil was relatively sensitive to look-direction (or ascending or descending mode) as well as incidence angle while the cropland was almost independent of antenna look-direction and small incidence angle changes. Although the look-directional variation of bare surface sigma nought was observed in this study, the look-direction anisotropic nature of the surface was not well examined. A further study would be required to account for this feature with various SAR observation configurations.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.73.2-73.2
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• 2016

We present the results of KaVA SgrA* observation together with Takahagi(32m), Yamaguchi(32m) and Nobeyama(45m) telescopes at 22 and 43GHz, respectively. In early 2014, G2 cloud was expected to encounter with SgrA* and to make a significant flux variation, but it has not been measured yet. So it's worth to check our amplitude calibration method to confirm if we have a missing flux caused by uncertainty in measuring it. We have tested both a standard method using system noise temperature(Tsys) with antenna gain information, and a template method in order to calibrate antenna gain using nearby maser source. As a result, we found that the latter method is useful for antennas which have inaccurate gain information or poor Tsys measurements, and is especially effective for sources at low elevation like SgrA*. In addition, the comparison shows that the amplitude calibration by standard method can be improved up to 10% with a correction factor using a template method. This result implies we can get more accurate flux from a standard method when any maser source not exists around target.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.34 no.6
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• pp.198-210
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• 2022

The southern strait of Jeju is a divergence point of the Tsushima Warm Current (TWC), and it is the starting point of the thermohaline circulation in the waters of the Korean Peninsula, affecting the size and frequency of marine disasters such as typhoons and tsunamis, and has a very important oceanographic impact, such as becoming a source of harmful organisms and radioactively contaminated water. Therefore, for an immediate response to these maritime disasters, real-time ocean observation is required. However, compared to other straits, in the case of southern Jeju, such wide area marine observations are insufficient. Therefore, in this study, surface current field of the southern strait of Jeju was calculated using High-Frequency radar (HF radar). the large surface current field is calculated, and post-processing and data improvement are carried out through APM (Antenna Pattern Measurement) and FOL (First Order Line), and comparative analysis is conducted using actual data. As a result, the correlation shows improvement of 0.4~0.7 and RMSE of about 1~19 cm/s. These high-frequency radar observation results will help solve domestic issues such as response to typhoons, verification of numerical models, utilization of wide area wave data, and ocean search and rescue in the future through the establishment of an open data network.