A 50 MHz System for GMRT

N. Udaya Shankar

K. S. DwarakanathRaman Research Institute, Bangalore, INDIA

MWA Meeting, Canberra,

18-23 Jan 2009

TEAM S. Amiri,

R. Somasekar, B.S. Girish, Arvind Nayak, Wenny Laus, S. Kasturi, Sujatha.

Outline

Objectives of a 50 MHz system for GMRT.

Feed design.

Summary of observations with the new feed.

Direct voltage recording system.

Experiments with the direct voltage recording system in the interferometric mode.

Conclusions.

Objectives of a 50 MHz system

Design and build feeds for operation at frequencies below about 100 MHz.

Carry out a low frequency survey of the sky visible to GMRT with arcmin resolution and unprecedented surface brightness sensitivity.

Make the survey results available to the astronomy community as an online resource.

Make the observing system available to GMRT user community.

Survey Parameters

Assuming: – Bandwidth: 10 MHz, – System temperature: 4000 °K,– Synthesized beam: 1’, and – RMS sensitivity (4 hr, dual polarization):

2 mJy/beam (Thermal Noise)10 mJy/beam (Dynamic Range Limited)

A mosaic of about 300 pointings to cover the Northern sky requires 1200 hrs of observation– Primary beam area of 0.03 steradians.

NameSky

coverage(steradians)

Telescope time (hrs)

Surveying speed (hrs/sr)

RMS noise

(in mJy)

Equivalent RMS noise at 50 MHz (1-σ in mJy)

GMRT 50 MHz

3 1200 128 2 (10) 2 (10)

GMRT-MWA 5.9 7200 364 0.14 0.35

NVSS 10.3 2700 262 0.15 7.2

WENSS 960 305 4 18

VLSS 3 900 95 100 137

Survey comparison

Larger collecting area and higher antenna efficiency means the low frequency GMRT system will be more sensitive than the 74 MHz VLA system.

Sensitivity Vs Angular size

55 confusion confusion

Development of high dynamic range, interference tolerant receivers.

Detection and mitigation of radio frequency interference.

Calibration issues arising out of ionospheric effects.

Problems of imaging wide-fields with non-coplanar baselines.

Deconvolution of images with large scale features.

Main challenges

A Low Frequency Feed for GMRT

Wish list: Covering the frequency range 30 MHz to 90

MHz.

Sky noise dominates the system temperature (Tsky> 5 Tsys).

Reasonable aperture efficiency (0.5 < ap < 0.7).

Symmetrical E & H plane patterns.

Physical dimension is suitable for mounting it along with one of the existing feeds on the GMRT antenna turret, with minimum interference to its operation.

Feed Design

Simulations were done to determine the configuration which cause minimum interference to the operation of the existing feed.

After field trials, it was decided to co-locate the new feed with the existing 327 MHz feed.

The desired frequency of operation (30 to 90 MHz) was chosen to:– Facilitate imaging in the band protected for Radio

Astronomy around 38 MHz,– Have an overlap with the 74 MHz system on VLA,

and – Minimize the RFI due to the FM Radio band

starting around 90 MHz.

Feed Design

Boxing ring configuration chosen since it has more symmetric radiation characteristics (E and H plane patterns) than a single dipole like feed.

Following parameters were tuned:– Distance between two parallel dipoles in the

boxing ring.– Shape of the dipoles to achieve the required

bandwidth.– Height of dipoles above the reflector.– Impedance mismatch at frequencies away from

the resonance.

Present Status

The first phase of the project :

A system for 4 GMRT antennas has been designed, built and installed and its performance has been examined.

The design would be developed further, if necessary, based on our experience.

Four folded V-dipoles each of length ~2.4m. Its feed point is positioned 1m above a 3m X 3m square reflector with a 50mm mesh.

The feed co-located with existing 327 MHz feed

Summary of observations

GMRT antennas C04, C11, E02 & W02 equipped with new feeds.

3C sources observed: Cyg-A, Cas-A and a few other sources.

Maximum EW baseline: 6 km.

C04-C11 NS baseline: ~0.5 km.

Aperture efficiency:

– ~65% (55 MHz)

– ~30% (40 MHz)

C11

C04

E02W02

Observed several 3C sources.

Over an hour of observing of Cyg-A, the cross correlation coefficients showed:– A steady behaviour as has been seen at other

observing bands of GMRT. – Measured rms of ~6% on the visibilities at 55 MHz

implies an rms of ~12° in closure phases and ~12% in closure amplitudes.

