Browsing by Author "Tseng, Tzu-Pang"

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A Near-Real-Time Automatic Orbit Determination System for COSMIC and Its Follow-On Satellite Mission: Analysis of Orbit and Clock Errors on Radio Occultation
(2014-06-01)
Li, Yi-Shan
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Hwang, Cheinway
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Tseng, Tzu-Pang
;
Huang, Cheng-Yung
;
Bock, Heike
The COSMIC-2 mission is a follow-on mission of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) with an upgraded payload for improved radio occultation (RO) applications. The objective of this paper is to develop a near-real-time (NRT) orbit determination system, called NRT National Chiao Tung University (NCTU) system, to support COSMIC-2 in atmospheric applications and verify the orbit product of COSMIC. The system is capable of automatic determinations of the NRT GPS clocks and LEO orbit and clock. To assess the NRT (NCTU) system, we use eight days of COSMIC data (March 24-31, 2011), which contain a total of 331 GPS observation sessions and 12 393 RO observable files. The parallel scheduling for independent GPS and LEO estimations and automatic time matching improves the computational efficiency by 64% compared to the sequential scheduling. Orbit difference analyses suggest a 10-cm accuracy for the COSMIC orbits from the NRT (NCTU) system, and it is consistent as the NRT University Corporation for Atmospheric Research (UCAR) system. The mean velocity accuracy from the NRT orbits of COSMIC is 0.168 mm/s, corresponding to an error of about 0.051 mu rad in the bending angle. The rms differences in the NRT COSMIC clock and in GPS clocks between the NRT (NCTU) and the postprocessing products are 3.742 and 1.427 ns. The GPS clocks determined from a partial ground GPS network [from NRT (NCTU)] and a full one [from NRT (UCAR)] result in mean rms frequency stabilities of 6.1E-12 and 2.7E-12, respectively, corresponding to range fluctuations of 5.5 and 2.4 cm and bending angle errors of 3.75 and 1.66 mu rad.
Assessing antenna field of view and receiver clocks of COSMIC and GRACE satellites: lessons for COSMIC-2
(2014-04-01)
Tseng, Tzu-Pang
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Zhang, Kefei
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Hwang, Cheinway
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Hugentobler, Urs
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Wang, Chuan-Sheng
We provide suggestions for the approved COSMIC-2 satellite mission regarding the field of view (FOV) and the clock stability of its future GNSS receiver based on numerical analyses using COSMIC GPS data. While the GRACE GPS receiver is mounted on the zenith direction, the precise orbit determination (POD) antennas of COSMIC are not. The COSMIC antenna design results in a narrow FOV and a reduction in the number of GPS observations. To strengthen the GPS geometry, GPS data from two POD antennas of COSMIC are used to estimate its orbits. The phase residuals of COSMIC are at the centimeter level, compared to the millimeter level of GRACE. The receiver clock corrections of COSMIC and GRACE are at the microsecond and nanosecond levels, respectively. The clock spectra of COSMIC at the frequencies of 0-0.005 Hz contain significant powers, indicating potential systematic errors in its clock corrections. The clock stability, expressed by the Allan deviation, of COSMIC ranges from 10(-9) to 10(-11) over 1 to 10(4) s, compared to 10(-12) to 10(-14) for GRACE. Compared to USO-based clock of GRACE, the clock of COSMIC is degraded in its stability and is linked to the reduction of GPS data quality. Lessons for improvement of COSMIC-2 over COSMIC in FOV and receiver clock stability are given.
