TECHNICAL DEVELOPMENT CENTER NEWS No.11
International Earth Rotation Service - VLBI Technical Development Center News

published by

Communications Research Laboratory

COMMUNICATIONS RESEARCH LABORATORY

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

Serial No. 11
April 9, 1998

November 1997

CONTENTS
Overview of the Eleventh TDC Meeting : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 2
Technical Reports
Evaluation of Daily Repeatability of Baseline Lengths
in the Key Stone Project VLBI Network : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 7
Performance of KSP Real-time VLBI Correlation Processing Software (RKATS) : : 11
Keystone Project SLR: Kashima Accuracy Validation Test
{Kashima Colocation{ : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 13
Tie of the Key Stone Project VLBI Network
to the International Terrestrial Reference Frame : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 15
Evaluation of a GPS Time and Frequency Reference Receiver
as a VLBI Frequency Standard : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 19
Millisecond Pulsar Observation System at CRL : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 23
Construction of New LF Station for Time and Frequency Service : : : : : : : : : : : : : : : : : : : : 27
Basic Research of Key Technologies for Next Generation Global
Navigation Satellite System : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 28
Giga-bit VLBI, Progress on G-bit Sampler : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 29
News-News-News
Message to the Key Stone Project : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 30
1

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IERS TDC News No.11

Overview of the Eleventh TDC
Fujinobu Takahashi, the vice-director of IERS
TDC at Communications Research Laboratory
Meeting(CRL), opened the meeting with an introductory

Tetsuro Kondo (kondo@crl.go.jp)
Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, JapanThe eleventh meeting of the Technical Development Center was held on September 18, 1997 at
the Communications Research Laboratory.

Attendance
CRL members

statement of the technical development center at
CRL.

2. Reorganization of CRL and the Key
Stone Project (Taizoh Yoshino)
Taizoh Yoshino represented a brief history of the
IERS emphasizing its relation to the technical development center at CRL. After the reorganizationof the VLBI participants in the IERS, the CRL wasnominated again as one of the Technical Development Centers in September 1996. In the CRL, ateam system was newly introduced in July 1997,
where each team devotes to a speci c subject in alimited period. The Keystone Project Team wasborn to run one of the major projects in the CRL.
In the Keystone Project Team, crustal deformation in the Tokyo metropolitan area is studied using
VLBI and SLR technology. Technical improvement
is also expected as one of the activities. Relationship between the existing research sections and theteam to perform technical developments was also
explained.

Kenichi Okamoto, Michito Imae, Mizuhiko Hosokawa, Yuko Hanado, Yukio Takahashi, Chihiro
Miki, Hitoshi Kiuchi, Akihiro Kaneko, Shin'ichi
Hama, Taizoh Yoshino, Hiroo Kunimori, Jun
Amagai, Hideyuki Nojiri, Toshimichi Otsubo,
Masato Furuya, Fujinobu Takahashi (KSRC:
Kashima Space Research Center), Noriyuki Kuri3. Activity Reports by the Special Members
hara (KSRC), Yasuhiro Koyama (KSRC), Ryuichi
Ichikawa (KSRC), Mamoru Sekido (KSRC), JuEach special member reported on the current
nichi Nakajima (KSRC), Tadahiro Gotoh (KSRC), status of the activities of each organization.
Tetsuro Kondo (KSRC)

Special members

Nobeyama Radio Observatory, National Astronomical Observatory (Noriyuki Kawaguchi)

Noriyuki Kawaguchi (National Astronomical Observatory), Hideo Hanada (National Astronomical
Observatory), Mikio Tobita (Geographical Survey
Institute), Masayuki Fujita (Hydrographic Department, Maritime Safety Agency) Seiichi Shimada(National Research Institute for Science and Disaster Prevention), Kachishige Sato (Tokyo Gakugei
University) Masayuki Takemura (Kobori ResearchComplex, Kajima Corporation)
Following special members could not attend:
Tetsuo Sasao (National Astronomical Observatory), Shuhei Okubo (Earthquake Research Institute, University of Tokyo),

Noriyuki Kawaguchi reported on the current status of Nobeyama Radio Observatory and space
VLBI project (VSOP) as follows. The VSOP is
successfully going on, this is greatly owing to an international collaboration. He also mentioned a format converter developed by himself, which makes
it possible to correlate the data taken by di erent
recording systems. So far it is possible to convert
among three kinds of recording formats. At the endof his report he expressed his hope that the technical development center at CRL will be a center
able to produce an international standard.

MinutesMizusawa Astrogeodynamics Observatory,
National Astronomical Observatory (Hideo
Hanada)1. Opening Greeting

Hideo Hanada reported that RISE (Research
In SElenology) in the SELENE (SELenologicaland ENgineering Explorer) project, which will belaunched in 2003 to the Moon, and a lander will

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November 1997

make a soft landing on the Moon while an orbiter
stays in lunar orbit, has entered the development
phase this scal year. However for budgetary reasons, the SELENE project is obliged to change its
initial form to decrease costs. As a result of replanning the lunar lander comes to play the role ofan engine for the orbiter, even though both had independent engines in the initial plan. Di erential
VLBI using signals from the lunar lander and a relay satellite which will be released from the orbiteris therefore postponed to one year after launch.

years shows motion of the island consistent withthat obtained by others, such as VLBI measurements. Presently the position of Ishigakijima Island is being measured. The Hydrographic Department plans to start GPS observations on an
uninhabited island next scal year.
Deployment of D-GPS network consisting of
more than 20 sites in Japan promoted by the Aids
to Navigation Department will be completed in thenext scal year, which is earlier than the originalschedule by one year.

Geographical Survey Institute (Mikio Tobita) National Research Institute for Science andDisaster Prevention (Seiichi Shimada)

Mikio Tobita reported Project'97 promoted by
the Geographical Survey Institute (GSI), theCommunications Research Laboratory (CRL), andthe Hydrographic Department, Maritime Safety
Agency (JHD) as a joint project. The Project'97
aims (1) to tie GPS, VLBI, and SLR, and to connect them to the origin of Japan geographic coordinates for the GIS campaign observations taken
in October, 1997, (2) to make collocation observations of GPS, VLBI and SLR for the IERS, and (3)to tie KSP-VLBI/CRL, SLR and GPS/JHD, anda 26m antenna at Kashima/GSI to a GPS network
operated by GSI.
He also expressed his thanks to the technicaldevelopment center for continuous technical support during installation of the correlation processing system at GSI. Now a K-4 type correlator,
available to process three baselines, is in operation
at the GSI.
As for the experiment, proposed by Tobita at the
9th TDC meeting held in September 1996, with
KSP antennas being utilized as ducial points in
SAR images by pointing them to an expected satellite direction, he reported that it did not get goodresults, i.e., no antenna was identi ed in the SAR
images. Antennas were merely pointed to the satellite direction without any special treatment, suchas placing a metal plane in front of the antenna feedhorn to increase re ecting signals. This result wastherefore partly expected before the experiment.
Tobita however asked a continuous cooperation of
CRL group to improve this experiment to obtainreasonable results because it is an excellent idea.

Hydrographic Department, Maritime Safety
Agency (Masayuki Fujita)
Masayuki Fujita reported on the current status of SLR and GPS observations carried out by
the Hydrographic Department, Maritime Safety
Agency as follows.
SLR measurements made at Chichijima Islandon the Philippine sea plate separated by several

After a brief self-introduction Seiichi Shimada
talked about the current tendency of GPS analysis software to emphasize treatment of atmosphericdelay e ects in the analysis. They consider the effect of horizontal gradients in the zenith excess delays, as well as taking the mapping function into
consideration. As for water vapor e ects, a trial
using numerical weather prediction data is starting
to produce good initial conditions in the analysis.

Tokyo Gakugei University (Kachishige Sato)
Kachisige Sato, who moved from Mizusawa Astrogeodynamics Observatory, National Astronomical Observatory, introduced his recent studies. He
has been investigating plate motion and deformation using space geodetic data. Comparing theresults from VLBI and GPS data, he suggested
that VLBI data are consistent with a plate motion model based on geological evidence, while GPSdata seem to show some inconsistency with themodel. The same inconsistency can be seen in SLRdata, he said. He plans to investigate this discrepancy more detail.

Kobori Research Complex, Kajima Corporation (Masayuki Takemura)
Masayuki Takemura, a specialist of strong
ground motions of earthquakes, said that he wants
to contribute to the TDC/CRL from a unique position, because his major eld is slightly di erent from that of other special members. Then
he introduced his recent work investigating the
1995 Hyogo-ken Nanbu earthquake in comparison
with historical large earthquakes. This comparative study shows that the Hyogo-ken Nanbu earthquake is not a special one but a common one often
occurring in Japan as an intra-plate earthquake, hesaid. He pointed out that the local ground condition is an important factor for damage caused by

4earthquakes. It can be learned from the groundmotion estimated from the directions of overturning tombstones. In the case of large earthquakes
like the 1923 Kanto earthquake, damage becomesmuch larger and wider in area. We should recognize that we cannot adopt all results learned fromthe Hyogo-ken Nanbu earthquake to much larger
earthquakes.