– Closure phases showed an rms of 1.4° and the closure amplitudes showed an rms of 7%.

– Observed rms in the closure quantities are much less implying that a significant part of the rms in the visibilities are due to systematics.

Summary of observations

Closure phases

Looking at one good channel across 1 hr time.

RMS: ~1.5°.

Ph

ase

(Deg

rees)

Baseline: C04-E02

Baseline: C11-E02

Baseline: C04-C11

Time (Hours)

Direct voltage recording system

Digitize a signal of ~5 MHz bandwidth centred at 55 MHz.

Data rate: ~80 GB/hr.

Systems synchronized using signals from GPS-disciplined Rubidium oscillators.

Two direct voltage recording systems were installed at C04 and C11 antenna bases.

Direct voltage recording system

Laboratory Test Setup

Setup At GMRTAntenna Base

8-bit A/DConverter

8-bit A/DConverter

Stored OnHard Disk

Stored OnHard Disk

DelayCorrection

Offl

ine D

ata

Pro

cessin

g

DelayCorrection

Visibilities PostIntegration

Rf Rf

8192-pointFourier

Transform

8192-pointFourier

Transform

~80 GB/Hr

Typical cross power dynamic spectra ~4000 channels. Spectral resolution of 1.36 KHz.

~4 min of data. Each time frame with ~1 second integration.

Notice the RFI stripes.

Only ~2000 central channels marked in plot used for analysis.

~2000 central channels used for analysis.

Typical Self & Cross spectra

1200 1400 1600 1800 2000 2200 2400 2600 2800 3000-30

-25

-20

CHANNEL INDEX

SELF ANT-1

1200 1400 1600 1800 2000 2200 2400 2600 2800 3000-30

-25

-20

CHANNEL INDEX

SELF ANT-2

1200 1400 1600 1800 2000 2200 2400 2600 2800 3000-34

-32

-30

-28

CHANNEL INDEX

Un-NORM CROSS

Integrated for ~4 min, only central ~2000 channels shown.

Notice the comb in self power of Antenna-2 (C11) and is relatively affected more by RFI.

RFI Scenario in ~1s integrated visibility for ~4 min data.

~0.5% RFI points above 4-σ.

RFI Scenario

100

101

102

100

200

300

400

500

600

Duration of interference (seconds)

Number of interference points

Dynamic spectra after RFI excision

Central ~2000 channels.

Notice, the vertical RFI stripes (across time) have disappeared.

Structures in Time-Frequency

Period: ~ 45 s

Variations: ± 5%

~4 min integrated spectra.

RFI channels: ~20%

Our experience shows that a multi-resolution filter provides a possible optimal solution for interference mitigation.

Using this approach we are in the process of analysing simultaneous observations carried out using:

– GMRT hardware correlator,

– New GMRT software correlator which is under testing, and

– RRI direct voltage recording system.

We hope these investigations will throw more light on the effect of systematics on thermal noise.

Conclusions

We have successfully designed, installed and tested a 50 MHz system on 4 GMRT antennas.

Aperture efficiency is ~65% in the 50 MHz band.

Test observations indicate satisfactory performance of the feed system.

Investigations are under way to understand the mitigation of systematics and RFI to achieve performance limited by thermal noise.

Acknowledgements

We thank our colleagues at GMRT site and NCRA for their excellent support.

Integrated for ~4 min, only central ~2000 channels shown.

First row:

Second row: Variations across time.

Structures in Time-Frequency

Aperture efficiency Cyg-A (Sep 07 observations)

Baseline- C04:C11

Centre frequency: 55 MHz

Aperture efficiency: ~65 %

Aperture efficiency Cyg-A (Sep 07 observations)

Baseline- C04:C11

Centre frequency: 38 MHz

Aperture efficiency: ~30 %

Surface Brightness Sensitivity of other surveys

Surface Brightness sensitivity of MRT

7 X 10 (-22) Watts/m2/Hz/ Ster (Theoretical)

2 X 10(-21) Watts/m2/Hz/ Ster (Achieved)

4.2 X 10 (-22) Watts/m2/Hz/ Ster (Eq at 1.4 GHz)

Typical SB of SNR = 1.5 X 10 (-21) Watts/m2/Hz/ Ster

SNR cat complete = 10(-20) Watts/m2/Hz/ Ster at 1 GHz

VLA 4mass 100 mJy/beam

Beam Size 80’’