Tropospheric Effects Research Articles

An observation operator for ground-based GPS slant delays

An observation operator for the Global Positioning System (GPS) Slant Total Delay (STD) observations is developed. The operator is implemented in the High Resolution Limited Area Model (HIRLAM) for data assimilation of tropospheric humidity information contained in the geodetic estimates of microwave signal phase delays.The observation operator integrates refractivity along the signal path between the satellite and the receiver to obtain STD. The signal path is solved using the geometrical path, as given by the geometrical azimuth and zenith angles of the satellite as viewed at the receiver, as the starting point. The tropospheric effect on the signal path is accounted for.A preliminary validation of the operator has been performed against observational data. Model counterparts are compared with the geodetic estimates of Zenith Total Delay (ZTD) and STD. The bias and standard deviation between modelled and observed STD are −1.0 and 1.7 cm, respectively, at zenith, and −7.2 and 8.6 cm, respectively, at zenithangle 80◦.

Performance of Fade Mitigation Techniques in a Fixed 48 GHz HAPS Network

The 48 GHz band has been allocated world-wide for fixed service with HAPS. High-capacity broadband access networks can be implemented in this band. The main drawback is the troposphere effects on the propagation, particularly rain attenuation, that may limit the link availability. In this paper, the performance of HAPS-ground links working in this band is analysed by means of simulation. Experimental slant-path propagation data, gathered at 49.5 GHz, are used in the simulations, so that attenuation levels and their temporal evolution may be considered realistic. The simulated system is defined in ITU-R Rec. F-1500. Only urban coverage links are simulated, as the propagation data correspond to a 40° elevation link. The use of Fade Mitigation Techniques (FMT) is shown to be useful to improve the link performance. The improvement is evaluated for two different FMTs: Power control and adaptive coding. Results are presented regarding link availability and other parameters related to the particular FMT, as well as outage dynamics.

Ambient levels of UV radiation

The levels of solar ultraviolet (UV) radiation at the Earth's surface vary widely, depending on the atmospheric and environmental conditions of the observation site. Under clear sky conditions, the most important parameters are solar zenith angle, total ozone content, amount and type of aerosols, altitude above sea level and albedo of the ground [1]. In addition, attenuation and in special cases amplification by clouds has to be considered. Based on detailed measurements under a large number of different conditions, the effects of the individual parameters on global UV irradiance are quantified. Most of these effects depend strongly on wavelength and many of the effects are most pronounced in the erythemally weighted UV-range. The effects of the most important parameters are: 1% decrease in ozone results in an increase of erythemally weighted UV irradiance of about 1.1% [2]; aerosols can attenuate erythemally weighted UV irradiance by up to 30%, but this depends strongly on amount and type of aerosols [3]; the increase of erythemally weighted UV irradiance with an increase in altitude of 1000 m amounts between 15% and 25%, for UVA-radiation this increase is about half that much [4]; an increase of ground albedo by 0.5 will increase erythemally weighted irradiance by about 17%, UVA irradiance by about 10% [5]; a complete cloud cover attenuates UV irradiance about 40% less than total irradiance [6]. Long-term changes of solar UV radiation are a complex combination of various effects, where stratospheric ozone depletion, increase of tropospheric ozone levels and effects of global climate change work together. Therefore predictions for future levels of UV radiation are limited to the assumptions of specific scenarios. Future measurements will be necessary to monitor any changes on a local and on a global scale.

Effect of the troposphere on surface neutron counter measurements

Surface neutron counter data are often used as a proxy for atmospheric ionisation from cosmic rays in studies of extraterrestrial effects on climate. Neutron counter instrumentation was developed in the 1950s and relationships between neutron counts, ionisation and meteorological conditions were investigated thoroughly using the techniques available at the time; the analysis can now be extended using modern data. Whilst surface neutron counts are shown to be a good proxy for ionisation rate, the usual meteorological correction applied to surface neutron measurements, using surface atmospheric pressure, does not completely compensate for tropospheric effects on neutron data. Residual correlations remain between neutron counts, atmospheric pressure and geopotential height, obtained from meteorological reanalysis data. These correlations may be caused by variations in the height and temperature of the atmospheric layer at ∼100 hPa. This is where the primary cosmic rays interact with atmospheric air, producing a cascade of secondary ionising particles.

