Research on sea surface temperature retrieval by the one-dimensional synthetic aperture microwave radiometer, 1D-SAMR

Due to the low spatial resolution of sea surface temperature(T_S)retrieval by real aperture microwave radiometers,in this study,an iterative retrieval method that minimizes the differences between brightness temperature(T_B)measured and modeled was used to retrieve sea surface temperature with a one-dimensional synthetic aperture microwave radiometer,temporarily named 1 D-SAMR.Regarding the configuration of the radiometer,an angular resolution of 0.43°was reached by theoretical calculation.Experiments on sea surface temperature retrieval were carried out with ideal parameters;the results show that the main factors affecting the retrieval accuracy of sea surface temperature are the accuracy of radiometer calibration and the precision of auxiliary geophysical parameters.In the case of no auxiliary parameter errors,the greatest error in retrieved sea surface temperature is obtained at low T_S scene(i.e.,0.7106 K for the incidence angle of 35°under the radiometer calibration accuracy of0.5 K).While errors on auxiliary parameters are assumed to follow a Gaussian distribution,the greatest error on retrieved sea surface temperature was 1.3305 K at an incidence angle of 65°in poorly known sea surface wind speed(W)(the error on W of 1.0 m/s)over high W scene,for the radiometer calibration accuracy of 0.5 K.

Spatial-temporal variations in near-surface soil freeze-thaw cycles in the source region of the Yellow River during the period 2002–2011 based on the Advanced Microwave Scanning Radiometer for the Earth Observing System(AMSR-E) data

Detecting near-surface soil freeze-thaw cycles in high-altitude cold regions is important for understanding the Earth's surface system, but such studies are rare. In this study, we detected the spatial-temporal variations in near-surface soil freeze-thaw cycles in the source region of the Yellow River(SRYR) during the period 2002–2011 based on data from the Advanced Microwave Scanning Radiometer for the Earth Observing System(AMSR-E). Moreover, the trends of onset dates and durations of the soil freeze-thaw cycles under different stages were also analyzed. Results showed that the thresholds of daytime and nighttime brightness temperatures of the freeze-thaw algorithm for the SRYR were 257.59 and 261.28 K, respectively. At the spatial scale, the daily frozen surface(DFS) area and the daily surface freeze-thaw cycle surface(DFTS) area decreased by 0.08% and 0.25%, respectively, and the daily thawed surface(DTS) area increased by 0.36%. At the temporal scale, the dates of the onset of thawing and complete thawing advanced by 3.10(±1.4) and 2.46(±1.4) days, respectively; and the dates of the onset of freezing and complete freezing were delayed by 0.9(±1.4) and 1.6(±1.1) days, respectively. The duration of thawing increased by 0.72(±0.21) day/a and the duration of freezing decreased by 0.52(±0.26) day/a. In conclusion, increases in the annual minimum temperature and winter air temperature are the main factors for the advanced thawing and delayed freezing and for the increase in the duration of thawing and the decrease in the duration of freezing in the SRYR.

Verification and recalibration of HY-2A microwave radiometer brightness temperature

HY-2A is the first one of the Chinese HY-2 ocean satellite series carrying a microwave radiometer(RM)to measure sea surface temperature,sea surface wind speed,atmospheric water vapor,cloud liquid water content,and rain rate.We verified the RM level 1B brightness temperature(T B)to retrieve environmental parameters.In the verification,TB that simulated using the ocean-atmosphere radiative transfer model(RTM)was used as a reference.The total bias and total standard deviation(SD)of the RM level 1B TB,with reference to the RTM simulation,ranged-20.6-4.38 K and 0.7-2.93 K,respectively.We found that both the total bias and the total SD depend on the frequency and polarization,although the values for ascending and descending passes are different.In addition,substantial seasonal variation of the bias was found at all channels.The verification results indicate the RM has some problems regarding calibration,e.g.,correction of antenna spillover and antenna physical emission,especially for the 18.7-GHz channel.Based on error analyses,a statistical recalibration algorithm was designed and recalibration was performed for the RM level 1B TB.Validation of the recalibrated TB indicated that the quality of the recalibrated RM level 1B TB was improved significantly.The bias of the recalibrated T B at all channels was reduced to<0.4 K,seasonal variation was almost eradicated,and SD was diminished(i.e.,the SD of the 18.7-GHz channel was reduced by more than 0.5K).

