ISSN 1882-3432

CONTENTS
Proceedings of the 16th NICT TDC Symposium (Kashima, June 21, 2017)
A numerical study on a photonic assisted analog-to-digital converter operat- . . . . . . 1
ing at 100 GSa/s
K. Furusawa, I. Morohashi, N. Sekine and I. Hosako
Water Vapor Radiometer for Radio Astronomy and Volcano Monitoring
Noriyuki Kawaguchi...... 5

Radio Frequency Interference in GNSS Observations by Artificial Signals for . . . . . 10
Mobile Phones
Yoshihiro Fukuzaki, Tomokazu Nakakuki, Yuji Miura, Asuka Sato, Hiromichi Tsuji
Look back on 18 years of Tsukuba 32-m antenna
Midori Fujiwara, Shinobu Kurihara , Masayoshi Ishimoto, Takahiro Wakasugi,
Michiko Umei
Phase-up twin antennas for high-sensitivity maser observations
Kazuhiro Takefuji, Koichiro. Sugiyama, Yoshinori Yonekura, Tagiru Saito, Kenta
Fujisawa, and Tetsuro Kondo. . . . . 14

Development of Wideband Antenna
Hideki Ujihara. . . . . 21

’Node - HUB’ Style VLBI with Broadband System.
Mamoru Sekido. . . . . 22

. . . . . 17

Technical Reports
Holographic Measurement for Kashima 34 meter Antenna
Kazuhiro Takefuji, Masanori Tsutsumi, Eiji Kawai, and Mamoru Sekido. . . . . 26

Recent progress of K5 software correlation system
Tetsuro Kondo. . . . . 29

1

December 2017

A numerical study on a pho- Photonic ADCs (p-ADC) have rather a long hissince 1970’s and various architectures have sotonic assisted analog-to-digital tory
far been proposed. An excellent review was given
converter operating at 100 by Valley[3]. Recent successful developments in pADCs include use of the time-stretch principle[4],GSa/spioneered by Jalari et al., with which the sampling
K. Furusawa (kfurusawa@nict.go.jp),
I. Morohashi, N. Sekine and I. Hosako,
Frontier Research Laboratory,
National Institute of Information andCommunications Technology (NICT),
4-2-1, Nukuikitamachi, Koganei, Tokyo, Japan
Abstract: The feasibility of a photonic-assisted
analog-to-digital converter (p-ADC) based on a frequency comb source operating at 10 GHz in thetelecommunication C band is numerically investigated. It is found that the effective number of bits
(ENOB) monotonically decreases with the input
RF frequencies. Nevertheless, ENOB > 4 can still
be anticipated at the Nyquist sampling rate of 50
GHz using only 10 WDM channels by managing
the temporal allocation of spectral channels.
1.

Introduction

rate beyond 1 TSa/s has been reported[5]. In viewof integrating the entire system on a silicon photonics chip, Kärtner et al. demonstrated 7 ENOB
for a 40 GHz signal[6], employing off-the-shelf components as a preliminary result, where the discrete
time-frequency mapping scheme was used in conjunction with narrowband detection. These studies
have clearly demonstrated the potential of p-ADCs
in scaling the sampling speeds. On the other hand,
there are a number of issues such as sensitivity andrealitimeliness. The latter is particularly relevant
to telecom applications since the signal capturedby the ADC should eventually be regenerated fromthe DAC output. In addition, the perfect reconstruction condition is not warranted for this typeof ADCs due partly to the complexity of the photonic analog link, which may call for new types of
equalization schemes.
In this study, we specifically consider utilizationof a frequency comb generator (FCG)[7] operating at 10 GHz in a system based on the discrete
time-frequency mapping scheme (see Fig.1). By
employing a high repetition rate light source, thenumber of optical components in the ADC system
can be reduced, thereby greatly simplifying the system architecture. Furthermore, since the external
RF clock source determines the timing jitter performance of the FCG, the timing jitter issues in
p-ADCs can be diverted to the accuracy of theclock: as we improve the clock performance, thejitter performance of the system can also be improved without modifying the entire system. However, our preliminary study has shown that thistype of system is spectrally hungry due to the crosstalks in both temporal and spectral domains. It
becomes increasingly difficult to retain a low level

Analog-to-digital converters are ubiquitously
used at the frontends in varieties of signal processing systems. The performance measures are typically characterized by sampling rates and effective
noise of bits (ENOB). Despite constant progress ofdigital electronics speeds, the progress in ultrafast
ADCs (> 10 GHz) used for dedicated applications
such as optical / ultrawideband wireless communications and data acquisitions in astronomy andgeodesy are rather slow; typically 1.5 bits per 8
years, as reported by Walden[1]. One of the major
reasons is an aperture jitter of temporal sampling.
In general, ENOB degrades 4 bits / octave for agiven amount jitter. At present, a clock jitter isabout 100 fs for laboratory instruments. This implies that the achievable ENOB is less than 3 at
100 GHz. Then, it is evident that the improvement in timing jitter is the key issue for analog
signal sampling beyond 100 GHz. In addition, finite transit time of the transistors used in the comparators leads to so-called comparator ambiguities,
which impose a more serious impediment for highspeed sampling in electronic ADCs (typically beyond 10 GHz). On the other hand, extremely low
timing jitter has already been achieved in modelocked solid-state lasers, owing to very short temporal duration of optical pulses and a high intracavity pulse energy[2]. This fact has motivated a Figure 1. Schematic of p-ADC system based on
renewed interest in photonic assisted form of ADCs discrete time-frequency mapping scheme.
for high speed applications.

IVS NICT-TDC News No.37

of timing jitter as the spectral bandwidth is increased since additional nonlinear spectral broadening may seriously deteriorate the original jitter
performance[8]. In order to make a system more
spectrally efficient, we propose a frequency hopping(FH) approach similarly to optical code-divisionmultiplexing systems[9]. By using appropriate allocation of the spectral channels in time domain,
the cross talks originated from the adjacent spectral channels can be reduced significantly. We discuss the design criteria of the proposed system andnumerically show how the spectral bandwidth requirement can be relaxed in a system operating at
100 GSa/s.
2.

2

Figure 2. Time-frequency representation of a sampling pulse train in (a) linear and (b) FH assignment, generated by taking cross correlations witha transform-limited gaussian pulse with 500 GHz
bandwidth (i.e. crosscorrelation frequency resolved
optical gating (XFROG) trace[14]).

Description of the system(∆fsource ) can be estimated to 9.2N/TS , where
TS is the sampling period, assuming the gaussian
profiles of the spectral channels. This suggeststhat the source bandwidth spanning from C to L
bands (wavelength of 1.53 ∼ 1.61µm) is required
to achieve the sampling rate of 100 GSa/s (TS =10ps). To relax this bandwidth requirement, we consider an allocation method of the spectral channels
in time domain. We label spectral channel numbers(1 ∼ 10) from the low frequency side. If we solely
assign them in order in the time domain (linear assignment: see Fig.2 (a)), the spectral cross talks
should also appear in the neighboring time slots inthe detected signals at the photodiodes. Then, thenatural strategy would be to separate the temporal
distance of adjacent spectral channels as much as
possible. For instance, in the case of N = 10, theminimum channel separation ∆N can be set to 3
(frequency hopping (FH) assignment: e.g. [1 8 5 2
9 6 3 10 7 4], see Fig.2(b)). Then, it becomes possible to eliminate the spectral cross talks by temporal gating (with the gating duration of ∼ TS ∆N ),which can be implemented using slightly faster eADCs (with their sampling rates of 1/(TS ∆N )).
Note that this approach can be even more effective in a system consisting of many channels witha low repetition rate light source since ∆N can beincreased by increasing N although the total bandwidth required for the source is increased (since the
3. Frequency-hopping (FH) scheme
channel width should be retained to avoid temporal
In designing the above p-ADC system, each spec- cross talks).
tral channel has to be wide enough to avoid sampling the adjacent time slots (temporal cross talks). 4. Numerical example
Similarly, flat phase responses of the channels are
To verify the effectiveness of the FH scheme, we
also important to generate a train of nearly Fourier- calculated the ENOB as a function of input RF
transform limited pulses since any pedestals due to frequency. The errors in p-ADC output signals dethe residual chirp penetrating to the adjacent time pend on the input frequency as well as the phase.
slots results in temporal cross talks. On the other By average the phase dependence, nominal ADC
hand, spectral cross talks should also be minimized errors ∆ν were first calculated, for which intrinsic
for signal detection. From these simple arguments, signal-to-noise ratio (SNR) is defined. Then, thethe total bandwidth required for the light source
We assume a FCG operating at 10 GHz repetition rate, for which timing jitter as low as 11 fs
has been reported[10]. This suggests potentiallyachievable ENOB of 7 for an input frequency of
50 GHz. We assume a number of spectral channels N = 10 so as to accomplish the sampling rateof 100 GSa/s. The FCG output was spectrally
sliced by an arrayed waveguide grating (AWG) followed by the delay lines, with which each spectral
channel is assigned to a specific timing within therepetition interval. After combining all channels
using another AWG, the RF signal is opticallysampled using a Mach-Zehnder electro-optic modulator (MZM)[11] with two complementary output
ports. These outputs are spectrally separated by
the AWGs again and each channel is individuallydetected by the photodiodes. Then, the electrically converted signals are quantized by electronic
ADCs (e-ADCs). This system allows us to scale thesampling speed by a factor of N while maintaining
the bandwidth requirement for the e-ADCs to therepetition frequency of the FCG. Detection of twocomplementary outputs allows for linearization of
MZM[12]. The details of the model are describedin Ref. [13]. The transmission profiles of the spectral channels through the AWGs were assumed tobe hyper-gaussian.

3

December 20175.

Summary

We have numerically investigated the performance of a p-ADC system operating at 100 GSa/s
based on discrete time-frequency mapping scheme
using a 10 GHz frequency comb generator lightsource. Assuming a realistic amount of cross talks,
it is shown that the ENOB degrades with an increase in input RF frequency (∼ 3.4 bits / octave).
Furthermore, it is also demonstrated that appropriate assignment of the spectral channels in time
domain is useful for relieving the bandwidth requirement with the aid of fast e-ADCs. As a result,
it is anticipated that ENOB > 4 can be obtainedat the Nyquist frequency of 50 GHz.
Figure 3. ENOB as a function of input RF frequency for linear (L) and frequency hopping (FH)
assignment with different integration time.

Acknowledgement
K.F. wishes to acknowledge Drs. A. Kanno, Y.
Yoshida, and T. Sakamoto (Network systems laboratory, NICT) for fruitful discussions.

ENOB was calculated by the standard formula[3]:References
SN R − 1.76
EN OB =
6.02

(1)

In AWGs, hyper-gaussian channels with theirbandwidths of 400 GHz and a uniform side mode
suppression ratio (SMSR) of 30 dB were assumed.
The channel spacing was also set to 400 GHz.
Then, the total bandwidth of the source amounts
to 4 THz that can be covered in the telecommunication C band. Residual phases were neglected.
Three cases were studied: 1) 100 ps and 2) 33 ps
integration time with linear assignment, and 3) 33
ps integration time with FH assignment. The results are compared in Figure 3. In all cases, ENOB
monotonically decreases with the RF frequency due
to the temporal cross talks since the amplitudedifference between the adjacent time slots increasingly becomes large for high frequency inputs. For
the case 1), additional spectral cross talks further
degrade the ENOB performance, consequently resulting in the ENOB less than 2 at the Nyquist
frequency of 50 GHz. Even when the integration
time is shortened to 33 ps (case 2)), the improvement in ENOB is subtle since the elimination of
the spectral cross talks is not effective due to thelinear spectral allocation. By contrast, the ENOB
can drastically be improved by using the FH assignment (case 3)), as anticipated, resulting in the
ENOB of > 4.3 at 50 GHz. Note that a broadband FCG spanning over 4 THz has already beenrealized in the experiment[15]. Therefore, it seems
feasible to achieve such p-ADC performance, provided that its timing jitter is sufficiently low (< 80fs).