Accurate measurements of tropospheric effects in volcanic areas from SAR interferometry data: application to Sakurajima volcano (Japan)

The accuracy of surface deformation maps derived from DinSAR (differential interferomery synthetic aperture radar) in volcanic areas is limited by temporal variations of the vertical refractivity profile in the troposphere between the radar data acquisitions. Due to their dependence on altitude, such variations may produce topography-related fringes in the computed interferograms. The variations should be carefully estimated in a given area so that they can be discriminated with real ground deformation signals. In this paper, we present a clear example of tropospheric induced fringes (reaching up to 2.9 fringes over 1200 m), evidenced on Sakurajima volcano (Japan) from ERS-1 and ERS-2 data. The analysis of phase–altitude correlations based on network adjustments of the interferometric dataset leads to a simple linear model of the tropospheric excess path delay (with an rms accuracy of 0.4 cm) that might be used for further corrections. A more detailed investigation of these phase variations providing accurate measurements of linear and non-linear contributions of tropospheric effects is also presented and shows that tropospheric phase delays can be better modeled on Sakurajima with a non-linear model.

GPS PHASE MEASURE CYCLE-SLIP DETECTING AND GPS BASELINE RESOLUTION BASED ON WAVELET TRANSFORM

The errors in GPS measurement generally consist of systematic errors (such as clock bias, iono - sphere, troposphere effects etc.), random errors (such as measuring error) and outliers. Systematic errors can be canceled by differencing techniques or by adding parameters into the equation system. Outliers may be detected by adding parameters or by statistical methods such as expectation shifting or variance inflating.Because wavelet analysis has many good natures both in the time domain and in the frequency domain, as can automatically zoom in or out with different scales (frequencies), the arbitrary details of a signal can be observed and analyzed by the aid of wavelet analysis. Based on the above features, wavelet analysis is reputed as “a mathematical microscope”.The paper presents a wavelet based outlier detecting and filtering algorithm. GPS phase observables are decomposed into different components by wavelet transform. The content at scale 1 mainly consists of high frequency components including cycle-slips or outliers. Because the amount of cycle-slips is zoomed in by wavelet transform at the original location, so the locations of cycle slips are easily detected after wavelet transform.Based on the properties of outliers, cycle slips and random noises under wavelet transform, a socalled á trouS reconstruction algorithm is selected. The noises in double difference phase observables are filtrated by removing some components from scale-land reconstructing original signal. Then an algorithm for GPS base-line resolution using phase double difference based on wavelet filtering is put forward in this paper. By using the proposed algorithm, the ambiguity searching space is reduced, and the validation of fixed ambiguities is improved as well. The efficiency for the proposed algorithm is approved by three examples.

GPS PHASE MEASURE CYCLE-SLIP DETECTING AND GPS BASELINE RESOLUTION BASED ON WAVELET TRANSFORM

The errors in GPS measurement generally consist of systematic errors (such as clock bias, iono - sphere, troposphere effects etc.), random errors (such as measuring error) and outliers. Systematic errors can be canceled by differencing techniques or by adding parameters into the equation system. Outliers may be detected by adding parameters or by statistical methods such as expectation shifting or variance inflating.Because wavelet analysis has many good natures both in the time domain and in the frequency domain, as can automatically zoom in or out with different scales (frequencies), the arbitrary details of a signal can be observed and analyzed by the aid of wavelet analysis. Based on the above features, wavelet analysis is reputed as “a mathematical microscope”.The paper presents a wavelet based outlier detecting and filtering algorithm. GPS phase observables are decomposed into different components by wavelet transform. The content at scale 1 mainly consists of high frequency components including cycle-slips or outliers. Because the amount of cycle-slips is zoomed in by wavelet transform at the original location, so the locations of cycle slips are easily detected after wavelet transform.Based on the properties of outliers, cycle slips and random noises under wavelet transform, a socalled á trouS reconstruction algorithm is selected. The noises in double difference phase observables are filtrated by removing some components from scale-land reconstructing original signal. Then an algorithm for GPS base-line resolution using phase double difference based on wavelet filtering is put forward in this paper. By using the proposed algorithm, the ambiguity searching space is reduced, and the validation of fixed ambiguities is improved as well. The efficiency for the proposed algorithm is approved by three examples.

