Correlation Processing System for the Key-Stone Project

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

Abstracts

We have developed an XF type VLBI correlation processor by making use of Field Programmable Gate Arrays. This correlator was specially designed for the Key-Stone Project, which is concerned with measuring crustal deformation in the Metropolitan area. The outline of the correlator is (1) Automatic bit synchronization during multi-baseline processing, (2) Network Filing System for storing correlated data, (3) maximum data rate is 512 Mbps, (4) 2-bit and higher sampled data processing capability, (5) 16ch high speed (32 Mbps/channel) processing, (6) Compact and light-weight, (7) Signal provided by VME back plane. Using this new correlator, we can improve the precision of the geodetic VLBI and also contribute to radio astronomy VLBI.

1. Correlation Processing System

The periodicity of the time code is not required for spectrum analysis. Only the sampled data are needed. The K-4 recorder has helical data tracks, two longitudinal annotation tracks and a control track (Fig. 1). The Track Set ID numbers are recorded on the control track, and can be read at any tape speed even during fast forward or rewind. The output interface unit can control the synchronous replay of several Data Recorders, convert the Track set ID for the data clock, and send data to a correlator. There is an obvious relationship between the Track Set ID (which is a head control signal written on the control track) and the time code block. It is possible to manage the time code using the Track set ID and Time code block. For a few seconds the time code data is over written on the data train in a pre-observation header block. After the time code block, data timing is checked by the Track Set ID, which means that the output data is only digitized raw data during an observation.

Figure 1. Tape format and data format



Figure 2. Block diagram of correlation processing system

In the multi-baseline correlation processing, all the output interface units are daisy-chain connected via GPIB and a timing control line. A block diagram of the correlation system is shown in Fig. 2. Therefore, the tape position data and the status data of all the data recorders can be exchanged via the output-interface units. The replayed data from the data recorder is written into a buffer memory. The measured phase difference between the replayed and the external 1 PPS signal sent from the Main Replay system is monitored by the clock. The measured data is then sent to the main replay system, and used for the bit synchronization (fine synchronization) between the main and sub replay systems. The main replay system (the main output interface unit and the data recorder) and the sub replay system (the sub output interface unit and the data recorder) can be synchronized in one-bit steps. The delay adjustment is done by controlling the track set ID position control and subsequent programmable memory. The signal (raw data) is unformatted instead of in the Mark-III format. We are developing an XF type VLBI correlation processor for the K-4 system by making use of Field Programmable Gate Arrays. The outline of the correlator is:

And specifications are; This correlator was specially designed for the Key-Stone Project, which is concerned with measuring crustal deformation in the Metropolitan area. The detected fringes are shown in Fig.3, and geodetic results are reported in this issue. The project utilizes 4 stations in the Metropolitan area, and each station has a new K-4 system and an 11-m antenna. Using this new correlator, we can improve the precision of the geodetic VLBI and also contribute to radio astronomy VLBI. The multi-baseline correlation processor (4-station) is shown in Fig.4. It is also possible to use this correlator for real-time VLBI.

Figure 3. Detected fringes



Figure 4. Developed multi-baseline correlation processing system. From left : 4 DMS (Digital Mass-strage System: automatic tape changer)

2. Conclusion

The KSP (new K-4) data acquisition mode includes VLBA and VSOP data modes. Multi-baseline correlation processors using Field Programmable Gate Array has been developed.

References

[1] Cooper, "Correlators with Two-bit Quantization", Ast. J. Phys., vol.23, pp.5 21-527, 1970.
[2] A.E.E.Rogers, A.E.E.; "Coherence Limits for Very-Long-Baseline Interferome try", IEEE Trans. Instrum. Meas., vol. 30, pp. 283- 286, December 1981.



Updated on June 25, 1995. Return to CONTENTS