Antenna Temperature Research Articles

Research on Antenna Temperature Contrast of Metal Solid Objects

It is necessary to study the radiation characteristic of metal solid objects for millimeter wave passive guidance. On basis of discussing the grounded theory, the antenna temperature contrast formula of metal solid objects is presented. Furthermore equivalent radiometric section coefficient based on scale-shrinking measuring theory is proposed in favor of engineering applications. The data error between 8mm theoretical calculation and actual measurement are mostly below 1K. So equivalent radiometric section coefficient gives a virtual way for engineering calculation and measurement of metal solid objects.

Jason Microwave Radiometer Performance and On-Orbit Calibration

Results are presented from the on-orbit calibration of the Jason Microwave Radiometer (JMR). The JMR brightness temperatures (TBs) are calibrated at the hottest and coldest ends of the instrument's dynamic range, using Amazon rain forest and vicarious cold on-Earth theoretical brightness temperature references. The retrieved path delay values are validated using collocated TOPEX Microwave Radiometer and Radiosonde Observation path delay (PD) values. Offsets of 1–4 K in the JMR TBs and 8–12 mm in the JMR PDs, relative to TMR measurements, were initially observed. There were also initial TB offsets of 2 K between the satellite's yaw state. The calibration was adjusted by tuning coefficients in the antenna temperature calibration algorithm and the antenna pattern correction algorithm. The calibrated path delay values are demonstrated to have no significant bias or scale errors with consistent performance in all nonprecipitating weather conditions. The uncertainty of the individual path delay measurements is estimated to be 0.74 cm ± 0.15, which exceeds the mission goal of 1.2 cm RMS.

Thirty-Element Active Antenna Array as a Prototype of a Huge Low-Frequency Radio Telescope

An effective wide-band (10 to 60 MHz) active antenna element has been developed. The cost of one short (3 m), thin dipole with built-in amplifier and metal construction is less than 45 euro. It was shown both theoretically and experimentally that the upper limiting frequency is at least 60 MHz, the dynamic range is 90 dB/μV and the share of the amplifier noise to the background antenna temperature is about 10%. The developed active dipole was tested by building a 30-element antenna array and comparing its parameters with one of the subpart of the UTR-2 radio telescope having passive dipoles of 8.6 m in length and 1.8 m in diameter. The 3C461 ionospheric scintillation spectra observed in the experiments show that the sensitivities and noise-immunities of both antennas are close. This proves the availability using of a short cheap active dipole in new generation giant radio telescopes.

Temperature compensation of total power radiometers

This paper describes a new technique to compensate output variations of total power radiometers due to physical temperature changes of the instrument. This technique performs the correction without the addition of expensive microwave hardware required in Dicke switching or many other widely used methods. A characterization period, over which the input antenna temperature is known, indicates the appropriate output adjustment needed for a change in physical temperature of the radiometer. The method effectively corrects the output in an example radiometer system built with inexpensive commercially available parts. For a 30-K variation in physical temperature, the measured data shows an improvement from 60-K peak-to-peak error to 6.9 K with an average absolute error of 1.1 K.

Submillimeter Wave Astronomy SatelliteMapping Observations of Water Vapor around Sagittarius B2

Observations of the 110-101 556.936 GHz transition of ortho-water with the Submillimeter Wave Astronomy Satellite have revealed the presence of widespread emission and absorption by water vapor around the strong submillimeter continuum source Sagittarius B2. An incompletely sampled spectral line map of a region of size 26' × 19' around Sgr B2 reveals three noteworthy features. First, absorption by foreground water vapor is detectable at local standard of rest (LSR) velocities in the range -100 to 0 km s-1 at almost every observed position. Second, spatially extended emission by water is detectable at LSR velocities in the range 80-120 km s-1 at almost every observed position. This emission is attributable to the 180 pc molecular ring identified from previous observations of CO. The typical peak antenna temperature of 0.075 K for this component implies a typical water abundance of (1.2-8) × 10-6 relative to H2. Third, strong absorption by water is observed within 5' of Sgr B2 at LSR velocities in the range 60-82 km s-1. An analysis of this absorption yields a HO abundance ~(2-4) × 10-7 relative to H2 if the absorbing water vapor is located within the core of Sgr B2 itself, or, alternatively, a water column density of ~(2.5-4) × 1016 cm-2 if the water absorption originates in the warm, foreground layer of gas proposed previously as the origin of ammonia absorption observed toward Sgr B2.

Microwave brightness temperature prediction of plane targets by a neural network

Studies on microwave radiation of many targets, such as air, ocean, ice, snow, vegetation, rock, sand, and so on, lead to the radiometric models of the targets. The model uses one or more formulas to represent the radiation of one target. A neural network (NN) is introduced to represent the antenna temperature (AT) or brightness temperature (BT) of the seven types of plane targets: water, concrete road, asphalt road, loess, grassland, crushed stone, and vegetation. The same NN can simulate the relationship of AT (or BT) to observation angle, surface temperature, and polarization of the seven types of plane targets. The agreement between the prediction of NN and the measured AT (or inverted BT) shows that the same NN can give good prediction of the AT (or BT) of the seven types of plane targets.