Neutral atmospheric delay in interferometric synthetic aperture radar applications: Statistical description and mitigation

Variations in the refractive index of the atmosphere cause variations in satellite‐based interferometric synthetic aperture radar (InSAR) observations. We can mitigate tropospheric effects by averaging N‐independent interferograms. Because the neutral atmosphere is uncorrelated at timescales longer than 1 day, using this technique statistically reduces the variance, σ2, of the noise by a factor of N. Using zenith neutral atmospheric delays from Global Positioning System (GPS) data from the Southern California Integrated GPS Network, we find that the average variance depends on the distance between observations, L, and height difference, H, as σ = c Lα + kH with estimated values for c, α, and k of about 2.5, 0.5, and 4.8, respectively, where σ is in mm and L and H are in km. We expect that the value of α is largely site‐independent but the value of c will depend on the water vapor variability of the area of interest. This model is valid over a range of L between approximately 10 and 800 km. Height differences between 0 and 3 km have been used in this analysis. For distances of 100 and 10 km with negligible height differences, σ is estimated to be approximately 25 and 8 mm, respectively. For a given orbit revisit time and image archive duration, we calculate the number and duration (assumed constant) of interferograms required to achieve a desired sensitivity to deformation rate at a given length scale. Assuming neutral atmosphere is the dominant source of noise, a 30° look angle, and an image revisit time of 7 days, detection of a deformation rate of 1 mm yr−1 over distances of 10 km requires about 2.2 years of continuous observations. Given our results, we suggest a data covariance structure to use when using InSAR data to constrain geophysical models.

Radiometer‐based calibration of the relation between radar reflectivity and microwave attenuation

The use of higher and higher frequencies in telecommunication systems requires the investigation of the effects of troposphere on electromagnetic wave propagation, and particular attention must be paid to the attenuation induced by raindrops along the radio path. For this purpose the direct measurements approach, in many cases, is not feasible or convenient, and the use of indirect measuring technique is now a common practice. In this respect, meteorological radar operating at nonattenuating frequencies can be of great help owing to their ability in collecting data over wide areas in a short time. The drawback is the incomplete knowledge of the relation between radar reflectivity (Z) and the quantity of direct interest so that an external source of “calibration” is required. In this paper we show that radiometers are proper instruments to calibrate single parameter radar and specifically to derive, on event basis, the relation to convert Z into specific attenuation (α). For this purpose, brightness temperatures at 13 GHz collected by a radiometer along the 30.6° slant path toward the Olympus satellite, direct attenuation measurements, and radar reflectivity profiles have been jointly worked out. First, the choice of the average effective medium temperature to estimate attenuation from brightness measurements has been addressed. Second, the best α ‐Z relation on an event basis has been evaluated through a best fit procedure that forced the radar predicted attenuation to agree with radiometer “measured” ones. The results are very encouraging and confirm the possibility to make use of radiometers as an external source of radar calibration.

Arctic Oscillation response to the 1991 Mount Pinatubo eruption: Effects of volcanic aerosols and ozone depletion