[1] Walden, R. H., ”Analog-to-digital converter
survey and analysis”, IEEE Journal on selected areas in communications, Vol. 17, No.
4, pp. 539-550, 1999.
[2] Haus, H. A., and Mecozzi, A., ”Noise ofmode-locked lasers”, IEEE Journal of Quantum Electronics, Vol. 29, No. 3, pp. 983-996,
1993.
[3] Valley, G. C., ”Photonic analog-to-digital converters”, Optics Express, Vol. 15, No. 5, pp.
1955-1982, 2007.
[4] Mahjoubfar, A., Churkin, D. V., Barland, S.,
Broderick, N., Turitsyn, S. K., and Jalali, B.,”Time stretch and its applications”, Nature
Photonics, Vol. 11, No. 6, pp. 341-351, 2017.
[5] Chou, J., Boyraz, O., Solli, D., and Jalali, B.,”Femtosecond real-time single-shot digitizer”,
Applied Physics Letters, Vol. 91, No. 16, p.
161105, 2007.
[6] Kim, J., Park, M. J., Perrott, M. H.,
and Kärtner, F. X., ”Photonic subsampling
analog-to-digital conversion of microwave signals at 40-GHz with higher than 7-ENOB resolution”, Optics Express, Vol. 16, No. 21, pp.
16509-16515, 2008.
[7] Sakamoto, T., Kawanishi, T., and Izutsu, M.,”Widely wavelength-tunable ultra-flat frequency comb generation using conventional
dual-drive Mach-Zehnder modulator”, Electronics Letters, Vol. 43, No. 19, pp. 1039-1040,
2007.

IVS NICT-TDC News No.37

4[8] Ishizawa, A., Nishikawa, T., Mizutori, A., [12] Sefler, G. A., Chou, J., Conway, J. A., andTakara, H., Takada, A., Sogawa, T., andValley, G. C., ”Distortion correction in a highKoga, M., ”Phase-noise characteristics of a 25resolution time-stretch ADC scalable to conGHz-spaced optical frequency comb based on
tinuous time”, Journal of Lightwave Technola phase- and intensity-modulated laser”, Opogy, Vol. 28, No. 10, pp. 1468-1476, 2010.
tics Express, Vol. 21, No. 24, pp.29186-29194,[13] Khilo, A., ”Integrated optical analog-to2013.
digital converter” (Doctoral dissertation, Mas[9] Fathallah, H.,
Rusch, L. A. andsachusetts Institute of Technology) 2008.
LaRochelle, S.,”Passive optical fast
frequency-hopCDMAcommunications [14] Trebino, R., ”Frequency-resolved optical gating: the measurement of ultrashort laser
system”, Journal of Lightwave Technology,
pulses”, Springer Science Business Media,
Vol. 17, No. 3, pp.397-405, 1999.
2012.
[10] Metcalf, A. J., Quinlan, F., Fortier, T. M.,
Diddams, S. A., and Weiner, A. M., ”Broadly [15] Morohashi, I., Sakamoto, T., Sotobayashi, H.,
Kawanishi, T., Hosako, I., and Tsuchiya, M.,
tunable, low timing jitter, high repetition rate
”Widely repetition-tunable 200 fs pulse
optoelectronic comb generator”, Electronics
source using a Mach-Zehnder-modulatorletters, Vol 51, No. 20, pp. 1596-1598, 2015.
based flat comb generator and dispersion[11] Taylor, H. F., Taylor, M. J. and Bauer P. W.,
flattened dispersion-decreasing fiber”, Optics”Electro-optic analog-to-digital conversion usletters, Vol. 33, No. 11, pp.1192-1194, 2008.
ing channel waveguide modulators”, Appl.
Phys. Lett. Vol. 32, No. 9, pp. 559-561, 1978

5

December 2017

Water Vapor Radiometer forRadio Astronomy and Volcano
Monitoring
Noriyuki Kawaguchi (kawagu.nori@nao.ac.jp),
National Astronomical Observatory of Japan,
2-21-1 Osawa, Mitaka Tokyo, Japan
Abstract: A water vapor radiometer has been used
for monitoring delay and absorption caused on aradio wave travelling through the troposphere, primarily caused by water vapor and liquid water.
The measured data is used as delay calibration on
geodetic VLBI observations and source brightness
calibration on radio astronomy observations. The
water vapor in a volcano fume also shall be measured to monitor the volcano activity, which is crucially important in Japan of volcanic islands where
many volcanos are located in active, and some are
erupting.
In this paper the importance of water vapor monitoring in radio astronomy, geodetic VLBI measurements and monitorings of volcano activities.

Figure 1. Radio spectrum of PKS0202+129

of a radio astronomy telescope unacceptably high.
The liquid water content in a cloud is impossible tomeasure without a water vapor radiometer. Thesebad impacts will be discussed in the next section.
The water vapor content in a volcano fume is important to understand the state of the volcano activity and to predict the eruption. So far the watercontent in the fume has been indirectly estimatedby measuring SO2 or other constituents with an
1. Introduction
optical laser. In the measurement the mixing raNAOJ, National Astronomical Observatory oftio of H2 O/SO2 is assumed as is empirically known
Japan has started the development of a water va(Burton 2000)[2] (Writter 2012)[3] . Direct measurepor radiometer (WVR) for precise radio astromment of water vapor in the volcanic fume will beetry of VERA (Kawaguchi 2016) [1] . The develdiscussed in chapter 3. In chapter 4 the importance
opment aims at obtaining precise delay correctionof a water vapor radiometer will be noted.
for VERA observations to get better geodetic andastrometric solutions. The radiometer is also ex- 2. Impacts on Radio Astronomy and VLBI
pected to be used in flux calibration for a radio
Geodesy
astronomy observation to measure the true brightFig.1 shows the radio flux spectrum ofness of a target source.
The radiometer is designed to measure total col- PKS0202+129 referenced to the the NASA/IPAC
umn density of water vapor toward the sky cov- Extragalactic Database (NED). As clearly seen on
ered by a single beam of the WVR antenna. The the figure, flux densities above 10GHz is largely
air in the beam contains not only water vapor but scattered, probably caused by large uncertainty
also liquid water, Oxygen, Nitrogen and minor con- in the atmospheric absorption. The absorption
stituents. The Oxygen and Nitrogen (Dry Compo- changes day by day depending on the weather connent) is uniformly distributed in the atmosphere dition on the day, water vapor and liquid waterand is easy to predict the delay and absorption content in the air. If a dark cloud passes over theby monitoring the slowly changing ground pressure radio telescope, a large fluctuation on the brightmeasured by a ground weather station. The water ness of a target source may happen in a short time
vapor and liquid water (Wet Component) are heav- scale. It is uncorrectable, even if we observe a wellily localized in the atmosphere. The delay and the calibrated source as a flux standard.
It varies with time as is shown in Fig.2 (Forkman
absorption is rapidly changing in time. The hu[4]2003). Radfio astronomers recently pay great
midity measurements on a ground is helpless toattentionon the intra day variable sources ofmeasure a total amount of the wet components in[5]
AGN(IDV). They need to observe the sources
the atmosphere. A rain meter provides us good
under
a
clear
sky
of stable atmosphere or to makeinformation if the liquid water drops down on the
carefulcorrection
on the atmospheric absorption,
ground at a time of raining. The rain causes heavy
otherwise
they
may
misjudge the variability withattenuation and makes system noise temperature

IVS NICT-TDC News No.37

6

see in Fig. 4 that astrometry errors in the declination are much larger than those in the right ascension. The position in declination is much moresensitive to the elevation angle and the water vapor content than that in the right ascension. Wa-

Figure 2. Radio spectrum of PKS0202+129
fluctuations of the atmospheric absorption.
Figure 4. Large astrometry error in declination
Geodeticist nowadays succeeds in positioning a
VLBI station in a few millimeters on a local horizontal plane (Jike 2017)[6] . Fig.3 shows the 3D formal error diagram. The vertical error is almost six
times larger than the horizontal error. In theory,the vertical error is twice the horizontal error. The
excess in the vertical error might be caused by theuncertainty in EPL, Excess Path Length, of theatmospheric propagation path. The major error
source is the water vapor in the atmosphere. Liquid water does not cause the delay on radio wave
propagation.

ter vapor in the atmosphere shall be measured andcorrected in geodesy and radio astronomy but has
never been tried to use a WVR in Japanese observatories except the K3 WVR (Kawaguchi 2016)[1] .
The NAOJ has started the development of a new
WVR in 2017 which will be briefly introduced in
the section 4.
3.

Volcano Monitoring

In 2014, Ontake, one of Japanese active volcanoes, suddenly exploded and many mountainclimbers more than 50 were dead (See Fig.5). No
abnormal phenomenon were perceived before theeruption.

Figure 5. Disarster in Ontake in 2014
Figure 3. Geodetic VLBI measurement error
Like as Geodeticists, the influence of water vapor in the atmosphere is also a serious problem
for radio astronomical observers working on precise astrometry. Fig.4 shows annual parallax measured on G48.61+0.02 to study the galaxy rotation
by VLBI in right ascension and declination (Nagayama 2011)[7] . Equivalent path length strongly
depends on a total amount of water vapor in the
propagation path and an elevation angle. We canVolcanologists pointed out a fact that water vapor content in the volcanic fume quickly decrease
as shown in a table of Fig.6 (Burton 2007)[8] . In
my private opinion the decrease of the water vapor
is a serious sign of eruption, indicating the orificeof gas discharging is closed and the internal pressure is going high and high. In other words, if wemonitor a continuous leak of water vapor from a
volcanic mountain, we can recognize the internalpressure does not reach at a dangerous level of the
explosion.

7

December 2017

Figure 6. Decrease in the water vapor, is it a signof eruption?
Figure 8. Trial of direct H2 O imaging

Figure 7. Indirect H2 O monitoring
The volcano monitoring for water vapor is important and was tried and presented by some authors(Witter 2012)[9] (Shinohara 2008)[10] . The Wittershows an indirect way of measurements by using an
infrared camera (See Fig.7) and the Shinohara suggests a mass ratio of H2 O against CO2 and SO2 .
They estimated a total content of water vapor bymeasuring CO2 and SO2 by using the mixing ratio
estimated in advance.
Direct detection of water vapor was first tried by(Brian 2016)[10] with a radiometer of frequencies of
22/183/232.5GHz. Excess brightness against thebackground emission observed in the volcanic fumeat 22GHz is 1.5K as shown in Fig.8. The 183-GHz
spectrum is inversely folded downward due to theextremely high sensitivity at the frequency, which
is probably a good tracer for thin water vapor athigh altitude or at a freezing cold place. The 232.5GHz emission is too weak to measure the water vapor content in volcanic fumes. The water vapor
emission at around 22GHz is probably the best.
A new WVR was designed on the frequency range
from 16GHz to 24GHz. A theoretical water vapor
spectrum is shown in Fig.9, which indicates a total

Figure 9. A H2 O emission spectrum around 22GHz
at the water vapor content of 30hPa.
absorption through a line of sight. The peak froma base indicates the attenuation by the water vapor
and the base is the attenuation caused by the Oxygen and the liquid water. The wideband spectrum
makes possible to extract the water vapor content
separated from the attenuation caused by the dry
air and the liquid water. The attenuation spectrum is easily converter the emission spectrum byconsidering a height profile of temperature and by
introducing a radiation transfer equation.
4.

A compact and cost-effective WVR designed for a radio telescope

Commercial available WVRs are shown in
Fig.10. The upper WVR is from Radio Physics
co. ltd. in Germany and the lower from Radiometerics co. ltd. in USA. Both WVR are equippedwith pointing drives on both azimuth and elevation
axes, weather sensors and a GPS receiver. These
ancillaries makes expensive in the cost. A radio
telescope has already being equipped with thesefacilities so that a WVR on the radio telescope ispossible to make as much more simple as possible.

IVS NICT-TDC News No.37

8

Figure 12. Frequency Specrum and the time stability of a simplified WVR
and as good as commercial WVRs. The measurement error depends on the receiver noise and the
bandwidth as an equation below,
√
2
Tsys
·
,
(1)
Error(rms) = √q
2BτFigure 10. Cimmercial available WVRs: the upperfrom Radio Physics and the lower from Radiometrics.