Design of a compact C-band receive-only horn for Earth station antenna G/T/sub A/ performance

The optimization of compact C-band corrugated horns to replace an existing C-band feed system while maintaining the overall G/T/sub A/ performance of the associated Cassegrain antenna is presented. G (dBi) is the gain of the antenna and T/sub A/ (K) is the antenna temperature. The cost-saving over the original system is discussed.

Simultaneous analysis of downlink beacon dynamics and sky brightness temperature. Part I. Remote sensing of a slant path

This paper investigates the remote sensing of height and displacement velocity of turbulent clouds, composed mainly of liquid water, using a combination of simultaneous radiometric and geostationary satellite beacon measurements. Because a radiometer integrates the sky brightness temperature distribution observed within the solid angle subtended by the antenna beamwidth, the paper shows how this process can be modeled as a low-pass filter using the small angle approximation and the frozen-in hypothesis. The 3-dB cutoff frequency of that filter is used in conjunction with the Fresnel frequency of the amplitude scintillations power spectrum to derive the cross-path wind speed and the height of the turbulent layer/cloud. In order to quantify the 3-dB cutoff frequency of the filter-radiometer, a dual aperture radiometer was constructed and was used in conjunction with a ground station receiver monitoring the 39.5-GHz beacon transmissions of ITALSAT F1 and measuring amplitude scintillations. The experimental results derived on cloud heights are then compared with the vertical profiles obtained by a 94-GHz radar, indicating agreement except in those cases where the thickness of the cloud structure becomes important. This aspect is then discussed in the paper and a radiometric definition is given for the cloud height. Finally, the important aspect of instrumentation is described including the effect of the radiation pattern of a radiometer antenna upon the cross correlation and cross spectrum between two coaxial measured antenna temperatures.

Anisotropy of Hectometric Cosmic Background

The results of investigation of the low-frequency cosmic background by the WIND spacecraft near the L1 point during the deepest phase of the last solar activity minimum are presented. The antenna temperature modulation index and the ecliptic longitude of the primary area of radiation for frequencies 260, 516, 772, and 1028 kHz were measured by means of a dipole on the spacecraft which is rotated in ecliptic plane. Our results correct and complement the conclusions of the previous measurements made with IMP-6.

Post-perihelion SWAS Observations of Water Vapor in the Coma of Comet C/1999 H1 (Lee)

We have monitored the 1 10–1 01 pure rotational transition of water vapor near 557 GHz toward Comet C/1999 H1 (Lee) over the period 1999 September 5 through 1999 December 8, using the Submillimeter Wave Astronomy Satellite. The measured antenna temperatures implied water production rates that declined from 3.2×10 28s −1 on 1999 September 5–8 to 1.2×10 28s −1 on 1999 September 25–28. No statistically significant detections of water emission were obtained after 1999 September 28. These post-perihelion observations are consistent with a steady decline in the water production rate, Q(H 2O), with increasing heliocentric distance, R h , that can be fit with a power law Q(H 2O)=1.45×10 29( R h /AU) −5.5s −1 over the range R h =1.3–1.7 AU.

Centaurus A: Molecular gas shells or large-scale outflow?

In order to test if the molecular \shells observed by Charmandaris et al. (2000) could be due to a molecular outflow, we have mapped CO inJ =1 ! 0a nd 2! 1 along the jet axis of Centaurus A. Where our map coincides with their observed positions, like them, we obtain 4 detections with a similar antenna temperature for CO 1 ! 0, although both transitions appear to be somewhat wider in velocity dispersion than theirs. As well as these, we have several tentative detections at distances of >5 kpc from the nucleus, and although these results are of too poor quality in order to verify or refute the shell model in favour of a molecular outflow, our observations of the inner kpc do suggest that at least a small-scale outflow is a possibility. Whether this would be a component of a larger-scale outflow or exists in conjunction with the molecular gas shells will have to wait for a much more extensive mapping of the large-scale gas distribution in Centaurus A, for which these and the results of Charmandaris et al. (2000) will hopefully provide sucient motivation.

