Antenna Temperature Research Articles

Normality Analysis for RFI Detection in Microwave Radiometry

Radio-frequency interference (RFI) present in microwave radiometry measurements leads to erroneous radiometric results. Sources of RFI include spurious signals and harmonics from lower frequency bands, spread-spectrum signals overlapping the “protected” band of operation, or out-of-band emissions not properly rejected by the pre-detection filters due to its finite rejection. The presence of RFI in the radiometric signal modifies the detected power and therefore the estimated antenna temperature from which the geophysical parameters will be retrieved. In recent years, techniques to detect the presence of RFI in radiometric measurements have been developed. They include time- and/or frequency domain analyses, or time and/or frequency domain statistical analysis of the received signal which, in the absence of RFI, must be a zero-mean Gaussian process. Statistical analyses performed to date include the calculation of the Kurtosis, and the Shapiro-Wilk normality test of the received signal. Nevertheless, statistical analysis of the received signal could be more extensive, as reported in the Statistics literature. The objective of this work is the study of the performance of a number of normality tests encountered in the Statistics literature when applied to the detection of the presence of RFI in the radiometric signal, which is Gaussian by nature. A description of the normality tests and the RFI detection results for different kinds of RFI are presented in view of determining an omnibus test that can deal with the blind spots of the currently used methods.

Redshifted intergalacticHe+38.7 GHz hyperfine absorption

Motivated by recent interest in redshifted 21 cm emission of intergalactic hydrogen, we investigate the 8.7 GHz ^2S_{1/2} F=0-1 hyperfine transition of ^3He^+. While the primordial abundance of 3He relative to hydrogen is 10^-5, the hyperfine spontaneous decay rate is 680 times larger. Furthermore, the antenna temperature is much lower at the frequencies relevant for the ^3He^+ transition compared to that of z>6 21 cm emission. We find that the spin temperature of this 8.7 GHz line in the intergalactic medium is approximately the cosmic microwave background temperature, such that this transition is best observed in absorption against high-redshift, radio-bright quasars. We show that intergalactic 8.7 GHz absorption is a promising, unsaturated observable of the ionization history of intergalactic helium (for which HeII->HeIII reionization is believed to be complete at z~3) and of the primordial 3He abundance. Instruments must reach ~1 \mu Jy RMS noise in bands of 1 MHz on a 1 Jy source to directly resolve this absorption. However, in combination with HI Ly\alpha forest measurements, an instrument can statistically detect this absorption from z > 3 with 30 \mu Jy RMS noise in 0.1 MHz spectral bands over 100 MHz, which may be within the reach of present instruments.

RFI Mitigation in Microwave Radiometry Using Wavelets

The performance of microwave radiometers can be seriously degraded by the presence of radio-frequency interference (RFI). Spurious signals and harmonics from lower frequency bands, spread-spectrum signals overlapping the “protected” band of operation, or out-of-band emissions not properly rejected by the pre-detection filters due to the finite rejection modify the detected power and the estimated antenna temperature from which the geophysical parameters will be retrieved. In recent years, techniques to detect the presence of RFI have been developed. They include time- and/or frequency domain analyses, or statistical analysis of the received signal which, in the absence of RFI, must be a zero-mean Gaussian process. Current mitigation techniques are mostly based on blanking in the time and/or frequency domains where RFI has been detected. However, in some geographical areas, RFI is so persistent in time that is not possible to acquire RFI-free radiometric data. In other applications such as sea surface salinity retrieval, where the sensitivity of the brightness temperature to salinity is weak, small amounts of RFI are also very difficult to detect and mitigate. In this work a wavelet-based technique is proposed to mitigate RFI (cancel RFI as much as possible). The interfering signal is estimated by using the powerful denoising capabilities of the wavelet transform. The estimated RFI signal is then subtracted from the received signal and a “cleaned” noise signal is obtained, from which the power is estimated later. The algorithm performance as a function of the threshold type, and the threshold selection method, the decomposition level, the wavelet type and the interferenceto-noise ratio is presented. Computational requirements are evaluated in terms of quantization levels, number of operations, memory requirements (sequence length). Even though they are high for today’s technology, the algorithms presented can be applied to recorded data. The results show that even RFI much larger than the noise signal can be very effectively mitigated, well below the noise level.

