Vl.lta.Tin Aatronomy, Vol. 31, pp. ~47--~52, 1988 0083-6656/88 $0.00+ .SO

Copyright © 1988 Science Press & Pergamon Journals Ltd.

THE JAPANESE VLBI PROJECT VERA (VLBI FOR THE EARTH ROTATION STUDY AND ASTROMETRY)

T. H a r a i . O k a m o t o T. S a s a o I n t e r ~ l a t i o n a l L a t i t u d e O b s e r v a t o r y o f M l z u s a ~ a , Jal)ax~

ABSTRACT

A brief description is made for the aims and specific features of a planned VLBI project "VLBI for the Earth Rotation study and Astrometry" (VERA).

I. SCIENTIFIC OBJECTIVES

VERA(VLBI for the Earth Rotation study and Astrometry) is a

domestic VLBI system under contemplation dedicated to high precisiot,

determinations of the Earth orientation and position of the radio

source positions. Two antennas with stable mounts will be installed

on granite bedrocks near Mizusawa-city in lwate prefecture (North-

east Japan main island) and at one of the South-west Islands in

Okinawa prefecture, spanning approximately 2300km.

As a major tool for Earth rotation research in Japan, VERA will

participate in the International VLBI network (IRIS/IERS) for the

long range monitoring of the variabilities in the rotation of the

Earth. VERA could improve both the accuracy and stability of the

international network. As a dedicated domestic system, VERA will

also conduct intensive observations of short term variations of the

Earth rotation. High frequency UT] fluctuations with time scale less

than a day could be studied in relation,with atmospheric exitation

processes. Accurate nutation series could be obtained and yield

information about the physical properties and dynamics of the fluid

core. It is hoped that VERA could unambiguously detect the free

core nutation.

A fairly large collecting area (currently we plan for a 35m

and 15m diameter antenna pair), the capability of wide frequency

coverage and enough machine time for VLBI will make VERA a powerful

instrument for the rapidly develiping field of radio astrometry.

647

648 T. Hara et al.

Distance measurements of star forming regions and late-type stars

with the maser line sources, tying maser source positions with

quasar positions, determination of positions and proper motions of

compact galactic radio sources such as pulsars, X-ray and gamma-ray

stars or black holes, searches for gravitational lens images of the

super-luminal quasar jets, establishment of radio reference systems,

and many other proposals have been made in VERA project meetings.

VERA will also cooperate with a wide range of astrophysical

and geodetic researches.

In order to monitor and eliminate systematic errors in the VLBI

measurement and also to conduct collaborative observations,

geophysical instruments such as an absolute gravimeter, a superconduc-

tive gravimeter and a wide band seismometer will be installed at

the VERA sites.

2. BASIC ISSUES

For the purposes of VERA, the most important issue is how to

minimize systematic errors.

The intended accuracy goal is: suppression of systematic errors to

a level less than 0.; milli-arc-sec when periodic components

of the Earth rotation irreqularities or continuum radio source

positions are estimated by statistical analysis of a large

number of measured group delay values. And 0.0] milli-arc-sec

accuracy in the relative phase (differential VLBI) measurements.

Every effort in the system design, site selection and plans for

collaboration observations of the environment must be directed

towards this goal.

From an operational point of view, VERA assumes two rather

distinct modes of operation. One is the highly regular and long-

lasting routine of Earth rotation observation using a fixed number

of quasar sources and fixed receiver systems with 2 and 8 GHz dual

frequency bands. Another is a variety of observations for radio

astrometry and other purposes, which will be conducted mainly on

the proposal/selection basis with all possible combinations of

objects and instrumentation. Finding an optimum way to meet the

requirements of both modes and to maximize the scientific outcome

The Japanese VLBI Project VERA 649

is one of the major issues of the VERA project.

3. TENTATIVE FEATURES OF THE VERA HARDWARE SYSTEM

3.1 Antennas and Feed Systems

Collecting area : should be sufficient to receive fluxes > several

tens of mJy (for continuum sources) and >a few Jy (for line

sources) for up to I000 sec integration time

Surface accuracy: 0.3mm rms

Pointing accuracy: better than 0.0025 deg

Deformations: Short term (less than lO00sec) fluctuations of the

optical path length and reference point (crossing of the

azimuth and elevation axes) should be less than Imm rms. Long

term deformations should be monitored by a specially designed

system with accuracy better than lmm.

Azimuth sky coverage: 0 to 90 deg

Elevation sky cov.: -270 to 270 deg

Drive speed: 3 deg/sec in azimuth; 2 deg/sec in elevation

Base: should be stably fixed to the granite bedrocks.

Feed systems: 2/8 GHz dual feed; 22GHz feed; 1.6 GHz feed (under

consideration) auxiliary feed horn connector (1.35-43.5 GHz)

Feed systems should easily be switched with the aid of computer

control.

Near Mizusawa station

Antenna aperture: 35m class

Wind vel. to stow: 30m/sec

Survival in stow: 60m/sec

South-west Islands station

Antenna aperture: 15m class

Wind vel. to stow: 30m/sec

Survival in stow: 90m/sec

3.2. Front-ends

The frequency range of low noise amplifiers:

1.6GHz band 1.33-1.75GHz (under consideration)

2 GHz band 2.15-2.35GHz,

8 GHz band 7.86-8.80GHz,

22 GHz band 22.2-24.6 GHz.

A feed horn and a receiver for arbitrary frequency from 1.33

650 T. Hara et al.

to 43.5GHz could be attached to the auxiliary feed horn connector.

Wide frequency ranges in 8GHz and 22 GHz bands are designed for

better determination of the group delay via the bandwidth synthesis.

The front-end system should be switched under computer control.

Front-end system for the 2/8 GHz routine observations and those

for non-routine observations should be in separate rooms to meet

different requirements of the two observing modes.

3.3 Back-ends

K-3 or Mark III system with high density recorders. Two IF distribu-

tors are needed for the wide-range bandwidth synthesis.

3.4. Correlator

Mark III A type or equivalent. Should be able to process 5 stations

(]0 baselines) simultaneously. Should have spectral-line processing

and pulsar-gating capabilities. Should be able to process the space

VLBI data.

3.5. Frequency Standards

A hydrogen maser frequency standard should be appropriate for VLBI

observations with the coherent loss of as little as two percent for

integration times of up to several hundred seconds.

Time keeping systems consist of cesium frequency standards,

GPS,'receivers and clock comparison systems.

3.6. Computer Control System

Computers for supervising the VERA system will be introduced at both

antenna sites. They will monitor and control the whole system

including the K-3 or Mark III data acquisition and control system.

Commercial high-speed communication lines should be used to link the

computers at the two sites to facilitate quicker transfer of the

schedule files and synchronized operation of the distant stations.

3.7. Delay Calibration System

High precision delay calibrators, capable of measuring cable delays

with ten pico-second level accuracy, should be developed and

equipped for both 2/8 GHz and 22 GHz receiving systems.

The Japanese VLBI Project VERA 651

V E R A C o n s i s t s o f

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652 T. Hara et al.

S y s t e m of VERA

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