Tie of the Key Stone Project VLBI Network to the International Terrestrial Reference Frame

Yasuhiro Koyama1(koyama(AT)nict.go.jp), Ryuichi Ichikawa1, and Jun Amagai2

1 Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan

2Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan

1. Introduction

In the Key Stone Project, site coordinates of three VLBI stations at Koganei, Miura, and Tateyama are estimated from the data obtained in each experiment while the site coordinates of the 11m antenna at Kashima for the Key Stone Project (KASHIM11) is fixed in the process of least-square estimations [Koyama, 1996]. The a-priori information of KASHIM11 coordinates should be given in the ITRF94 reference frame to make them consistent with other a-priori parameters. The source positions and Earth Orientation Parameters are taken from ICRF94 and EOP(IERS)90C04 series and its extensions reported in regular bulletins from IERS, respectively, which are consistent with ITRF94. It is also important to give accurate and reliable ITRF94 coordinates of KASHIM11 to tie the VLBI Network of the Key Stone Project with the global terrestrial reference frame. Estimated site coordinates of three VLBI stations in the Key Stone Project can be used as accurate reference points in the ITRF94 reference frame if the site coordinates of KASHIM11 station can be determined in the frame with sufficient accuracy. Two geodetic VLBI experiments have been performed to determine the KASHIM11 site coordinates in the ITRF94. The first one was performed for 24 hours on January 17, 1995 with 34m antenna station at Kashima (KASHIM34) and two Key Stone Project sites at Kashima and Koganei. The second experiment was performed for 23.5 hours on May 1, 1997 with KASHIM34 and four Key Stone Project sites. The estimated KASHIM11 positions are compared in Figure 1. In the figure, estimated positions are shown by ellipses which express the one-sigma uncertainty either in the horizontal plane or in the vertical plane.


Figure 1. Comparison of KASHIM11 position estimated from two joint experiments. The ellipses express one sigma uncertainties of the estimated position (a) in the horizontal plane and (b) in the vertical plane.

Uncertainties in the results from the first experiment are larger than the results from the second experiment mainly because KASHIM34-KASHIM11 baseline data were not correlated in the first experiment. From the comparison, vertical position and North-South component of the KASHIM11 station is consistent between the two experiments, while there is a significant discrepancy in the East-West component. The cause of this discrepancy is not known but it might have been introduced when the vertical alignment of the pedestal of the antenna at Kashima was corrected in February 1996. The movement of the intersection of the Azimuth and Elevation axes of the antenna was measured at the time of correction. From the measurements, the position of the KASHIM11 station moved by 4.4 mm in the direction of N25oW. The discrepancy in Figure 1 is about 12 mm mainly in the westward direction and is not fully explained by the correction made in February 1996. The situation will be made clearer in the future when more joint VLBI experiments are performed with KASHIM34 and Key Stone Project VLBI Network.

Table 1. Estimated VLBI site positions R on January 1, 1997, and velocities V defined in the ITRF94 coordinate system.
Site X Y Z
KoganeiR (mm) -3941937398.4+/-5.8 3368150858.3+/-4.7 3702235261.4+/-5.0
V (mm/year) 7.5+/-0.7-5.2+/-0.6-15.8+/-0.6
KashimaR (mm) -3997505622.1+/-6.1 3276878350.2+/-4.8 3724240665.8+/5.2
V (mm/year) 5.1+/-2.3-3.6+/-2.0-19.8+/-2.5
Miura R (mm) -3976129918.1+/-6.9 3377927833.6+/-5.7 3656753813.7+/-5.9
V (mm/year) 24.2+/-1.1-6.7+/-0.9-15.5+/-1.0
Tateyama R (mm) -4000983352.7+/-5.7 3375275900.1+/-4.7 3632213145.2+/-5.0
V (mm/year) 33.5+/-1.9-13.3+/-1.6-24.0+/-1.7

Table 1 shows the ITRF94 coordinates of four VLBI stations in the Key Stone Project VLBI Network estimated from the data obtained in the second joint experiment on May 1, 1997. The uncertainties of the site coordinates are one-sigma standard deviations and the uncertainty of the site coordinate of KASHIM34 given in the ITRF94 is not added to the uncertainties in the table. Site velocities in the ITRF94 are also shown in the table. Site velocity of the KASHIM11 station is assumed to be the same as the site velocity of KASHIM34 given in the ITRF94. The site velocities of the three other stations are the results obtained from the Key Stone Project VLBI experiments until the end of August 1997. While the uncertainty of the KASHIM11 site velocity is the value for KASHIM34 site velocity in the ITRF94, the uncertainties of site velocities of the three other stations are calculated from the results of Key Stone Project VLBI experiments and the site velocity uncertainties of the KASHIM11 station are not added.

