• Korean Journal of Remote Sensing
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• v.34 no.4
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• pp.671-680
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• 2018

Compared to the imagery produced by traditional satellites, PlanetScope satellite imagery has made it possible to easily capture remotely-sensed imagery every day through dozens or even hundreds of satellites on a relatively small budget. This study aimed to detect changed areas and update a land cover map using a PlanetScope image. To generate a classification map, pixel-based Random Forest (RF) classification was performed by using additional features, such as the Normalized Difference Water Index (NDWI) and the Normalized Difference Vegetation Index (NDVI). The classification result was converted to vector data and compared with the existing land cover map to estimate the changed area. To estimate the accuracy and trends of the changed area, the quantitative quality of the supervised classification result using the PlanetScope image was evaluated first. In addition, the patterns of the changed area that corresponded to the classification result were analyzed using the PlanetScope satellite image. Experimental results found that the PlanetScope image can be used to effectively to detect changed areas on large-scale land cover maps, and supervised classification results can update the changed areas.

• Journal of Positioning, Navigation, and Timing
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• v.12 no.1
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• pp.11-22
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• 2023

In this paper, we propose an efficient satellite selection method for multi-constellation GNSS. The number of visible satellites has increased dramatically recently due to multi-constellation GNSS. By the increased availability, the overall GNSS performance can be improved. Whereas, due to the increase of the number of visible satellites, the computational burden in implementing advanced processing such as integer ambiguity resolution and fault detection can be increased considerably. As widely known, the optimal satellite selection method requires very large computational burden and its real-time implementation is practically impossible. To reduce computational burden, several sub-optimal but efficient satellite selection methods have been proposed recently. However, these methods are prone to the local optimum problem and do not fully utilize the information redundancy between different constellation systems. To solve this problem, the proposed method utilizes the inter-system biases and geometric assignments. As a result, the proposed method can be implemented in real-time, avoids the local optimum problem, and does not exclude any single-satellite constellation. The performance of the proposed method is compared with the optimal method and two popular sub-optimal methods by a simulation and an experiment.

• Journal of Space Technology and Applications
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• v.3 no.4
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• pp.322-332
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• 2023

Satellite attitude-control actuators are equipped with a reaction wheel for three-axis attitude control. The reaction wheel rotates a motor inside the actuator to generate torque in the vector direction. When using the reaction wheel, there are restrictions on the torque values generated as the motor rotates. The torque value of the reaction wheels mounted on small satellites is approximately 10 mNm, and high values are not used. Therefore, three-axis attitude control of a small satellite is possible using a reaction wheel, but this method is not suitable for missions that require rapid attitude control at a specific time. As a technology to overcome the small torque value of the reaction wheel, the control moment gyro (CMG) is currently in wide use as a rapid attitude-control actuator in space satellites. The CMG has an internal gimbal mounted at a right angle to the rotation motor and generates a large torque value. In general, when the gimbal operates, a torque value approximately 100 times greater is generated, making it suitable for rapid posture maneuvering. Currently, we are developing a technology for mounting a controlled moment gyro on a small satellite, and here we share the development status of an 800 mNm CMG.

• Aerospace Engineering and Technology
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• v.12 no.2
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• pp.57-63
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• 2013

The mission of a lot of satellites on geostationary orbit is the communication and/or the broadcasting. These satellites need a big power, so these have a large solar array. Recently, the new satellite for Earth environment monitoring is developing on geostationary orbit. The payload of Earth monitoring satellite requires better thermal condition on detector. Therefore this satellite uses a boom for the attitude stability instead of rejecting one-side solar array as a heat source. The other hand, it uses some momentum wheels being a more momentum capacity to control the large disturbance by solar pressure due to the asymmetric solar array configuration. In this paper, the analysis on the wheel allocation and the wheel off-loading for COMS is summarized and the results are verified by telemetry of COMS. COMS has no boom and a perfectly asymmetric solar array configuration, and it is operating well on geostationary orbit.

• Korean Journal of Remote Sensing
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• v.35 no.5_2
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• pp.773-779
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• 2019

We developed the automatic ground deformation monitoring system using Sentinel-1 satellites which is operating by European Space Agency (ESA) for the Korea Peninsula's ground disaster monitoring. Ground deformation occurring over a long-term period are difficult to monitoring because it occurred in a wide area and required a large amount of satellite data for analysis. With the development of satellites, the methods to regularly observe large areas has been developed. These accumulated satellite data are used for time series ground displacement analysis. The National Disaster Management Research Institute (NDMI) established an automation system for all processes ranging from acquiring satellite observation data to analyzing ground displacement and expressing them. Based on the system developed in this research, ground displacement data on the Korean Peninsula can be updated periodically. In the future, more diverse ground displacement information could be provided if automated small regional analysis systems, multi-channel analysis method, and 3D analysis system techniques are developed with the existing system.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.50 no.3
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• pp.203-214
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• 2022