0-10 KM TEMPERATURE AND HUMIDITY PROFILES RETRIEVAL FROM GROUND-BASED MICROWAVE RADIOMETER

Deviation exists between measured and simulated microwave radiometer sounding data. The bias results in low-accuracy atmospheric temperature and humidity profiles simulated by Back Propagation artificial neural network models. This paper evaluated a retrieving atmospheric temperature and humidity profiles method by adopting an input data adjustment-based Back Propagation artificial neural networks model. First, the sounding data acquired at a Nanjing meteorological site in June 2014 were inputted into the Mono RTM Radiative transfer model to simulate atmospheric downwelling radiance at the 22 spectral channels from 22.234 GHz to 58.8 GHz, and we performed a comparison and analysis of the real observed data; an adjustment model for the measured microwave radiometer sounding data was built. Second, we simulated the sounding data of the 22 channels using the sounding data acquired at the site from 2011 to 2013. Based on the simulated rightness temperature data and the sounding data, BP neural network-based models were trained for the retrieval of atmospheric temperature, water vapor density and relative humidity profiles. Finally, we applied the adjustment model to the microwave radiometer sounding data collected in July 2014, generating the corrected data. After that, we inputted the corrected data into the BP neural network regression model to predict the atmospheric temperature, vapor density and relative humidity profile at 58 high levels from 0 to 10 km. We evaluated our model's effect by comparing its output with the real measured data and the microwave radiometer's own second-level product. The experiments showed that the inversion model improves atmospheric temperature and humidity profile retrieval accuracy; the atmospheric temperature RMS error is between 1 K and 2.0 K; the water vapor density's RMS error is between 0.2 g/m^3 and 1.93 g/m3; and the relative humidity's RMS error is between 2.5% and 18.6%.

Retrieval of Tropospheric CO Profiles Using Correlation Radiometer:I.Retrieval Experiments for a Clear Atmosphere

This paper discusses the retrieval scheme associated with the gas correlated rariometer MOPITT which will be on board of EOS-AM1 to measure the global vertical profiles of carbon monoxide. The vertical resolution and retrieval errors caused by errors in the temperatureprofiles and in the surface temperature have been assessed. The main results are:a. Assuming the noise equivalent radiance (NER) of 1 .8×105 Wm-2 sr-1, the surface temperature can be deduced from the wide band signals with uncertainty less than 1 K, and theatmospheric term of t he modulated signal can be deduced with errors almost equal to the NERwhich does not significantly increase errors in the retrieved CO profiles.b. With typical uncertainty in temperature profiles, errors in the retrieved profiles at latitudes lower than 70° are generally less than 20% with the first guess of 100 ppbv. (If a betterfirst guess was used, the errors may decrease).c. By incorporating the total column CO amonut derived from the reflected solar radiationin 2.3 tim spectral region into the retrieval,the accuracy of the retrieved CO profile below 6 kmma y be greatly improved.d. In the retrieval experiment with 10 CO profiles representing the typical CO profiles, ther. m.s. relative/absolute errors of the retrieved CO profiles are about 10%/15-20 ppbv.

Retrieval of Tropospheric CO Profiles Using Correlation Radiometer.Ⅱ: Effects of Other Gases and the Retrieval in Cloudy Atmosphere

The effects of methane, ozone, water vapor and nitrous oxide on the retrieval of tropospheric CO profiles using correlatinn radiometer have been assessed. The scheme of the retrieval in the presence of solid clouds have been proposed. The effect of methane and nitrous oxide can be well accounted by their mean profile, and that of ozone can be represented by a typical middle latitude ozone profile, while for water vapor, less than 50% uncertainty is required. With the assumption of blackbody for cloud surface, the CO profile may be retrieved for low and middle solid clouds.However, the retrieval of CO profile will lose quality for high clouds.

Sensitivity Investigation and Optimization for Solar Irradiance Absolute Radiometer

Since the calibration accuracy decreases with the reduction of irradiance power,the nonlinearity of sensitivity is experimentally investigated and corrected in order to improve the precision of calibration for Solar Irradiance Absolute Radiometer( SIAR). The optimization method is proposed based on the interpolation approximation principle. The real-time correction sensitivities are repeatedly measured at each radiant power.The relative system error of sensitivity nonlinearity is 2.2%. The impact of sensitivity nonlinearity is analyzed.The radiant powers are measured by the optimization method and the traditional method. The comparison experiment results illustrate that the optimization method improves the measure accuracy to 0. 1%. The sensitivity nonlinearity is compensated by the real-time correction. Thus,the optimization method significantly improves the calibration precision of ground-based comparison experiment. The investigation of sensitivity nonlinearity also provides the theoretical and experimental basis for optimizing the on-orbit calibration precision of Total Solar Irradiance Monitor( TSIM).