Figure 11. A wideband digital specrometer on the
KEK WVR

A new WVR developed by NAOJ consists of afeed horn, a simple receiver and a high speed digitalspectrometer. In the first step the digital spectrometer was attached on the high-end receiver of the
KEK, High Energy Accelerator Research Organization in Japan. The amplifier of the KEK receiveris a low noise amplifier cooled down below 50K and
the frequency coverage is wide enough from 18GHz
to 32GHz. The receiver noise temperature is 40K.
The digital spectrum is measured at the time interval of one second. The one-second integrated spectrum at adjacent one-second samples is shown in
Fig.11. The rms deviation between two one-second
samples was 0.106K which corresponds to 0.45mm
in EPL, Excess Path Length, and 0.07mm in PW,
Precipitable Water. The precision is good enoughwhere Tsys is a system noise temperature, a sumof a receiver noise temperature (40K in case of the
KEK receiver) and a sky noise (∼60K), B is bandwidth (72.75MHz), and τ is an integration time(one second). All values in parenthesis are the best
estimated value for the sky noise temperature andthe setting parameters at a time of this experiment.
The q is a digital quantization loss (0.85 here for
3-bit quantization). The theoretical expected error
is 0.05K. The actual observed error is twice larger
than the expected error, which shall be studied in
the later work.
As we can see in the equation, even if the receivernoise increases from 40K to 180K in case that an
amplifier is not cooled and exposed in the room
temperature environment, the system noise temperature increases only by a factor of 2 for the sky
noise temperature of 90K under humid weather.
Thus a new compact WVR uses a room temperature amplifier to keep the cost reasonable. The
new WVR was tested in a factory and the resultis shown in Fig.12. The water vapor emiision isshown in the spectrum (frequency profile) in leftof the figure and the time profile of the spectrum
peak at 22.26GHz in the right. The rms error isabout 0.1K corresponding to the theoretically expected error. The simplified WVR will be mounted
on the Mizusawa 20-m telescope in Japanese fiscal
year of 2017.
5.

Cocluding Remarks

A water vapor radiometer, WVR, is useful toestimate the water vapor content in the atmosphere and to calibrate delay and absorption forgeodetic/astronomical observations. A new compact WVR is now under development in Japan withfinancial support of NAOJ, which will be installed

9

December 2017

in the Mizusawa 20m radio telescope in 2017 andexpected to improve the astrometric accuracy better.
The WVR will be a powerful tool in predicting volcanic eruption if the single beam receiver would beassembled into a multi beam phased array. The
array will be draw water vapor distribution overvolcanoes. An imaging technique developed by radio astronomers will effectively applied to draw themap. The water vapor is invisible so that the ejection hole is hard to be specified (See Fig.13). Radio
detection of water vapor emission makes possible toidentify a place where explosion would happen in
future. The sudden decrease of H2 O in a volcanic
fume would be a serious sign of volcanic eruption,
which might be useful to alert dangerous explosions.

[3] Witter, J. B., Hernandez, P., Harris, A. J. L.
and Preéz, N., Quantification of the Mass Fluxof H2O Gas (Steam) at Three Active Volcanoes Using Thermal Infrared Imagery, Pure
Appl. Geophys., vol. 169, pp. 1875-1889, 2012
[4] Forkman, P., Erikssonb, P., Winnberg, A.,
The 22 GHz radio-aeronomy receiver at Onsala Space Observatory, Journal of Quantitative Spectroscopy & Radiative Transfer, vol.
77, pp. 23-42, 2003
[5] Taeseok Lee, Sascha Trippe, Junghwan Oh,
Do-Young Byun, Bong-Won Sohn, and SangSung Lee, A SEARCH FOR AGN INTRADAY VARIABILITY WITH KVN, Journal of
the Korean Astronomical Society, vol. 48, pp.
313-323, 2015
[6] Adoption of K-band in the VERA Internal
Geodetic VLBI and its Effectiveness, Jike,
T., Manabe, S., Tamura, Y., Journal og theGeodetic Society of Japan, in printing, 2017.

Figure 13. Which is a real fumarole position?

6.

Acknowledgments

The author acknowledges to research collaborators, Dr. Yusuke Kohno and Dr. Tomoaki Oyama
of NAOJ, National Astronomical Observatory ofJapan, Dr. Taketo Nagasaki of KEK, High Energy Accelerator Research Organization, and Dr.
Takashi Maeda of JAXA, Japan Aerospace Exploration Agency. A new water vapor radio meter is
now being developed with them under a financial
support of NAOJ. In the KEK experiment and thefactory tests I received great helps of Mr. Kenichi
Harada and Mr. Yuichi Chikahiro, engineers in
Elecs co. ltd.. Here I would like to express mygreat thanks to them.
References[7] Takumi Nagayama, Toshihiro Omodaka,
Toshihiro Handa, Mareki Honma, Hideyuki
Kobayashi, Noriyuki Kawaguchi, and Yuji
Ueno1, Astrometry of Galactic Star-Forming
Region G48.61+0.02 with VERA, Publ. Astron. Soc. Japan, vol. 63, pp. 719- 725, 2011[8] Mike Burton, Patrick Allard, Filippo Murée,
Alessandro La Spina, Magmatic Gas Composition Reveals the Source Depth of SlugDriven Strombolian Explosive Activity, SCIENCE, VOL. 317 No. 13, pp. 227-230, JULY
2007[9] J. B. WITTER, P. HERNANDEZ, A. J. L.
HARRIS and N. PEéREZ, Quantification of
the Mass Flux of H2 O Gas (Steam) at Three
Active Volcanoes Using Thermal Infrared Imagery, Pure Appl. Geophys. vol. 169, pp.18751889, 2012[10] H. Shinohara, A. Aiuppa, G. Giudice, S. Gurrieri, and M. Liuzzo, Variation of H2O/CO2[1] Kawaguchi, Water Vapor Radiometer: Chaland CO2/SO2 ratios of volcanic gases dislenge Again, Technology Development Centercharged by continuous degassing of Mount
News, National Institute of Information andEtna volcano, Italy, JOURNAL OF GEOCommunications Technology, Serial No. 36,
PHYSICAL RESEARCH, VOL. 113, B09203,
page 4, November 20162008
(http : //www2.nict.go.jp/sts/stmg/
Space-Time Standards Group, NGNC, NICT,
ivstdc/news3 6/tdcn ews36.pdf ).
Kashima 34-m Radio Telescope 2007 Annual[2] MBurton, M. R., Oppenheimer, C., Horrocks,
Report, 2008 (in Japanese).
L. A., Francis, P. W., Remote sensing of CO2
and H2O emission rates from Masaya volcano,Geology, vo. 28, No. 10, pp. 915-918, October
2000.

IVS NICT-TDC News No.37

10Radio Frequency Interference mations, location-based service, meteorology, ionoresearch, and so on.
in GNSS Observations by Arti- sphere
Recently radio communication using mobile
ficial Signals for Mobile Phones phones (mobile telecommunications) is becoming
more popular in Japan, and radio waves are transmitted in several frequency bands for the mobile
telecommunications. One of the assigned frequency
band is 1.475–1.511 GHz, which is located near the
Yoshihiro Fukuzaki
frequencies assigned for GNSS, L1 (1.57542 GHz)
(fukuzaki-y96pe@mlit.go.jp),
and L2 (1.2276 GHz). Since strong radio waves
Tomokazu Nakakuki, Yuji Miura, Asuka
are transmitted from the radio towers for mobile
Sato, Hiromichi Tsuji
telecommunications, Radio Frequency Interference(RFI) occurs at several GEONET stations with
Geospatial Information Authority of Japan,
a specific receiver that are located near the radio
1 Kitasato, Tsukuba Ibaraki, Japantower and apparent height variations are detected.
Abstract: The Geospatial Information Authority ofThis paper gives the apparent height variations
Japan installed Continuously Operating Reference of GEONET stations caused by RFI, discussion on
Stations (CORSs) to receive GNSS signals in Japan its generation mechanism and solution to solve thisand has maintained since 1996, and their number is problem.
more than 1,300 at present. The CORS network inJapan is now called GEONET (GNSS Earth Obser- 2. Apparent Height Variations
vation NETwork system). Recently radio commuThe time series of the height of ’Hakodate’ that
nication using mobile phones (mobile telecommuis one of the GEONET station are plotted in Figure
nications) is becoming more popular in Japan, and
1. On May 27 in 2013 Signal-to-Noise ratios (SNR)
radio waves are transmitted in several frequencyof L1 and L2 signals became lower (L1: 45.1→42.7,
bands for the mobile telecommunications. One of
L2: 31.9→30.2), and after this day obvious perithe assigned frequency band is 1.475–1.511 GHz,
odic variations were detected. Because no actual
which is located near the frequencies assigned formovement was detected by leveling measurement
GNSS, L1 (1.57542 GHz) and L2 (1.2276 GHz).
performed at the site, it is considered that the peSince strong radio waves are transmitted from theriodic variations are apparent. The similar heightradio towers for mobile telecommunications, Ravariations are also detected at several GEONET
dio Frequency Interference (RFI) occurs at several
stations.
GEONET stations with a specific receiver that arelocated near the radio tower and apparent heightvariations are detected.
We briefly report the apparent height variations
of GEONET stations caused by RFI, discussion on
its generation mechanism, and solution to solve thisproblem.
1.

Introduction

The Global Navigation Satellite System (GNSS)
including Global Positioning System (GPS) developed and maintained by the Unites States andQuasi-Zenith Satellite System (QZSS) by Japanis one of the space geodetic technologies. The
Geospatial Information Authority of Japan (GSI)
installed Continuously Operating Reference Stations (CORSs) to receive GNSS signals in Japanand has maintained since 1996, and their number
is now more than 1,300. GSI’s CORS network inJapan is now called GEONET (GNSS Earth Observation NETwork system). GEONET data areprovided to public and private sectors and used
for a lot of purposes in Japan, for example, public
and private surveys, monitoring of crustal defor-

Figure 1. Time series of the height of the Hakodate
station from Jan. 2013 to Dec. 2015.
At Hakodate, according to a mobile phone company, radio waves for the fourth generation of mobile telecommunications technology, which is called
Long Term Evolution (LTE), started to be transmitted in those days before May 27, 2013. Thissuggests that the cause of the apparent variations
is RFI by the strong radio waves for LTE. In thecase of the other stations at which the similar vari-

11

December 2017ations are detected, there are also radio towers near
the GEONET stations, so it is considered that thecause of the apparent height variation is the RFI
by the strong radio waves for LTE.
It should be noted that a splitter that acted as
the attenuator of –7 dB was inserted at the Hakodate station in September 2014 and replaced withthe attenuator of –18 dB in October 2016.
3.

Discussion on Generation Mechanism

The most serious influence of the strong RFI signals is saturation of amplifiers in the GNSS receiving system (GNSS antenna and receiver). Eachamplifier has its own limit of the output level, andthe amplifier saturation occurs when the strong radio wave whose level is over the amplifier limit is
input. A conceptual figure of the amplifier saturation is shown in Figure 2. The input signals of
the GNSS receiving system consist of a lot of wave
components, and if the amplifier is saturated by thestrong signal the saturated parts of all input waves
are affected to be lack of their information and replaced with the highest values of their amplitude
in both plus and minus sides. A schematic figureis shown in Figure 3. Because the frequencies of
the LTE signal, L1, and L2 are slightly different,
the number of the affected parts is different in theirown periods.

Figure 2. Conceptual figure of the amplifier saturation.

In the GNSS receiver, Carrier Phase Locked
Loop (CPLL) circuits are installed to reconstruct
the carrier waves of GPS/QZSS signals, which areused for baseline analysis [1]. A block diagram of
CPLL is depicted in Figure 4. In order to reconstruct the original carrier wave using the received
carrier wave modulated by a Binary Phase Shift
Keying (BPSK) technique, the square of the modulated carrier wave is generated in the CPLL circuit,and the reconstructed carrier wave is output.
If the amplifier before the CPLL circuit is saturated by the strong RFI signals, the squares ofthe modulated carrier wave contain high amplitude
pieces quasi-periodically and the output voltage
level of the Low–Pass Filter (Vcont ) in the circuit
becomes slightly higher than the normal case in order to lock the loop (Figure 5). This causes a slightshift of the frequency of the reconstructed carrier
wave since the output frequency of the CPLL circuit is controlled by the voltage level of the inputof the Voltage–Controlled Oscillator in the circuit.
Here this phenomenon easily occurs in the case of
the L1 signal because the frequency of the L1 signal
is very near that of LTE. On the other hand, thefrequency of the L2 signal is different from that of
LTE and the CPLL circuit works as a narrow bandpass filter, so the only circuit for the L1 signal isaffected.
L1 and L2 carrier phases are employed when thebaseline analysis is done to determine the positionof the GEONET station. Because the frequency of
the L1 signal is slightly shifted when the amplifieris saturated and the shifted L1 frequency causes
the periodic L1 phase bias, the height obtained as
the result of the baseline analysis has a periodic
offset.
More detailed mechanism is still under investigation.
4.