TRENDS IN MOLECULAR EMISSION FROM DIFFERENT EXTRAGALACTIC STELLAR INITIAL MASS FUNCTIONS

Banerji et al. (2009) suggested that top-heavy stellar Initial Mass Functions (IMFs) in galaxies may arise when the interstellar physical conditions inhibit low-mass star formation, and they determined the physical conditions under which this suppression may or may not occur. In this work, we explore the sensitivity of the chemistry of interstellar gas under a wide range of conditions. We use these results to predict the relative velocity-integrated antenna temperatures of the CO rotational spectrum for several models of high redshift active galaxies which may produce both top-heavy and unbiased IMFs. We find that while active galaxies with solar metallicity (and top-heavy IMFs) produce higher antenna temperatures than those with sub-solar metallicity (and unbiased IMFs) the actual rotational distribution is similar. The high-J to peak CO ratio however may be used to roughly infer the metallicity of a galaxy provided we know whether it is active or quiescent. The metallicity strongly influences the shape of the IMF. High order CO transitions are also found to provide a good diagnostic for high far-UV intensity and low metallicity counterparts of Milky Way type systems both of which show some evidence for having top-heavy IMFs. We also compute the relative abundances of molecules known to be effective tracers of high density gas in these galaxy models. We find that the molecules CO and CS may be used to distinguish between solar and sub-solar metallicity in active galaxies at high redshift whereas HCN, HNC and CN are found to be relatively insensitive to the IMF shape at the large visual magnitudes typically associated with extragalactic sources.

RE-EXAMINING LARSON'S SCALING RELATIONSHIPS IN GALACTIC MOLECULAR CLOUDS

The properties of Galactic molecular clouds tabulated by Solomon etal (1987) (SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey of 13CO J=1-0 emission. These new data provide a lower opacity tracer of molecular clouds and improved angular and spectral resolution than previous surveys of molecular line emission along the Galactic Plane. We calculate GMC masses within the SRBY cloud boundaries assuming LTE conditions throughout the cloud and a constant H2 to 13CO abundance, while accounting for the variation of the 12C/13C with Galacto-centric radius. The LTE derived masses are typically five times smaller than the SRBY virial masses. The corresponding median mass surface density of molecular hydrogen for this sample is 42 Msun/pc^2, which is significantly lower than the value derived by SRBY (median 206 Msun/pc^2) that has been widely adopted by most models of cloud evolution and star formation. This discrepancy arises from both the extrapolation by SRBY of velocity dispersion, size, and CO luminosity to the 1K antenna temperature isophote that likely overestimates the GMC masses and our assumption of constant 13CO abundance over the projected area of each cloud. Owing to the uncertainty of molecular abundances in the envelopes of clouds, the mass surface density of giant molecular clouds could be larger than the values derived from our 13CO measurements. From velocity dispersions derived from the 13CO data, we find that the coefficient of the cloud structure functions, vo=sigma_v/R^{1/2}, is not constant, as required to satisfy Larson's scaling relationships, but rather systematically varies with the surface density of the cloud as Sigma^{0.5} as expected for clouds in self-gravitational equlibrium.

Numerical simulation on antenna temperature field of complex structure satellite in solar simulator

A thermal network model is developed for studying the temperature variation of complicated structure satellite surfaces. The solar incident areas, the infrared and solar radiation transfer coefficients among surfaces are numerically simulated by means of Monte Carlo ray tracing (MCRT) method in model. The non-uniformity and the instability of solar radiation, which plays the important roles in simulating outer-space heat flux designation parameters by solar simulator, are studied for analysis variation of antenna temperature fields in detail. Results showed non-uniformity irradiation effects are greater than those of instability for this kind of geometry sheltering structure.