4. Technical Development Reports

IERS TDC News No.11

rate rather than an o set, isn't it? Is it actuallynecessary for a hydrogen maser frequency standard
to adjust clock o set frequently? Sekido replied asfollows. RKATS has both a rate and an o set adjustment function. For a practical use it is enoughto adopt only an o set correction. A 1 pps signal of a station clock was sometimes reset to keepa time di erence suciently small compared with
the UTC.

4.3 Analysis of KSP GPS Observations froma Meteorological Point of View (Ryuichi
4.1 Preliminary Report of 120 hour Contin- Ichikawa)uous Observation on the Key Stone Project Ryuichi Ichikawa reported on results obtained(Crustal Deformation Observation System through the comparison between VLBI and GPS
in the Tokyo Metropolitan Area) VLBI Net- measurements which were simultaneously conwork (Tetsuro Kondo)
ducted with an experimental continuous observa-

CRL has been carrying out daily observations
using the real-time KSP VLBI network to monitor
crustal deformation around the Tokyo metropolitan area. Tetsuro Kondo reported on an experiment carried out on the KSP VLBI network over
120 hours continuously from July 28 to August 1,
1997. Each session lasted for 24 hours and consisted of about 600 scans. Five sessions are included in the experiment. The main purpose ofthis experiment is to evaluate the accuracy limit
achieved by the current system. Repeatability ofmeasured daily baseline lengths represents a considerable improvement compared with that for routine daily observations of 6 hour duration. Thus
daily 6-hour observations on the KSP network
are modi ed to include 24-hour experiments every other day to increase the accuracy of measurements. One of the special members asked, how can
the number of scans be increased to such a large
number? Kondo replied that this is made possible
by real-time correlation processing, which does not
require an extra period in each scan which is necessary for tape synchronization in the case of tapebased VLBI. Thus we can reduce the time for eachscan, resulting in an increase in the total numberof scans.

tion from July 28 to August 1, 1997. The results
showed that time variations can be seen in a geodetic solution of GPS results that are similar to thoseseen in VLBI results. He also mentioned that scatter in the east-west components obtained by GPS
observation became larger when low elevation angles were included in an analysis, and this may berelated to an inhomogeneous distribution of water
vapor around a station. Regarding a systematic
error seen in the position measured by GPS for
Tateyama station, one of the KSP stations, he suspected the radome of the GPS antenna of in uencing the results.

4.4 Current Status of KSP SLR System (Hiroo Kunimori)
Hiroo Kunimori reported on the current statusof SLR system in the KSP as follows. A calibration for optic-electronic packages was carried out
using colocation optics at Kashima con guring all
four connected to a single telescope and one oflasers transmitting to the ground targets. Results
demonstrated system stability to about 2 mm.

4.5. Tie of the KSP Network to the ITRF
(Yasuhiro Koyama)

4.2 Report on Real-time Correlation Pro- Yasuhiro Koyama reported on the result of excessing Software RKATS (Mamoru Sekido)
periment using the KSP VLBI network and theKashima 34 m antenna as follows. The position
Real-time correlation processing software RKATS of Kashima 34 m antenna is well determined indeveloped for KSP-VLBI system was introduced the international terrestrial reference frame (ITRF)
by Mamoru Sekido. Because of an automatic through a number of international VLBI sessions.
fringe search and a dynamic clock o set adjust- By connecting the KSP network with the Kashima
ment, which are RKATS's important functions, 34 m antenna, station positions of KSP network incontinuous unmanned operation is achieved, he em- the ITRF were determined within a discrepancy ofphasized. One of the special members asked him, 1 cm for horizontal components and 3 cm for verit is important for clock compensation to adjust a tical components. Comparing these results with

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November 1997

ITRF coordinates of GPS ground bench marks, it
is shown that the positions of KSP stations measured by VLBI and GPS are coincident with eachother within 2 cm for horizontal and 5 cm for vertical components. However the discrepancies are notso small and the reasons for this are under investigation.

Junichi Nakajima reported on the current status of development of next generation VLBI system based on a gigabit recorder (GBR-1000) as
follows. Performance test of the sampler portionwas carried out at the Nobeyama Radio Observatory. Auto-correlation was successfully detected by
a GICO (GIgabit COrrelator).

4.6 PCAL System at Urumqi Station 4.10 An Evaluation of GPS Time/Frequency(Noriyuki Kurihara)
Receiver as a VLBI Frequency Standard(Tetsuro Kondo)
Noriyuki Kurihara visited Urumqi VLBI station,
China to improve the phase calibration (PCAL)
system in July, 1997. He reported on the current
status of Urumqi station and a preliminary result of
VLBI observation carried out just after he installed
an improved PCAL system. The new PCAL system seems to be working well but some problems
still remain in the total system, he pointed out.

4.7 Current Status of Multimedia Virtual
Laboratory Project (Yukio Takahashi)

Tetsuro Kondo presented the results of an evaluation of GPS time/frequency receiver as a VLBI
frequency standard. He measured the phase di erence of 10 MHz signals from two independent GPS
time and frequency reference receivers which arelocked to GPS satellites. He then calculated Allan
variances and coherence loss using the measured
data. He said, as a result it is demonstrated that
the GPS Time and Frequency Reference Receivercan be used as a frequency standard for frequencies
lower than 1 GHz. He also mentioned that if we expand the fringe search process to include searches
at least up to a third order phase change against
time, then a GPS Time and Frequency Reference
Receiver is adoptable as a frequency standard for
VLBI operated at even higher frequencies, up to 8
GHz.

Yukio Takahashi reported on applications for
VLBI use in the Multimedia Virtual Laboratory(MVL) project. As for the VLBI applications, highspeed transmission of a huge amount of data anda technique for real-time data processing on distributed stations become key items in system development. He showed an example of ideas about
correlation processing as carried out at multiple
4.11 Millisecond Pulsar Observation Using
stations simultaneously with sharing the data.

34 m Antenna at Kashima (Yuko Hanado)

4.8 A Next Generation VLBI Terminal (Hi- Yuko Hanado introduced a millisecond pulsar obtoshi Kiuchi)
servation system and reported on recent observaHitoshi Kiuchi presented his ideas about a next
generation VLBI terminal. According to his idea,
a data recorder will consist of 16 channels andeach channel will record data at a rate of either
128 Mbps or 256 Mbps. The video bandwidth will
be wider than 32 MHz (up to 64 MHz), which isrealizable even with current technology. The input/output interface will be uni ed to use an optical interface.
By adapting parallel processing logic to a correlator, the speed of processing will reach 1 Gps perchannel. An FPGA instead of a custom-made LSI
will be used for correlator development because thedebugging process becomes easier than that with
LSI. The di erences of recording and data format
among VLBI systems throughout the world will beabsorbed at the correlator.

tion results. The results show good repeatability
due to an upgrade of the software TEMPO which
estimates the pulse phase of pulser. A new longterm-drift appears in the results, however. Comments on this report follow. Use of TEMPO as a
black box may limit this study in the future. A
pulse phase estimation software should be developed along with system development. As for an
improvement of signal to noise ratio, a burst sampling technique is suggested.

4.12 Construction of New Station Disseminating Japanese Standard Time and Frequency at LF Band (Michito Imae)

Michito Imae introduced the new station transmitting standard radio waves at the LF band, under construction in Fukushima Prefecture, Japan.
4.9 Current Status of Development of Next CRL is responsible for keeping Japan Standard
Generation VLBI System Based on Gigabit Time (JST) and dissemination of standard timeand frequency. At present, CRL disseminates the
Recorder (Junichi Nakajima)

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IERS TDC News No.11

JST and frequency using JJY (HF) and JG2AS(LF). However, interference is increasing at HF
band, and JG2AS is not a permanent station. The
investigation committee organized in CRL for dissemination of more stable and practical signals permanently recommended to unify both stations as asingle LF station. Thus construction of the new LF
station started. It will transmit standard signals at
40 kHz with a power in excess of 10 kW. Service
will start in April, 1999.

use was brie y introduced by Michito Imae. He
showed topics which should be studied at CRL as
follows: development of an on-board atomic frequency standard, a time keeping technique for agroup of on-board clocks, and a technique for precise real-time orbit determination. He also presented his plan to include an atomic clock on theEngineering Test Satellite-VIII (ETS-VIII) which
will be launched in 2002 by the National Space Development Agency of JAPAN (NASDA).

