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A new method of retrieving water vapor content using ground-based rdiometer with single band
A NEW M E T H O D OF RETRIEVING WATER VAPOR CONTENT USING GROUND-BASED R A D I O M E T E R WITH SINGLE BAND Shun-Peng Shih and Yen-Hsyang Chu
Institute of Space Science, National Central University, Chung-Li, Taiwan, R.O.C.
ABSTRACT Using a single-frequency ground-based microwave radiometer with steerable horn antenna at frequency 19.5 GHz, we measure sky radiometric temperature over Taiwan area during the period from May 1997 to September 1999. A method of retrieving integrated precipitable water vapor content (IPWV) from the observed brightness temperature made by a single-frequency radiometer at 19.5 GHz is developed and examined. Comparing the retrieved and measured IPWVs shows a good agreement between them, validating the method proposed in this article. INTRODUCTION The microwave radiometer has been proved to be an effective instrument in the remote sensing of atmosphere. The applications of ground-based microwave radiometer to the measurements of meteorological parameters have been widely made in the community of atmospheric remote sensing for years (e.g., Janssen, 1993, and references therein). It is possible to establish a useful relation between the magnitude of the radiation intensity from a specific terrestrial or atmospheric object and corresponding parameter of interest. Once the relation is established, the desired parameter can thus be obtained from the measurements of microwave radiometer. Since the pioneered work of Westwater (1965), atmospheric temperature profiles were retrieved successfully by many scientific workers from the radiometry measurements of gaseous emission in the use of various inversion techniques (Snider, 1972; Westwater et al., 1975; Decker et al., 1978; Askne and Westwater, 1982). The precipitable water vapor and cloud liquid can also be retrieved from the observation of microwave radiometer operated at selected frequencies (Guiraud et al., 1979; Snider et al., 1980). The retrieved accuracy of precipitable water vapor made by microwave radiometer is believed to be the same as or even better than those estimated by radiosonde (Guiraud et al., 1979). In addition to the atmospheric parameters, atmospheric attenuation of radiowave due to absorptions of atmospheric gases, i.e., water vapor and oxygen molecule, can also be estimated from the observed brightness temperature made with a microwave radiometer. The atmospheric attenuation is an essential parameter in designing the earth-satellite communication link (Ippolito, 1986). Westwater et al. (1990) measured the atmospheric attenuations from the conversion of observed brightness temperatures at 20.6, 31.65, and 90 GHz and found that measurement and theory generally agree to within 30%.
- 307-
S.-P. Shih and Y.-H. Chu A portable microwave radiometer with steerable horn antenna is employed for the measurement of brightness temperature in this study. This ground-based radiometer is a single linearly polarized, superheterodyne, and Dicke-switched instrument. The central frequency of the radiometer is 19.5 GHz with IF bandwidth of 500 MHz. The half-power beam width of the horn antenna is approximately 6 degrees and the sensitivity is 0.5 K for an integration time of 1 sec. The linear polarized radiation, either horizontal or vertical, is capable of being received through orienting the feed horn vertically or horizontally. The horizontal or vertical voltage signal from the radiometer is fed into the low pass filter/amplifier which determines the integration time of the radiometer. The low pass filter used for the measurements has a various cutoff frequency for the integration time between 4 and 1024 ms. The horizontal or vertical signal from the low pass filter is sent to a printer/recorder for viewing the data in real time and to a data collection board connected to the bus of the host personal computer that controls the data acquisition. The data collection board digitizes the radiometer output with 12 bits precision. The positions of the antenna in azimuth and elevation are recorded at each time when the radiometer signals are sampled. In reality, the noise temperature measured by a radiometer contains not only true sky noise temperature, but also the system temperatures. Therefore, proper calibration of the radiometer is required to derive constants for use in a radiometer equation to estimate the absolute sky noise temperature. The 19.5 GHz radiometer employed in this experiment has a Dicke reference load and an avalanche noise diode that serves as an internal calibration source. In addition to the internal calibration, the radiometer is also calibrated using external loads before the deployment of field experiment. The hot and clod calibrations are, respectively, performed in this experiment by using 0~ (273 K) ice water and -196~ (77 K) liquid nitrogen. These two points permit an absolute calibration of the radiometer. RETRIEVAL OF