Observations show that strong equatorial volcanic eruptions have been followed by a pronounced positive phase of the Arctic Oscillation (AO) for one or two Northern Hemisphere winters. It has been previously assumed that this effect is forced by strengthening of the equator‐to‐pole temperature gradient in the lower stratosphere, caused by aerosol radiative heating in the tropics. To understand atmospheric processes that cause the AO response, we studied the impact of the 1991 Mount Pinatubo eruption, which produced the largest global volcanic aerosol cloud in the twentieth century. A series of control and perturbation experiments were conducted with the GFDL SKYHI general circulation model to examine the evolution of the circulation in the 2 years following the Pinatubo eruption. In one set of perturbation experiments, the full radiative effects of the observed Pinatubo aerosol cloud were included, while in another only the effects of the aerosols in reducing the solar flux in the troposphere were included, and the aerosol heating effects in the stratosphere were suppressed. A third set of perturbation experiments imposed the stratospheric ozone losses observed in the post‐Pinatubo period. We conducted ensembles of four to eight realizations for each case. Forced by aerosols, SKYHI produces a statistically significant positive phase of the AO in winter, as observed. Ozone depletion causes a positive phase of the AO in late winter and early spring by cooling the lower stratosphere in high latitudes, strengthening the polar night jet, and delaying the final warming. A positive phase of the AO was also produced in the experiment with only the tropospheric effect of aerosols, showing that aerosol heating in the lower tropical stratosphere is not necessary to force positive AO response, as was previously assumed. Aerosol‐induced tropospheric cooling in the subtropics decreases the meridional temperature gradient in the winter troposphere between 30°N and 60°N. The corresponding reduction of mean zonal energy and amplitudes of planetary waves in the troposphere decreases wave activity flux into the lower stratosphere. The resulting strengthening of the polar vortex forces a positive phase of the AO. We suggest that this mechanism can also contribute to the observed long‐term AO trend being caused by greenhouse gas increases because they also weaken the tropospheric meridional temperature gradient due to polar amplification of warming.

Anatomy of apparent seasonal variations from GPS‐derived site position time series

Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the “peering” approach. Peering is a way to depict the contributions of the comparatively well‐known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that ∼40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales.

Calibration of atmospheric effects on SAR interferograms by GPS and local atmosphere models: first results

A comparison between the zenith path delay (ZPD), obtained from GPS measurements, and the expected delay, derived from models used to compensate tropospheric effects on SAR interferograms, is made. The results of the two methods are comparable, though the available data set is not large enough for a complete statistical validation of the methods. The results of this preliminary study suggest a possible integration of GPS-based ZPD data with cheap and standard meteorological data, since the estimated atmospheric component proved to be similar. Furthermore, the impact on volcanology of the effects measured by GPS, and in particular on the determination of the depth of the volcanic sources, is discussed.

Determination of semi‐diurnal ocean tide loading constituents using GPS in Alaska

During the past years, the accuracy of relative positioning using differential GPS (DGPS) has been improved significantly. The present accuracy of DGPS allows us to directly estimate the differential amplitudes and Greenwich phase lags of the main semi‐diurnal ocean tide loading constituents (S2, K2, M2 and N2). For this purpose a test is carried out using two GPS stations in Alaska. One station, Chi3, is located on an island in the Gulf of Alaska, while the second station, Fair, is located far away from the coastal areas. Processing hourly GPS solutions for the baseline between Fair and Chi3 during 49 days gives differential amplitudes of 23.21 mm and 4.71 mm for M2 and N2, respectively, while the theoretically differential amplitudes of M2 and N2 are 20.90 mm and 4.21 mm, respectively (using the GOT99.2 ocean tide model). The diurnal ocean tide loading constituents are not considered, because unmodeled troposphere effects increase the noise level near the diurnal frequency band and prevent us from obtaining significant results.

Instantaneous geodetic positioning at medium distances with the Global Positioning System

We evaluate a new method of Global Positioning System (GPS) data analysis, called instantaneous positioning, at spatial scale lengths typical of interstation spacings in a modern crustal motion network. This method is more precise and versatile than traditional GPS static and kinematic processing of multi‐epoch batches of data. The key to instantaneous positioning is the ability to resolve integer‐cycle phase ambiguities with only a single epoch of dual‐frequency phase and pseudorange data, rendering receiver cycle slips irrelevant. We estimate three‐dimensional relative coordinates and atmospheric zenith delay parameters independently every 30 s over a 12‐week period for baseline distances of 50 m, 14 km, and 37 km. Horizontal precision of a single‐epoch coordinate solution is about 15 mm and vertical precision is about 7–8 times worse. Removing that component of each time series which repeats with a period of exactly 1 sidereal day, and thus manifests signal multipath, reduces the scatter by about 50% in all components. Solution averaging of the high‐frequency time series can be performed using any number of measurement epochs to further improve coordinate precision. We demonstrate that the daily coordinates estimated with instantaneous positioning are more precise (by 20–50% per coordinate component) than those estimated with 24‐hour batch processing. Spectral analysis of the single‐epoch solutions indicates that the flicker noise characteristic of GPS time series observed in lower‐frequency bands also affects GPS solutions in the frequency band 0.01 mHz to 10 mHz. We argue that the flicker noise is induced by tropospheric effects. Since modern GPS receivers are capable of observing at frequencies as high as 10 Hz, our technique significantly overlaps and complements the frequency band of broadband seismology and benefits other research areas such as earthquake geodesy, volcanology, and GPS meteorology.