4.13 Study on the Basic Technique of Satel- 5. Closing Greetinglite Positioning System (Michito Imae)

The closing greeting was delivered by Kenichi
A basic technique of a satellite positioning sys- Okamato, the director of IERS TDC at Communitem investigated at CRL, in particular in the eld cations Research Laboratory.
of frequency standards, for the purpose of future

The eleventh TDC meeting held at the headquaters of Communications Research Laboratory
on September 18, 1997

November 1997

Evaluation of Daily Repeatability of Baseline Lengths in
the Key Stone Project VLBI
Network7

system using an ATM (asynchronous transmittingmode) network which combines four stations witha high speed digital link (maximum speed is 2.4Gbps). 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 inroutine operations since June, 1997.
Tetsuro Kondo1(kondo@crl.go.jp), Kohichi
As VLBI/KSP enters a stable operation phase,
Sebata2 , Jun Amagai2, Masato Furuya2 ,we have begun an evaluation of total system perNoriyuki Kurihara1, Hitoshi Kiuchi2 ,formance in terms of measurement accuracy. How
Yasuhiro Koyama1 , Mamoru Sekido1 ,accurately can we measure baseline length among
Akihiro Kaneko2, Yukio Takahashi2,
KSP stations using the current system, which
Ryuichi Ichikawa1 and Taizoh Yoshino2
means both hardware and analysis software? To
1 Kashima Space Research Centeranswer this inquiry, we conducted continuous observation over 120 hours on the KSP network from
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan July 28 to August 1, 1997. These observations were
2 Communications Research Laboratory
successfully nished and the formal error of baseline length estimation for the last day is 0.7 mm,
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
which is the champion value of KSP at present
Japantime. In this paper, we make an evaluation of
results of the 120-hour-observation by comparing
Abstract: VLBI measurements using four xed them with KSP results taken at other times.
VLBI stations around the Tokyo metropolitan area
are producing continuous data of station positions
and baseline lengths. Accuracy of baseline length 2. Observations
measurements is evaluated on the basis of their
Continuous 120 hour observation was carried out
repeatability in terms of root mean square vari- from July 28 to August 1, 1997 and was divided
ance in ve adjacent sessions. Five day continuous into 5 sessions. Each session lasts for 24 hours andobservation sessions demonstrate that typical re- includes about 600 scans of radio sources (quasars).
peatability of about 1-2 mm in baseline length is Unmanned observations according to the same obachieved on our VLBI network.
servation schedule distributed from the KSP central station, Koganei, were carried out at the all
1. Introduction
four stations. Observation status is always monitored at Koganei automatically. An observation
We have been carrying out routine daily VLBI and its correlation processing for 6 baselines isobservations (6 hours per day) to monitor crustal carried out simultaneously using data transmitted
deformation around the Tokyo metropolitan area from each station through an ATM network whichusing four stations: Kashima, Koganei, Miura and connects all stations to the Koganei central station.
Tateyama (Figure 1). The project is named theTable 1 summarizes scheduled number of scans
Key Stone Project (KSP) and is promoted by the and valid scan number by session by baseline. Valid
Communications Research Laboratory. Each VLBI scan number means the number used for a baseline
station is equipped with the same VLBI facility, analysis. During the second session some problems
i.e., a parabolic antenna with 11 m diameter and occurred on the correlation processor, and Miuraa highly automated data acquisition system dedi- station was down for about 5 hours on the fourthcated to KSP. The longest distance between KSP session. These problems resulted in the low values
stations is about 135 km (Kashima-Tateyama), so of valid scans during the second and fourth sessionthat the KSP network is very compact as a VLBI compared with the other sessions.
network. The KSP started regular observations
A baseline analysis is made on the basis of eachusing Kashima and Koganei stations in January, session.
1995. Later other stations joined and daily observations using all four stations started in September,
1996. Since then the observation system has expe- 3. Method
rienced some re nements to improve its total perWe use baseline length analysis results to evalformance. In parallel with the daily observations
uate the accuracy of measurements, because theusing a conventional tape recording method, we
estimation of baseline length is robust against thewere establishing a real time correlation processing

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IERS TDC News No.11

KOGANEI
109km
58km

KASHIMA

92km 123km

MIURA

135km
35km
TATEYAMA

Figure 1. Key stone project VLBI network.
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, wetake 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 in uence 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 ofbaseline lengths of the 5 sessions, with its formal
error de ned 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  means correlation coecient andN is the number of samples plotted in the gure.
Weak correlation between rms variances and formal errors can be seen. Open triangles, results of
120-hour-observations, are mostly located at thelower-left edge of the population of samples. Thismeans that an improvement in both repeatability

and formal error can be seen for the 5 sessions when
we compare them with other periods.
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 aroundTokyo 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, therepeatability 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 isdicult to improve the repeatability by improving only the formal error, even though there ex-9November 1997

Table 1. Summary of observations.
Session(Date)
97JUL28XX(97/07/28)
01:10{24:51

97JUL29XX(97/07/29)
01:13{24:51

97JUL30XX(97/07/30)
01:10{24:51

97JUL31XX(97/07/31)
01:13{24:54

97AUG01XX(97/08/01)
01:17{24:52

Baseline
KASHIMA{KOGANEIKASHIMA{MIURAKASHIMA{TATEYAMAKOGANEI{MIURA
KOGANEI{TATEYAMAMIURA {TATEYAMAKASHIMA{KOGANEIKASHIMA{MIURAKASHIMA{TATEYAMAKOGANEI{MIURA
KOGANEI{TATEYAMAMIURA {TATEYAMAKASHIMA{KOGANEIKASHIMA{MIURAKASHIMA{TATEYAMAKOGANEI{MIURA
KOGANEI{TATEYAMAMIURA {TATEYAMAKASHIMA{KOGANEIKASHIMA{MIURAKASHIMA{TATEYAMAKOGANEI{MIURA
KOGANEI{TATEYAMAMIURA {TATEYAMAKASHIMA{KOGANEIKASHIMA{MIURAKASHIMA{TATEYAMAKOGANEI{MIURA
KOGANEI{TATEYAMAMIURA {TATEYAMA

Scheduled
593
593
593
593
593
593
598
598
598
598
598
598
598
598
598
598
598
598
599
599
599
599
599
599
597
597
597
597
597
597

Number of Scans
Valid for Analysis Valid/Scheduled (%)
487
82.1
550
92.7
517
87.2
518
87.4
510
86.0
546
92.1
390
65.2
386
64.5
419
70.1
417
69.7
395
66.1
462
77.3
507
84.8
496
82.9
519
86.8
489
81.8
507
84.8
556
93.0
483
80.6
446
74.5
532
88.8
426
71.1
504
84.1
455
76.0
526
88.1
568
95.1
567
95.0
540
90.5
526
88.1
576
96.5

Table 2. Summary of Comparison.
Baseline
KASHIMA{KOGANEIKASHIMA{MIURAKASHIMA{TATEYAMAKOGANEI{MIURA
KOGANEI{TATEYAMAMIURA {TATEYAMA

Repeatability(mm)for 1997/6/1
for{1997/8/13 5 sessions
4.361.5
1.6
3.761.4
2.5
4.761.8
4.1
5.262.5
1.3
6.262.6
1.6
4.661.9
2.0

Mean Formal Error(mm)for 1997/6/1
for{1997/8/13 5 sessions
2.260.7
1.2
2.160.5
1.4
2.360.5
1.4
2.560.9
1.1
2.761.0
1.1
2.160.5
1.0

ists a slight possibility. Improvement of the formal physical model should be rather investigated.
error is mainly due to system hardware or observation schedule. It is considered that a physicalmodel, such as propagation delay model for theatmosphere, is related to an improvement of therepeatability in 5 days. Thus an improvement in a

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IERS TDC News No.11

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.  is a correlation coecients between
parameters.

11

November 1997

Performance of KSP Real-time 2. Di erences between Tape-based VLBIVLBI Correlation Processing and Real-time VLBI
The main di erences of correlation processing
Software (RKATS)
between tape-based VLBI and real-time VLBI arelisted in Table 1.
1
Real-time VLBI is advanced from the standpoint
Mamoru Sekido (sekido@crl.go.jp), Tetsuro
of automatic operation and rapidity of output, but
Kondo1 , Hitoshi Kiuchi2 , Hiroto Sato3,
Yasuhiro Koyama1 , and Tetsuo Masubuchi3 interruption of correlation processing due to any
problems leads to direct loss of data. Therefore,
1 Kashima Space Research Center
non-stop operability is strongly requested of realtime correlation processing software.
Communications Research Laboratory
Except for hardware errors and software hang893-1 Hirai, Kashima, Ibaraki 314-0012, Japan
2 Communications Research Laboratory
ups, most probable cause of data loss would bewrong clock parameters. Wrong clock parameters
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
will move fringes out of the lag window of the corJapan
3 Kety Co. Ltd.
relator, and such data cannot be used for base4 CosmoResearch Co. Ltd.
line analysis. Also, fringe monitoring and adjustment of clock parameters by an operator is dicult,because VLBI experiments may start at midnight
and the operators are not supposed to be familiar
1. Introduction
with VLBI. Therefore we implemented \Dynamic
Key Stone Project(KSP) real-time VLBI ob- Clock Adjustment(DCA)" and \Automatic Fringe
servations started in June of 1997 by using Search(AFS)" functions in RKATS. By using these
Asynchronous data Transfer Mode(ATM) network. functions, fully automatic real-time daily VLBI obReal-time VLBI correlation processing software servation has realized.
\RKATS" has developed for fully automatic realtime VLBI data processing. Overview of RKATS 3. Automatic Fringe Search and Dynamic
was reviewed in TDC News No.9 p.15. In this Clock Adjustment
report, \Dynamic Clock Adjustment" and \Automatic Fringe Search" functions are introduced as
key functions for RKATS.