Assessments of F16 Special Sensor Microwave Imager and Sounder Antenna Temperatures at Lower Atmospheric Sounding Channels

The main reflector of the Special Sensor Microwave Imager/Sounder (SSMIS) aboard the Defense Meteorological Satellite Program (DMSP) F-16 satellite emits variable radiation, and the SSMIS warm calibration load is intruded by direct and indirect solar radiation. These contamination sources produce antenna brightness temperature anomalies of around 2 K at SSMIS sounding channels which are obviously inappropriate for assimilation into numerical weather prediction models and remote sensing retrievals of atmospheric and surface parameters. In this study, antenna brightness temperature anomalies at several lower atmospheric sounding (LAS) channels are assessed, and the algorithm is developed for corrections of these antenna temperature anomalies. When compared against radiative transfer model simulations and simultaneous observations from AMSU-A aboard NOAA-16, the SSMIS antenna temperatures at 52.8, 53.6, 54.4, 55.5, 57.3, and 59.4 GHz after the anomaly correction exhibit small residual errors (<0.5 K). After such SSMIS antenna temperatures are applied to the National Center for Environmental Prediction Numerical Weather Prediction (NWP) model, more satellite data is used and the analysis field of the geopotential height is significantly improved throughout troposphere and lower stratosphere. Therefore, the SSMIS antenna temperatures after the anomaly correction have demonstrated their potentials in NWP models.

Brightness-Temperature Retrieval Methods in Synthetic Aperture Radiometers

Brightness-temperature retrieval techniques for synthetic aperture radiometers are reviewed. Three different approaches to combine measured visibility and antenna temperatures, along with instrument characterization data, into a general equation to invert are presented. Discretization and windowing techniques are briefly discussed, and formulas for reciprocal grids using rectangular and hexagonal samplings are given. Two known techniques are used to invert the equation, namely, inverse Fourier transform and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> -matrix pseudoinverse. The proposed preprocessing approaches combined with these two inversion methods are implemented with real data measured by an airborne Y-shaped interferometric radiometer over land and water, and are compared. The images indicate that best results are obtained when inverting an incremental visibility obtained after substracting a term that includes the individual antenna temperatures, the physical temperatures of the receivers, and a flat-target response directly measured from cold-sky looks.

Assessment of the topography impact on microwave radiometry at L‐band

The TuRTLE 2006 field experiment aimed at evaluating, from a ground‐based point of view, the topography impact on microwave radiometry at L‐band. Observations of a mountainous area 50 km north of Barcelona, Spain, were acquired using the fully polarimetric radiometer LAURA (1.4135 GHz working frequency). The look angle was changed in elevation and azimuth to observe a wide area which included almost bare surfaces, shrubs growing at the valley descending slope, and mixed woodland present at the ascending slope of the mountain in front of LAURA (at the other side of the valley). Ground‐truth soil moisture and temperature, vegetation water content, and meteorological parameters were also sampled. Radiometric measurements have been compared to antenna temperatures simulated using a facet model which takes into account the high‐resolution digital elevation model and the land cover map of the experiment site, the polarization mixing due to surface tilting, and the radiometer's antenna radiation pattern. Four different scenarios have been simulated: (1) a hypothetical scenario without vegetation and without topography, (2) a hypothetical scenario without topography but with the actual land cover map of the experiment site, (3) a hypothetical scenario without vegetation but with the digital elevation map of the experiment site, and (4) the actual experiment site, including both the digital elevation model and the land cover map. Simulations using a facet model agreed with measurements only when both topography and the land cover map were considered. The largest discrepancies occur for almost bare soils at large local incidence angles, which suggests that further modeling work is still needed for this case.