2. Ground Survey Measurements

The relative positions of reference points of the SLR and GPS facilities with respect to the reference point of the VLBI antenna were measured by ground surveys. Reference points of VLBI and SLR are defined by intersection of azimuth axis and elevation axis of the antenna and the telescope, respectively. Reference point of GPS is defined by the phase center of the antenna and it is assumed to be the center of the top plane of the base plate of the antenna. The results are tabulated in Table 2 along with the results obtained from VLBI and GPS observations. Geographical locations of the three techniques at sites in the Key Stone Project Network are shown in Figure 2. VLBI results are the coordinates estimated from the experiment on May 1, 1997. GPS results were obtained by averaging three independent estimates from three days of observations from July 30, 1997. GPS observations were performed with the Ashtech receivers at four stations in the Key Stone Project Network and at Tsukuba (TSKB) which is an IGS site operated by the Geographical Survey Institute. Data were collected at 30 second intervals for 24 hours a day, and the observed data were analyzed by using Bernese Version 4.0 software which has been developed in Bern University. In the GPS data analysis, the coordinate of TSKB was constrained to an IGS solution based on the ITRF94 coordinate system. Thus the results from VLBI and GPS are based on the same reference frame.

Table 2. Positions of SLR and GPS reference points seen from VLBI reference point obtained by (1) ground survey measurements and (2) VLBI and GPS observations.
Site Eastward (mm)Northward (mm)Upward (mm)
(1)KoganeiSLR 17420.8 -31359.6 -1975.0
GPS 3493.0 -23843.2 -1769.9
KashimaSLR 12387.9 76693.2 7725.0
GPS 18285.7 -24052.4 -4471.4
MiuraSLR 16603.7 -74138.4 -2240.0
GPS -17031.7 6363.9 -4489.0
TateyamaSLR -37205.4 -15229.3 -2056.0
GPS -34009.6 -19653.5 -297.0
(2) KoganeiGPS 3477.3+/-1.7 -23859.1+/-1.7 -1774.7+/-8.6
KashimaGPS 18283.3+/-1.6 -24068.3+/-1.8 -4450.4+/-9.1
MiuraGPS -17041.9+/-2.0 6347.7+/-2.2 -4454.5+/-10.3
TateyamaGPS -34010.9+/-1.6 -19672.9+/-1.8 -285.5+/-8.6

The comparison between ground survey results and two space geodetic measurements of VLBI and GPS showed an agreement within 20 mm in horizontal components, and 35 mm in the vertical. The discrepancies were larger than expected from the accuracies of the ground survey measurements and estimated uncertainties from VLBI and GPS. But the discrepancies can be decreased if we consider that there are inconsistencies between the results from VLBI and GPS. It is possible that either the KASHIM34 position or the TSKB position provided in the ITRF94 coordinate system may have a significant error which corresponds to a part of the discrepancies in the comparison in Table 2. If the KASHIM34 coordinates used in the VLBI data analysis had in fact an error of 8.5 mm in Eastward direction, 17.6 mm in Northward direction, and -14.9 mm in Upward direction, then the discrepancies do not exceed $8mm$ in the horizontal components and 20 mm in the vertical component, and these values seem to be reasonable. These comparisons demonstrated the importance of the precise ground survey measurements for the collocation studies to tie different space geodetic techniques. Further VLBI experiments and GPS observations will improve the tie between VLBI and GPS as well as the tie of the Key Stone Project VLBI Network to the ITRF coordinate system.

3. Concluding Remarks and Future Plans

The Key Stone Project VLBI Network has been tied to the ITRF94 coordinate system through two joint VLBI experiments with KASHIM34 or other global VLBI stations such as the 26m antenna at Kashima Space Research Center. The tie will be improved by repeating such experiments in the future. Regular and extensive observations of VLBI, SLR, and GPS will be compared with each other to improve consistencies and accuracies by using the information obtained from the tie measurements.

Acknowledgments

The authors would like to express deep appreciations to colleagues in Geographical Survey Institute for GPS observations at Tsukuba and data correlation of a joint VLBI experiment of Key Stone Project and 34m antenna at Kashima.

Reference

Koyama, Y., Automated Remote Operation System and Data Analysis System for the Key Stone Project, Proceedings of the Technical Workshop for APT and APSG 1996, pp.139, 1996.


Figure 2. Geographical locations of three space geodetic measurement facilities at (a) Koganei, (b) Kashima, (c) Miura, and (d) Tateyama sites.



Updated on November 20, 1997. Return to CONTENTS