INTEGRATED PRECIPITABLE WATER VAPOR According to the radiative transfer theory, the brightness temperature measured by a ground-based microwave radiometer is the integration of the radiation emitted by the absorptive gases in the antenna beam. Once the brightness temperatures are measured, the corresponding total atmospheric attenuation due to gaseous absorption can be converted directly. It is well known that oxygen molecule and water vapor both give rise to the radiowave absorption, computation shows that in Taiwan area the contribution of water vapor to the absorption is much more significant than that of oxygen molecule by a factor of about 12, where the meteorological data employed for the calculation are taken from Pan-Chiao rawinsonde station over the period from 1 st August 1994 to 30 November 1999. This result implies that oxygen absorption can be reasonably neglected in retrieving water vapor content from radiometer-observed brightness temperature. Ideally, the radiometer-measured attenuation should be identical to the calculated one if the expression of specific attenuation coefficient is accurate enough and the total amount of water vapor content responsible for the measured and calculated attenuations is the same. Therefore, with the help of a relevant approximation equation connecting the gaseous absorption and water vapor concentration, it is possible to estimate the integrated precipitable water vapor content (IPWV) from the brightness temperature measured by a single-frequency ground-based radiometer. An approximation equation as a complicate function of temperature, pressure and water vapor concentration has been recommended by ITU to calculate the gaseous attenuation. However, the ITU-recommended equation is still so complicate that the use of it to retrieve water vapor content from the measurement of single-frequency
- 308 -
A New Method of Retrieving Water Vapor Content... radiometer is difficult and impractical. By appropriately adjusting the constants in the ITU-recommended approximation equation (ITU Recommendation, 1997), a very simple approximation expression at the frequency of 19.5 GHz is proposed as follows O~w= 0.00036 + 2.1432608x10-3pw
(1)
(dB/km)
where Pw is the water vapor concentration in unit of g/m 3. An examination of the accuracy of the approximation equation as shown in (1) is made and the results are presented in Fig.l, where 69 months radiosonde data from January 1, 1994, to September 30, 1999 are employed for the examination9 As indicated, the attenuations Aa calculated from the approximation equation agree very well with those estimated from ITU-recommended equation with RMS of less than 0.0061. This good
1994/01 ~1999/09 0.25
i
|
i
i
Aatru=0.00071733+0.98696AaMode j
/
RMSE=O.O061276 dB /
0.2 C.C.=0.97009
/
/
/
/
...
/
.....-
..//
/. / "
/ /
0.15
,
133
"o ..
0.1
-
".;'"
,.
/.
%,
~..
,+
9
;r
~
0.05
//./
00
+
0.05 '
0 i1
0.15 '
0 i2
0.25
Aa,,~,l~t (dB)
Fig. 1 Comparison between gaseous attenuations calculated by approximation equation shown in Eq.4 and ITU-recommended equation. agreement validates the usefulness of Eq. 1. Note that Aa is defined as
A,, = ~o aw(z)d z (dB)
(2)
where r0 is the maximum height that the water vapor extends up to and can be detected by the radiosonde. Assume that the concentration of water vapor decreases with height exponentially in the form of
- 309 -
S.-P. Shih and Y.-H. Chu
Pw = Pws e x p ( - ~ ww)
(g/m3)
(3)
where lOwsand Hw are, respectively, surface concentration (g/m 3) and scale height (km) of water vapor concentration. Note that the integrated precipitable water vapor content (IPWV) hv can be estimated in accordance with following equation tr0
hv=
MY
(cm)
= ~PwdZ
10pL
(4)
10p~.
where p~. is the density of liquid water (equal to approximately 1 g/cm3), lOw is water vapor concentration (in unit of g/m3), ro (in unit km) is the highest altitude that radiosonde can reach, and M v (g km/m 3) is the total mass of atmospheric water vapor contained in a vertical column of unit cross section. It is obvious that in practice the measured profile of 9w may not necessarily decrease with height exponentially. However, data analysis indicates that hv estimated from real profile of lOw recorded by radiosonde is very close to that calculated by using exponential model, as shown in Fig.2. Therefore, it is justified to estimate hv in terms of exponential decrease model.
10
,
,
.
1994/01~1999/09 . . .
.
/ /
IPWVuoesured = 0 2 0 6 9 7 + 0 . 9 5 7 x
IPWV;,It,m
RMSE--0.1714 cm /9 ~.\, 9
9
~6 .)
1
."
15 > 2., 9
:r
. .. ./
./
IPWVe~ Imi~(cm)
Fig. 2. Comparison between integrated prcipitable water vapor IPWV estimated from measured profile of water vapor density and calculated by best fitting exponential model to the measured profile Substituting Eq.3 into Eqs. 1 and 2, we have A,, = Bro + C p ~ s H , , ( 1 - e -r~
(dB)
(5)
where B=0.00036 dB km -~ and C=2.1432608x10 -3 dBm 3 g-1 km-~. From Eqs.4 and 5, the IPWV hv can thus be derived as follow
-310-
A New Method o f Retrieving Water Vapor C o n t e n t . . .