Assessing attitude error of FORMOSAT-3/COSMIC satellites and its impact on orbit determination
(2012-05-01)
Tseng, Tzu-Pang
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Hwang, Cheinway
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Yang, Shan Kuo
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土木工程學系
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Department of Civil Engineering
An attitude determination and control system (ADCS) is critical to satellite attitude maneuvers and to the coordinate transformation from the inertial frame to the spacecraft frame. This paper shows specific sensors in the ADCS of the satellite mission FORMOSAT-3/COSMIC (F3/C) and the impact of the ADCS quality on orbit accuracy. The selection of main POD antenna depends on the beta angles of the different F3/C satellites (for FM2 and FM4) during the inflight phase. In particular, under the eclipse, alternative attitude sensors are activated to replace the Sun sensors, and such a sensor change leads to anomalous GPS phase residuals and a degraded orbit accuracy. Since the nominal attitude serves as a reference for ADCS, the 3-dimensional attitude-induced errors in reduced dynamic orbits over selected days in 2010 show 9.35, 10.78, 4.97, 5.48, 7.18, and 6.89 cm for FM1-FM6. Besides, the 3-dimensional velocity errors induced by the attitude effect are 0.10, 0.10, 0.07, 0.08, 0.09, and 0.10 for FM1-FM6. We analyze the quality of the observed attitude transformation matrix of F3/C and its impact on kinematic orbit determination. With 249 days of GPS in 2008, the analysis leads to the following averaged 3-dimensional attitude-induced orbit errors: 2.72, 2.62, 2.37, 1.90, 1.70, and 1.99 cm for satellites FM1-FM6. Critical suggestions of geodetic payloads for the follow-on mission of F3/C are presented based on the current result. (C) 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.
Geocenter motion estimated from GRACE orbits: The impact of F10.7 solar flux
(2017-06-01)
Tseng, Tzu-Pang
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Hwang, Cheinway
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Sosnica, Krzysztof
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Kuo, Chung-Yen
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Liu, Ya-Chi
We assess the impact of orbit modeling on the origin offsets between GRACE kinematic and reduced-dynamic orbits. The origin of the kinematic orbit is the center of IGS network (CN), whereas the origin of the reduced-dynamic orbit is assumed to be the center of mass of the Earth (CM). Theoretically, the origin offset between these two orbits is associated with the geocenter motion. However, the dynamic property of the reduced-dynamic orbit is highly related to orbit parameterizations. The assessment of the F10.7 impact on the geocenter motion is implemented by using different orbit parameterization setups in the reduced-dynamic method. We generate two types of reduced-dynamic orbits using 15 and 240 empirical parameters per day from 2005 to 2012. The empirical parameter used in Bernese GNSS Software is called piece-wise constant empirical acceleration (PCA) and is mainly to absorb the non-gravitational forces mostly related to the atmospheric drag and solar radiation pressure. The differences between kinematic and dynamic orbits can serve as a measurement for geocenter. The RMS value of the geocenter measurement in the 15-PCA case is approximately 3.5 cm and approximately 2 cm in the 240-PCA case. The correlation between the orbit difference and F10.7 is about 0.90 in the 15-PCA case and -0.10 to 0 in the 240-PCA case. This implies that the reduced-dynamic orbit modeled with 240 PCAs absorbs the F10.7 variation, which aliases to the 15-PCA orbit solution. The annual amplitudes of the geocenter motion are 3.1, 3.1 and 2.5 mm in the 15-PCA case, compared to 0.9, 2.0 and 1.3 mm in the 240-PCA case in the X, Y and Z components, respectively. The 15-PCA solution is thus closer to the geocenter motions derived from other space-geodetic techniques. The proposed method is limited to the parameterizations in the reduced-dynamic approach. (C) 2016 COSPAR. Published by Elsevier Ltd. All rights reserved.
Low-degree gravity change from GPS data of COSMIC and GRACE satellite missions
(2012-01-01)
Lin, Tingjung
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Hwang, Cheinway
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Tseng, Tzu-Pang
;
Chao, B. F.
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土木工程學系
This paper demonstrates estimation of time-varying gravity harmonic coefficients from GPS data of COSMIC and GRACE satellite missions. The kinematic orbits of COSMIC and GRACE are determined to the cm-level accuracy. The NASA Goddard's GEODYN 11 software is used to model the orbit dynamics of COSMIC and GRACE, including the effect of a static gravity field. The surface forces are estimated per one orbital period. Residual orbits generated from kinematic and reference orbits serve as observables to determine the harmonic coefficients in the weighted-constraint least-squares. The monthly COSMIC and GRACE GPS data from September 2006 to December 2007 (16 months) are processed to estimate harmonic coefficients to degree 5. The geoid variations from the GPS and CSR RL04 (GRACE) solutions show consistent patterns over space and time, especially in regions of active hydrological changes. The monthly GPS-derived second zonal coefficient closely resembles the SLR-derived and CSR RL04 values, and third and fourth zonal coefficients resemble the CSR RL04 values. (C) 2011 Elsevier Ltd. All rights reserved.