Dynamic Clock Adjustment Function

Automatic Fringe Search Function
0.6
KOGANEIMIURATATEYAMA

Clock Parameter (μsec.)

Clock Parameter (μsec.)

100
Fringe Search–100Normal Correlation
5

10

OBS Number

15

0.4

0.2

0

50

100

150

OBS Number

Figure 1. Performance test of AFS(left) and DCA(right) function. Clock parameter ofKashima is xed as reference. For the performance test, clock parameters of Koganei, Miura,and Tateyama are arti cially biased by 10sec, 010sec, and 05sec respectively. At rst,
correlation has started in \Normal Mode" but fringes were not detected. Then RKATS
changed the mode to \Fringe Search Mode" and detected fringes automatically. After thatclock parameter was adjusted, the mode was returned back to \Normal Mode". In the \Normal Mode" operation, DCA function adjusts the clock parameter and keeps the fringes atthe center of lag window.

12

IERS TDC News No.11

Table 1. Di erences between tape-base VLBI and real-time VLBI
Data transportation
and replay
Log information
Trouble recovery
Rapidity of output

TAPE-BASE
Human operation is inevitable atwork of Magnetic tape transportation and tape mount on recorders
Observation information and Clockparameter is provided by log le.
Trouble at data processing can be recovered by re-processing of recorded
data.
Analysis result comes out 1-2 days
after observation.

REAL-TIME
Real-time data transfer by ATM enables automatic operation.
Information is collected by computer
network just before the observation.
Stop of data processing due to troubles leads to data loss directly.
Analysis result comes out just 10-20
minutes after observation.

Table 2. Parameter for AFS and DCA
Parameter name
Fringe Search Mode transition
threshold
Averaging Number
SNR threshold
Source list for DCA

Meanings
If ratio of fringe detection (#detected/#total) become lower
than this value, correlation mode is changed to FSM.
Averaging number for fringe detection ratio.
SNR threshold to judge fringe detection.
radio source list, on which fringe is expected to be detectedcertainly.

Figure 1 shows the clock parameters are adjusted
by AFS and they are kept almost within 60:1sec
by DCA function. AFS and DCA works as follows:

dow.
Even though the clock o set is changed by abouta few hundred nanoseconds, no di erences in theanalysis results were observed.
Data processed in \Fringe Search Mode (FSM)"
is unusable for baseline analysis. The frequency oftransition to FSM must be as small as possible. For
stabilization of AFS and DCA, RKATS has several
parameters. They are listed in Table 2.
By optimizing these parameters empirically,
RKATS can be tuned to operate as reliably as possible.

1. AFS: In the case that the station clock information is wrong and fringes are shifted outsideof the lag window of the correlator ( 61sec
at 16Mbps/ch, 64sec at 4Mbps/ch), correlation mode is changed from \Normal Mode"
to \Fringe Search Mode". Then \Fringe
Search Mode" arranges all lags of 16 channels sequentially and makes the lag window
wider (616sec at 16Mbps/ch, 664sec at
4Mbps/ch) to get fringes. After fringes are detected, the station clock parameter is adjusted 4. Summary
and a return is made to \Normal Mode".
AFS and DCA are introduced as key functions
2. DCA: Position of fringes within the lag winof RKATS. Under the combination of RKATS and
dow is monitored by the software. Clock paautomated KSP system, baseline vectors among
rameters of each station are adjusted so thatfour stations are being measured automatically
fringes are kept at the center of the lag windaily.

13

November 1997

Keystone Project SLR :Kashima Accuracy Validation
Test-Kashima Colocation-

Hiroo Kunimori(kuni@crl.go.jp) and SLR
group
Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
Japan
Abstract: The Keystone SLR network has been installed and is starting test observation on a single baseline from February 1997. The role of SLR
is to provide a combination of techniques to contribute to baseline accuracy up to a few mm in 3-D.
The calibration of the measured range is the mostimportant work for SLR. It is normally achievedby using the laser ranging system itself to measure
the distance to one or more local ground targets
whose distances from the instrumental reference
point have been accurately measured by geodetic
survey. The di erence between the laser ranging
value and the survey value is then applied to satellite ranging measurements. In order to demonstrate stability and di erence of the ranges within
millimeters, we set up colocation optics for calibration of four optic-electronic packages, con guring all four packages connected to a single telescope and one of laser transmitting to the groundand satellite targets to receive simultaneous signals. Ranging to ground target in four hours results in a single shot precision of 4-6mm rms, and15-second (normal point) precision of 2-3mm rms.

Figure 1. Schematic View of Keystone SLR.

Figure 2. Kashima Colocation Optics.

Figure 3. Kashima Colocation Ranges (50Hz, 15 seconds averaging).

14

IERS TDC News No.11

Figure 4. Kashima Colocation Real time calibration.

Figure 5. Kashima Colocation MINICO results.
We see a small common drift possibly due to laser
characteristics, and demonstrated stability of 10
minutes. System delay reaches 1.5mm rms by applying real time calibration that uses a corner cubeon the telescope aperture. MINICO (Mini Coloca-tion) procedure which ensures consistency among
survey and system delay is within 6 3mm, foundsigni cant discrepancies among ground target survey results.

15

November 1997

1 Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan
2 Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
Japan

1. Introduction

1997.5.1

10

10

North

20 (mm)

20 (mm)In the Key Stone Project, site coordinatesof three VLBI stations at Koganei, Miura, andTateyama are estimated from the data obtained ineach experiment while the site coordinates of the
11m antenna at Kashima for the Key Stone Project(KASHIM11) is xed in the process of least-squareestimations [Koyama, 1996]. The a-priori information of KASHIM11 coordinates should be given in

1995.1.19

Yasuhiro Koyama1 (koyama@crl.go.jp),
Ryuichi Ichikawa1 , and Jun Amagai2

the ITRF94 reference frame to make them consistent with other a-priori parameters. The source
positions and Earth Orientation Parameters aretaken from ICRF94 and EOP(IERS)90C04 series
and its extensions reported in regular bulletinsfrom IERS, respectively, which are consistent withITRF94. It is also important to give accurate andreliable ITRF94 coordinates of KASHIM11 to tie
the VLBI Network of the Key Stone Project withthe global terrestrial reference frame. Estimatedsite 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 sucient accuracy. Two
geodetic VLBI experiments have been performed todetermine the KASHIM11 site coordinates in the
ITRF94. The rst one was performed for 24 hours
on January 17, 1995 with 34m antenna station at
Kashima (KASHIM34) and two Key Stone Projectsites 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 comparedin Figure 1. In the gure, estimated positions areshown by ellipses which express the one-sigma uncertainty either in the horizontal plane or in thevertical plane.

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

0

0

1995.1.19

-10

-10

1997.5.1

East
-20

-20

North

-20

-10

0
(mm)

10

20

-20

-10

0
(mm)

10

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 planeand (b) in the vertical plane.

