Kyoung-Min Roh

Mutation in the Transcriptional Regulator PhoP Contributes to Avirulence of Mycobacterium tuberculosis H37Ra Strain

Attenuated strains of mycobacteria can be exploited to determine genes essential for their pathogenesis and persistence. To this goal, we sequenced the genome of H37Ra, an attenuated variant of Mycobacterium tuberculosis H37Rv strain. Comparison with H37Rv revealed three unique coding region polymorphisms. One polymorphism was located in the DNA-binding domain of the transcriptional regulator PhoP, causing the proteins diminished DNA-binding capacity. Temporal gene expression profiles showed that several genes with reduced expression in H37Ra were also repressed in an H37Rv phoP knockout strain. At later time points, genes of the dormancy regulon, typically expressed in a state of nonreplicating persistence, were upregulated in H37Ra. Complementation of H37Ra with H37Rv phoP partially restored its persistence in a murine macrophage infection model. Our approach demonstrates the feasibility of identifying minute but distinct differences between isogenic strains and illustrates the consequences of single point mutations on the survival stratagem of M. tuberculosis.

Orbit Determination Using the Geomagnetic Field Measurement via the Unscented Kalman Filter

OrbitdeterminationofspacecraftusingonlymagnetometermeasurementsviatheunscentedKalman filter(UKF) is presented. An algorithm was formulated by adopting the UKF and an adequate dynamic model developed for processing geomagnetic field measurements. The paper consists of the analysis of force and estimation models, the dependencyofposition accuracyonorbittype,andmeasurement errors,as wellas acomparisonof theUKFandthe extended Kalman filter (EKF). Finally, the developed algorithm used actual magnetometer flight data from the Magnetic Field Satellite (MAGSAT). The results obtained from the MAGSAT data demonstrate that the achieved position error was approximately 2 km. The UKF performs similar to the EKF in position accuracy for a sampling interval of less than 20 s. In contrast, for a sampling interval of 40 s, the EKF yields lower position accuracy than the UKF.Inparticular,thedecomposedpositionerrorsarenotbiasedbecausetheUKFisnotaffectedbylinearizationof the measurement function. This improvement makes the magnetometer-based orbit determination method more robust and reliable as a real-time orbit determination system for small satellites that require moderate position accuracy, and also as a backup orbit determination system for large satellites. Nomenclature adrag = acceleration due to atmospheric drag, m=s 2 ageo = acceleration due to geopotential, m=s 2 aSRP = acceleration due to the solar radiation pressure, m=s 2 asun=moon = acceleration due to the sun and moon’s gravity, m=s 2 Br, B� , B� = the Earth magnetic field vector components, nT B � = inverse of the ballistic coefficient, m 2 =kg e = eccentricity

Orbit Determination of Spacecraft Using Global Positioning System Single-Frequency Measurement

The dynamic orbit determination of a low Earth orbiter using global positioning system single-frequency measurements has been implemented. Currently two methods are being applied to eliminate or reduce ionospheric path delay in single-frequency measurement. One is a group and phase ionosphere calibration technique using code pseudorange and L1 carrier phase, and the other is application of total electron content values from an ionospheric model using only L1 carrier phase to determine the orbit. A new method based on the latter has been developed, which estimates the scale factors of total electron content values in the location of a low Earth orbiter once per each measurement time. Orbit determination using actual global positioning system measurements of the TOPEX/POSEIDON and the Challenging minisatellite payload was conducted to verify the accuracy of the new method. It is verified that, if the total electron contents scale factor estimation technique were applied, 1-m level position accuracy (1σ) for low Earth orbit below 500-km altitudes could be achieved using precision orbit determination based on the global positioning system double-differencing method.

Reconstruction of KOMPSAT-l GPS navigation solutions using GPS data generation and preprocessing program

Abstract GPS navigation solutions and GPS-related telemetry points are analyzed using one-day playback data from the KOMPSAT-1 Spacecraft. Then, the GPS navigation solutions are reconstructed using simulated GPS raw measurements such as pseudo-ranges between the KOMPSAT-1 and GPS satellites constellation. GPS data generation and preprocessing computer programs are developed and used for the generation of the GPS raw measurement and reconstruction of the GPS navigation solutions. The playback GPS data and the reconstructed data are compared with each other.

Comparison of the Characteristics of Precipitable Water Vapor Measured by Global Positioning System and Microwave Radiometer

Korea Astronomy and Space Science Institute, Daejeon 305-348, KoreaIn this study, global positioning system (GPS)-derived precipitable water vapor (PWV) and microwave radiometer (MWR)-measured integrated water vapor (IWV) were compared and their characteristics were analyzed. Comparing those two quantities for two years from August 2009, we found that GPS PWV estimates were larger than MWR IWV. The average difference over the entire test period was 1.1 mm and the standard deviation was 1.2 mm. When the discrepan-cies between GPS PWV and MWR IWV were analyzed depending on season, the average difference was 0.7 mm and 1.9 mm in the winter and summer months, respectively. Thus, the average difference was about 2.5 times larger in summer than that in winter. However, MWR IWV measurements in the winter months were over-estimated than those in the sum -mer months as the water vapor content got larger. The results of the diurnal analysis showed that MWR IWV was under-estimated in the daytime, showing a difference of 0.8 mm. In the early morning hours, MWR IWV has a tendency to be over-estimated, with a difference of 1.3 mm with respect to GPS PWV.

