6,876 research outputs found
A Preliminary Study of Three-Point Onboard External Calibration for Tracking Radiometric Stability and Accuracy
Absolute calibration of radiometers is usually implemented onboard using one hot and one cold external calibration targets. However, two-point calibration methods are unable to differentiate calibration drifts and associated errors from fluctuations in receiver gain and offset. Furthermore, they are inadequate to characterize temporal calibration stability of radiometers. In this paper, a preliminary study with linear radiometer systems has been presented to show that onboard external three-point calibration offers the means to quantify calibration drifts in the radiometer systems, and characterize associated errors as well as temporal stability in Earth and space measurements. Radiometers with three external calibration reference targets operating two data processing paths: i.e., (1) measurement path and (2) calibration validation path have been introduced. In the calibration validation data processing path, measurements of one known calibration target is calibrated using the other two calibration references, and temporal calibration stability and possible calibration temperature drifts are analyzed. In the measurement data processing path, the impact of the calibration drifts on Earth and space measurements is quantified and bounded by an upper limit. This two-path analysis is performed through calibration error analysis (CEA) diagrams introduced in this paper
Microwave limb sounder
Trace gases in the upper atmosphere can be measured by comparing spectral noise content of limb soundings with the spectral noise content of cold space. An offset Cassegrain antenna system and tiltable input mirror alternately look out at the limb and up at cold space at an elevation angle of about 22. The mirror can also be tilted to look at a black body calibration target. Reflection from the mirror is directed into a radiometer whose head functions as a diplexer to combine the input radiation and a local ocillator (klystron) beam. The radiometer head is comprised of a Fabry-Perot resonator consisting of two Fabry-Perot cavities spaced a number of half wavelengths apart. Incoming radiation received on one side is reflected and rotated 90 deg in polarization by the resonator so that it will be reflected by an input grid into a mixer, while the klystron beam received on the other side is also reflected and rotated 90 deg, but not without passing some energy to be reflected by the input grid into the mixer
Planck pre-launch status: Low Frequency Instrument calibration and expected scientific performance
We give the calibration and scientific performance parameters of the Planck
Low Frequency Instrument (LFI) measured during the ground cryogenic test
campaign. These parameters characterise the instrument response and constitute
our best pre-launch knowledge of the LFI scientific performance. The LFI shows
excellent stability and rejection of instrumental systematic effects;
measured noise performance shows that LFI is the most sensitive instrument of
its kind. The set of measured calibration parameters will be updated during
flight operations through the end of the mission.Comment: Accepted for publications in Astronomy and Astrophysics. Astronomy &
Astrophysics, 2010 (acceptance date: 12 Jan 2010
SMMR Simulator radiative transfer calibration model. 2: Algorithm development
Passive microwave measurements performed from Earth orbit can be used to provide global data on a wide range of geophysical and meteorological phenomena. A Scanning Multichannel Microwave Radiometer (SMMR) is being flown on the Nimbus-G satellite. The SMMR Simulator duplicates the frequency bands utilized in the spacecraft instruments through an amalgamate of radiometer systems. The algorithm developed utilizes data from the fall 1978 NASA CV-990 Nimbus-G underflight test series and subsequent laboratory testing
Tropospheric Phase Calibration in Millimeter Interferometry
We review millimeter interferometric phase variations caused by variations in
the precipitable water vapor content of the troposphere, and we discuss
techniques proposed to correct for these variations. We present observations
with the Very Large Array at 22 GHz and 43 GHz designed to test these
techniques. We find that both the Fast Switching and Paired Array calibration
techniques are effective at reducing tropospheric phase noise for radio
interferometers. In both cases, the residual rms phase fluctuations after
correction are independent of baseline length for b > b_{eff}. These techniques
allow for diffraction limited imaging of faint sources on arbitrarily long
baselines at mm wavelengths. We consider the technique of tropospheric phase
correction using a measurement of the precipitable water vapor content of the
troposphere via a radiometric measurement of the brightness temperature of the
atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz
for the MMA, and 120 mK for the VLA at 22 GHz. The minimum gain stability
requirement is 200 at 185 GHz at the MMA assuming that the astronomical
receivers are used for radiometry. This increases to 2000 for an uncooled
system. The stability requirement is 450 for the cooled system at the VLA at 22
GHz. To perform absolute radiometric phase corrections also requires knowledge
of the tropospheric parameters and models to an accuracy of a few percent. It
may be possible to perform an `empirically calibrated' radiometric phase
correction, in which the relationship between fluctuations in brightness
temperature differences with fluctuations in interferometric phases is
calibrated by observing a celestial calibrator at regular intervals.Comment: AAS LATEX preprint format. to appear in Radio Science 199
Planck pre-launch status: calibration of the Low Frequency Instrument flight model radiometers
The Low Frequency Instrument (LFI) on-board the ESA Planck satellite carries
eleven radiometer subsystems, called Radiometer Chain Assemblies (RCAs), each
composed of a pair of pseudo-correlation receivers. We describe the on-ground
calibration campaign performed to qualify the flight model RCAs and to measure
their pre-launch performances. Each RCA was calibrated in a dedicated
flight-like cryogenic environment with the radiometer front-end cooled to 20K
and the back-end at 300K, and with an external input load cooled to 4K. A
matched load simulating a blackbody at different temperatures was placed in
front of the sky horn to derive basic radiometer properties such as noise
temperature, gain, and noise performance, e.g. 1/f noise. The spectral response
of each detector was measured as was their susceptibility to thermal variation.