20

16

IERS TDC News No.11

Table 1. Estimated VLBI site positions R on January 1, 1997, and velocities V de ned in the ITRF94
coordinate system.
Site
KoganeiKashima
Miura
Tateyama

R
V
R
V
R
V
R
V

(mm)
(mm/year)(mm)(mm/year)(mm)(mm/year)(mm)(mm/year)X

03941937398:4 6 5:8
7:5 6 0:7
03997505622:1 6 6:1
5:1 6 2:3
03976129918:1 6 6:9
24:2 6 1:1
04000983352:7 6 5:7
33:5 6 1:9

Uncertainties in the results from the rst experiment are larger than the results from the second experiment mainly because KASHIM34-KASHIM11
baseline data were not correlated in the rst experiment. From the comparison, vertical position andNorth-South component of the KASHIM11 stationis consistent between the two experiments, while
there is a signi cant discrepancy in the East-Westcomponent. The cause of this discrepancy is notknown but it might have been introduced whenthe vertical alignment of the pedestal of the antenna at Kashima was corrected in February 1996.
The movement of the intersection of the Azimuthand Elevation axes of the antenna was measured
at the time of correction. From the measurements,
the position of the KASHIM11 station moved by
4:4mm in the direction of N25 W. The discrepancy
in Figure 1 is about 12mm mainly in the westward
direction and is not fully explained by the correction made in February 1996. The situation will bemade clearer in the future when more joint VLBI
experiments are performed with KASHIM34 and
Key Stone Project VLBI Network.
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 thethree other stations are the results obtained fromthe Key Stone Project VLBI experiments until
the end of August 1997. While the uncertaintyof the KASHIM11 site velocity is the value for
KASHIM34 site velocity in the ITRF94, the uncertainties of site velocities of the three other stations

Y
3368150858:3 6 4:7
05:2 6 0:6
3276878350:2 6 4:8
03:6 6 2:0
3377927833:6 6 5:7
06:7 6 0:9
3375275900:1 6 4:7
013:3 6 1:6Z
3702235261:4 6 5:0
015:8 6 0:6
3724240665:8 6 5:2
019:8 6 2:5
3656753813:7 6 5:9
015:5 6 1:0
3632213145:2 6 5:0
024:0 6 1:7

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 byground surveys. Reference points of VLBI and SLR
are de ned by intersection of azimuth axis and elevation axis of the antenna and the telescope, respectively. Reference point of GPS is de ned 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 thethree 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 daysof 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 aday, 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.
The comparison between ground survey results
and two space geodetic measurements of VLBI and
GPS showed an agreement within 20mm in horizontal components, and 35mm in the vertical. The
discrepancies were larger than expected from theaccuracies of the ground survey measurements and17

November 1997

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.
(1)

Site
KoganeiKashima
Miura
Tateyema(2)

KoganeiKashima
Miura
Tateyama

SLR
GPS
SLR
GPS
SLR
GPS
SLR
GPS
GPS
GPS
GPS
GPS

Eastward (mm)17420:83493:012387:918285:7
16603:7017031:7
037205:4
034009:6
3477:3 6 1:7
18283:3 6 1:6017041:9 6 2:0
034010:9 6 1:6

estimated uncertainties from VLBI and GPS. But
the discrepancies can be decreased if we consider
that there are inconsistencies between the resultsfrom VLBI and GPS. It is possible that either the
KASHIM34 position or the TSKB position provided in the ITRF94 coordinate system may havea signi cant error which corresponds to a part ofthe discrepancies in the comparison in Table 2. If
the KASHIM34 coordinates used in the VLBI data
analysis had in fact an error of 8:5mm in Eastward direction, 17:6mm in Northward direction,and 014:9mm in Upward direction, then the discrepancies do not exceed 8mm in the horizontal
components and 20mm 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 di erent 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.

Northward (mm)031359:6
023843:2
76693:2024052:4
074138:4
6363:9015229:3
019653:5023859:1 6 1:7024068:3 6 1:8
6347:7 6 2:2
019672:9 6 1:8

Upward (mm)01975:001769:97725:004471:402240:004489:002056:00297:001774:7 6 8:604450:4 6 9:1
04454:5 6 10:3
0285:5 6 8:6

joint VLBI experiments with KASHIM34 or otherglobal 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 toimprove consistencies and accuracies by using theinformation obtained from the tie measurements.

Acknowledgments
The authors would like to express deep appreciations to colleagues in Geographical Survey Institutefor 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 Systemand Data Analysis System for the Key Stone Project,
Proceedings of the Technical Workshop for APT and
APSG 1996, pp.139, 1996.

3. Concluding Remarks and Future Plans
The Key Stone Project VLBI Network has beentied to the ITRF94 coordinate system through two

18

IERS TDC News No.11(a) Koganei(b) Kashima
VLBI Antenna
31

32

30
31

29

30

SLR Observation Shelter

Observation Control& Data Analysis Building

SLR Telescope
29
28
27

GPS Antenna

26
25
21
22
23
24

VLBI Observation
Control Building

SLR Telescope
SLR Observation Shelter

VLBI Antenna(c) Miura

GPS Antenna
VLBI Observation
Control Building(d) Tateyama

VLBI Antenna
GPS Antenna
VLBI Antenna

VLBI Observation
Control Building

SLR Telescope
GPS Antenna
SLR Observation Shelter
SLR Telescope
SLR Observation Shelter

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

19

November 1997

Evaluation of a GPS Time andFrequency Reference Receiver
as a VLBI Frequency Standard

ventionally used for VLBI, its lower cost is attractive when we consider wide deployment of VLBI
and VLBI-like techniques. We have therefore evaluated the performance of the GPS receiver and thepossibility of its adoption as a frequency standard
in VLBI observations.

Tetsuro Kondo1(kondo@crl.go.jp) and Jun
Amagai2

2. Method and Results

1 Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan
2 Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
Japan

1. Introduction
In recent years a GPS time and frequency reference receiver has shown remarkable progress inperformance, and has come to be widely used as afrequency standard supplying highly stable signals(1 2 10012 /day) at low cost. Although its stability
is less than that of H-maser( 1 2 10014 /day) con-

An HP58503A is one of the GPS receivers commercially sold as a time and frequency reference
receiver. It provides 1 pps signals synchronized to
UTC within about 100 nsec as well as stable 10
MHz signals. We located two GPS antennas closely
to each other; separation between them was only
about 7 meters. Two receivers (HP58503A) wereconnected to the antennas independently. Aftersucient running of receivers (3 days) according
to the instructions attached with the receiver, wemeasured the phase di erence between 10 MHz signals from two receivers for 6 days starting from
June 29, 1997. Analog output from a phase comparator (HP K34-59991A) is converted into digital
signals with a sampling period of 1 sec. Digitizeddata are stored in a notebook PC (Figure 1).

GPS signals
HP58503A

Antenna
HEWLET
PACKARD

10MHz

58503AGPS TIME AND FREQUENCY REFERENCE RECEIVER

HP58503A

Antenna

HEWLET
PACKARD

58503AGPS TIME AND FREQUENCY REFERENCE RECEIVER

Phase
Sensitive
Detector

10MHz

GP‑IB

Penrecorder

Data acquistion

Figure 1. Schematic block diagram of observation system.

20

IERS TDC News No.11

Figure 2. Observed phase data for 3 hours. Discontinuities due to a comparator are already removed
There are 72 discontinuities in raw phase dataspanning 6 days due to inherent characteristics ofthe comparator, i.e., a trip of 360 degree occurs
when the phase crosses the 360 or 0 degree boundary. These discontinuities should be removed before further statistical analysis is carried out. Figure 2 shows the observed phase after removal of
discontinuity. Small data discontinuities still remain in the data.
The Allan variance is a good measure for evaluating stability of reference signals. We calculated
the Allan variance at an averaging time  from thephase data at 10 MHz as follows,y2 ( ) hy2(2; ;  )i = h(yk+1 0 yk )2i=2

where
yk =

0

(tk +  ) (tk )20 

Figure 3. Root Allan variance calculated for 24 sets

(1) of phase data. Each data set spans 6 hours. Mea(2)where (tk ) is the phase at time tk and 0 is thefrequency at which the phase measurement is made(=10 MHz). We divide phase data into 24 sets of 6hour spans. Figure 3 shows the Allan variances calculated for each phase data set. The results show
stability of better than 10011 for an averaging time
range less than 1000 sec. This is almost the same
performance expected from speci cations of the receiver. Scatter on the plots seen for time range lessthan 1000 sec is attributed to small discontinuities
still left in the phase data as mentioned before.
System noise level is sometimes larger than the receiver performance for the time range less than 10
sec. However we can say that it reaches the leveldescribed as typical characteristics in the speci cations of the receiver, i.e.,  2 2 10012 at 10 sec.

suring system noise lebel is represented by symbol\2".

Next we evaluate coherence loss when this frequency standard is applied to S (2.2 GHz) andX (8.8 GHz) band observations. Using measured
phase data, coherence after N sec integration is
calculated as follows,
1NX

n = C 2 (tn )(3)=1where C is a ratio of observation frequency and
10 MHz, i.e., 220 and 880 for 2.2 GHz and 8.8 GHz
respectively, and (tn ) is a phase at time tn . We
assume no coherence loss for 1 sec integration. Coherence calculated this way is shown in Figure 4.
In this calculation phase data are divided into 48
sets of 3 hour span. As shown in the gure coherence decreases as the integration period increases.
Even at 10 sec integration they are down to about
rN =

N

n

ein ;

21

November 1997

0.7 and about 0.3 for 2.2 GHz and 8.8 GHz, respectively. They are further decreased down to 0.1 for
100 sec integration, which is a typical accumulation
period in a geodetic VLBI. Therefore it seems to
be dicult to adopt for VLBI observations.
Fringe searches in VLBI correlation processing
are relevant to this problem. Correlator outputs
integration results every second or every several
seconds. These time segmented data are further
integrated with phase change being adjusted by atime-linear function to maximize integrated correlation amplitude. This process is called a \fringe
search".
We adopt a similar technique in a calculationof coherence. Firstly phase variation during accumulation period is tted by a polynomial function
using a least squares method. Then we calculate
coherence using residual phase after tting. We use
a polynomial of degree 1 to 3.
Figure 5 shows results in case of the rst orderpolynomial being adopted. This is the case just likea \fringe search" in geodetic VLBI processing. We
can see remarkable increase in coherence. At 100
sec, almost no loss can be seen for 2.2 GHz. Evenfor 8.8 GHz, coherence keeps value of 0.3. Figure
6 presents results when a third order polynomial isadopted. It is seen that an integration period without loss can extend to about 300 sec for 2.2 GHz
and about 100 sec for 8.8 GHz. Thus adopting thefringe search process for integration, the GPS Timeand Frequency Reference Receiver can be used asa frequency standard for VLBI observations madeat S and X bands. However, the observed delay

time obtained this way should be carefully examined for utilization in further analysis such as abaseline length analysis, because clock stability is
an important factor to connect a scan, which is acontinuous observation of a radio source, between
neighboring observations in further analysis, anductuation of the clock between observations remains as an unknown parameter.