Volcano‐wide fringes in ERS synthetic aperture radar interferograms of Etna (1992–1998): Deformation or tropospheric effect?

Mount Etna (3300 m) is the volcano that has been first and most studied by differential synthetic aperture radar. Previous papers gave evidence for a large‐scale deformation of the entire edifice consistent with unrest episodes but with a poor fit with classical elastic models. Also, atmospheric effects on mountainous areas are known to be very significant. Accordingly, interferograms may reflect both deformation and tropospheric effects. We investigate here the possibility of evaluating and correcting tropospheric effects directly from the interferograms. From 38 ERS synthetic aperture radar ascending scenes taken from 1992 to 1999, we have computed 238 interferograms. The amount of data allows us to analyze how the coherence is maintained over long periods of time and thus how it reveals the location of permanent scatters. From a simple analysis of phase‐elevation relation, we give evidence for the clear difference in pattern between stratified tropospheric effects and large‐scale deformation produced by a source in a Threedimensional elastic body with topography. Using combination of inversion processes, we analyze both deformation source and tropospheric effects for the complete set of 238 interferograms. Finally time evolution of the parameters and modeling probability estimation are retrieved for each SAR image. Estimated relative change in tropospheric delay varies in time from −2.7 to +3.0 (±1.2) fringes. They are consistent with other tropospheric estimations at Etna based on ground met and Global Positioning System observations on the same period of time. Magmatic volume variations remain below error bars and thus cannot be estimated properly. Finally, some of the interferograms may be entirely explained by the difference in tropospheric conditions between the two acquisition periods, without the need to invoking a deformation of the volcano.

A Diagnosis of Tropospheric Effects upon Surface Precipitation Amount for a Sample of East Coast Snowstorms

Abstract Upper-air variables have been correlated with near-simultaneous precipitation observations from 35 winter storms that produced heavy snowfall in the eastern coastal region of the United States. Standard radiosonde observations (raob’s) were used to calculate upper-air variables at mandatory levels, while liquid-equivalent precipitation amounts were gathered from surface reports at five stations within the region. This study seeks to identify those synoptic-scale variables, individually and in combination with other variables that may be useful to estimate short-term precipitation amount. Using both simple and multiple correlation analyses, the selected variables (e.g., thermal and vorticity advections, wind speeds, moisture convergence and advection, divergence, etc.) were correlated with precipitation amounts occurring over 1-, 3-, and 6-h periods near raob times. The best correlated upper-air variables (using the 6-h duration) are 850-mb temperature advection (0.48), 850-mb equivalent potential...

The Role of Clouds, Water Vapor, Circulation, and Boundary Layer Structure in the Sensitivity of the Tropical Climate

The physical mechanisms that affect the tropical sea surface temperature (SST) are investigated using a two-box equilibrium model of the Tropics. One box represents the convecting, warm SST, high humidity region of the Tropics, and the other box represents the subsidence region with low humidity, boundary layer clouds, and cooler SST. The two regions communicate by energy and moisture fluxes that are proportional to the strength of the overturning circulation that couples the two regions. The boundary layer properties in the subsiding region are predicted with a mixing line model. Humidity above the inversion in the subsiding region is predicted from moisture conservation. The humidity above the inversion in the subsiding region increases rapidly with temperature, but this has less effect on the sensitivity than expected, because the inversion lowers as the humidity above the inversion is increased. Some of the increased greenhouse effect of the free troposphere can be offset by decreased greenhouse effect of the boundary layer. Increasing the area of the warm, convective region increases the SSTs, because of the greenhouse effect of the greater upper-tropospheric water vapor in the convective region. The circulation strength is constrained by radiative cooling in the cold pool. The strength of the circulation decreases with increasing convective area, because the increase in dry static stability overwhelms the increase in cooling rate. Although they have strong individual effects on longwave and shortwave radiation, high clouds in the convective region do not affect the tropical SSTs strongly, because their net radiative forcing at the top of the atmosphere is small. Low clouds in the subsidence region have a strong cooling affect on the tropical SST, because they strongly reduce net radiative heating at the top of the atmosphere. A negative feedback is produced if the low clouds are predicted from the observed relationship between stratus cloud amount and lower-tropospheric stability.

