Evaluation of Daily Repeatability of Baseline Lengths in the Key Stone Project VLBI Network

Tetsuro Kondo1 (kondo(AT)nict.go.jp), Kohichi Sebata2, Jun Amagai2, Masato Furuya2, Noriyuki Kurihara1, Hitoshi Kiuchi2, Yasuhiro Koyama1, Mamoru Sekido1, Akihiro Kaneko2, Yukio Takahashi2, Ryuichi Ichikawa1 and Taizoh Yoshino2

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

Abstract: VLBI measurements using four fixed VLBI stations around the Tokyo metropolitan area are producing continuous data of station positions and baseline lengths. Accuracy of baseline length measurements is evaluated on the basis of their repeatability in terms of root mean square variance in five adjacent sessions. Five day continuous observation sessions demonstrate that typical repeatability of about 1-2 mm in baseline length is achieved on our VLBI network.

1. Introduction

We have been carrying out routine daily VLBI observations (6 hours per day) to monitor crustal deformation around the Tokyo metropolitan area using four stations: Kashima, Koganei, Miura and Tateyama (Figure 1). The project is named the Key Stone Project (KSP) and is promoted by the Communications Research Laboratory. Each VLBI station is equipped with the same VLBI facility, i.e., a parabolic antenna with 11 m diameter and a highly automated data acquisition system dedicated to KSP. The longest distance between KSP stations is about 135 km (Kashima-Tateyama), so that the KSP network is very compact as a VLBI network. The KSP started regular observations using Kashima and Koganei stations in January, 1995. Later other stations joined and daily observations using all four stations started in September, 1996. Since then the observation system has experienced some refinements to improve its total performance. In parallel with the daily observations using a conventional tape recording method, we were establishing a real time correlation processing system using an ATM (asynchronous transmitting mode) network which combines four stations with a high speed digital link (maximum speed is 2.4 Gbps). Digitized signals observed at each station are transmitted to Koganei, where a KSP correlator is located, in real time through the ATM network. This real time processing has been used in routine operations since June, 1997.

Figure 1. Key stone project VLBI network.

As VLBI/KSP enters a stable operation phase, we have begun an evaluation of total system performance in terms of measurement accuracy. How accurately can we measure baseline length among KSP stations using the current system, which means both hardware and analysis software? To answer this inquiry, we conducted continuous observation over 120 hours on the KSP network from July 28 to August 1, 1997. These observations were successfully finished and the formal error of baseline length estimation for the last day is 0.7 mm, which is the champion value of KSP at present time. In this paper, we make an evaluation of results of the 120-hour-observation by comparing them with KSP results taken at other times.

2. Observations

Continuous 120 hour observation was carried out from July 28 to August 1, 1997 and was divided into 5 sessions. Each session lasts for 24 hours and includes about 600 scans of radio sources (quasars). Unmanned observations according to the same observation schedule distributed from the KSP central station, Koganei, were carried out at the all four stations. Observation status is always monitored at Koganei automatically. An observation and its correlation processing for 6 baselines is carried out simultaneously using data transmitted from each station through an ATM network which connects all stations to the Koganei central station.

Table 1 summarizes scheduled number of scans and valid scan number by session by baseline. Valid scan number means the number used for a baseline analysis. During the second session some problems occurred on the correlation processor, and Miura station was down for about 5 hours on the fourth session. These problems resulted in the low values of valid scans during the second and fourth session compared with the other sessions.

A baseline analysis is made on the basis of each session.

Table 1. Summary of observations.
BaselineNumber of Scans
ScheduledValid for AnalysisValid/Scheduled(%)

3. Method

We use baseline length analysis results to evaluate the accuracy of measurements, because the estimation of baseline length is robust against the model uncertainty such as earth rotation parameters. Root mean square variance of continuous 5 samples of baseline lengths by baseline is compared with adjacent 5 samples in other period. As for the formal error corresponding to 5 sessions, we take a simple average of that of each session concerned. KSP observation participated by all four stations started in January, 1996. Initially a video bandwidth of 2 MHz was used. After a system reliability check, this was expanded to 8 MHz on June 1, 1997. To avoid any influence that this may cause we limit the period to June 1, 1997 to August 13, 1997 for an evaluation study.

4. Results

Figure 2 presents scatter plots of rms variance of baseline lengths of the 5 sessions, with its formal error defined as a simple average of each session's formal error. An open triangle in each panel represents a result for 5 sessions from July 28 to August 1, 1997, i.e., the period of continuous observations. In each panel rho means correlation coefficient and N is the number of samples plotted in the figure. Weak correlation between rms variances and formal errors can be seen. Open triangles, results of 120-hour-observations, are mostly located at the lower-left edge of the population of samples. This means that an improvement in both repeatability and formal error can be seen for the 5 sessions when we compare them with other periods.

Figure 2. Repeatabilities and formal errors by baseline for the period between June 1, 1997 to August 13, 1997. Open triangles represent data for 120-hour observations. Rho is a correlation coefficients between parameters.

Table 2. Summary of Comparison.
Baseline Repeatability (mm) Mean Formal Error (mm)
for 1997/6/1
for 5 sessions for 1997/6/1
for 5 sessions

Table 2 summarizes comparison results between the entire period and 5 sessions for repeatability and mean formal error.

Repeatability of the 5 sessions varies from 1.3 mm to 2.5 mm. However we can see a clear improvement in repeatability with one exception in the case of the Kashima-Tateyama baseline. These results demonstrate that 24-hour observation in a day gives better repeatability than 6-hour a day observation.

5. Conclusion

We have monitored the deformation around Tokyo metropolitan area by using four VLBI stations dedicated to this purpose. It is important to know the limitation of accuracy achieved by the current measurement system for discriminating anomalies in crustal deformation.

According to an experimental observation lasting over 120 hours which consists of 5 sessions, the repeatability achieved by the current system is approaching 1 mm for baseline lengths.

We can see faint correlation between repeatability and formal error. This suggests that it is difficult to improve the repeatability by improving only the formal error, even though there exists a slight possibility. Improvement of the formal error is mainly due to system hardware or observation schedule. It is considered that a physical model, such as propagation delay model for the atmosphere, is related to an improvement of the repeatability in 5 days. Thus an improvement in a physical model should be rather investigated.

Updated on November 20, 1997. Return to CONTENTS