Power spectrum of the polarized diffuse Galactic radio emission

We have analyzed the available polarization surveys of the Galactic emission to estimate to what extent it may be a serious hindrance to forthcoming experiments aimed at detecting the polarized component of Cosmic Microwave Background (CMB) anisotropies. Regions were identied for which independent data consistently indicate that Faraday depolarization may be small. The power spectrum of the polarized emission, in terms of antenna temperature, was found to be described by C P ' ' (1:2 0:8) 10 9 ('=450) 1:80:3 (=2: 4G Hz) 5:8 K 2 , from arcminute to degree scales. Data on larger angular scales (' 100) indicate a steeper slope' 3 . We conclude that polarized Galactic emission is unlikely to be a serious limitation to CMB polarization measurements at the highest frequencies of the MAP and Planck-LFI instruments, at least for ' 50 and standard cosmological models. The weak correlation between polarization and total power and the low polarization degree of radio emission close to the Galactic plane is interpreted as due to large contributions to the observed intensity from unpolarized sources, primarily strong Hii regions, concentrated on the Galactic plane. Thus estimates of the power spectrum of total intensity at low Galactic latitudes are not representative of the spatial distribution of Galactic emission far from the plane. Both total power and polarized emissions show highly signicant deviations from a Gaussian distribution.

Recalibration of the NOAA microwave sounding unit

The microwave sounding unit (MSU) prelaunch thermal‐vacuum chamber test data for eight MSU flight models that flew on TIROS‐N, NOAA 7 through NOAA 12, and NOAA 14 were reanalyzed using an improved calibration algorithm, originally designed for the advanced microwave sounding unit‐A (AMSU‐A) operations. The new calibration algorithm can automatically adjust for any channel gain shift in operation. Adoption of this calibration algorithm as the MSU calibration procedure in the recalibration project will make the data sets from the MSU and the AMSU‐A more consistent. This will be useful for future blending of the climate trends generated from the MSU and AMSU‐A data. A single nonlinearity parameter u, which appears in the new calibration algorithm, was obtained for each channel from analysis of the prelaunch calibration test data. A software package for implementing this new calibration algorithm was developed and applied to calculate the MSU time series for improvement of the accuracy of the climate record. Sample calculations of MSU antenna temperatures with the new calibration algorithm were made for two satellites and are compared with similar results obtained from the old MSU calibration algorithm. Significant differences are observed, and strong evidence indicates that the new calibration procedure will provide a more accurate quantification of climate trends.

Search for CO isotopes in cores of massive molecular clouds

Using the newly installed SIS receiving system of the 13.7 m telescope of PMO, the CO isotope line pair, 13CO and C 18O J = 1 − 0, were observed in a search for dense cores in 64 IRAS sources with high color indices or H 2O masers. Maps were obtained for some of the sources. The line pair was detected in all the target sources. The emission of 13CO J = 1 − 0 is strong showing that the emission regions are dense. The pair lines of the CO isotopes were measured for the first time in at least 60 sources. Antenna temperatures, center velocities and line widths are obtained with Gaussian fitting. Preliminary analyses were made for the emission intensities and line profiles.

Methods and constraints for the correction of the error beam pick-up in single dish radio observations

The beam pattern of a single dish radio telescope is given by the main beam and additional components at larger angles, usually called error beam or stray pattern. The latter have relatively small peak amplitudes (typ. below -25 dB), depending on the rms surface error of the primary reflector. However, because of their large angular extent, they are sensitive to extended sources, and a significant fraction of the observed intensity can result from error beam pick-up. For (sub-)mm observations suffering from error beam pick-up we introduce a new temperature scale for the corrected data, the corrected main beam brightness temperature T mb,c , which provides a better approximation to the intensity detected by the main beam than the commonly used antenna temperature and main beam brightness temperature . We consider two different correction methods. The first method uses complementary observations obtained with a smaller telescope. Smeared to the angular resolution of the error beam pattern they provide an estimate of the error beam pick-up in the observations of the large telescope. For the second method, the error beam pick-up is de-convolved from the observed map in Fourier space. The requirements for both correction methods and their advantages and limitations are discussed in detail. Both correction methods require additional observations, unless the full spatial extent of the emission is observed. We find that the de-convolution method is attractive for the correction of fully sampled maps with an angular extent much larger than the error beam pattern. For smaller maps and more sparsely sampled observations, the subtraction method is favorable, because the additional observations with a small telescope are less time consuming.

ITAL]Submillimeter Wave Astronomy Satellite[/ITAL] Observations of Water Vapor toward Comet C/1999 H1 (Lee)

We have detected the 110-101 pure rotational transition of water vapor toward comet C/1999 H1 using the Submillimeter Wave Astronomy Satellite. Over the period 1999 May 19.01-23.69 UT, the average integrated antenna temperature was 1.79 ± 0.03 K km s-1 within a 33 × 45 (FWHM) elliptical beam. For an assumed ortho-to-para ratio of 3, we estimate the total water production rate as 8 × 1028 s-1. This value lies approximately 50% above the value estimated by Biver et al. from contemporaneous radio observations of hydroxyl molecules. The observed line width of 1.8 km s-1 (FWHM) is broader than the instrumental profile and suggests an intrinsic line width of about 1.4 km s-1 (FWHM). The data, taken during a portion of every 97 minute spacecraft orbit over a 4.68 day period, provide no evidence for variability.