3. Conclusion
We have evaluated the performance of GPS Timeand Frequency Reference Receiver (HP58503A) to
determine whether it is adoptable as a frequency
standard in VLBI observations. Firstly we measured phase di erence between 10 MHz signals output from two independent receivers. Then we calculated the Allan variance to evaluate the stability
at average times ranging from 1 to 10000 sec. Coherence loss was also calculated from the actual
phase data. As a result it is demonstrated that
the GPS Time and Frequency Reference Receivercan be used as a frequency standard if we expand
the fringe search process to account for third ordervariations with time.

Acknowledgments
The authors would like to thank Hiroo Kunimori,
KSP/SLR group leader, for o ering two GPS timeand frequency reference receivers (HP58503A) for
this study.

Figure 4. Coherence calculated from the phase data versus integration time for 2.2 GHz (leftpanel) and 8.8 GHz (right panel).

22

IERS TDC News No.11

Figure 5. Coherence calculated using residual phase data after a linear drift is removed versus integration
time for 2.2 GHz (left panel) and 8.8 GHz (right panel).

Figure 6. Coherence calculated using residual phase data after a third order polynomial t versus integration time for 2.2 GHz (left panel) and 8.8 GHz (right panel).

23

November 1997

Millisecond Pulsar Observa- veloped a basic observation system using our 34m
antenna at Kashima in 1992 [Hanado et al., 1993],
tion System at CRL
and succeeded in detecting PSR1937+21. Based on

Yuko Hanado1 (yuko@crl.go.jp), Michito
Imae1 , Mizuhiko Hosokawa1 , and Mamoru
Sekido2
1 Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
Japan
2 Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan

1. Introduction
It is known that millisecond pulsars having a millisecond pulse rate maintain extremely stable pulse
timings over the long term. As for the rst discovered millisecond pulsar PSR1937+21 with 1.6 ms
pulse period, fractional frequency stability was reported on the order of 10014 for 2.6 years averaging
times [Taylor,1991], which is comparable to a cesium clock. With such characteristics, millisecond
pulsars are expected to be used as probes for detecting the background of gravitational waves anductuations in the interstellar medium, and as stable frequency standards [Fruchter et al., 1995].
Communications Research Laboratory (CRL) is
the national institute of time and frequency standards in Japan, and we aim to apply the millisecond pulsars to the frequency standards. We de-this result, we develop a more sensitive system using an Acousto-Optic Spectrometer (AOS) [Goutzoulis and Abramovitz, 1988]. This report introduces this system and current observation resultsof PSR1937+21.

2. Concept of System Design
It is dicult to measure precise pulse timing of
millisecond pulsars because their signals are quite
weak and signal-to-noise ratios are not good. Theux density of PSR1937+21, which is one of thestrong millisecond pulsars, is only about 4mJy at
2GHz (calculated from Foster et al.[1991]). Because our 34m antenna is not so large for millisecond pulsar observation, we require a highly sensitive pulse-detecting system.
One way to improve a system's sensitivity is toexpand its observing bandwidth. For pulsar observations, however, we must note the dispersion e ectcaused by interstellar plasma. Owing to this e ect,observed pulse shape is broadened as the receiving
bandwidth increases. In order to avoid this problem, a wide-band signal must be divided into narrow bands at rst, and recombined after cancelingthe dispersion delay in each narrow band. Such a
process is called de-dispersion [Lyne and GrahamSmith, 1990]. A lter-bank method is popular forde-dispersion, but we use an AOS instead of a lterbank method in our system.

Figure 1. Block diagram of millisecond pulsar observation system at CRL.

24Æëñâëðæñö

IERS TDC News No.11­«­ ­«¯ ­«° ­«² ­«³ ­«µ ­«¶ÑæêâE<B®«® ®«¯ ®«±®«³Figure 3. Pulse pro le for PSR1937+21 at 2GHz
band. Integration time is about 14 minutes and receiving bandwidth is 150MHz.
obtain the averaged data almost in real time.

3. Observation System

Figure 2. Data ow from the AOS to the host computer.
AOS is a spectrometer using an acousto-optic device such as a single crystal of TeO2 for spectrum
analyzing. This spectrometer can divide a wideband signal into many narrow channels simultaneously by a small crystal, so it makes the system
simple and compact.
Long integration time is another way to improve sensitivity, which is achieved by accumulating many pulses. For this purpose we developed
a high speed averaging processor which can average pulses 224 times without data transportation
to the host computer. It can eliminate dead timein data processing by performing internal calculations during the next data acquisition, and we canFigure 1 is a block diagram of our observation
system using the 34m antenna at Kashima Space
Research Center of CRL, and Figure 2 shows its
data ow. The system's parameters are listed inTable 1. We use 2GHz band for pulsar timing observations. An IF signal with 200MHz bandwidth
is divided to 50MHz x 4units at video converter.
Each 50MHz bandwidth is divided to 200kHz x
256ch by the AOS, then transported to the video
averaging processor serially. This transportingtime for one line is 12.8  sec (=50ns 2 256ch),
which limits time resolution. When the transportation trigger is set to 1/100 of the pulsar period, the time resolution becomes about 16  sec
for PSR1937+21. The video averaging processor
works as an 8-bit A/D converter, and an averager
which allows 224 pulses' addition (= 7 hours' integration for PSR1937+21) in each channel. At thehost1, the averaged data of each channel are combined after the dispersion-delay calibration carriedout in 1/1000 steps of pulsar period, and nal pulse
pro le is de ned. From this pro le, the peak phaseis de ned as arrival pulse timing. Host2 calculates
the a-priori pulse period, and supplies it in realtime to the synthesizer which controls the averaging trigger clock of the timing signal generator.
For this calculation, we use the program TEMPO
which is the Princeton pulsar timing analysis package [Foster and Backer, 1990]. The reference clockof this system is synchronized with UTC via theGPS satellites. The di erence between UTC and
the internal clock of the timing signal generator ismonitored by a time interval counter.

25

November 1997

Table 1. Parameters of the millisecond pulsar observation system at CRL.
Antenna diameter
Observation frequency
Total bandwidth
Frequency resolution
Time resolution
A/D converter resolution
Number of pulse-addition

34m
2120 - 2320 MHz
200 MHz (50 MHz 2 4 units)
200 kHz (50 MHz / 256 ch)
16  sec
8 bit
20 - 224(a)­ÏâðæáòÞé

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Figure 4. Residuals of peak phase for PSR1937+21. (a) Observation data using a database calculated
by VAX TEMPO. (b) The same days' data corrected by a database calculated by UNIX TEMPO.

4. Observation of PSR1937+21
We carried out the preliminary observations of
PSR1937+21 at 2GHz band. Figure 3 shows thepulse pro le made after averaging 524288 pulses (= integrating about 14 minutes). From such averaged pro les, peak phases are de ned. Figure
4 shows the residuals R0(t)s calculated as follows,which shows the phase uctuation:
R0 (t)R(t)= R(t) 0 Rave
= obs(t) 0 calc (t)here obs(t) is an observed peak phase, calc (t) isa calculated a-priori phase, and Rave is an averageof all R(t)s over three days. Residuals observed on
May 30, June 2, and June 4 tend to increase in
one day (Fig. 4(a)). We suppose it is due to the26

IERS TDC News No.11

mismatch of a-priori calculation, because we used
the old database made by old TEMPO (VAX version) in these observations. Then we corrected theobserved data by using new TEMPO (UNIX version). The UNIX version uses a newer ephemeris
compared with that of the VAX version. After
this correction, the residuals' drift seems to be at(Fig.4(b)), and the standard deviation is improved
to 4.8  sec from 7.2  sec. From these results, it
seems to be better to use UNIX TEMPO.
After June 19, we use UNIX TEMPO for theobservations, remain. Figure 5 shows the obser-¯­

References®­

Foster, R. S. and D. C. Backer, Constructing a pulsar

Ïâðæ
ðæá
áòÞé

Ï·¥ñ
¥ñ¦¦



B

°­

of the smallest antennas for millisecond pulsar observation in the world. Using this system, continuous observations have been carried out for
PSR1937+21. The standard deviation of observed
peak phases is about 4.8  sec which shows thecurrent observation precision. This value is reasonable in terms of the system's performance andthe observation conditions.
However some problems exist in our system yet.
We must check the a-priori pulse period calculation
because the observed peek phases show some systematic trend. In addition, some noise exists in thepulse pro le which may make the observation precision worse. We are trying to solve these problems
in order to get better observation precision.

timing array,­ª®­study of four millisecond pulsars,

ª¯­
ª°­
®´­

Astrophys.J., 361, 300-308, 1990.