Analysis of radiometer effect on proof mass in purely gravitational orbit

Spacecrafts with the pure gravity environment are of great significance in precision navigation,gravity field measurement for celestial bodies,and basic physics experiments.The radiometer effect is one of the important interfering factors on the proof mass in a purely gravitational orbit.For the gravity field measurement system based on the inner-formation flying,the relationship between the radiometer effect on the innersatellite and the system parameters is studied by analytical and numerical methods.An approximate function of the radiometer effect suitable for the engineering computation and the correction factor are obtained.The analytic results show that the radiometer effect on the inner-satellite is proportional to the average pressure while inversely proportional to the average temperature in the outer-satellite cavity.The radiometer effect increases with the temperature difference in the cavity,and its minimum exists when the cavity radius increases.When the minimum of the radiometer effect arrives,the ratio of the cavity radius to the inner-satellite radius is 1.189 4.This constant is determined by the spherical cavity configuration and independent of the temperature and pressure distributions.When the ratio of the cavity radius to the inner-satellite radius is more than 10,it is believed that the cavity is large enough,the radiometer effect is approximately proportional to the square of the inner-satellite radius,and the influence of the outer-satellite cavity radius on the radiometer effect can be ignored.

TWO NEW TYPES OF MICROWAVE RADIOMETER: A COMPUTER GAIN COMPENSATIVE DESIGN AND A REAL-TIME CALIBRATED DESIGN

Long term stability is the main factor that influences the minimum detectable signal of microwave radiometers. Two new types of microwave radiometer were studied: a computer gain compensative microwave radiometer and a real-time calibrated microwave radiometer. The long term stability of both designs was optimal because they were insensitive to system gain fluctuations. The continuous calibrated microwave radiometer was also insensitive to system noise fluctuations. The minimum detectable signals were 0.13 and 0.19K respectively under an integration time of 0.6s.

Sea surface temperature retrieval based on simulated microwave polarimetric measurements of a one-dimensional synthetic aperture microwave radiometer

Compared with traditional real aperture microwave radiometers,one-dimensional synthetic aperture microwave radiometers have higher spatial resolution.In this paper,we proposed to retrieve sea surface temperature using a one-dimensional synthetic aperture microwave radiometer that operates at frequencies of 6.9 GHz,10.65 GHz,18.7 GHz and 23.8 GHz at multiple incidence angles.We used the ERA5 reanalysis data provided by the European Centre for Medium-Range Weather Forecasts and a radiation transmission forward model to calculate the model brightness temperature.The brightness temperature measured by the spaceborne one-dimensional synthetic aperture microwave radiometer was simulated by adding Gaussian noise to the model brightness temperature.Then,a backpropagation(BP)neural network algorithm,a random forest(RF)algorithm and two multiple linear regression algorithms(RE1 and RE2)were developed to retrieve sea surface temperature from the measured brightness temperature within the incidence angle range of 0°-65°.The results show that the retrieval errors of the four algorithms increase with the increasing Gaussian noise.The BP achieves the lowest retrieval errors at all incidence angles.The retrieval error of the RE1 and RE2 decrease first and then increase with the incidence angle and the retrieval error of the RF is contrary to that of RE1 and RE2.

CALCULATION OF THE SPATIAL RESOLUTION OF A SYNTHETIC APERTURE MICROWAVE RADIOMETER

The synthetic aperture microwave radiometer (SAMR) has been proved to be an interesting system for the remote sensing of the earth from space with a high spatial resolution. This paper presents the formulas for computing the spatial resolution of the SAMR. Some practical examples are also presented, and it is proved that the spatial resolution becomes more inferior with the increment of the off-nadir angle.

Retrieval of Sea Surface Wind Speed by One-Dimensional Synthetic Aperture Microwave Radiometer