Discussion to Solve this Problem

Because it is considered that the main cause ofthis problem is the amplifier saturation, it shouldbe tried to avoid the saturation of the amplifierin the GNSS receiving system in order to solve
this problem. The simplest method is insertionof an attenuator in the front of the receiver that
includes the saturated amplifier. By installing attenuators with –18 dB at GEONET stations thatwere affected by the RFI, apparent variations became much small actually. The insertion of a large
amount of attenuators causes the worse SNR valueof the observables of L1 and L2 carrier phases. In
Figure 3. Schematic figure of the affected signals of order to avoid increase of the noise level, the total
L1 and L2. The parts saturated by the RFI signal gain before the GNSS receiver (a pre-amplifier, theare replaced with the highest values in both plus and attenuator and cables) should be more than 30 dB.
In addition, it is expected that necessity of theminus sides.

IVS NICT-TDC News No.37

12

Figure 4. Block diagram of Carrier Phase Locked Loopmobile telecommunications will increase nationwide in Japan in the near future because the mobile
telecommunications is being employed to achieve
the ’Internet of Things’ concept. The most drastic
method is to introduce the antenna and the receiver that are designed to be employed for the severe radio condition like Japan. Insertion of a filter
that eliminates the frequency range for the mobile
telecommunications (1.475–1.511 GHz) also seems
to be available to use the present receiving systemwith small modification. This type of filter is called’notch filter.’ The basic specifications of the notch
filter are listed in Table 1. Since the L1 frequency(1.57542 GHz) is located near the elimination frequency range, the characteristics of the filter (cut–off frequency, temperature stability, and so on) are
important. Especially the phase delay characteristic is very important because the phase instability
directly influences the position estimation in baseline analysis. The necessary characteristic of the
cut-off frequency of the filter to design the higher
frequency side is listed in Table 2 and depicted in
Figure 6.
5.

Table 1. Basic specifications of the notch filter toeliminate mobile telecommunications signals
Freq. (MHz)1160–1290
1475–1515
1535–1590

Condition
PassedAttenuated
PassedTable 2. List of the necessary characteristic of thefilter to design the higher frequency side
Freq. (MHz)
1515
1516
1518
1520
1523
1525
1528
1530
1535

Att. (dB)
–60
–55
–47
–38
–25
–18
–8
–3
0

Summary

Because of strong RFI signals, which were originally transmitted for mobile phones, amplifier sat- ent height variations of several GEONET stations
uration occurred in the GNSS receivers, and appar- with a specific receiver were detected. Although

13

December 2017

Figure 5. Explanation of the frequency shift caused by the saturated signal in CPLL. High amplitude
pieces originated from the saturation cause the shift of Vcont , which makes the output frequency shifted.
6.

Acknowledgments

The authors would like to thank GSI’s colleagues
who have worked for the RFI problem at GEONET
stations.
References[1] Geodetic Society of Japan, GPS –Precise Positioning System using Satellites–, 1986, published by the Japan Association of Surveyors(in Japanese).
Figure 6. Graph of the necessary characteristic of
the filter to design the higher frequency side
its generation mechanism was still not clear, theapparent variations became much small by inserting the attenuator to avoid the amplifier saturationin the GNSS receiver. The detail of the generation
mechanism of the apparent height variations is still
under investigation.

IVS NICT-TDC News No.37

14

Look back on 18 years of
Tsukuba 32-m antenna
Midori FUJIWARA(fujiwara-m96qu@mlit.go.jp),
Shinobu KURIHARA , Masayoshi
ISHIMOTO, Takahiro WAKASUGI,
Michiko UMEI
Geospatial Information Authority of Japan,
1 Kitasato, Tsukuba Ibaraki, Japan
Abstract: The Geospatial Information Authority of
Figure 1. View of the Tsukuba antenna.
Japan (GSI) has implemented VLBI operation inorder to determine the precise position of Japanon the earth and to monitor plate motion in and Table 1. Tsukuba specifications (Wakasugi et al.,around Japan since 1984. Tsukuba 32-m VLBI (2015))
antenna which was built in 1998 was a leading
Year of construction
1998
VLBI antenna in the world, and contributed toRadio telescope mount type Az-El
the rapid determination of earth orientation paAntenna optics
Cassegrain
rameter (EOP) in cooperation with foreign obserDiameter of main reflector
32 mvatories under the International VLBI Service for
Az/El drive velocity
3◦ /sec
Geodesy and Astrometry (IVS) and the revision of
SEFD(X/S)
320 Jy/360 Jy
the Japanese Geodetic Datum after the 2011 off theFrequency range (S-band)
2.10 ∼ 2.50 GHz
Pacific coast of Tohoku Earthquake. In this article,
Frequency range (X-band)
7.78 ∼ 8.98 GHz
Tsukuba’s several contributions are reviewed.
1.

Introduction

years from 1998 through the end of 2016. At the
beginning of its operation, the open-reel type magnetic tapes called Mark4 system for international
observations, and the cassette cartridge type magnetic tapes called K4 system for domestic observations were used as recording media. The Mark4system needed a few manual tape changes in one
24-hr observation. From around 2005, the magnetic tapes have taken the place of the pc-based
data acquisition system with hard disk drives (K5system). As a result of this replacement, the manual tape change became unnecessary, and the automatic unmanned operation and data transfer via
network became possible. This caused the increaseof the number of sessions per year from around 100
in the magnetic tape era to around 200. The K5
system and data transfer via network produces a
new VLBI application. The actual measurementof UT1-UTC is practically and technically possible
by only VLBI, however, for its rapid determination, a long latency of VLBI, which is caused bydata transfer and correlation processing, had beena consideration for a long time. GSI, NICT, On2.
The chief events of Tsukuba 32-m an- sala Space Observatory in Sweden, and Metsähovi
Radio Observatory in Norway worked on an expertenna
iment for rapid determination of UT1-UTC, andThe chief events of Tsukuba 32-m antenna for automated the whole process from data transfer
18 years are shown in Table. 2 Tsukuba 32-m an- through analysis. In February 2008, we derived
tenna was involved in about 2,600 sessions in 18
From 1998 through 2002, GSI used to implement VLBI operation by using Kashima 26-m antenna owned by the Radio Research Laboratory (atpresent The National Institute of Information andCommunications Technology; NICT). When theJapanese Geodetic Datum 2000 (JGD2000) was realized, the coordinates of more than 110 thousandof control points in all over Japan were calculated
based on the position of Kashima 26-m antenna
which was derived from its international VLBI observations. Then Kashima 26-m antenna built in1968 was replaced by Tsukuba 32-m antenna of
which construction budget was approved in the
2nd supplemental budget for the Great HanshinAwaji Earthquake in 1995 (Tsuji et al., 2004).
Table 1 shows the specification of Tsukuba 32m antenna. Its design was optimized for geodetic observation, and it had a large parabolic dishand Azimuth/Elevation motors to slew the antenna
quickly. Therefore, it could acquire high quality
data efficiently.

15

December 2017

Table 2. The chief events of Tsukuba 32-m antenna.
1998.3

Antenna construction was completed.

1998.6First international experiment was carried out.

2002.5Intensive UT1 session series was started.

2000.5K4/Mark4 systems were replaced by K5 system completely.

2003.5The experiments for the orbit determination of the space probe ’Hayabusa’ (Joint
research with Japan Aerospace Exploration Agency (JAXA)/ Institute of Spaceand Astronautical Science (ISAS) )2005.9The IVS 15-day campaign session CONT05 was implemented.
CONT08, CONT11, and CONT14. ).

2008.2

GSI and NICT succeeded in measurement of UT1-UTC within only 3 min 45 sec
after the end of the session.

2011.5

GSI revised the coordinates of 438 CORS stations in east Japan due to the crustal
deformation caused by the 2011 off the Pacific coast of Tohoku Earthquake. These
coordinates were calculated based on VLBI observations during one month justafter the earthquake.

2014.3Ishioka VLBI station was completed.

2016.12

The final international experiment was done.

2017.3

Dismantlement of Tsukuba 32-m antenna ended.

(Followed by

UT1-UTC from the experiment in 3 min 45 sec
from the end of the observation. (Kurihara andKokado, 2009). On the other hand, Tsukuba 32-m
antenna contributed to the realization of the national geodetic reference frame after the 2011 off
the Pacific coast of Tohoku Earthquake. The earthquake caused the large crustal deformation over awide area of Eastern Japan. The coordinates ofnational control points such as Continuously Operating Reference Station (CORS) and triangulation
points in the area became unavailable due to thecrustal deformation. Though Tsukuba 32-m antenna put off restarting the VLBI operation after
the earthquake for a while due to aftershocks withan intensity of lower 5 or greater on the Japaneseseismic scale, on April 4th Tsukuba came back to
the international observation. On May 31st GSI
revised and published the new coordinates of 438
CORS stations. The coordinates of these stations
were calculated based on the international VLBI
observation at Tsukuba on May 10. (Hiyama et Figure 2. The process for destroying the antenna.
(1) Removing the main reflector. (2) After deal., 2011).
taching the main reflector and sub reflector. (3),(4) Detaching the center hub by cutting into small
3. Dismantlement
parts by gas cutting method. (5) The frameworks
In 2009, a Committee of IVS showed the new were dismantled. (6) The site after dismantlement
specification of next generation VLBI system called of the antenna.
VLBI Global Observing System (VGOS) (Petrachenko et al., 2009). The committee said that thecurrent VLBI system will be replaced by VGOS inIVS NICT-TDC News No.37

future. Thus, it was necessary for GSI to build the
new VGOS-compliant VLBI antenna in order tocontinue maintaining the precise position of Japanon the global geodetic reference frame. The government of Japan approved the establishment of the
VGOS antenna system for enhancement of monitoring crustal deformation including plate motionsin global scale on the supplemental budget for theGreat Tohoku Earthquake. The new VGOS station was completed in 2014 at Ishioka, Ibaraki andhad been implemented parallel observation until
the end of 2016 with Tsukuba 32-m antenna forsmooth succession of the result of Tsukuba. GSI
decided the future VLBI operation with one observatory Ishioka. It meant that Tsukuba 32-m
antenna was dismantled and removed. The dismantlement started from January 2017. The mainreflector, the sub reflector, and center hub of themain reflector were detached in this order from themain body of the antenna (Fig. 2). The dismantlement ended at the end of March 2017
4.

Conclusion

16

Hiyama, Y., A. Yamagiwa, T. Kawahara, M. Iwata,
Y. Fukuzaki, Y. Shouji, Y. Sato, T. Yutsudo,
T. Sasaki, H. Shigematsu, H. Yamao, T. Inukai,
M. Ohtaki, K. Kokado, S. Kurihara, I. Kimura,
T. Tsutsumi, T. Yahagi, Y. Furuya, I. Kageyama,
S. Kawamoto, K. Yamaguchi, H. Tsuji ,S. Matsumura, “Revision of the Results of Control Points
after the 2011 off the Pacific coast of Tohoku Earthquake”, Bulletin of the Geospatial Information Authority of Japan, 59, 2011.
Kurihara, S., Kokado, K., “Ultra-Rapid UT1 Experiment Using e-VLBI Technique”, Bulletin of the
Geospatial Information Authority of Japan,57,2009
Petrachenko, B., A. Niell, D. Behrend, B. Corey,
J. Böhm, P. Charlot, A. Collioud, J. Gipson,
R. Haas, T. Hobiger, Y. Koyama, D. MacMillan,Z. Malkin, T. Nilsson, A. Pany, G. Tuccari,
A. Whitney, J. Wresnik, “Design Aspects of the
VLBI2010 System”, Progress Report of the IVS
VLBI2010 Committee, 2009.
Tsuji, H., T. Tanabe, H. Kawawa, K. Takashima,
K. Miyagawa, S. Kurihara, S. Matsuzaka, ”Achievements of Outgoing Kashima 26m VLBI Antenna
for Geodesy and Surveying; Journal of the Geospatial Information Authority of Japan, 103, 2004. (inJapanese)

This report briefly reviewed the 18-year history
of Tsukuba 32-m antenna. It participated in many
sessions and contributed to monitoring earth ori- Wakasugi, T,R. Kawabata, S. Kurihara, Y. Fukuzaki,
J. Kuroda, K. Wada, S. Mizuno, T. Ishida, “Tsukuba
entation and crustal deformation as well as main32-m VLBI Station”, International VLBI Service fortaining geodetic reference frame. Through the opGeodesy and Astrometry 2014 Annual Report, 2015.
erations, the know-hows of operations, such as automated operation, data transfer and stable observation with less failure were accumulated. Suchknow-hows are utilized in the operation of Ishioka
VLBI antenna.