Foster, R. S., L. Fairhead, and D. C. Backer, A spectral

Astrophys.J., 378,

687-695, 1991.

®´²

®µ­

®µ²

®¶­

ÁÌÖ 30
30HH

®¶²

¯­­

¯­²

¯®­

Figure 5. Peak phase residuals for PSR1937+21
observations using UNIX TEMPO.

Fruchter, A.

S., M. Tavani, and D. C. Backer, Mil-

Astronomical
Society of the Paci c Conference Series, 72, 345-356,
lisecond pulsars: A decade of surprise,

1995.
Goutzoulis, A. P. and I. J. Abramovitz, Digital electronics meets its match,

IEEE Spectrum, 21-25, Au-

gust 1988.

vation results from June 19 to July 25, and theresiduals show a systematic trend. Perhaps it isbecause of the misuse for the program or hardware
problem, and we are now trying to clarify the cause.

Lyne, A. G. and F. Graham-Smith, Pulsar astronomy,

5. Conclusion

Taylor, J. H., Millisecond Pulsars: Nature's Most Sta-

We developed a millisecond pulsar timing observation system using the Kashima 34m antenna,and succeed in detecting PSR1937+21. It is one

Hanado,Y.,

M. Imae,

H. Kiuchi,

S. Hama,A.

Kaneko,andMillisecond pulsar observation at CRL,

J. Commun. Res. Lab., 40, 55-62, 1993.

Cambridge Univ. Press, 27-28, 1990.

ble Clocks, Proc.IEEE, vol.79, No.7, pp.1054- 1062,
July, 1991.

27

November 1997

Construction of New LF Station for Time and Frequency
ServiceMichito Imae (imae@crl.go.jp)
Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
Japan

1. Background of New LF Station

 High stability, about 10011 to 10012 , in thearea of ground wave There is no interference time service signal using Japanese LF band (40 kHz) around Japan There is a big market, such as radio controlled

clocks and watches, using LF time signals
In the global view, the ground standard time service is shifting to LF band from HF band, such as
DCF-77 and MSL in England, increased power of
WWVB in USA.
Of course this is the age of GPS, and using GPS
signals everyone can make easily obtain positioning
to better than 100 m and get time signals better
than 1 microsecond. But in our policy, the LF
time signal can coexist with GPS for the following
reasons:(1) the frequency stability of LF signal is of thesame order as GPS in the case of SA,(2) the receiver of LF signal is more simple compared to a GPS receiver,(3) LF signal can be received inside buildings.

Communications Research Laboratory (CRL)
has the responsibility for national frequency andtime standards in Japan. Depending on this charter CRL has been engaged in research and services
in this domain, such as(1) Development of primary frequency standards,(2) Time keeping,(3) International precise time transfer,
3. Outline of CRL's New LF Station(4) Dissemination of standard frequency and time
signals.
Location : southern part of Fukushima prefec(5) Related basic researches.
ture (about 200 km north of Tokyo)
Concerning the dissemination of standard fre- Antenna : top loading umbrella type antenna
quency and time, CRL employs several dissemina- type
mounted on a 250 m main towertion ways, such as standard time signal transmission using HF and LF bands, dial-up time service Carrier Frequency : 40 kHz: 50 kW (solid state type)using telephone lines (called Telephone JJY) and Transmitter
Network Time Protocol stratum-1 server for com- E ective radia- : > 10 kW (more than tention power
times greater than present
puter networks.
The construction of a new LF station for time
station)and frequency service is one of the activities forConstruction schedule
dissemination service at CRL.
Fiscal year 1997 : building and bases for antenna
2. Why LF Time Service?
Fiscal year 1998 : antenna tower, transmitters, etc.
CRL started transmission of JJY (HF time signal service) in 1940. But its quality or frequency
stability is not enough for recent needs, because HF signal has about 1007 or 1008 frequency 4. Applications of LF Signal
stability at receiving point due to ionosphere
while the transmission signal has 10012 toApplication areas of the new LF standard signal
10013 stability.
are expected to be the following: around Japan area there are several HF stan- For time signal:
Radio controlled clocks and watches
dard time service stations, such as BPM from
China, BSF from Taiwan, HLA from Korea,
Time control of public transportationand WWVH from Hawaii. Due to these forTime maintenance of meteorological equipeign standard time signals, it is dicult to disment and seismometers, etc.
criminate JJY from the others, especially in
For frequency standard:
the western part of Japan.
Keep tractability of the frequency standard to
the national frequency standard for industry
On the other hand LF signal has following advanRadio frequency regression
tages over the HF signal

28

Basic Research of Key Technologies for Next Generation Global Navigation Satellite System

Michito Imae (imae@crl.go.jp)

IERS TDC News No.11

started development of an accurate time andfrequency transfer technique between spaceand ground station. They are planning to
use two way transmission mode to cancel thee ects of propagation medium, such as ionosphere and troposphere, and use carrier phaseof the signal to obtain precise measurement.

Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184-8795,
Japan(3) Highly precise and real time orbit determination technique. Concerning this item, CRL
has just started discussions concerning themethod which should be most suitable for the
next generation GNSS system.

1. Introduction

3. Relation to the ETS-VIII Satellite

GPS (Global Positioning System) is one of themost popular navigation systems and it is usedin very diverse application areas, such as precise geodesy, time comparison, meteorological purposes.
On the other hand discussions for the next generation global navigation system (GNSS), and how
Japanese government can contribute to this system
has been made on several occasions.
Depending on such discussion, the committee ofspace development of Japan made a scenario fornext generation GNSS.
Its basic policy is:Japan should establish basic research and development items to contribute to the next generation
GNSS, such as on board atomic clock, precise timeand frequency transfer between ground clock andsatellite on board clock and precise and real timeorbit determination technique.
CRL has started research and development on
the above items during this scal year.

ETS-VIII satellite is \Experimental Technology
Satellite VIII" which will be launched in 2002.
NASDA (National Space Development Agency)
has plans to equip this satellite with atomic clocks.
The on-board atomic clocks will be commercial
type Cesium clocks. By using these atomic clocks
NASDA plans to carry out basic experiments innavigation.
A working group for the satellite navigation system was organized by the members of CRL and
NASDA to discuss the function of the ETS-VIII
on-board atomic clock equipment and experiments
using it.

2. R & D Items for Next Generation GNSSCRL has initiated the following main researchand development items this year:(1) Development of on board Hydrogen Maser
type frequency standard; Atomic standard
section is in charge of this item. They
started development of Hydrogen Maser type
frequency standard in cooperation with aJapanese private company. They are going toconstruct a prototype within four years. It will
be have almost same frequency stability as alaboratory type and its weight will be 50 - 70kg.
(2) Highly accurate time and frequency transfer
technology between ground station and satellites: Frequency and time standards section

Figure 1. TDS784A Gigabit samplers under test.

29

November 1997

Giga-bit VLBI,
Progress on Giga-bit Sampler

Table 1. TDS-spec Comparison between the VLBI
& Spectrometer

Junichi Nakajima(nakaji@crl.go.jp)
Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan

1. Status
Giga-bit samplers (see Figure 1 on page 28) were
tested at the Nobeyama radio observatory (see theprevious issue of TDC news for details of the VLBI
giga-bit sampler). Both CRL-VLBI and NROspectrometer group carried out the test. The DSO(Digital Sampling Oscilloscope) sampler consists ofa modi ed DSO part and an interface for successive
processor. The interface unit is working as a demultiplexer of the data stream for each purpose.
Nobeyama 25-multi-beam receiver for 45m radio
telescope will use the DSO sampler with 1 Gsps
(sampling per second) 2-bit 4-channel. Although
there is a di erence in the interface unit channel
con guration, we agreed to unify the DSO sampling part. This will simplify the maintenance procedure at the manufacture. Radio astronomical society will be able to accommodate each other withsampler on site. In September whole astronomicalgiga-bit sampler had been gathered at Nobeyama.
We measured all DSO sampler at the same bench.
Several static and dynamic parameters to evaluate AD performance had been measured. Table 1
shows the uni ed astronomical sampler major speci cation.