All eleven LFI RCAs were calibrated. Instrumental parameters measured in these
tests, such as noise temperature, bandwidth, radiometer isolation, and
linearity, provide essential inputs to the Planck-LFI data analysis.Comment: 15 pages, 18 figures. Accepted for publication in Astronomy and
Astrophysic
Observational issues in radiometric and interferometric detection and analysis of the Sunyaev-Zel'dovich effects
This review discusses the techniques used in single-dish and interferometric
radiometric observations of the Sunyaev-Zel'dovich effects, the pitfalls that
arise, the systematic and other sources of error in the data, and the
uncertainties in the interpretation of the results.Comment: 46 pages, 23 figures. To appear in Background Microwave Radiation and
Intracluster Cosmology, Proceedings of the International School of Physics
"Enrico Fermi", Eds. Melchiorri, F. & Rephaeli, Y., 200
A Dynamic Approach to Linear Statistical Calibration with an Application in Microwave Radiometry
The problem of statistical calibration of a measuring instrument can be
framed both in a statistical context as well as in an engineering context. In
the first, the problem is dealt with by distinguishing between the 'classical'
approach and the 'inverse' regression approach. Both of these models are static
models and are used to estimate exact measurements from measurements that are
affected by error. In the engineering context, the variables of interest are
considered to be taken at the time at which you observe it. The Bayesian time
series analysis method of Dynamic Linear Models (DLM) can be used to monitor
the evolution of the measures, thus introducing an dynamic approach to
statistical calibration. The research presented employs the use of Bayesian
methodology to perform statistical calibration. The DLM's framework is used to
capture the time-varying parameters that maybe changing or drifting over time.
Two separate DLM based models are presented in this paper. A simulation study
is conducted where the two models are compared to some well known 'static'
calibration approaches in the literature from both the frequentist and Bayesian
perspectives. The focus of the study is to understand how well the dynamic
statistical calibration methods performs under various signal-to-noise ratios,
r. The posterior distributions of the estimated calibration points as well as
the 95% coverage intervals are compared by statistical summaries. These dynamic
methods are applied to a microwave radiometry data set.Comment: 26 pages, 10 figure
SARAS 2: A Spectral Radiometer for probing Cosmic Dawn and the Epoch of Reionization through detection of the global 21 cm signal
The global 21 cm signal from Cosmic Dawn (CD) and the Epoch of Reionization
(EoR), at redshifts , probes the nature of first sources of
radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the
signal is predicted to be extremely weak, of wide fractional bandwidth, and
lies in a frequency range that is dominated by Galactic and Extragalactic
foregrounds as well as Radio Frequency Interference, detection of the signal is
a daunting task. Critical to the experiment is the manner in which the sky
signal is represented through the instrument. It is of utmost importance to
design a system whose spectral bandpass and additive spurious can be well
calibrated and any calibration residual does not mimic the signal. SARAS is an
ongoing experiment that aims to detect the global 21 cm signal. Here we present
the design philosophy of the SARAS 2 system and discuss its performance and
limitations based on laboratory and field measurements. Laboratory tests with
the antenna replaced with a variety of terminations, including a network model
for the antenna impedance, show that the gain calibration and modeling of
internal additives leave no residuals with Fourier amplitudes exceeding 2~mK,
or residual Gaussians of 25 MHz width with amplitudes exceeding 2~mK. Thus,
even accounting for reflection and radiation efficiency losses in the antenna,
the SARAS~2 system is capable of detection of complex 21-cm profiles at the
level predicted by currently favoured models for thermal baryon evolution.Comment: 44 pages, 17 figures; comments and suggestions are welcom
Ice/frost detection using millimeter wave radiometry
A series of ice detection tests was performed on the shuttle external tank (ET) and on ET target samples using a 35/95 GHz instrumentation radiometer. Ice was formed using liquid nitrogen and water spray inside a test enclosure containing ET spray on foam insulation samples. During cryogenic fueling operations prior to the shuttle orbiter engine firing tests, ice was formed with freon and water over a one meter square section of the ET LOX tank. Data analysis was performed on the ice signatures, collected by the radiometer, using Georgia Tech computing facilities. Data analysis technique developed include: ice signature images of scanned ET target; pixel temperature contour plots; time correlation of target data with ice present versus no ice formation; and ice signature radiometric temperature statistical data, i.e., mean, variance, and standard deviation
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