17

December 2017

Phase-up Twin Antennas for is considered to be performed. This is the secgoal of our research. Thus, we attempt to use
High-Sensitivity Maser Obser- ond
the above techniques with the Hitachi 32 m radio
vations
telescope and the Takahagi 32 m radio telescope
Kazuhiro Takefuji1 (takefuji@nict.go.jp)
Koichiro. Sugiyama2 , Yoshinori Yonekura3 ,Tagiru Saito4 , Kenta Fujisawa5 , andTetsuro Kondo1
1Kashima Space Technology Center, National
Institute of Information and Communications
Technology, 893-1 Hirai, Kashima, Ibaraki
314-8501, Japan
2
Mizusawa VLBI Observatory, National
Astronomical Observatory of Japan, 2-21-1
Osawa, Mitaka, Tokyo 181-8588, Japan
3
Center for Astronomy, Ibaraki University, 2-1-1
Bunkyo, Mito, Ibaraki 310-8512, Japan 4 Collegeof Science, Ibaraki University, 2-1-1 Bunkyo,
Mito, Ibaraki 310-8512, Japan 5 The ResearchInstitute for Time Studies, Yamaguchi University,
1677-1 Yoshida, Yamaguchi, Yamaguchi
753-8511, Japan
Abstract: For the sake of high-sensitivity 6.7 GHz
methanol maser observations, we developed a newtechnology for coherently combining the two signals from the Hitachi 32 m radio telescope andthe Takahagi 32 m radio telescope. Furthermore,
we have applied newly developed digital position
switching (DPS), which is a similar technology tothat used in noise-cancelling headphones, and we
confirmed the improvement the sensitivity of DPS
rather than that of the VLBI method. See moredetail in Takefuji el al., 2017.
1.

Introduction

The establishment of a synthesis technology using two signals from two stations with a maser
observation as a reference is our first goal. Once
this technology is realized, the phase difference ofthe synthesis parameters can be determined by themaser observation with two antennas. As a result, the observation efficiency is expected to be
improved. Moreover, in the case of spectral observation, the position switching of a single dish is performed by moving the antenna physically. Then,the system noise and other background noises canbe removed and only the maser signal will remain.
Once two antennas are phased, it is expected to create a virtual-off source by changing the phase of thesynthesis parameters similarly to noise-cancelling
headphones. Without physically moving the antenna, the digital position switching observationof the Japanese VLBI network (JVN), which areseparated by about 260 m. Furthermore, we perform VLBI experiments simultaneously with the
Kashima 34 m radio telescope at a distance ofabout 50 km and the Yamaguchi 32 m radio telescope at a distance of about 1000 km to confirm
the improved sensitivity of a combined signal.
2.

Parameters for phasing two radio telescopes by maser observation

If we define Pa as the signal power of antenna
A and Pb as the signal power of antenna B, thenPsum , the signal power after combining antennas A
and B with phase difference ∆θ, can be expressed
asPsum= |Pa + Pb |q
Pa2 + Pb2 + 2Pa Pb cos ∆θ.
=

(1)
(2)

The maximum value of the combined signal is
Pa + Pb and the minimum value is Pa − Pb (whenPa ≥ Pb ). If the signal powers of the two antennas are equal, the power is doubled. However, depending on the phase difference ∆θ, the signal will
decrease and disappear in the worst case. The extinction state can be used as an off-source observation. By adding an offset of 180◦ to the perfectly
matched angle ∆θ, it can be applied for digital position switching.
By focusing on the phase, the rotation of thephase difference ∆θ, which is caused by the delaydifference between the two antennas, shifts very
rapidly in our experiment. The predicted values
such as the geometric delay and delay rate are calculated using a VLBI software in advance and arereasonably accurate. However, it is necessary to
determine the phase difference between the antennas by actual observation. On the other hand,the bandwidth covering maser emission is generally on the order of 100 kHz. Therefore, the accuracy of the delay determined by interferometry is
not so high. In addition, if the phase at the frequency of the peak intensity of the maser emission
in the cross-spectrum is determined, it has to beconverted to the phase at the sky frequency, i.e.
the phase at 0 MHz of the intermediate frequency(IF). In this conversion, the precise delay information is also required. It is difficult to synthesize thesignals of two telescopes without any corrections of
the phase difference.
Here, we will determine the phase difference ∆θby the following procedure. Firstly, we observe

IVS NICT-TDC News No.37

18

φm using Eq. 3. In addition, the delay rate can be
determined by tracking the phase φm of the maserπemission line. Thus, the parameters for phasing(f , φ )two telescopes can be obtained.
Figure 2 shows a schematic diagram of thefphased telescopes. Firstly, the Fourier transform isperformed for two signals from the two telescopes(f , φ )and then the delay with reference to the antennaπ
A is corrected in the frequency domain. We actually performed a 2048-length fast Fourier transform(FFT) for the signals of Hitachi and Takahagi. AfFigure 1. Relationship of the initial phase differ- ter the FFT, the obtained phase, delay, and delay
ence φ0 at the sky frequency f0 and the phase dif- rate were corrected in a frequency domain. The sigference φm at the maser emission line fm against nal of the antenna B at this time can be regarded as
an intermediate frequency on cross-spectrum after observed at the position of the antenna A. Therea correlation. The discontinuous solid lines with after, signals were synthesized and returned to thethe slope of 2πτg represent the delay τg between time-series data by inverse FFT (2048 points). Fitwo telescopes determined beforehand by the obser- nally, the combined signal was generated.
vations of the radio continuum sources.
3. Observations
φ

+

m

m

0

0

0

-

a strong quasar to determine the clock offset between two antennas. If the reference clocks ofthe telescopes are the same, the difference in theclock rate between the telescopes is small. Otherwise, it would be better to observe the quasars
in the latter scan to estimate the clock rate. Secondly, we observe a series of maser sources with thetwo telescopes. Thirdly, we correlate the recorded
data between the two telescopes whose geometricor clock delay were adjusted to be identical so thatthe phase difference ∆θ was involving the whole
delay. Finally, we convert the obtained phase difference ∆θ at the maser line to the phase at thesky frequency.
Figure 1 shows a conceptual phase on a crossspectrum after performing a cross-correlation forthe phase conversion. A maser emission line ispresent at the frequency and the fringe phase(fm , φm ) at the sampling frequency of fsps [Hz].
The initial phase at the sky frequency (f0 , φ0 ) is
required to synthesize two telescopes. Here, thetotal delay comprising the geometric and clock delays is represented as the discontinuous slopes due
to an ambiguity of 2π of the cross-spectrum. Now,
we define the total delay as τg [s] and the initial
phase is expressed as

We carried out the maser observation from 03:47
to 16:32 (UTC) on 26 Oct 2016 (DOY 300). Before the maser observation, we observed the quasar
NRAO512 to determine the geometric and clock
delays among telescopes and performed a series ofmaser observations. The telescopes used in eachobservation are described in table 1. The bandfiltered radio frequency (RF) signals (from 6664 to
6672 MHz) of antennas were down-converted andthe intermediate frequency (IF) signals (from 0 to
8 MHz) were sampled by using the analogue to digital sampler K5/VSSP32 with a 16 MHz sampling
speed and 4-bit quantization
4.

Results

4.1

Phased telescopes

In accordance with the aforementioned procedure, we observed a total of 56 masers using the
Hitachi and Takahagi telescopes as the phased antennas. The recorded data of the Takahagi 32m
telescope were shifted to those of the Hitachi 32m
telescope position. Figure 3 shows the spectra of
the maser source G26.52-0.26 with the phased andunphased antennas with 60 s integration and 1 kHz
resolution. We removed the trends of the bandpass profiles by the least-squares method with aquadratic function and give the offset for comparφ0 = φm − 2πfm τg (mod 2π).
(3) ison. The SNRs of each spectrum were estimatedto be about 6.4, 4.8, and 4.6 from top to bottom.
In contrast to the continuum source observation,
By adding an offset of 180◦ to the perfectly
the phase can be determined only at the frequenmatched phase of the phased antenna, it cancies where the maser emission exist. Therefore, disbe considered as a virtual off-source and applied
continuous slopes in Fig. 1 do not appear, so thatfor digital position switching. If digital position
it is difficult to convert φm into φ0 . However, once
switching is realized, the off-source observations
the delay τg is obtained by the previous quasar obcan be skipped and therefore it is expected that
servation, it becomes possible to determine φ0 from

19

December 2017

Figure 2. Schematic diagram of the procedure of the phased telescopes.
Table 1. Parameters of the experiment (f c = 6668 MHz) on 26 Oct 2016 (DOY 300).
Station
Diameter [m]Tsys*[K]Efficiency [%]Polarization
Weather

Hitachi32
29.5(@EL=85 deg)
55-75
LHCP
Fine

Takahagi
32
Typ 30-40
55-75
LHCP
Fine

Kashima34
Typ 150
Typ 40
VerticalFine

Yamaguchi
32
110(@EL=46 deg)
Typ 65
LHCP
Cloudy140000

10000

56000

120000

Hitachi-5000

Takahagi-10000-15000

54000
52000

0

Hitachi + Takahagi

Relative power

Relative power

5000

100000

50000
48000

80000

46000
1. 5

1. 6

1. 7

1. 8

1. 9

2

2. 1

2. 2

60000
40000
20000

-200000-25000
1.6

1.8

2

2.2

2.4

Intermediate Frequency [MHz]

0

1

2

3

4

5

6

7

8

Intermediate Frequency [MHz]Figure 3. Spectra of the maser source G26.520.26 obtained by the Hitachi telescope (top), by the
Takahagi telescope (middle) and by the phased Hitachi and Takahagi telescopes (bottom).

Figure 4. The on-source and the off-source spectra of maser G12.025-0.031 calculated by the digital position switching of the phased Hitachi and
Takahagi antennas.

the observation efficiency will be increased. Figure
4 shows the ON-OFF spectra calculated using thephased Hitachi and Takahagi antennas. The onsource data contains the maser signal of G12.0250.031 for the in-phase combination with 60 s integration and 1 kHz resolution. On the other hand,the off-source data is obtained by suppressing themaser signal by applying a phase difference of 180◦
to the in-phase combination. From the figure, theband characters without the maser region have thesame curvature and were consistent between the
on-source and the off-source. Here, we provided asingle parameter for the phasing procedure. The

amplitude of the on-source can be increased owing
to the more accurate accumulation. Figure 5 is obtained by multiplying the ratio (ON-OFF)/OFF by
the system equivalent flux density (SEFD), which
we assumed to be 85 Jy. Since the SEFD of boththe Hitachi and Takahagi telescopes is approximately 170 Jy, when the efficiency and the system temperature are 70% and 30 K, respectively,
we provided the half value of the SEFD. Here, theobtained flux density of the maser sources is consistent with our recent monitoring result. Since
the power, when the maser did not exist, is close
to zero, the common components such as the bandcharacter were removed.

IVS NICT-TDC News No.37

20

140
Ratio of the SNR
by digital position switching
to the SNR by VLBI

3

120

14

Flux density [Jy]

12

100

10
8
6

80

4
2

60

0
-2

40

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

20
0

2.5
2
1.5
1
0.5
0

-20
0

1

2

3

4

5

6

7

02

8

Figure 5. Spectrum formed from the ratio (ONOFF)/OFF of maser G12.025-0.031.

Finally, Fig. 6 shows a comparison of the SNRsof the detected maser sources with 60 s integration obtained using VLBI and the phased telescope.
Only the data whose SNR was over 15 were plotted. The mean SNR of the phased antenna relative to that of VLBI is 1.597±0.444. In the calculation of the SNR, the 1σ noise was commonly
defined for both methods from the scan, in the frequency range where no maser lines were detected.
Then, the SNR was obtained by dividing the peak
power of the maser line by the defined σ. The
spectra obtained by the digital position switchingwere produced without the requantization effect.
Thus, the obtained SNRs could be directly compared. Since some maser scans had a factor of 2 or
3 unexpectedly, the results may have been overestimated (the median of the SNR ratio was 1.410).
We assume that the radio continuum emission toward the maser source is one possibility.

