The S3 VLBI data record and playback system and future Space VLBI missions

Vol. 26, No. 4, pp. 747-750, 2000 Adv. SpaceRes. COSPAR. Published by Elsevier Science Ltd. All rights reserved Printedin Great Britain 0273-1177/00 $20.00 t 0.00 PII: SO273-1177(99)01210-7 0 2000

Pergamon www.elsevier.nl/locate/asr

THE S3 VLBI DATA RECORD AND PLAYBACK SYSTEM AND FUTURE SPACE VLBI MISSIONS W.H.

Cannon’

’ Department

of Physics and Astronomy,

York University,

Toronto,

Canada

ABSTRACT to conduct VLBI observations on Earth-t&pace and SpaceThe use of “orbiting radio observatories” t*Space baselines is the only means by which the fundamental limitations on the angular resolution of images afforded by the technique of VLBI which result from the finite physical dimensions of the Earth and the opacity of the atmosphere at millimeter and sub-millimeter wavelengths may be circumvented. other

For continuum

technical

future Space (1 - 2 Gb/sec)

support

Mb/set (SGL)

observations,

considerations

of the Center

generate

VLBI missions. “S3” VLBI system for Research

the loss of correlated

flux on long baselines

a requirement for very wide bandwidth This report presents a brief description

in Earth

under

development

and Space

as well as

VLBI systems to of the 1024 - 2048

at the Space Geodynamics

Laboratory

Technology (CRESTech). 0 2000 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION The angular resolution provided by the technique directly proportional to the length of the baseline the observed at microwave the finite

radiation. frequencies

dimensions

ter wavelengths.

The angular (centimeter

of the Earth

The only

means

of Very Long and inversely

Baseline Interferometry (VLBI) proportional to the wavelength

resolution of VLBI conducted to sub-millimeter wavelengths)

and the opacity of achieving

from the surface of the Earth is limited fundamentally by

of the atmosphere

angular

resolutions

is of

at millimeter that

exceed

and submillime-

these

limitations

at

microwave frequencies is to place at least one interferometer element in Earth orbit and conduct “Space VLBI” observations on Earth-to-Space baselines. The technical feasibility of Space VLBI observations at S-band (2.3 GHz) and K-band (15 GHz) was demonstrated in 1986-88 using the TDRSS communications satellite with a 4.9 m diameter antenna (Levy, G.S. et. al. 1989). The recent ISAS-led, from Kagoshima

VSOP Space VLBI mission using Japan, carrying an 8 m diameter

5 GHz, and 22 GHz, constitutes for use by the international radio of HALCA

and the success

be characterized

the first operational, astronomy community

of the VSOP

by a permanent

presence

acting as space-borne VLBI as Space-to-Space baselines.

elements

THE

WIDE

REQUIRMENT

To date missions technical

FOR

the HALCA spacecraft launched in February I997 antenna with observing frequencies of 1.67 GHz,

with

mission

dedicated Space VLBI observatory available (Hirabayashi, H., et. al. 1998). The launch

opens

in space observations

BANDWIDTHS

a new era in radio

of one or more being

IN SPACE

orbiting

conducted

VLBI

astronomy, “radio

hopefully

to

observatories”

on Earth-to-Space

as well

OBSERVATIONS

all orbiting radio observatories, past, present, and future, intended for use in space VLBI were placed, and are planned to be placed, in orbit by a single launch vehicle. This is a issue that serves to restrict the size of the aperture of the space-borne antenna on such

missions. The TDRSS observations used a 4.9 m antenna, the HALCA antenna is 8 m, the future Russian-led “Radioastron” space VLBI antenna is 10 m (Kardashev, N. and Slysh, V., 1988). The 747

148

W. H.Cannon

planned use of inflatable technology in the future NASA ARISE mission (Ulvestad, deploy a 25 m antenna may serve to mitigate this restriction somewhat. In addition appear to be the option of multi-launch space from sub-assemblies offer the possibility

delivered

of constructing

to Earth

apertures

of continuum

range achievable

While the multi-launch in the weightless

future we must anticipate

sources

in imaging

case, fundamental

considerations

of space, it

will be conducted

standards. of space VLBI

will be limited

and the

resolution.

In this

systems can only be

longer coherent

integration

systems

by the use of small orbital

from the high angular

of reduced system temperatures,

Of these options longer coherent

in

option would seem to

imply that the recovery of SNR in space VLBI

by a combination

and wider bandwidths.

flux resulting

is assembled

environment

that space VLBI

the signal to noise ratio by such systems

as well as the loss of correlated

accomplished

in which the antenna

that are rather small (25 m or less) by terrestrial

For observations dynamic

orbit.

missions

very large apertures

would appear that for the foreseeable with antennas

space VLBI

J., 1998) to there would

integration

times, while technically

times, feasible

by Earth-to-Space phase links, are undesirable (and even unhelpful) due to the smearing of the image transform in the uv plane that results from the rotation of the baseline during long integration periods.

Amplifier systems used in modern VLBI

and as a result very large improvements

applications

are approaching

in system temperatures

quantum noise limits

of space VLBI

systems

are ruled

out by fundamental physics. Apart from larger apertures, one of the principal options for achieving significant (factors of 5 or 10) improvement in space VLBI observations of continuum sources is increased

bandwidths.