As the global demand for small satellites weighing less than 500 kg increases, the development and operation of dedicated small launch vehicles increase significantly. The responsiveness of a launch vehicle that puts a small satellite into a target orbit at the desired time is attracting attention. As a result, interest in the air launch is increasing in the rapid establishment of a constellation. As the demand for small satellites in south Korea increases, this study performed analyses on the applicability of an air launch vehicle using a large civil aircraft considering the geographical environment. In terms of responsiveness, mission response times were compared and analyzed for air launch vehicles and ground small and large vehicles. In addition, an air vehicle and a small ground vehicle were quantitatively compared and analyzed for the orbital insertion performance. As a result of the analysis, the air launch vehicle has limited responsiveness in Korea regarding rapid satellite constellation establishment. However, it can be an effective alternative for low inclination angle orbit insertion with the benefit of a fast turnaround time. Furthermore, the performance of the orbital injection is close to that of the ground small launch vehicle, and the high efficiency in terms of the required propellant mass is possible, so air launch can be an effective launch means for putting small satellites into orbit in Korea.

• Journal of Aerospace System Engineering
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• v.16 no.4
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• pp.28-34
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• 2022

In 2021, Virgin Orbit converted a 747-400 aircraft into an air launch platform, and successfully launched it twice in February and July. Compared to the existing ground launch, interest in the air launch is increasing due to its great utility, such as its independence from the launch location or weather, cost reducing factor, shorter launch preparation time, and its benefit pursuant to altitude and speed. Additionally, as small satellites have similar performance to mid/large satellites in the past due to the miniaturization and precision of electronic equipment, small satellite launches are expected to dominate in the future. In this paper, institutional certification methods such as domestic, overseas, civilian and military airworthiness certification regulations/procedures are reviewed to ensure flight safety of aerial projectiles using large domestic civil aircraft, and applicable civil and military airworthiness certification technology standards are reviewed and analyzed. Additionally, we will review and suggest effective airworthiness certification application plans that reflect the reality, and present airworthiness certification standards (draft) for aerial launch vehicles, by analyzing applicable airworthiness certification technical standards when remodeling aerial launch vehicles.

• Aerospace Engineering and Technology
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• v.7 no.2
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• pp.76-81
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• 2008

The vibration test system of satellite environment test dept. has been used successfully for the vibration tests of a majority of korean space programs. To meet the recent needs of large size test facility available for the vibrational tests of the huge launch vehicles and satellites, KARI have developed the large size multi-electromagnetic shaking system with $3{\times}3m$ head expander system. The new system will consist of three electromagnetic shakers which has 160 kN thrust force individually, and be able to sustain up to 8 tons test load and 300 kNm overturing moment. This paper describes the design components in the development process of multi-excitation shaker system.

• Journal of Korean Society for Atmospheric Environment
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• v.20 no.4
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• pp.437-450
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• 2004

Extensive forest fire activities occurred across the border in Russia, particularly east of Lake Baikal between the Amur and Lena rivers in May 2003. These forest fires released large amounts of particulates and gases into the atmosphere, resulting in adverse effects on regional air quality and the global radiation budget. Smoke pollution from the Russian fires near Lake Baikal was transported to Korea through Mongolia and eastern China. On 20 May 2003, a number of large fires were burning in eastern Russian, producing a thick, widespread pall of smoke over much of Northeast Asia. In this study, separation technique was used for aerosol retrieval application with imagery from MODIS aboard TERRA satellites. MODIS true-color image shows the location of fires and the grayish color of the smoke plumes over Northeast Asia. Aerosol optical thckness (AOT) retrieved from the MODIS data were compared with fire hot spots, ground-based radiation data and TOMS -based aerosol index data. Large AOT, 2.0-5.0 was observed on 20 May 2003 over Korea due to the influence of the long range transport of smoke aerosol plume from the Russian fires, while surface observed fine mode of aerosol size distribution increased.

• Korean Journal of Remote Sensing
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• v.38 no.6_1
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• pp.1231-1244
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• 2022

The frequency of exposure of field crops to stress situations is increasing due to abnormal weather conditions. In South Korea, large-scale diseases in representative paddy rice cultivation area were happened. There are limits to field investigation on the crop damage due to large-scale. Satellite-based remote sensing techniques are useful for monitoring crops in cities and counties, but the sensitivity of vegetation index measured from satellite under abnormal growth of crop should be evaluated. The goal is to evaluate satellite-based normalized difference vegetation index (NDVI) retrieved from different spatial scales using drone imagery. In this study, Sentinel-2 and Landsat-8 satellites were used and they have spatial resolution of 10 and 30 m. Drone-based NDVI, which was resampled to the scale of satellite data, had correlation of 0.867-0.940 with Sentinel-2 NDVI and of 0.813-0.934 with Landsat-8 NDVI. When the effects of bias were minimized, Sentinel-2 NDVI had a normalized root mean square error of 0.2 to 2.8% less than that of the drone NDVI compared to Landsat-8 NDVI. In addition, Sentinel-2 NDVI had the constant error values regardless of diseases damage. On the other hand, Landsat-8 NDVI had different error values depending on degree of diseases. Considering the large error at the boundary of agricultural field, high spatial resolution data is more effective in monitoring crops.