Development of the Kinematic Global Positioning System Precise Point Positioning Method Using 3-Pass Filter

Kinematic global positioning system precise point positioning (GPS PPP) technology is widely used to the several area such as monitoring of crustal movement and precise orbit determination (POD) using the dual-frequency GPS observations. In this study we developed a kinematic PPP technology and applied 3-pass (forward/backward/forward) filter for the stabilization of the initial state of the parameters to be estimated. For verification of results, we obtained GPS data sets from six international GPS reference stations (ALGO, AMC2, BJFS, GRAZ, IENG and TSKB) and processed in daily basis by using the developed software. As a result, the mean position errors by kinematic PPP showed 0.51 cm in the east-west direction, 0.31 cm in the north-south direction and 1.02 cm in the up-down direction. The root mean square values produced from them were 1.59 cm for the east-west component, 1.26 cm for the south-west component and 2.95 cm for the up-down component.

Radiosonde Sensors Bias in Precipitable Water Vapor From Comparisons With Global Positioning System Measurements

In this study, we compared the precipitable water vapor (PWV) data derived from the radiosonde observation data at Sokcho Observatory and the PWV data at Sokcho Global Positioning System (GPS) Observatory provided by Korea Astronomy and Space Science Institute, for the years of 2006, 2008, 2010, and analyzed the radiosonde seasonal, diurnal bias according to radiosonde sensor types. In the scatter diagram of the daytime and nighttime radiosonde PWV data and the GPS PWV data, dry bias was found in the daytime radiosonde observation as known in the previous study. Overall, the tendency that the wet bias of the radiosonde PWV increased as the GPS PWV decreased and the dry bias of the radiosonde PWV increased as the GPS PWV increased. The quantitative analysis of the bias and error of the radiosonde PWV data showed that the mean bias decreased in the nighttime except for 2006 winter, and in comparison for summer, RS92-SGP sensor showed the highest quality.

The Effect of Equatorial Spread F on Relative Orbit Determination of GRACE Using Differenced GPS Observations

In this paper, relative orbit of Low Earth Orbit satellites is determined using only GPS measurements and the effects of Equatorial Spread-F (ESF), that is one of biggest ionospheric irregularities, are investigated. First, relative orbit determiation process is constructed based on doubly differenced GPS observations. In order to see orbit determination performance, relative orbit of two GRACE satellites is estimated for one month in 2004 when no ESF is observed. The root mean square of the achieved baselines compared with that from K-Band Ranger sensor is about 2~3 mm and average of 95% of ambiguities are resolved. Based on this performance, the relative orbit is estimated for two weeks of two difference years, 2003 when there are lots of ESF occurred, and 2004 when only few ESF occurred. For 2003, the averaged baseline error over two weeks is about 15 mm. That is about 4 times larger than the case of 2004 (3.6 mm). Ionospheric status achieved from K-Band Ranging sensor also shows that more Equatorial Spread-F occurred at 2003 than 2004. Investigation on raw observations and screening process revealed that the ionospheric irregualarities caused by Equatorial Spread-F gave significant effects on GPS signal like signal loss or enhancement ionospheric error, From this study, relative orbit determination using GPS observations should consider the effect of Equatorial Spread-F and adjust orbit determination strategy, especially at the time of solar maximum.

Real-time GPS Ionospheric TEC Estimation over South Korea

Ionosphere is one of the largest error sources when the navigational signals produced by Global Positioning System (GPS) satellites are transmitted. Therefore it is very important to estimate total electron contents (TEC) in ionosphere precisely for navigation, precise positioning and some other applications. When we provide ionospheric TEC values in real-time, its application can be expanded to other areas. In this study we have used data obtained from nine Global Navigation Satellite System (GNSS) reference stations which have been operated by Korea Astronomy and Space Science Institute (KASI) to detect ionospheric TEC over South Korea in real-time. We performed data processing that covers converting 1Hz raw data delivered from GNSS reference stations to Receiver INdependent Exchange (RINEX) format files at intervals of 5 minutes. We also analyzed the elevation angles of GPS satellites, vertical TEC (VTEC) values and their changes.

The Effects of the IERS Conventions (2010) on High Precision Orbit Propagation

The Earth is not perfectly spherical and its rotational axis is not fixed in space, and these geophysical and kinematic irregularities work as dominant perturbations in satellite orbit propagation. The International Earth Rotation Service (IERS) provides the Conventions as guidelines for using the Earth’s model and the reference time and coordinate systems defined by the International Astronomical Union (IAU). These guidelines are directly applied to model orbital dynamics of Earth satellites. In the present work, the effects of the latest conventions released in 2010 on orbit propagation are investigated by comparison with cases of applying the previous guidelines, IERS Conventions (2003). All seven major updates are tested, i.e., for the models of the precession/nutation, the geopotential, the ocean tides, the ocean pole tides, the free core nutation, the polar motion, and the solar system ephemeris. The resultant position differences for one week of orbit propagation range from tens of meters for the geopotential model change from EGM96 to EGM2008 to a few mm for the precession/nutation model change from IAU2000 to IAU2006. The along-track differences vary secularly while the cross-track components show periodic variation. However, the radial-track position differences are very small compared with the other components in all cases. These phenomena reflect the variation of the ascending node and the argument of latitude. The reason is that the changed models tested in the current study can be regarded as small fluctuations of the geopotential model from the point of view of orbital dynamics. The ascending node and the argument of latitude are more sensitive to the geopotential than the other elements. This study contributes to understanding of the relation between the Earth’s geophysical properties and orbital motion of satellites as well as satellite-based observations.