2. Measurement
Each channel of the sampler works 1024 Msps
2-bit for scienti c data sampling. We have checked
AD performance. Although the TDS784 is completed as the time-domain measurement instrument, longterm frequency-domain AD characteristics should be examined for astronomical purposes.
We have measured following parameters for the
rst step .

Di erential Liniarity Error In 2-bit sampling,

AD deviation from ideal transition spacing(DLE) exists. Improper code-width between
the transitions will give bad AD linearity.
Statical distribution of output-code bin will
not represent actual level distribution. In our
astronomical data processing, there is no DLE
calibrating hardware. DLE is expected to bevery small. Following equation,(TDS784 modi ed)
Sample rate(M)Channel Input
AD Quantaization
Output Datarate
Total Datarate(Interface)
Input Datarate
Handle rate(M)Select channels
Quantaization
Output connector
Total datarate(Processor)
Instrument
Datarate

VLBI

Spectrometer

1024/512/256
4/4/4 (*1)
2
128MHz x64
8Gbps

1024/512/256
4/4/4
2128MHz x6
8Gbps

128MHz x64
1024/512/256
1/2/4
1 or 2(*2)
D-Sub
2Gbps

128MHz x64
1024/512/256
4/4/4
2
SCSI-halfpitch
8Gbps

Recorder
1Gbps (x2) (*2)

Auto-correlator
2Gbps (x4)Notes: 1: uni ed version. 2: single recorder use MSB bit
only.

Table 2. Di erential Linearity Measurement Resultof two VLBI sampler#1#2
DLE2 DLE1 DLE2 DLE1
ch-1 0.072 0.082 0.010 0.030
ch-2 0.032 0.068 0.040 0.004
ch-3 0.074 0.046 0.038 0.016
ch-4 0.074 0.076 0.042 0.004

DLE =

0

Tn+1 T n
Vfs =2N

0 1(LSB )de nes DLE measured with LSB. Table 2
shows the measured TDS784 DLE between 3transitions under 2-bit sampling. Measured
DLE is small enough to neglect in the latterpart.
O set Error DC o set also results in the deviation of code appearance. Trouble will occur inthe latter part digital processing especially incorrelation. Few mV DC o set had con rmed
in the AD raw data. To cancel out this, additional o set can be applied from the TDS
front panel. Rather important longterm drift
and temperature dependence will be measured
separately.
Spectrum Auto-correlation result of a period
was used to check the spectral characteristics. UWBC correlator in the Nobeyama observatory was used for preliminary test. 64
MHz harmonics was observed in cetain channel data. The bug was xed.

30

IERS TDC News No.11-News-News-News-News-News-News-News-News-News-News-News-News-

Message to the Key Stone ProjectProf. T. A. Herring of Massachusetts Institute of Technology kindly gave his message to the Key
Stoneproject Team when he nished his stay at the Communications Reserach Laboratory as a gusest
scientist in September, 1997. We are happy to open it as an article of the TDC News getting his
acceptance since it is full of suggestions for the technical develoments.(T.Y.)

Dear Taizoh-san and Keystone Project Team,I would like to thank you and the Keystone Project Team for a most enjoyable visit. I was very
impressed by the facilities here at CRL and at Kashima. The Keystone Project is unique in theworld and it seems that it has the potential to make great contributions to our understanding of
the Earth and modern geodesy. The VLBI system in the Keystone Project is the only current
system running real-time and on such short baselines. The SLR system is also unique inthe proximity of the systems and its collocation with VLBI and GPS. As such, you have aunique opportunity to address a number of critical issues in Earth sciences. To me, one of themost important issues to address is the \stability" of the Earth. While our representation of
the motions of points on the Earth surface is through linear motions with occasional episodic
displacements and post-seismic motions, there is mounting evidence, mainly from GPS, that theactual behavior of the Earth may be more complicated. The deviations from secular motion are
generally less than 10 mm in horizontal components and seem to be of large scale (i.e., correlated
over may hundreds of kilometers). But so far, there is little evidence that two di erent geodetic
systems see the same deviations. In this regard, the collocated Keystone systems are uniquein a tectonically active region. Because the non-secular motions appear correlated over large
distances, the expected signal sizes on a small network are likely to be only a few millimeters.
This is the case in California where the RMS scatter of daily positions estimates for many of
the GPS stations separated by <100 km is between 1 and 2 mm for horizontal components and5-10 mm in height. Of course in California there is only a GPS network and it is not possible
to compare GPS to other space geodetic signals.
For the Keystone Project I see several tasks that would greatly increase our understanding ofgeodetic systems and the Earth. With this system, it should be possible to have millimeter
accuracy results available from all three geodetic systems and the detailed comparison between
them should reveal much about these systems and the Earth. For VLBI, I think you should
investigate using phase delays which should improve the delay precision by a factor of ten
or so; it will be interesting to see how much the station position estimates improve with thisincreased data quality. For GPS, you could potentially improve the phase center models forthe antennas through the comparison of the position estimates from VLBI, GPS and SLR, andthrough detailed studies of the phase residuals. One interesting approach here would be to use
the PRESTAR system with its clock shared with a nearby GPS receiver using an omnidirectional
antenna. Such as system is being developed in the US for in-situ calibration of GPS antennas.
One of the largest uncertainties in GPS measurements arises from the phase center variationsof the GPS antennas. SLR has, in principle, a major advantage over GPS and VLBI due
to its much lower sensitivity of atmospheric water vapor. There is also an intrinsic strength
in making a direct range measurement rather than the biased range measurements implicitly
make by GPS and VLBI. The challenge in SLR is to realize these advantages; at the moment it
appears that SLR generates geodetic results which are not of higher quality than GPS and VLBI
even in the height component. Understanding why this is so, and improving the performance of
SLR seems to be a high priority. The Keystone system is again unique in being able to addressthese fundamental issues.

November 1997

For all systems, it seems that the availability of the JMA 10 km resolution atmospheric data
sets will allow detailed studies of the e ects of atmospheric propagation on geodetic systems.
Establishing how large the e ects of inhomogeneity in the atmosphere are is an important task.
Of great utility to the whole geodetic community would be the development a parameterizationof these inhomogeneities so that they could be estimated from the geodetic data themselves thus
extending the currently used zenith delay and gradient estimation strategies. If the VLBI/SLR
and GPS systems do not generate the same results between collocated stations then CRL is in aunique position to understand and correct the de ciencies in each of these systems. Ultimately,
if the systems can be shown to agree at the 1 mm level, at least in the changes of the positions
that they measure, then again the Keystone Project is in a unique position to improve theaccuracy of all three systems to better than 1 mm. Ultimately it is not clear what will set thenal accuracy achievable with any of these systems: it could be the atmosphere or the solid
Earth itself. The operation of these systems and the redundancy of the measurements systems
should de nitively establish the e ects to be expected before, during, and after earthquakes.
Motions after earthquakes should establish the rheological properties of the Earth and the forces
acting on it after an earthquake. Detection of a reliable pre-seismic signals, measured by three
independent systems, would be of fundamental importance in the study of earthquakes. Of
course, these signals may be too small to detect currently, but the Keystone Project is in aunique position to be able to address this issue with multiple techniques.
Again I thank you for a wonderful visit, and I look forward to seeing some great results coming
out of the Keystone Project.
Best Regards
Tom Herring.

31

32

IERS TDC News No.11

VLBI Technical Development Center (TDC) at the Communications Research Laboratory (CRL)
is supposed to do1) the development of new observation techniques and new systems for advanced Earth's rotation observations by VLBI and other space techniques,2) the promotion of research in Earth rotation by advanced methods in VLBI,
3) the distribution of new VLBI technology.
The TDC meeting, attended by the ordinary members from inside the CRL and the special
members from the outside, is held twice a year. The special members advise the committee,
concerning the plan of technical developments. The TDC newsletter is published biannually by
CRL to inform the IERS community its current activities.
This news was edited by Tetsuro Kondo and Yasuhiro Koyama, Kashima Space Research Center,
who are editorial sta members of TDC at the Communications Research Laboratory, Japan. The
editors wish to thank Dr. O. J. Sovers for his kind help in the correction of the news translated
from Japanese to English. Inquires on this issue should be addressed to T. Kondo, Kashima
Space Research Center, Communications Research Laboratory, 893-1 Hirai, Kashima, Ibaraki
314-0012, Japan, TEL : +81-299-84-7137, FAX : +81-299-84-7159, e-mail : kondo@crl.go.jp.
Summaries of VLBI and related activities at the Communications Research Laboratory areavailable from the home page of the Radio Astronomy Applications Section of the Kashima
Space Research Center on the World Wide Web (WWW). The URL to view the home page is :\http://www.crl.go.jp/ka/radioastro/" .

TECHNICAL DEVELOPMENT CENTER NEWS No.11, November, 1997
International Earth Rotation Service - VLBI Technical Development Center News
published by
Communications Research Laboratory, 4-2-1 Nukui-kita, Koganei, Tokyo 184-8795, Japan