Water and ammonia abundances in S140 with the Odin satellite

We have used the Odin satellite to obtain strip maps of the ground-state rotational transitions of ortho-water and ortho-ammonia, as well as CO(5-4) and 13CO(5-4) across the PDR, and H218O in the central position. A physi-chemical inhomogeneous PDR model was used to compute the temperature and abundance distributions for water, ammonia and CO. A multi-zone escape probability method then calculated the level populations and intensity distributions. These results are compared to a homogeneous model computed with an enhanced version of the RADEX code. H2O, NH3 and 13CO show emission from an extended PDR with a narrow line width of ~3 kms. Like CO, the water line profile is dominated by outflow emission, however, mainly in the red wing. The PDR model suggests that the water emission mainly arises from the surfaces of optically thick, high density clumps with n(H2)>10^6 cm^-3 and a clump water abundance, with respect to H2, of 5x10^-8. The mean water abundance in the PDR is 5x10^-9, and between ~2x10^-8 -- 2x10^-7 in the outflow derived from a simple two-level approximation. Ammonia is also observed in the extended clumpy PDR, likely from the same high density and warm clumps as water. The average ammonia abundance is about the same as for water: 4x10^-9 and 8x10^-9 given by the PDR model and RADEX, respectively. The similarity of water and ammonia PDR emission is also seen in the almost identical line profiles observed close to the bright rim. Around the central position, ammonia also shows some outflow emission although weaker than water in the red wing. Predictions of the H2O(110-101) and (111-000) antenna temperatures across the PDR are estimated with our PDR model for the forthcoming observations with the Herschel Space Observatory.

Resolution enhancement for microwave‐based atmospheric sounding from geostationary orbits

The purpose of this study is to develop and evaluate techniques that improve the spatial resolution of the channels already selected in the preliminary studies for Geostationary Observatory for Microwave Atmospheric Soundings (GOMAS). Reference high resolution multifrequency brightness temperatures scenarios have been derived by applying radiative transfer calculation to the spatially and microphysically detailed output of meteorological events simulated by the University of Wisconsin–Nonhydrostatic Model System. Three approaches, Wiener filter, Super‐resolution and Image‐fusion have been applied to some representative GOMAS frequency channels to enhance the resolution of antenna temperatures. The Wiener filter improved resolution of the largely oversampled images by a factor 1.5–2.0 without introducing any penalty in the radiometric accuracy. Super‐resolution, suitable for not largely oversampled images, improved resolution by a factor ∼1.5 but introducing an increased radiometric noise by a factor 1.4–2.5. The Image‐fusion allows finally to further increase the spatial frequency of the images obtained by the Wiener filter increasing the total resolution up to a factor 5.0 with an increased radiometric noise closely linked to the radiometric frequency and to the examined case study.

Search for spatial and spectral fluctuations of cosmic microwave background radiation with the ratan-600 radio telescope. The 2001–2006 observations

We report some of the results of the search for narrow-band spatial and spectral fluctuations of cosmic microwave background at the wavelength of 6.2 cmperformed with the RATAN-600 radio telescope in 2001–2006 in two 35′ × 7′ strips on the sky in the vicinity of the North Celestial Pole. We find the spectra of spatial fluctuations in the 12 MHz radio-frequency band and in the interval of spatial periods from 4′ to 16′ to exhibit power-law rises with exponents reaching −2.0±0.5, with a periodicity of 2–3 MHz. We also find two narrow-band (in terms of angular frequency) features at 4870.4 and 4871.5 MHz with the corresponding fluctuation amplitudes of 5±0.5 mK in terms of antenna temperature in the vicinity of angular periods of about 5′ with the frequency bandwidths of about 600 kHz. Standard tests performed using the spectra of the half-sum and half-difference of two groups of observations randomly drawn from a total sample of 23 records of the March 2002 observing set confirm the reality of the features of the angular spectrumof fluctuations mentioned above and so does the comparison with the spectra of cold matched load connected to the receiver input instead of the antenna. However, the nature of the features found remains unclear. Our attempt to link this radiation to rotational transitions 2Π1/2, J = 5/2 of the CH molecule, which has one of the components of its multiplet located inside the frequency interval of interest considered failed.

Brightness Temperature Reconstruction Using BGI

This paper departs from the popular usage of the Backus-Gilbert inversion (BGI) method as a tool for inversion of antenna temperature measurements in microwave radiometry. The BGI method is applied in this paper to enhance the information content of an existing set of oversampled brightness-temperature (TB) data. The purpose is to isolate the inversion process from its resolution enhancement counterpart. The advantage gained is that the resolution enhancement can be performed in a simplified way and in a different level of processing that starts with the scan-mode TB data product and simply requires with it the knowledge of the antenna gain pattern and the sensor's scan geometry. The technique is demonstrated with the 19.35-GHz Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) channel, which provides oversampled TB data. The radiometric resemblance of this channel with that of the 37 GHz and geocollocation of their TB footprints facilitate validation of the enhancement of features. The significance of oversampling the low-frequency (LF) radiometer channels is underscored in the process, which gives the authors the confidence to propose oversampling of the LF data for the forthcoming sensor Microwave Analysis and Detection of Rain and Atmospheric Structures (MADRAS) onboard the Megha_Tropiques mission, which is a joint ISRO-CNES collaboration (due for launch in 2009).