One-dimensional synthetic aperture microwave radiometers have higher spatial resolution and record measurements at multiple incidence angles.In this paper,we propose a multiple linear regression method to retrieve sea surface wind speed at an incidence angle between 0°65°.We assume that a one-dimensional synthetic aperture microwave radiometer operates at frequencies of 6.9,10.65,18.7,23.8 and 36.5 GHz.Then,the microwave radiative transfer forward model is used to simulate the measured brightness temperatures.The sensitivity of the brightness temperatures at 0°65°to the sea surface wind speed is calculated.Then,vertical polarization channels(VR),horizontal polarization channels(HR)and all channels(AR)are used to retrieve the sea surface wind speed via a multiple linear regression algorithm at 0°65°,and the relationship between the retrieval error and incidence angle is obtained.The results are as follows:(1)The sensitivity of the vertical polarization brightness temperature to the sea surface wind speed is smaller than that of the horizontal polarization.(2)The retrieval error increases with Gaussian noise.The retrieval error of VR first increases and then decreases with increasing incidence angle,the retrieval error of HR gradually decreases with increasing incidence angle,and the retrieval error of AR first decreases and then increases with increasing incidence angle.(3)The retrieval error of AR is the lowest and it is necessary to retrieve the sea surface wind speed at a larger incidence angle for AR.

MICROWAVE RADIOMETER AND ITS APPLICATION TO SNOW MEASUREMENT

The microwave radiactivproperty of snow is studied using self-developed three band microwave radiometers. Experiments were carried out repeatedly from 1986 to 1989 in Northeast China The relation between brightness temperature and observation angle was obtained with both vertical and horizontal polarizations. The curves corresponding brightness temperature and snow depth were also drawn out for different frequency bands. The results demonstrate the application potential of the multifrequency microwave radiometer in this area.

High-Resolution Radiometer for Remote Sensing of Solar Flare Activity from Low Earth Orbit Satellites

Solar flares, intense bursts of radiation, can disrupt the atmosphere and potentially affect communication, navigation and electrical systems. A newly developed miniaturised microwave radiometer used on a space-borne platform should offer astronomers unprecedented understanding of the largest explosive phenomena in our solar system. In this paper the activity and results of the EU funded research project FLARES are presented. Objective of FLARES has been the study, analysis and design of millimetre-wave system-on-chip (SoC) radiometers for space-borne detection of solar flares. Thanks to the proposed methodology, the power consumption and encumbrance (volume, weight) of the instrument can be sensibly reduced when compared to the devices currently in operation for observing and studying solar flares. In particular, the proposed SoC Dicke radiometer can achieve a ten-time better resolution. This in turn, allows detecting solar flares having relatively low intensity, about 100 times lower when compared to the flares currently detected by the existing systems, owing to space-borne operations and the microchip-level miniaturization through silicon technology under space qualification.

Application Concept of Zero Method Measurement in Microwave Radiometers

This article examined in detail microwave radiometer functioning algorithm with synchronously using of the two types of pulse modulation: amplitude pulse modulation and pulse-width modulation. This allows a zero-radiometer measurement method to realize when the fluctuation effect of the receiver gain and the influence of its own noise changes are minimized. A zero balance automatically maintains in radiometer. The antenna signal is indirectly determined through the signal duration that controls the pulse-width modulation. An analytical expression of the fluctuation sensitivity was obtained in a general form. From its analysis gain in sensitivity, conditions were defined by the optimizing of the radiometer input knot’s construction. Three modifications of the radiometer input knot were researched. Fluctuation sensitivity at different measurement range was determined for modification of the radiometer input knot.

The development of semiconductor detectors for radiometers of alpha‑radiation and the examination of the volumetric activity of radon in various areas

Purpose Development of highly sensitive semiconductor detectors of large diameter and manufacturing of a measuring complex—a radiometer based on the developed detectors for studying the activity of alpha particles and the volumetric activity of radon in various media.Methods The detectors were manufactured using surface-barrier and heterojunction technologies.Polishing etchant formulations for silicon have been developed.To obtain plane-parallelism of the plates during chemical etching,a special dynamic setup was used.The structure of the radiometer,electrical circuits,and device software have been developed.Results The results of the development of technology for the manufacture of detectors of large dimensions(30-100 mm in diameter)are presented.Studies of the electrical and radiometric characteristics of surface-barrier n detectors and detectors based on Al-αGe-pSi-Au heterojunctions were carried out.The principle of operation of the electronic components of a radiometer made using these detectors is also given.Conclusion The data of monitoring of radon content in soil air are given.Monitoring results showed that the concentration varies depending on temperature,humidity,and time of day.The GSM/SMS module allows the device to operate in real time.