Tropospheric corrections of SAR interferograms with strong topography. Application to Etna

The accuracy of spaceborne geodetic techniques, including SAR interferometry, is limited by the time and spatial variation and altitude dependance of the propagation delay of electomagnetic waves in the lower troposphere, particularly in mountainous areas. In this paper, we use a 1D model developed for tropospheric corrections of GPS and DORIS measurements to correct SAR data. The differential tropospheric delay is computed at each pixel of the interferogram from ground temperature, humidity and pressure using two empirical parameters calibrated from several radio‐soundings acquired in various latitude and climate conditions. It is shown that with such a model, given the 3300 meters topography of Etna, tropospheric variations can generate up to 4π phase rotations between the top and the bottom of the volcano. In 16 out of the 20 interferograms processed with images acquired between August 1992 and October 1993, correction of the tropospheric effect reduces the number of fringes associated with the 1991–93 eruption from previous estimates. The remaining deformation is consistant with a deforming source located at a depth of 14±1 km. During the second half of the eruption, the subsidence rate at the top of the volcano is roughly stable at 13±3 mm/month. These values are in good agreement with tiltmeter data collected on Etna during the same period and with the estimated volume of erupted material. No significant deformation can be observed during the last month of eruption. Inflation of the volcano seems to resume immediately after the end of the eruption at a rate of 3 mm/month.

Numerical simulation of compaction: O.Barter et al. (Laboratoire 35, Grenoble, France.)

Atmospheric effects represent one of the major error sources of repeat-pass Interferometric Synthetic Aperture Radar (InSAR), and could mask actual displacements due to tectonic or volcanic deformation. The tropospheric delays vary both vertically and laterally and can be considered as the sum of (i) a vertically stratified component highly correlated with topography and (ii) a turbulent component resulting from turbulent processes in the troposphere varying both in space and time. In this paper, we outline a framework to routinely use pointwise GPS data to reduce tropospheric effects on satellite radar measurements. An Iterative Tropospheric Decomposition (ITD) model is used and further developed to separate tropospheric stratified and turbulent signals and then generate high-resolution correction maps for SAR interferograms. Cross validation is employed to assess the performance of the ITD model and act as an indicator to users of when and where correction is feasible. Tests were carried out to assess the impact of GPS station spacing on the ITD model InSAR correction performance, which provides insights into the trade-off between station spacing and the achievable accuracy. The application of this framework to Sentinel-1A interferograms over the Southern California (USA) and Southern England (UK) regions shows approximately 45–78% of noise reduction even with a sparse (~ 50–80 km station spacing) GPS network and/or with strong and non-random tropospheric turbulence. This is about a 50% greater improvement than previous methods. It is believed that this framework could lead to a generic InSAR atmospheric correction model while incorporating continuous and global tropospheric delay datasets, e.g. numerical weather models.

Some studies on attenuation and atmospheric water vapour measurements in India

The rainrate distributions obtained from rapid response rain gauge measurements over five different locations in India are presented. On the basis of simultaneous observations of rainrate measured by rain gauges having integration times of 10 seconds and 15 minutes, a power law has been established. Rain attenuation characteristics at 11GHz were analysed and some interesting results are presented, which are also compared with International Radio Consultative Committee and other models. Attenuations due to water vapour are deduced for a frequency range of 1-350GHz at elevation angles 3 and 10 . The attenuation due to water vapour at a lower elevation angle is found to increase as the tropospheric effects become dominant. It can also be seen that the antenna temperature attains a peak value of about 160K in the monsoon months and a minimum of about 80 K during the winter. The monthly variation of antenna temperature reveals a similar variation for the attenuation and water vapour content.