The Search for Primordial3He Using the 8.665 GHz Hyperfine Spin‐Flip Transition of3He+

Frequency-switched observations of the 8.665 GHz hyperfine spin-flip transition of 3He+ have been made in eight Galactic H II regions in both the inner and outer Galaxy. For sources in the outer Galaxy, the observations reported here show no evidence for 3He+ to levels well below the level of detections claimed by Bania et al. using the position-switching technique. There is strong evidence that weak terrestrial interference is the explanation for the variability and relatively high level of 3He+ previously reported in several outer Galaxy sources. In the inner Galaxy, 3He+ is detected in W43, M17S, and W51 at abundance levels similar to those reported by Balser et al. with 3He+/H+~ (1-2) × 10-5. Contrary to what has been reported previously, the 3He+/H+ abundances are found to decrease rapidly with increasing distance from the Galactic center and fall below detectable levels by the Galactic radius of W49 (~8 kpc). This fall-off in second-generation 3He abundance with distance from the Galactic center is similar to what is predicted by the Pd = 0 model of Galli et al. but is at least a factor of 5 underabundant. The upper limit to the antenna temperatures obtained here for the 3He+ lines in Orion A, W49, and S209 gives an upper limit for the abundance of primordial 3He that is an order of magnitude below the upper limit suggested by Bania et al. This leads to a lower limit for the nucleon-to-photon ratio η that is at least an order of magnitude above the narrow limits set by 7Li and deuterium. Possible explanations for this are that there is a larger than expected uncertainty in determining 3He abundances or that some mechanism is destroying primordial 3He in the Galaxy over a Hubble time.

A microwave radiometer with an extended signal measurement range

A microwave radiometer for measuring high effective antenna temperatures using the modified zero receive method is described. This radiometer controls the input signal by adjusting the duration of the reference noise signal. Based on this signal duration, an antenna signal is determined. The modification of the radiometers is analyzed, and two signal combinations are chosen for modulation in the radiometer receiver, which is characterized by a linear depednence of the pulse-duration modulated reference signal on the antenna signal, allowing one to measure signals in different ranges. Block diagrams of the radiometer input and control units are given, allowing one to double the measurement range by an automatic selection of the modulation signal combinations without a decrease in accuracy.

A Super‐Alfvenic Model of Dark Clouds

Supersonic random motions are observed in dark clouds and are traditionally interpreted as Alfven waves, but the possibility that these motions are super-Alfvenic has not been ruled out. In this work we report the results of numerical experiments in two opposite regimes: A ~ 1 and A 1, where A is the initial Alfvenic Mach number—the ratio of the rms velocity to the Alfven speed. Our results show that models with A 1 are consistent with the observed properties of molecular clouds that we have tested (statistics of extinction measurements, distribution of integrated antenna temperature, Zeeman-splitting measurements of magnetic field strength, line width versus integrated antenna temperature of molecular emission-line spectra, statistical B-n relation, and scatter in that relation), while models withA ~ 1 have properties that are in conflict with the observations. We find that both the density and the magnetic field in molecular clouds may be very intermittent. The statistical distributions of the magnetic field and gas density are related by a power law, with an index that decreases with time in experiments with decaying turbulence. After about one dynamical time it stabilizes at B ∝ n0.4. Magnetically dominated cores form early in the evolution, while later on the intermittency in the density field wins out, and also cores with a weak field can be generated by mass accretion along magnetic field lines.

Supersonic Turbulence in the Perseus Molecular Cloud

We compare the statistical properties of J=1-0 13CO spectra observed in the Perseus Molecular Cloud with synthetic J=1-0 13CO spectra, computed solving the non-LTE radiative transfer problem for a model cloud obtained as solutions of the three dimensional magneto-hydrodynamic (MHD) equations. The model cloud is a randomly forced super-Alfvenic and highly super-sonic turbulent isothermal flow. The purpose of the present work is to test if idealized turbulent flows, without self-gravity, stellar radiation, stellar outflows, or any other effect of star formation, are inconsistent or not with statistical properties of star forming molecular clouds. We present several statistical results that demonstrate remarkable similarity between real data and the synthetic cloud. Statistical properties of molecular clouds like Perseus are appropriately described by random super-sonic and super-Alfvenic MHD flows. Although the description of gravity and stellar radiation are essential to understand the formation of single protostars and the effects of star formation in the cloud dynamics, the overall description of the cloud and of the initial conditions for star formation can apparently be provided on intermediate scales without accounting for gravity, stellar radiation, and a detailed modeling of stellar outflows. We also show that the relation between equivalent line width and integrated antenna temperature indicates the presence of a relatively strong magnetic field in the core B1, in agreement with Zeeman splitting measurements.