Basic Element for Square Kilometer Array Training (BEST): Evaluation of the Antenna Noise Temperature

In this paper, a rigorous procedure for calculating the antenna noise temperature is described, and applied to the antenna of BEST-1 (basic element for SKA training - version 1), which represents one of the SKA (square kilometer array) demonstrators. The SKA will be a new-generation radio telescope, with a collecting area 50 times larger than the area of today's largest radio telescope. BEST is the Italian reduced-scale SKA demonstrator, based on the re-instrumentation of about 8000 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of the Northern Cross radio telescope, built with cylindrical parabolic antennas. In order to perform the antenna-temperature analysis, an electromagnetic tool to accurately evaluate the antenna pattern in the whole space surrounding the antenna itself is required. We used the commercial software GRASP8, developed by TICRA, to characterize reflector antennas. The antenna temperature was evaluated using the guideline adopted by the Antenna Task Force of the SKA world consortium. For BEST-1 at 408 MHz, we found an antenna temperature equal to 30 K in the zenith direction and 60 K at the horizon. The numerical results have been verified through several celestial calibration radio sources.

Intercalibration Between Special Sensor Microwave Imager/Sounder and Special Sensor Microwave Imager

The F16 satellite was successfully launched on October 18, 2003, carrying the first special sensor microwave imager/sounder (SSMIS) onboard. In this paper, the SSMIS imaging channels 12-18 are intercalibated against the F15 special sensor microwave/imager (SSM/I) instrument using simultaneous conical overpassing (SCO) observations from both satellites. Results show that the SSMIS antenna temperatures have a mean bias as large as 1-2 K with a maximum of 3 K at 22.235 GHz with respect to F15. It appears that the mean biases at frequencies from 19.35 to 37 GHz do not strongly depend on the region and season, although the biases at the 91.655-GHz channels are slightly variable. The intercalibration analysis also shows that the nonlinearity may be one of the major sources resulting in differences between F15 SSM/I and F16 SSMIS measurements. For improved calibration and for the future SSM/I and SSMIS reprocessing, the SCO data are further utilized to resolve the SSMIS and SSM/I nonlinearity terms using a newly developed calibration algorithm. With the derived nonlinearity correction, the mean biases of the antenna temperatures between F15 and F16 are significantly reduced. To intercalibrate SSMIS to the same reference as SSM/I, SSMIS imaging channels can also be linearly mapped to the same and similar F15 SSM/I channels using the SCO matchup data. After the linear mapping, SSMIS snow-free land, snow, and sea ice surface emissivities are consistent with those derived from SSM/I.

Earth-Viewing L-Band Radiometer Sensing of Sea Surface Scattered Celestial Sky Radiation—Part I: General Characteristics

The ldquogalactic glitterrdquo phenomenon at L-band, i.e., the scattering of celestial sky radiation by the rough ocean surface, is examined here as a potential source of error for sea surface salinity (SSS) remote sensing. We begin by considering the transformations that must be applied to downwelling celestial noise in order to compute the eventual impact on the antenna temperature. Then, outside the context of any particular measurement system, we use approximate scattering models along with a model for the equilibrium wind wave spectrum to examine how the scattered signal at the surface might depend on the geophysical conditions and scattering geometry. It is found that, when the specular point lies far away from the galactic plane, where the incident celestial brightness is uniform, sea surface roughness has a negligible impact on the glitter. At such a point, variations in both the orientation of the incidence plane and the wind direction relative to the scattering azimuth have negligible impact. By contrast, when the specular point lies in the vicinity of a localized maximum of brightness, scattering by the roughened ocean surface may reduce the glitter by more than 30%, as compared to a perfectly flat surface, and the glitter amplitude may vary by up to 0.7 K with variations in wind direction and by up to 0.5 K with variations in incidence plane orientation. It is shown that accounting for the roughness impact on celestial noise contamination is of particular concern for the remote sensing of SSS.

