8,919 research outputs found
Non-linear Redundancy Calibration
For radio interferometric arrays with a sufficient number of redundant
spacings the multiplicity of measurements of the same sky visibility can be
used to determine both the antenna gains as well as the true visibilities. Many
of the earlier approaches to this problem focused on linearized versions of the
relation between the measured and the true visibilities. Here we propose to use
a standard non-linear minimization algorithm to solve for both the antenna
gains as well as the true visibilities. We show through simulations done in the
context of the ongoing upgrade to the Ooty Radio Telescope that the non-linear
minimization algorithm is fast compared to the earlier approaches. Further,
unlike the most straightforward linearized approach, which works with the
logarithms of the visibilities and the gains, the non-linear minimization
algorithm leads to unbiased solutions. Finally we present error estimates for
the estimated gains and visibilities. Monte-Carlo simulations establish that
the estimator is indeed statistically efficient, achieving the Cramer-Rao
bound.Comment: 9 pages, 5 figures. Accepted for publication in MNRAS. The definitive
version will be available at http://mnras.oxfordjournals.or
TDIR: Time-Delay Interferometric Ranging for Space-Borne Gravitational-Wave Detectors
Space-borne interferometric gravitational-wave detectors, sensitive in the
low-frequency (mHz) band, will fly in the next decade. In these detectors, the
spacecraft-to-spacecraft light-travel times will necessarily be unequal and
time-varying, and (because of aberration) will have different values on up- and
down-links. In such unequal-armlength interferometers, laser phase noise will
be canceled by taking linear combinations of the laser-phase observables
measured between pairs of spacecraft, appropriately time-shifted by the light
propagation times along the corresponding arms. This procedure, known as
time-delay interferometry (TDI), requires an accurate knowledge of the
light-time delays as functions of time. Here we propose a high-accuracy
technique to estimate these time delays and study its use in the context of the
Laser Interferometer Space Antenna (LISA) mission. We refer to this ranging
technique, which relies on the TDI combinations themselves, as Time-Delay
Interferometric Ranging (TDIR). For every TDI combination, we show that, by
minimizing the rms power in that combination (averaged over integration times
s) with respect to the time-delay parameters, we obtain estimates
of the time delays accurate enough to cancel laser noise to a level well below
the secondary noises. Thus TDIR allows the implementation of TDI without the
use of dedicated inter-spacecraft ranging systems, with a potential
simplification of the LISA design. In this paper we define the TDIR procedure
formally, and we characterize its expected performance via simulations with the
\textit{Synthetic LISA} software package.Comment: 5 pages, 2 figure
- …