Modeling Orbit Dynamics of FORMOSAT-3/COSMIC Satellites for Recovery of Temporal Gravity Variations
(2008-11-01)
Hwang, Cheinway
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Lin, Ting-Jung
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Tseng, Tzu-Pang
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Chao, Benjamin Fong
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土木工程學系
The precise GPS high-low tracking data from the joint Taiwan-USA mission FORMOSAT-3/COSMIC (COSMIC) can be used for gravity recovery. The current orbital accuracy of COSMIC kinematic orbit is 2 cm and is better than 1 cm for 60-s normal points. We model the perturbing forces acting on the COSMIC spacecraft based on standard models of orbit dynamics. The major tool for the numerical work of force modeling is NASA Goddard's GEODYN II software. Considering that COSMIC spacecrafts are not equipped with accelerometers, the accelerations due to atmospheric drag, solar radiation pressure, and other minor surface forces are modeled by estimating relevant parameters over one orbital period from COSMIC's kinematic and reduced dynamic orbits. We carry out experimental solutions of time-varying geopotential coefficients using one month of COSMIC kinematic orbits (August 2006). With the nongravity origin forces properly modeled by GEODYN II, residual orbital perturbations (difference between kinematic and reference orbits) are assumed to be linear functions of time-varying geopotential coefficients and are used as observations to estimate the latter. Both COSMIC and combined COSMIC and GRACE gravity solutions are computed. The COSMIC solution shows some well-known temporal gravity signatures but contains artifacts. The combined COSMIC and GRACE solution enhances some local temporal gravity signatures in the GRACE solution.
Precise orbit determination for the FORMOSAT-3/COSMIC satellite mission using GPS
(2009-05-01)
Hwang, Cheinway
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Tseng, Tzu-Pang
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Lin, Tingjung
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Svehla, Drazen
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Schreiner, Bill
The joint Taiwan-US mission FORMOSAT-3/ COSMIC (COSMIC) was launched on April 17, 2006. Each of the six satellites is equipped with two POD antennas. The orbits of the six satellites are determined from GPS data using zero-difference carrier-phase measurements by the reduced dynamic and kinematic methods. The effects of satellite center of mass (COM) variation, satellite attitude, GPS antenna phase center variation (PCV), and cable delay difference on the COSMIC orbit determination are studied. Nominal attitudes estimated from satellite state vectors deliver a better orbit accuracy when compared to observed attitude. Numerical tests show that the COSMIC COM must be precisely calibrated in order not to corrupt orbit determination. Based on the analyses of the 5 and 6-h orbit overlaps of two 30-h arcs, orbit accuracies from the reduced dynamic and kinematic solutions are nearly identical and are at the 2-3 cm level. The mean RMS difference between the orbits from this paper and those from UCAR (near real-time) and WHU (post-processed) is about 10 cm, which is largely due to different uses of GPS ephemerides, high-rate GPS clocks and force models. The kinematic orbits of COSMIC are expected to be used for recovery of temporal variations in the gravity field.
Quality assessment of FORMOSAT-3/COSMIC and GRACE GPS observables: analysis of multipath, ionospheric delay and phase residual in orbit determination
(2010-01-01)
Hwang, Cheinway
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Tseng, Tzu-Pang
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Lin, Ting-Jung
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Svehla, Drazen
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Hugentobler, Urs
The precise orbit determination antennas of F3/C and GRACE-A satellites are from the same manufacturer, but are installed in different configurations. The current orbit accuracy of F3/C is 3 cm at arcs with good GPS data, compared to 1 cm of GRACE, which has a larger ratio of usable GPS data. This paper compares the qualities of GPS observables from F3/C and GRACE. Using selected satellites and time spans, the following average values for the satellite F3/C and satellite A of GRACE are obtained: multipath effect on the pseudorange P1, 0.78 and 0.38 m; multipath effect on the pseudorange P2, 1.03 and 0.69 m; occurrence frequency of cycle slip, 1/29 and 1/84; standard error of unit weight, 4 and 1 cm; dynamic-kinematic orbit difference, 10 and 2 cm. For gravity determination using F3/C GPS data, a careful selection of GPS data is critical. With six satellites in orbit, F3/C's large amount of GPS data will make up the deficiency in data quality.