THE S3 VLBI

DATA RECORD

AND PLAYBACK

At the present time the standard VSOP space VLBI of 128 Mb/set. This bandwidth is provided by: 1. the VLBA/MkIV

VLBI

data recorders

Honeywell 96 tape recorder 2. the VSOP

VLBI

observations

in which recording

using many parallel

data recorder

SYSTEM

longitudinal

in which recording

are conducted

using bandwidths

is accomplished

on an open reel,

tracks

is accomplished

using a single ID-l

helical

scan tape recorder 3. the S2 VLBI

data recorder

in which recording

is accomplished

using an array of eight VHS

helical scan tape recorders The Space Geodynamics Laboratory has begun work on the development of the S3 VLBI data record and playback system which will resemble the S2 in that it will consist of an array of digital video tape transports.

Work to date has resulted

in the selection

of the tape transport

system and preliminary

design work has begun on the signal channel.

of eight tape transports

arranged

in two rack mountable

to be used in the

The S3 will consist of an array

or desktop Tape Transport

Array Modules

(TAMS) t o g e th er with a rack mountable or desktop Data, Signal, and Control Module (DSCM). The S3 will provide the user with a “user selectable” number of input data lines as well as a set of “user selectable” data rates per input signal line. Each tape transort at 128 Mb/set for an overall data rate of 1024 Mb/set. The unattended

record time at 1024 Mb/ set (1 Gb/sec)

in the array of eight will record data

will be of the order of 2.5 hours based on:

1. the use of 13 micron tape loaded into VHS form factor cassettes record time combined

with

with a “standard”

124 minute

The S3 VLBI Data System

149

2. the use of 16 micron

track pitch instead of the “standard” ‘20 micron track pitch. This is a aleady demonstrated capability of the tape transport selected for development of the S3.

Longer unattended record times will be achievable example the S3 can be expected to record:

l

512 Mb/set

for 5 hours

l

256 Mb/set

for 10 hours

l

128 Mb/set

for 20 hours

for data recording

at reduced

bandwidths.

For

The “cost of media” (tape costs) for operation of the S3 at the maximum bandwidth of 1024 Mb/set is expected to be $3.50 (US) p er minute based on present-day tape costs. Tape costs of operation at lower data rates is expected to be proportionally lower:

l

tape costs at 512 Mb/set:

l

tape costs at 256 Mb/ set: $0.88 (US) per minute

l

tape costs at 128 Mb/set:

THE “S3 EXTENDED”

$1.75 (US) per minute

$0.44 (US) per minute

(S3-E) VLBI DATA RECORD

AND PLAYBACK

SYSTEM

It is intended to design the S3 system to accommdate an extended version of the S3, the “S3-E”. The S3-E will consist of a “standard S3” system equipped with a second set of S3 Tape Transport Array Modules (TAMS) which will be “slaved” to the first set of S3 TAMS. The motivation for S3-E is two-fold:

1. The S3-E will offer double the unattended S3 data rates

record times of the S3 when operated

2. the S3-E will offer the capability of recording for higher sensitivity in VLBI applications

The unattended

data at rates of up to 2048 Mb/set

record times for the S3-E are expected

l

2048 Mb/set

for 2.5 hours

l

1024 Mb/set

for 5 hours

l

512 Mb/set

for 10 hours

l

256 Mb/set

for 20 hours

l

128 Mb/set

for 40 hours

at “standard”

to be:

(2 Gb/sec)

750

W. H. Cannon

ROBOTIC

TAPE

CHANGER

FOR THE

S3 AND S3-E VLBI

DATA

RECORD

AND PLAYBACK

SYSTEMS The S3 and S3-E VLBI cassettes

may be serviced

VHS cassettes A simple tended rates

record

and playback

by a rather

simple

tape

data

changer

record

of 1024 Mb/ skc or 2048

tape changer

front

system

VLBI

with

data

S3 and

record

S3-E

loading,

VHS form factor

which would remove

ejected

into the TAMS from a local supply.

in the S3 and S3-E systems

Unattended Mb/set

with their

new VHS cassettes

implemented

times.

systems

robotic

from the S3 TAMS and insert

robotic VLBI

objectives THE

data

times

would

enable

of 24 hours

respectively

would

lengthy

or longer

seem

unatat data

like reasonable

for such a system.

“S4” VLBI

The tape

DATA

transport

selected

conservatively

designed

version

same

of the

RECORD

and

tape

AND

PLAYBACK

by the Space

Geodynamics

there

is the possibility

transport.

If a double

SYSTEM Laboratory

for the development

of introducing head-to-tape

a double

speed

of the S3 is

head-to-tape

version

of this

speed

tape

transport

were available it would open the door to the development of the “S4” VLBI data record and playback system. The S4 would be a double data rate version of the S3. That is to say that the standard data rate for the S4 would S4-E would

be 2048 Mb/set

with an unattended

(2 Gb/sec)

record

time of 1.25 hours.

The

offer either:

1. VLBI

data

recording

rate of 2048 Mb/ set for an unattened

record

2. VLBI

data

recording

rate of 4096 Mb/set

an unattended

(4 Gb/sec)

with

time

of 2.5 hours record

time

of 1.25

hours

ACKNOWLEDGEMENTS We gratefully

acknowledge

Astronautical

Science

the VSOP Project, which is led by the Japanese Institute of Space and in cooperation with many organizations and radio telescopes around

(ISAS)

the world. REFERENCES

Hirabayashi, H., et.al., Overview of the Operation Interferometry Space Observatory Program, Kardashev, N., Slysh, V., The Radioastron Geophysics, edited by M. Reid and

and the Initial Science, 281,

Project, in The Impact of VLBI on Astrophysics and M.J. Moran, pp. 433-440, Kluwer Academic Publishers

(1967). Levy, G.S. et.al. Ulvestad,

Astrophysical

J., Personal

Journal,

communication

Results of the Very Long Baseline 1825 (1998).

336, (1998).

1098 (1989).