04

06

08

10

12

14

16

18

UTC hour of the day

Intermediate Frequency [MHz]Figure 6. Comparison of the maser SNR of
VLBI and the phased telescopes of Hitachi 32 m
and Takahagi 32 m as a function of time. The
SNRs by the VLBI method were extracted fromthe cross-spectrum of the VLBI made for the Hitachi and Takahagi telescopes. While, the SNRs by
the phased method were obtained by the spectrumof digital position switching (ON-OFF) with thephased two telescopes of Hitachi and Takahagi telescopes. The mean SNR ratio of the phased antenna
relative to that of VLBI is 1.597 ±0.444 (dashedline) and the median of the SNR ratio is 1.410.
5.

Summary

We developed phased-up technology using thetwin antennas Hitachi and Takahagi and applied
it to a series of 6.7 GHz methanol maser observations. Furthermore, with the newly developed
digital position switching, we confirmed that thephased telescopes operated over 12 hours with amaser observation as a reference without any offsource observations. Thus, the off-source observations can be skipped and therefore we expect thatthe observation efficiency will be increased.
References[1] Takefuji, K. et al., “Development of thePhase-up Technology of the Radio Telescopes:
6.7 GHz Methanol Maser Observations with
Phased Hitachi 32 m and Takahagi 32 m Radio Telescopes”, PASP, Vol 129, No 981, 2017

21

December 2017

Development of
Wideband Antenna
Hideki Ujihara (ujihara@nict.go.jp)
Kashima Space Technology Center,
National Institute of Information
and Communications Technology,893-1 Hirai, Kashima, Ibaraki 314-8501, Japan
1.

Status of development

The prototype of NINJA feed installed in the
Kashima 34m antenna in 2015 was replaced witha new NINJA feed developed for 34m. Previous
model was composed with a NINJA feed designed
for the primary focus of the previous MARBLE
with a 1.5m or 1.6m parabola dish. The newfeed was shortened and designed for 3.2-14.4 GHzwith reduced waveguide dimensions to set cutoff
frequency around 3 GHz, and was mounted again
nearby the IGUANA Daughter feed which is operated in 6.5-15 GHz band[1] as shown in Figure 2.
Also a orthogonal mode transducer(OMT) was developed for the new feed for capability of receiving
for both two linear polarizations. The main reflector of MARBLE 1 at AIST Tsukuba was enlarged
from 1.6 m to 2.4 m and its optics was changed to
the Cassegrain with the NINJA in the same way for
MARBLE 2 at NICT Koganei, as shown in Figure
3. CFRP pipes are used to support the sub reflector instead of aluminium frames to reduce thetotal load on the antenna mount. They are operated with 20-40 % aperture efficiency with OMT.

2.

Plans

New wideband OMT and NINJA feeds were developed for better performance, but RFI below
3GHz is not cut well for Kashima 34 m that causes

Figure 2. The NINJA feed and the IGUANA feedin the Kashima 34m receiver cabin.

Figure 3. MARBLE 2.4m on the roof top of the
AIST Tsukuba.
modulation noise to increase Tsys over 200K. So
further development will be done for OMT and
NINJA feeds. Also, wideband OMTs will be developed for the IGUANA feeds. Frames of MARBLE
2 will be replaced with CFRP pipes to reduce themass and to gain rigid surface of the main reflector.

References[1] Ujihara H., (2016) Development of wideband
feed for Kashima 34m Antenna, Radio Sci., 52,
doi:10.1002/2016RS006071

Figure 1. Return loss measurement of the new
wideband OMT with the NINJA feed.

IVS NICT-TDC News No.37

22’Node - HUB’ Style VLBI
with Broadband System.
Mamoru Sekido (sekido@nict.go.jp) , NICTSpace-Time Standards Laboratory,
Kashima Space Technology Center,893-1 Hirai, Kashima Ibaraki, Japan
Abstract: Our VLBI project is targeting distantfrequency comparison with broadband VLBI system. Transportable small VLBI stations are usedas the nodes of the measurements, and large diameter antenna is participating the VLBI observation
as a hub for boosting signal to noise ratio. Delayobservable between small antenna pair is not always expected to be directly measured by VLBI,
but indirectly derived by using closure delay relation including large diameter antenna. This observation scheme was not attractive because of larger
error in the past. Although it obtained a reality to
be used in geodesy by high delay precision withbroadband VLBI. Because broadband VLBI canachieve sufficiently high delay precision even withsmall diameter antenna. We named this observation scheme as ’Node - Hub style (NHS) VLBI’, inwhich multiple small antennas work as the nodes
and small number of large diameter antennas workas hub to enhance the SNR. The NH VLBI will hassome advantages in geodetic VLBI and frequency
transfer VLBI application. This paper describes algorithm and some aspect of the NHS VLBI scheme.
1.

Introduction

VLBI group of NICT have been working for application of broadband VLBI system to frequency
comparison between distant atomic frequency standards [1]. The concept of broadband VLBI system
VGOS [2] has been proposed by the IVS, and its
prototype system so called NASA Proof of Concept(PoC) is current standard of the VGOS system.
Our broadband data acquisition system named
GALA-V is another realization of the VGOS. By
implementation of originally developed broadband
feed [3, 4], Kashima 34m antenna and two small
Cassegrain antennas have gained broadband observation capability. The GALA-V system employeda unique data acquisition technique so called RFDirect sampling[5], which assisted precise broadband group delay measurements by wide bandwidth synthesis software [6]. Combination use ofsmall diameter antennas as nodes and high sensitivity antenna as a hub is another aspect of our
VLBI project.
Concept of transportable small VLBI station has
been investigated since 1980s to expand the capability of VLBI in global geodesy [7, 8, 9]. At present
the German Geographical Authority BKG has been
operating 6-m diameter VLBI station TIGO[10]Figure 1. Multiple small dimeter antennas are thenodes of observation and they makes VLBI observation with large dimeter antenna ’A’ in one scan(panel (a)). Small antennas can quickly change radio source for the next scan, and antenna ’B’ isused for hug of the next scan (panel (b)), which isprepared during previous scan. And antenna ’A’
will move to the next scan while antenna ’B’ is observing the current scan. In this way, large antennas ’A’ and ’B’ participate observation alternatively, and they play the role of hub in the network to enhance the SNR. Delay observables between small antenna pairs are derived from delay
observables of node and hub baselines by closure delay relation. This procedure has several benefits as
described in text.
in South America, which is a geodetic complex
equipped with multi space geodetic techniques.
Even though a small diameter antenna can work
as a geodetic VLBI station by combination observation with larger antennas, its delay precision has
been limited by the signal to noise ratio (SNR) in
legacy S/X observation. This situation has beenchanged in broadband VLBI case. As we see inlatter section, precision of group delay measurement was drastically improved by synthesizing thesignal over broad frequency range. Fringe detection is still matter by sensitivity, although if fringe
is detected, small VLBI station works as geodetic
VLBI station with sufficient geodetic precision.
The SNR of VLBI observation is proportional tosquare root of data acquisition rate and the products of diameters (sensitivity) of the two telescopesof the baseline, VLBI sensitivity can be improvedby higher data acquisition rate and joint observation with large diameter (higher sensitivity) antenna.
Let us suppose multiple small diameter stations
and some high sensitivity stations are available for
VLBI network observation (Fig. 1). Small stations
are regarded as node, whose position and clock parameters are to be measured. Large diameter highsensitivity stations play the role of hub to enhance
the SNR of observation. Small antenna can move
fast, then it can change observing radio source
quickly. Large diameter antenna will not move so
fast, although it has high sensitivity. We propose
a new VLBI observation scheme, which takes these23

December 2017

equations of (1) becomes
prop
prop
prop
τAB
(t2 ) = τRB(t1 ) − τRA(t1 ).

Figure 2. A wave-front of signal from a target radiosource is arriving to Station R, A, and B at t1 , t2 ,and t3 , respectively.
advantages of merits of both small and large diameter antennas. Small antennas (nodes) play the
key role in the measurement, where many radiosources are observed in a unit of time by quickly
changing radio sources. At least one large antennas(hubs) must be included in the network observation
to enhance the SNR of node stations network. All
the large diameter antenna need not participate all
the time, but they may participate alternatively.
Appropriate scheduling will enable to keep enoughtime for large diameters (hub) antenna to movelarge angular distances for the next scan while theother hub antenna is participating current scan. In
this case, sufficient SNR may not be gained for thesmall antenna (nodes) pairs. Thus, delay observables between small station (nodes) pairs are derived from delay observables of node-hub baselines
by using closure delay relation. We named this observation scheme as Node-Hub style (NHS) VLBI.
2.

The NHS VLBI observation scheme

2.1

Closure Delay

Utilizing virtual delay between node stations derived from node-hub baselines by using closure delay relation is unique technique of the NHS VLBI.
Closure delay relation is schematically depicted inFig. 2. Let us suppose three stations are observing
a common radio source, and a wave-front of radio
signal from the target radio source is arriving tostation R, A, and B at epoch of terrestrial time t1 ,t2 , and t3 , respectively. Here we define propagation
prop
geo
atm
delay as τXY
= τXY
+ τXY
, which is sum of signal
propagation delay due to geometry of XY baseline
and propagation in the atmospheric including ionosphere. Propagation delays are expressed as signal
arrival time difference, then propagation delays between three stations are represented as follows:

prop
τRA
(t1 ) = t2 − t1

prop
τAB
(t2 ) = t3 − t2
, (1)

prop
prop
τBR
(t3 ) = t1 − t3 = −τRB(t1 )where delay τXY is defined at reference epoch, when
the signal arrivals to station X. Summation of three

(2)

Since
current
VLBI
analysis
software
CALC/SOLVE[11] treat delay data of any
baselines in identical scan at a common integer
second epoch, then the reference epoch must be
adjusted to the epoch t1 , which is the reference
integer second epoch of this scan. By using
prop
t2 = t1 + τRAof eqn (1), delay of AB baseline at
epoch t2 is expressed by Taylor series expansion ofτAB (t) around epoch t1 as
prop
prop
τAB
(t2 ) = τAB(t1 ) +

∞
X
di

dti
i=1

prop iτAB · (τRA
) . (3)

Then we get
prop
prop
prop
τAB
(t1 ) = τRB(t1 ) − τRA
(t1 )

−

d
d2
prop
prop 2
τAB · τRA
− 2 τAB · (τRA
) − · · · (4)
dt
dt

prop
Note that theoretical a priori delay value τRA
and its derivative must be used, but not observed
obs
delay τRA
, for the correction term of eqn (4). Beobs
may contain clock offset
cause observed delay τRAof a few µsec. Thus, it does not correctly represent propagation delay. Theoretical a priori delay
has precision no worth than 1 × 10−8 sec. by using good a priori model such as CALC [11]. In thiscase, magnitude of the second order term of eqn (4)becomes less than 3 × 10−14 on any ground based
baseline, thus higher order correction terms can besafely eliminated.
Delay observable of RA baseline is composed ofcontribution of propagation delay including geomeprop
try and atmosphere (τRA), station dependent delay including clock and instrumental (τRstn , τAstn ),str).
and delay caused by radio source structure (τRA
prop
obs
str
τRA
= τRA
+ τAstn − τRstn + τRA,(5)where time argument (t1 ) is omitted for simplicity. Radio source structure effect of group delay
is derivative of visibility phase with respect to theradio frequency [12]. Visibility of VLBI is obtainedby integration of interferometry fringe weighted bybrightness distribution of radio source on the sky,thus it depends on radio source brightness distribution and the projected baseline.
Then taking difference of delays between RA andRB baselines with eqn (5) becomes
prop
prop
obs
obs
τRB
(t1 ) − τRA(t1 ) = τRB(t1 ) − τRA
(t1 )
str
str
+ τBstn − τAstn + τRB
− τRA
. (6)

Expecting delay observable of AB baseline corresponding to eqn (5) is
prop
strτAB (t1 )obs = τAB(t1 ) + τBstn − τAstn + τAB(7)

IVS NICT-TDC News No.37

24

prop
prop
By eliminating (τRA(t1 ), τRB(t1 ) ) with eqn (4)and (6), similar delay expression is defined as

d propτ· τRA (8)
dt AB
prop
str
str
= τAB
(t1 ) + τBstn − τAstn + τRB
− τRA
. (9)
def

obs∗
obs
obs
τAB
(t1 ) = τRB(t1 ) − τRA(t1 ) −

Eqn (8) is definition of virtual delay observableobs∗
τAB. Difference of virtual observable and true observable of AB baseline is derived by comparing
eqn(7) and (9) as
obs∗
obs
str
str
str
τAB
− τAB
= τRB
− τRA
− τAB
.