Stokes Antenna Temperatures

The growing importance of polarimetric radiometers has led to the need for a detailed theory for Stokes antenna temperatures. In this paper, we provide a full Stokes vector formulation of an antenna temperature that accounts for the entire antenna pattern, which includes polarization mixing in the main-beam and sidelobe effects. To derive the Stokes antenna temperatures, we follow the conventional methods in the Earth remote sensing literature while relying on a coherency algebra approach from radio astronomy. Connections and parallels to the conventional approaches are noted along the way. We also introduce generalizations of beam efficiency and cross polarization for use with polarimetric radiometers. These provide important metrics in the design of future systems.

A study of lunar contamination and on‐orbit performance of the NOAA 18 Advanced Microwave Sounding Unit–A

An algorithm for detection and correction of the lunar contamination in the Advanced Microwave Sounding Unit–A (AMSU‐A) data is applied to study the observed lunar contamination in the NOAA 18 AMSU‐A data. The algorithm is based on a physical model of the lunar surface brightness temperature and the AMSU‐A antenna pattern powers to detect the lunar contamination in cold space calibration counts over a large number of scans. The algorithm performs a scan‐by‐scan detection and correction of the effect of lunar contamination on the AMSU‐A data. It is found that the lunar contamination is significant only when the separation angle between the lunar disk and the antenna space viewing is less than 4°, beyond which no significant lunar contamination is detected, as the AMSU‐A antenna power drops below 40 dB from its peak. The parameters of the AMSU‐A antenna pattern powers are determined from least squares fit to the observed lunar contamination counts extracted from the cold space counts. Using the best fit parameters, we investigated the effect of the lunar contamination on the AMSU‐A scene temperatures. It is found that the differences ΔTS between the near‐nadir (field of view, FOV 16) scene antenna temperatures calculated with and without correction of lunar contamination vary with channels. It is about 1.5 K over ocean at channels 1 and 2 but only 0.38 K at channel 4. This trend in the ΔTS variation as a function of channels is easily understood by examining the two‐point calibration equation. The results presented in this study show that the lunar contamination in the AMSU‐A space calibration counts can be accurately detected and that its effect on the measured scene antenna temperatures can be corrected. The algorithm provides a practical approach for scan‐by‐scan correction of the lunar contamination in AMSU‐A data and improves the accuracy of operational calibration of NOAA Level 1B data. An assessment of the postlaunch instrument performance is also presented.

On the antenna beam shape reconstruction using planet transit

The calibration of the in-flight antenna beam shape and possible beamdegradation is one of the most crucial tasks for the upcoming Planck mission. We examine several effects which could significantly influence the in-flight main beam calibration using planet transit: the problems of the variability of the Jupiter's flux, the antenna temperature and passing of the planets through the main beam. We estimate these effects on the antenna beam shape calibration and calculate the limits on the main beam and far sidelobe measurements, using observations of Jupiter and Saturn. We also discuss possible effects of degradation of the mirror surfaces and specify corresponding parameters which can help us to determine these effects.

Effects of the Antenna Aperture on Remote Sensing of Sea Surface Salinity at L-Band

Remote sensing of sea surface salinity can be performed by means of microwave radiometry at L-band, but it requires high radiometric accuracy (e.g., on the order of 0.1 K). Since the variability of salinity in the open ocean exhibits large spatial scales and long temporal scales, it is possible to use antennas with large footprints and averaging to meet this goal. However, antennas with large footprints introduce other problems such as variations of the incidence angle and direction of the polarization vectors over the footprint. Examples of these effects are computed here using antennas that are representative of those that will be flown on the Aquarius/SAC-D mission being developed for remote sensing of salinity from space. It is shown that the antenna temperature (i.e., integrated over the antenna pattern) is biased relative to the value at boresight. In part, this is due to change in incidence angles across the field of view. Polarization mixing, because of the variations of the local plane of incidence across the footprint, also induces bias (peculiarly for the third Stokes parameter). Finally, large antenna footprints limit how close to land measurements can be made.