A Thorough Evaluation of the Passive Microwave Radiometer Measurements onboard Three Fengyun-3 Satellites

Microwave Radiometer Imager(MWRI) is a key payload of China’s second generation polar meteorological satellite, i.e., Fengyun-3 series(FY-3). Up to now, 5 satellites including FY-3A(2008), FY-3B(2010), FY-3C(2013), FY-3D(2018), and FY-3E(2021) have been launched successfully to provide multiwavelength, all-weather, and global data for decades. Much progress has been made on the calibration of MWRI and a recalibrated MWRI brightness temperature(BT) product(V2) was recently released. This study thoroughly evaluates the accuracy of this new product from FY-3B, 3C, and 3D by using the simultaneous collocated Global Precipitation Measurement(GPM)Microwave Imager(GMI) measurements as a reference. The results show that the mean biases(MBEs) of the BT between MWRI and GMI are generally less than 0.5 K and the root mean squares(RMSs) between them are less than1.5 K. The previous notable ascending and descending difference of the MWRI has disappeared. This indicates that the new MWRI recalibration procedure is very effective in removing potential errors associated with the emission of the hot-load reflector. Analysis of the dependence of MBE on the latitude and earth scene temperature shows that MBE decreases with decreasing latitude over ocean. Furthermore, MBE over ocean decreases linearly with increasing scene temperature for almost all channels, whereas this does not occur over land. A linear regression fitting is then used to modify MWRI, which can reduce the MBE over ocean to be within 0.2 K. The standard deviation of error of GMI, FY-3B, and FY-3D MWRI BT data derived by using the three-cornered hat method(TCH) shows that GMI has the best overall performance over ocean except at 10.65 GHz where its standard deviation of error is slightly larger than that of FY-3D. Over land, the standard deviation of error of FY-3D is the lowest at almost all channels except at 89V. MWRI onboard FY-3 series satellites would serve as an important passive microwave radiometer member of the constellation to monitor key surface and atmospheric properties.

Estimating global land surface broadband thermal-infrared emissivityusing advanced very high resolution radiometer optical data

An algorithm for retrieving global eight-day 5 km broadband emissivity (BBE)from advanced very high resolution radiometer (AVHRR) visible and nearinfrared data from 1981 through 1999 was presented. Land surface was dividedinto three types according to its normalized difference vegetation index (NDVI)values: bare soil, vegetated area, and transition zone. For each type, BBE at813.5 mm was formulated as a nonlinear function of AVHRR reflectance forChannels 1 and 2. Given difficulties in validating coarse emissivity products withground measurements, the algorithm was cross-validated by comparing retrievedBBE with BBE derived through different methods. Retrieved BBE was initiallycompared with BBE derived from moderate-resolution imaging spectroradiometer (MODIS) albedos. Respective absolute bias and root-mean-square errorwere less than 0.003 and 0.014 for bare soil, less than 0.002 and 0.011 fortransition zones, and 0.002 and 0.005 for vegetated areas. Retrieved BBE wasalso compared with BBE obtained through the NDVI threshold method. Theproposed algorithm was better than the NDVI threshold method, particularly forbare soil. Finally, retrieved BBE and BBE derived from MODIS data wereconsistent, as were the two BBE values.

A new algorithm for microwave radiometer remote sensing of sea surface salinity and temperature

The microwave radiation of the sea surface, which is denoted by the sea surface brightness temperature, is not only related with sea surface salinity (SSS) and temperature (SST), but also influenced by sea surface wind. The errors of wind detected by satellite sensor have significant influences on the accuracy of SSS and SST retrieval. The effects of sea surface wind on sea surface brightness temperature, i.e. △Th,v, and the relations among △Th,v, wind speed, sea surface tempera- ture, sea surface salinity and incidence angle of observation are investigated. Based on the investi- gations, a new algorithm depending on the design of a single radiometer with double polarizations and multi-incidence angles is proposed. The algorithm excludes the influence of sea surface wind on SSS and SST retrieval, and provides a new method for remote sensing of SSS and SST.

Absolute cryogenic radiometer for high accuracy optical radiant power measurement in a wide spectral range

An absolute cryogenic radiometer(ACR) with a detachable optical window was designed and built for high accuracy optical radiant power measurement and photodetector spectral responsivity calibration. The ACR receiver is an electroplated pure copper cavity with a 50-μm-thick wall and inner surface coated with a specular black polymer material mixed with highly dispersible carbon nanotubes. The absorptivity of the cavity receivers was evaluated to be ≥0.9999 in the 250 nm–16 μm wavelength range and ≥0.99995 in 500 nm–16 μm. The cavity receiver works at the temperature of ~5.2 K with nanowatt-level noise-equivalent power. The relative standard uncertainty is 0.041% for the measurement of ~100 μW optical radiant power(250 nm–16 μm) and 0.015% for^1 m W(500 nm–16 μm).