(10)

Then only radio source structure effect is difference
from expected true VLBI delay of AB baseline.
Figure 3. Histogram of post-fit delay residualof OA, OB, and AB baselines in the session
The NHS VLBI observation with small diame- of 10-11 July 2016, where delay of AB baseline
ter antenna has four major advantages: (1) lower was computed by the algorithm described in thecost, (2) fast slew rate, (3) smaller distortion, and text (O:Kashima34, A: 1.6m diameter antenna at(4) elimination of local effect of large diameter anTsukuba, and B:2.4m diameter antenna at Kotenna. Point (1) – (3) are obvious due to the properties of small antenna size. The point (4) is con- ganei). The WRMS of the residuals were 24.5 ps,
firmed from eqn (9), were station dependent effect 23.3 ps, and 22.2 ps for OA, OB, and AB baseline,of antenna R is eliminated. Point (2) – (4) are im- respectively.
portant aspect for improving accuracy of geodesy.
It is reported that gravitational deformation effectis significant and inevitable for large diameter an- Table 1. Observation parameters in the comparison
tenna [13, 14]. Thus, NHS VLBI scheme has a experiment between NHS and legacy VLBI observation.
potential to improve geodetic VLBI.
NHSLegacyThis NHS VLBI did not has reality for practical
VLBI
schemeBroadband
S/X
used before broadband VLBI became available, beBandwidthperchannel
1024
MHz
8MHz
cause delay precision with small diameter antenna
Number of Channels
416
has been limited by poor SNR due to small colEffective Bandwidth
1.8 ∼ 3.8 GHz
133MHzlecting area in legacy S/X observation. However,Integration time
1 sec.
9 sec.
owing to its large effective bandwidth, broadband
SNR
about 10
about 100
VLBI can achieve better than pico second delay
precision with minimum SNR of fringe detection.
RMS of delay residual
4 ∼ 7 psec
35 psec
Fig. 3 shows histogram of post-fit residual distribution as an example of NHS VLBI observation data.
It demonstrates that post-fit residual distribution 4. Comparison of Delay Precision betweenof AB baseline does not expand w.r.t. those of
Broadband NHS and Legacy VLBI
original OA, and OB. Weighted root mean square
An experiment was conducted to compare the(WRMS) of post-fit residual of AB baseline wasdelay
precision between legacy S/X mode andeven smaller than that of OA, and OB baselines. It
broadband
NHS VLBI. Legacy mode observation
indicates that some station dependent contribution
from large diameter antenna ’O’ was commonly re- was made with 11m diameter antenna pair by using
moved, then NHS scheme improved the observation frequency sequence of IVS-T2 session. Broadband
NHS VLBI observation data used in this compariprecision.
Since NHS VLBI scheme suppose that two nodes son was taken from the series of VLBI experiments
and hub have to observe common radio source si- in 2016 conducted among 1.6 m antenna installed
multainously, this is one of the limiting factor of at National Metrology Institute of Japan (NMIJ),
the baseline length. Another draw back is delay er- 2.4m antenna at NICT (Koganei), and 34m anror caused by radio source structure effect in group tenna at Kashima. In both cases, strong radio
delay represented by eqn (10). Although the NHS source were observed for a certain length of time.
VLBI cannot be used earth diameter scale VLBI, Then precision of delay observable is evaluated by
it is valuable to investigate its potential to improve scattering of data around the mean of detrended
the geodetic precision. Radio source structure is delay time series. Alan standard deviations were
subject to be solve in global VGOS observation, computed as an indicator of the precision (Fig. 4).
Observation parameters are listed in Table 1. This
then this issue have to be investigated.
3.

The NHS VLBI Observation Scheme.

25

December 2017

pp 476-489, DOI: 10.1002/2016RS006071,
2017.
[4] Ujihara, H., “Development of Wideband Antenna”, IVS NICT-TDC News (ISSN 18823432), No. 36., pp.13, 2016.
[5] Takefuji, K., “ Performance of Direct Sampler
K6/GALAS”, IVS NICT-TDC News(ISSN
1882-3432) No.36, pp.20-22, 2016.
[6] Kondo, T., & K. Takefuji, “An algorithm of wideband bandwidth synthesis for geodetic VLBI”, Radio Sci., 51,
doi:10.1002/2016RS006070, 2016.
Figure 4. Comparison of delay precision was madeby Alan standard deviation between NHS VLBI
with antenna group of 2.4m-34m-1.6m and legacy
S/X VLBI with 11m antenna pair. Alan standard
deviations were computed from detrended time series VLBI delay obtained by continuous tracking ofstrong radio source.
experiment result indicates an evidence that delay
precision of NHS scheme with 1.6m and 2.4m antenna shows higher delay precision than legacy S/X
VLBI observation with 11m diameter antenna pair.
5.

Summary

The node - hub style (NHS) VLBI with closure delay relation was introduced as a new VLBI
observation scheme. Due to large effective bandwidth, broadband NHS has got a reality to be usedin geodetic observation. Advantages of the NHS
scheme are (1) lower cost, (2) fast slew rate, (3)smaller distortion, and (4) eliminating the local effect of large diameter antenna. To be consideredin the NHS scheme is radio source structure effect,
which remains in the closure delay relation. Since
local effect of large diameter antenna is eliminatedin the NHS VLBI, it has a potential to improve theprecision of geodetic VLBI.
References[7] Niell, A. E., et al., “Geodetic Measurementswith a Mobile VLBI System, Radio Interferometry Techniques for Geodesy”, Proceedingsof a conference held June 19-21, 1979 at the
M. I. T., NASA CP-2115, 3. 1980
[8] Kaneko, A., et al., “II.5 The 11m Antenna
System for Minami-Daito Isl.Station and theHighly Transportable VLBI Station”, J. Communications Research Lab., 36 (8), pp65-74,
1990.
[9] Takashima, K., & M. Ishihara, “The History
and Transition of Very long Baseline Survery
at Geographical Survey Institute”, J. Geod.
Soc. Japan., 54(4),205-219,2008.
[10] Hase, H., et al., “The TIGO-Project”, New
Technologies in VLBI ASP Conf. Ser., 306,
347-360,2003.
[11] Gordon, D., & MacMillan, D., “Calc/Solve
Software”, https://lupus.gsfc.nasa.gov/
software_calc_solve.htm[12] Charlot, P., “Radio-Source Structure in Astrometric and Geodetic Very Long baseline Interferometry”, Astrn. J., 99(4), pp. 1309-1326,
1990.
[13] Pierguido, S., et al., “Height bias and scaleeffect induced by antenna gravitational deformations in geodetic VLBI data analysis”,
J. Geod., 85,pp-18, doi:10.1007/s00190-0100410-6, 2011.

[1] Sekido, M., et al., “Development Status of
GALA-V Broadband VLBI - Geodetic Solution and Clock Comparison -”, IVS NICT- [14] Artz, T., Springer, A., and Norhnagel, A.,
TDC News (ISSN 1882-3432), No. 36., pp.20“A complete VLBI delay model for deform22, 2016.
ing radio telescopes: the Effelsberg case”, J.
Geod., 88, pp.1145-1161, doi:10.1007/s00190[2] Petrachenko, B. , et al., “Design As014-0749-1, 2014.
pects of the VLBI2010 System”, NASA
Technical Memorandum, NASA/TM-2009214180, ftp://ivscc.gsfc.nasa.gov/pub/misc/V2C/PR-V2C_090417.pdf, 2009.
[3] Ujihara, H., “Development of wideband feed
for Kashima 34m antenna”, Radio Sci., Vol.52,

IVS NICT-TDC News No.37

26

Holographic Measurement for
Kashima 34 meter Antenna
Kazuhiro Takefuji1 (takefuji@nict.go.jp),
Masanori Tsutsumi1 , Eiji Kawai1 , andMamoru Sekido1
1

Kashima Space Technology Center, National
Institute of Information and Communications
Technology, 893-1 Hirai, Kashima, Ibaraki
314-8501, Japan
Abstract: We carried out the first VLBI style holographic observation for Kashima 34 meter radio
telescope at 12.25 GHz. To operate the holographic
observation, we made the reference antenna called
MARBLE0 and front/back end system capable toreceive 10 - 13 GHz frequency range. Broadband
receiver system developed for the GALA-V wasused to receive signal of broadcast satellite at 34m
antenna. After performing software correlation of
the recorded data from MARBLE0 and Kashima
34m, and two-dimensional Fourier transform of it,then we successfully obtained the illumination pattern and surface height distribution. The result
will be used for the turning the surface of the 34
meter telescope in near future.

Figure 1. Photo of the reference antenna, which
we called MARBLE0, and Kashima 34 m antennabehind the MARBLE0 directing toward a satellite.

is composed of 1.6m dish, Misumi aluminum frame
pillar parts, and receiver system. This reference
station can be easily raised up and fixed manually.
Inside the RF box at the focus of the reference
antenna, an ambient temperature LNA and amplifier, which works in a frequency range from 10 to 12
GHz, have been installed to receive satellite signals.
As for receiving system of Kashima 34m antenna,broadband observation system of GALA-V, which
1. Introduction
composed of NINJA feed and room temperature
Since the location of Kashima is close to the Pa- LNA, was used to receive signal of broadcastingcific Ocean and the salty wind from the sea blows satellite. Appropriate attenuation was inserted in
toward our station, then our antenna is suffered the receiver system to avoid saturation.
from corrosion. We are planning to make repair
work on the back-up structure of Kashima 34 me- 3. Observations
ter parabolic dish from the end of this fiscal year
Fig. 3 shows the signal received from the broadto next year. Some of the antenna panel might
have to be removed and restored in the repair work, cast satellite, JCSAT-3A. Each rectangular shapedThus, surface height measurement and adjustment spectrum corresponds to modulated digital signal
is required in that case. Then we planned surface of different channels. The lowest band of 8 MHz
height adjustment not only for removing panels in width was converted by analog frequency converter
the repair work, but also for whole antenna aper- and sampled by K5/VSSP32 with 8 bit quantiture area to improve antenna efficiency. For the zation at 16 MHz sampling rate. High dynamic
preparation of this project, we have made a holo- range is important for accurate holographic meagraphic surface height measurement of Kashima 34 surement. Observation procedure was as follows.
meter antenna in prior to the repair work in 2018. Reference station MARBLE0 continuously observe
the satellite signal with fixed azimuthal and elevation angle. Kashima 34-meter antenna is oper2. MARBLE0 as Reference antenna
ated by a snap schedule file of the Field system
In the spring of 2017, 1.6-meter diameter main 9 (FS9). The snap schedule controls the antennareflector of the compact antennas (MARBLE1) to obtain complex beam pattern in a rectangularwere replaced to 2.4-meter one to improve the sen- area (∆Az × ∆El) in the sky around the satellite.
sitivity. We reused the old 1.6-meter dish for the The antenna sweeps for a certain azimuthal anreference antenna of the holographic measurement. gle (∆Az) width fixed elevation in one step. Then
Fig 1 and Fig. 2 show the photo of the reference an- antenna sweeps back to the same azimuthal angle
tenna and block diagram of the observing system, in opposite direction with elevation angle differs by
respectively. The MARBLE0 reference antenna

27

December 2017

Antenna sideNINJA+OMTMARBLE0

RF Boxport1,
V-pol

minicircu
its
ALS03493
gain
11GHz:32.6dB
12GHz:27.3dB
Low cutoﬀ 5GHz

LNA
LNF-LNR4-14B
gain: 37 dB
NF: 0.65 dB
freq: 4-14 GHz

4 dB

ALS0612-25-20
6-12GHz, gain 25 dB

HPF

minicircu
its

E/O

Observation room

Coaxial cable

LPF
16 MHzZVA-183+
gain: 26 dB
P1dB: 24 dB
freq: 0.7-18 GHz

Optical ﬁber

R&K

K5/
VSSP32

16Msps, 8bit, 1ch
gain: 40 dB
freq: 0.01-3 GHz

SG 12.242 GHz, +6dBm

O/E

LPF
16 MHz

To obs room
ZVA-183+
gain: 26 dB
P1dB: 24 dB
freq: 0.7-18 GHz

R&K

K5/
VSSP32

16Msps, 8bit, 1ch
gain: 40 dB
freq: 0.01-3 GHz

Figure 2. Observing system block diagram of the reference antenna MARBLE0 and broadband Gala-V
for the holographic observation.
3. Extracting proper antenna position (0.05 deg
step as usual) from log file and matching itwith correlation results
4. Performing two dimensional Fourier transformwith obtained correlation amplitude and phasewith adjusting the peak position.
5. Removing phase slope and offset by leastsquare estimation, and obtaining residual
phase, which corresponds to twice of the antenna surface height.
4.
Figure 3. Example of received satellite signal at 12
GHz with the MARBLE0 system
δEl. By repeating this so called ’zigzag scan’, beam
pattern data is corrected in rectangular sky area.
The sweeping ranges ∆Az and ∆El are set to be
1.4 degrees so that number of points (∆El/δEl) becomes 34 with expecting 1 meter spatial resolution.
Sweeping in azimuthal direction is made by slow
speed slew mode without commanding azimuthal
angle position. Actual Az/El angle position data
is readout from antenna control unit (ACU) by every second and recorded in log file of FS9. One session of this observation takes about 1.5 hour. Afterthe observation, complex beam pattern is derived
by correlation processing with software correlator
GICO3. The data processing procedure is,
1. Performing software correlation of 8-bits data
by GICO3
2. Searching fringe between 34 m and Marble0 inevery 10ms

Results

Fig. 4 show the correlation amplitude and phasein the azimuth and elevation after the zigzag schedule. Different number of quantization bit was examined before the observation. By acquiring datawith changing quantization bit 1, 2, 4, and 8-bits,
correlation amplitude of 20, 26, 78.5, and 79.9 wereobtained, respectively. By comparing amplitude ofsignal between power measurement and correlation
processing of the data, 8-bit quantization has been
selected.
Fig. 5 show the two dimensional amplitude (illumination pattern) and phase (surface height distribution) obtained by two dimensional Fourier transform of the complex beam pattern. We have performed several sessions of this holographic observation for several reasons, such as to determine thecorrespondence between antenna surface positionand obtained surface map, to evaluate the measurement precision, and to confirm the height adjustment direction. Fig. 5 shows the illumination
pattern in case that we put a black body absorberon the surface. That absorber corresponds to theslightly dark point seen at the position of (5m, -8m)
in horizontal vertical coordinates. Consequently, itIVS NICT-TDC News No.37

28

Received corr-amp [dB] on Kas34-MARBLE0 on 2017/300 06:27

Received phase [rad] on Kas34-MARBLE0 on 2017/300 06:27

47

50

46.8
45
46.6

47

200

46.8

150

46.6

100

46.4

50

46.2

0

46

-50

45.8

-100

45.6

-150

46.4
46.2

35

46

Elevation [deg]Elevation [deg]

40

30
45.8
25
45.6
45.4
199.8 200 200.2 200.4 200.6 200.8 201 201.2 201.4 201.6 201.8

20

45.4
199.8 200 200.2 200.4 200.6 200.8 201 201.2 201.4 201.6 201.8

Azimuth [deg]-200

Azimuth [deg]Figure 4. Correlation amplitude(left) and phase (right) after the zigzag schedule
Amplitude after FFT on Kas34-MARBLE0 on 2017/314 06:25
20
3500

Phase[m] after FFT on Kas34-MARBLE0 on 2017/314 06:25
20

3000

15

6

2500

10

2000

5

2

0

0

1000

-5

-2

-15

500

-10

-4

-20

0

-15

-6

5
0

1500

-5
-10

-20 -15 -10 -5
0
5
Horizontal

10

15

20

Vertical [m]Vertical

10

4

-20

[mm]

15

8
6
5
4
3
2
1
0
-1
-2
-3
-4
-5
-6

-8
-20

-15

-10

-5

0
Horizontal [m]

5

10

15

20

Figure 5. Obtained illumination pattern (left) and surface height distribution (right) of the Kashima 34
meter telescope
was found out that surface is bumpy at the bottom and left side of surface. These height distribution is result of accumulated deformation for 29
years, since no measurement and adjustment havebeen performed after the construction. This is thefirst holographic height measurement result for the
Kashima 34 meter antenna.
5.

Summary

We have carried out the first holographic surface
height measurement for Kashima 34 meter radio
telescope. To operate the holographic observation,
we prepared a reference antenna named MARBLE0
and front/back end system. After software correlation of observed data at MARBLE0 and Kashima
34m, and two-dimensional Fourier transform, we
have successfully obtained the illumination patternand surface height distribution. This technique willused for the turning the surface of the 34 metertelescope in 2018.
Acknowledgement
We thank Jun Amagai of NICT Okinawa, Hiroshi Mikoshiba, and Kazuyuki Handa of NAO for
their kind advices of holographic technique.

29

Recent progress of K5 software
correlation system
Tetsuro Kondo1,2(kondo@shao.ac.jp, kondo@nict.go.jp)
1

Kashima Space Technology Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314-0012, Japan
2
Shanghai Astronomical Observatory
Chinese Academy of Science
80 Nandan Road, Shanghai 200030, China

December 2017

K5 SOFTWAREOBSERVATIONSAMPLER ACCESS
autoobs, sampling, monit, .

DATA CHECK& EDITskdchk, datachk, speana, four2one, .

FORMAT
CONVERSION
k5tom5, m5tok5, .

CORRELATION
apri_calc, cor, fx_cor, cor_new, fx_cor_new .

Abstract: K5 software correlation system consisting
FRINGE SEARCHBANDWIDTH SYNTHESIS
of software correlator and fringe search software insdelay, komb, .
cluding a bandwidth synthesis processing has beendeveloped to carry out correlation processing for
Figure 1. Contents of K5 software package.
K5/VSSP series data. Now K5 software correlator can process data with Mark5B and VDIF data
formats directly besides the K5 format data. A
wideband bandwidth synthesis function is newly K5/VSSP data (KASHIM11) and Mark-5B format
implemented in the K5 bandwidth synthesis soft- data (TOMAKO11). Coarse fringe search results
ware named KOMB.
are presented for 4 channels among 16 channels.
As for TOMAKO11, K5/VSSP data are converted
1. Introduction
to Mark-5B format data for checking software corNational Institute of Information and Commu- relator.
nications Technology (NICT) has developed theK5 software package dedicated to data obtainedby K5/VSSP series sampler (Kondo et al., 2002;
Kondo et al., 2004). The software package consists of three categories of processing, that is, 1)“observation”, 2) “correlation processing”, and 3)“fringe search and bandwidth synthesis” as shownin Fig. 1. K5 software correlator is a member of K5
software package that has been developed to carryout correlation processing. Early K5 software correlator could only process K5 format data. Therefore data format conversion software was developed
to process other format data, such as Mark-5B
format data. However, it can now handle Mark5B and VDIF data formats directly besides the
Figure 2. An example of correlation results beK5 format data. Furthermore, conventional bandtween K5/VSSP format (KASHIMA11) and Markwidth synthesis software KOMB has been improved
5B format (TOMAKO11) data.
to make possible to realize a wideband bandwidth
synthesis with a bandwidth of 10 GHz or more byusing an algorithm developed by Kondo and Takefuji (2016). These new functions are introduced in
3. Wideband bandwidth synthesis
this report.
Correlated data are processed by the K5 bandwidth synthesis software KOMB (Kondo et al.,K5 software correlator has been revised to han- 1999) to get precise residual delay by synthesizing
dle Mark-5B and VDIF format data directly. Cor- multi-channel correlated data. Now KOMB canrelation processing between different data for- make a bandwidth synthesis with a bandwidth ofmats is also supported. Fig. 2 shows an exam- 10 GHz or more. Figs. 3 and 4 shows a flowchart ofple of results of correlation processing between wideband bandwidth synthesis (WBWS) without
2. Revised software correlator

IVS NICT-TDC News No.37

30

the use of phase calibration signals. At first, a reference scan is selected to determine phase and delay calibration data for a WBWS processing. Theneach scan is processed. Differential total electron
content (TEC) is also estimated in the WBWS processing (see Kondo and Takefuji (2016) for details).

cross spectrum

search function

startselect a reference scanFigure 5. An example of wideband bandwidth synthesis without phase and delay corrections (leftpanel: cross spectrum, right panel: search function). Phases are not aligned and peak in searchfunction is spread in the delay direction.

process each band, and determine
correction data as follows,1) inter-band delay
2) inner-band phase characteristics
process other scansstopcross spectrum

search function

Figure 3. A flowchart of wideband bandwidth synthesis (WBWS) without phase calibration signals.

start
apply correction data tocross spectrum
estimate delay anddelay rate
calculate cross spectrum
after delay and delay rate
are correctedionosphericcorrection?
NOestimate total electron
content (TEC)
calculate cross spectrum
after TEC is corrected
re-estimate delay anddelay rate

YES

stop

Figure 4. A flowchart of the processing of each
scan.

Figure 6. An example of wideband bandwidth synthesis with phase and delay corrections.

References
Kondo, T., M. Sekido, and H. Kiuchi, KSP bandwidth
synthesizing software, J. Commun. Res. Lab., 46,
67-76, 1999.
Kondo, T., Y. Koyama, J. Nakajima, M. Sekido, R.
Ichikawa, E. Kawai, H. Okubo, H. Osaki, M. Kimura,Y. Ichikawa, and GALAXY team, Real-time Gigabit
VLBI system and Internet VLBI system, IVS 2002
General Meeting Proceedings, pp.142–146, 2002.

Fig. 5 shows an example of WBWS results with- Kondo,T., M. Kimura, Y. Koyama, and H. Osaki,
Current status of software correlators developed atout phase and delay corrections. Left panel shows
Kashima Space Research Center, IVS 2004 Generala cross spectrum and right panel a search function.
Meeting Proceedings, edited by N. R. Vandenberg
Lower panel in the cross spectrum shows a residual
and K. D. Baver, NASA/CP-2004-212255, pp.186–
phase. As shown in the figure, residual phases are
190, 2004.
rotating with respect to the frequency. This meansthat WBWS failed. Peak position denotes residual Kondo, T., and K. Takefuji, An algorithm of wideband
bandwidth synthesis for geodetic VLBI, Radio Sci.,delay and delay rate in the plot of search function,
Vol. 51, doi:10.1002/2016RS006070, 2016.
but in this case, the peak is spread in the directionof delay. However after phase and delay correction,
residual phases are aligned in the frequency direction and the peak in the search function becomes
clear as shown in Fig. 6.

“IVS NICT Technology Development Center News” (IVS NICT-TDC News) published by the
National Institute of Information and Communications Technology (NICT) (former the Communications Research Laboratory (CRL)) is the continuation of “IVS CRL Technology Development
Center News” (IVS CRL-TDC News). (On April 1, 2004, Communications Research Laboratory(CRL) and Telecommunications Advancement Organization of JAPAN (TAO) were reorganized
as “National Institute of Information and Communications Technology (NICT)”.)
VLBI Technology Development Center (TDC) at NICT is supposed
1) to develop new observation techniques and new systems for advanced Earth’s rotation observations by VLBI and other space techniques,2) to promote research in Earth rotation using VLBI,
3) to distribute new VLBI technology,4) to contribute the standardization of VLBI interface, and5) to deploy the real-time VLBI technique.
The NICT TDC newsletter (IVS NICT-TDC News) is published biannually by NICT.

This news was edited by Mamoru SEKIDO, Kashima Space Technology Center. Inquires on thisissue should be addressed to Mamoru SEKIDO, Kashima Space Technology Center, National
Institute of Information and Communications Technology, 893-1 Hirai, Kashima, Ibaraki 3148501, Japan, e-mail : sekido@nict.go.jp.
Summaries of VLBI and related activities at the National Institute of Information and Communications Technology are on the Web. The URL to view the home page of the Radio Astronomy
Applications Section of the Space-Time Measurement Group of Space-Time Standards Laboratory is : “http://www2.nict.go.jp/sts/stmg/ivstdc/news-index.html”.

IVS NICT TECHNOLOGY DEVELOPMENT CENTER NEWS No.37, December 2017
International VLBI Service for Geodesy and Astrometry
NICT Technology Development Center News
published by
National Institute of Information and Communications Technology, 4-2-1 Nukui-kita, Koganei,
Tokyo 184-8795, Japan