hv =
A, Br o (cm) lOCpL (1-e-rO/I~, ' )
(6)
-
Note that the mean values of ro and Hw over Taiwan area are, respectively, 10 and 2.2 km, producing that Bro=0.0036 dB and exp(-ro/Hw)-0.0106. In view of the fact that Aa is in general greater than Bro by 1-2 orders of magnitude and 1-exp(-ro/Hw)-0.99, Bro and exp(-ro/Hw) can be neglected reasonably in the use of Eq.6 to calculate IPWV from observed zenith opacity % by letting % be equal to Aa as defined in Eq.2. Fig.3 presents the comparison of IPWVs' estimated from radiosonde data and
1
i
i
1998/04-1999/o9 i i
i
1
i ~ , . ~
8 c.c.=0.88056
7
RMSE=O.96916 crn
~
Mean of Radiosonde=4.2667 cm
I
o
~6
o- 3
2
1
IPWV(Radiosonde)
(era)
Fig. 3. Comparison of IPWVs estimated from radiosonde data and retrieved from zenith opacity measured by a 19.5 GHz radiometer.
retrieved from zenith gaseous attenuation observed by a 19.5 GHz radiometer, where the data employed for the comparison are collected in the clear-air condition to avoid the effects of both cloud and precipitation on observed brightness temperature. As shown, the retrieved IPWVs are in good agreement with the measured ones with RMS error of about 0.97 cm. Note that Pan-Chiao rawinsonde station locates about 25 km northeast from the Chung-Li. It is reasonable to speculate that water vapor contents over Pan-Chiao and Chung-Li will be different due to spatial inhomogeneity. We believe that this is the major cause for a few points with large scatter from the line with slope of 1. Therefore, the method of retrieving IPWV from the measurement of single-frequency radiometer proposed in this section is applicable. CONCLUSION With the help of ITU-recommended equation, a method of retrieving integrated precipitable water vapor content using zenith opacity measured by a single-frequency radiometer at 19.5 GHz is developed in this article. A comparison of retrieved IPWV and observed IPWV by radiosonde shows good agreement between them, validating the proposed method.
-311 -
S.-P. Shih and Y.-H. Chu ACKNOWLEDGEMENT This work was supported by the National Space Program Office of the Republic of China (R.O.C.) on Taiwan under grants NSC89-NSPO-A-ECP-008-01 and NSC89-2111-M-008-029-A10. REFERENCES Askne, J., and E.R.Westwater, A review of ground-based remote sensing of temperature and moisture by passive microwave radiometers, IEEE Trans. Geosci. Remote Sens., GE-24, 340-352 (1982) Decker, M.T., E.R.Westwater, and EO.Guiraud, Experimental evaluation of ground-based microwave radiometric sensing of atmospheric temperature and water vapor profiles, J. Appl. Meteorol., 17, 1788-1795 (1978) Guiraud, EO., J.Howard, and D.C.Hogg, A dual-channel microwave radiometer for measurement of precipitable water vapor and liquid, IEEE Tran. Geosci. Electron., GE-17, 129-136 (1979) Ippolito Jr., L.J., Radiowave propagation in satellite communication, Van Nostrand Reinhold Company Inc., New York, pp.241 (1986) ITU Recommendation ITU-R P.676-3 (1997) Janssen, M., Atmospheric remote sensing by microwave radiometry, John Wiley & Sons, Inc., New York, pp.572 (1993) Snider, J.B., Ground-based sensing of temperature profiles from angular and multi-spectral microwave emission measurements, J. Appl. Meteorol., 11,958-967 (1972) Snider, J.B., F.O.Guiraud, and D.C.Hogg, Comparison of cloud liquid content measured by two independent ground-based systems, J. Appl. Meteorol., 19, 577-579 (1980) Van Vleck, J.H., and V.EWeisskopf, On the shape of collision broadened lines, Rev. Mod. Phys., 17, 227-236 (1945) Westwater, E.R., Ground-based passive probing using the microwave spectrum of oxygen, Radio Sci., 69D, 1201-1211 (1965) Westwater, E.R., J.B.Snider, and A.V.Carlson, Experimental determination of temperature profiles by ground-based microwave radiometry, J. Appl. Meterolo., 14, 524-539 (1975) Westwater, E.R., J.B.Snider, and M.J.Falls, Ground-based radiometric observations of atmospheric emission and attenuation at 20.6, 31.65, and 90.0 GHz: A comparison of measurements and theory, IEEE Trans. Antennas Propagat., 38, 1569-1580 (1990)
-312-
Institute of Space Science, National Central University, Chung-Li, Taiwan, R.O.C.
ABSTRACT Using a single-frequency ground-based microwave radiometer with steerable horn antenna at frequency 19.5 GHz, we measure sky radiometric temperature over Taiwan area during the period from May 1997 to September 1999. A method of retrieving integrated precipitable water vapor content (IPWV) from the observed brightness temperature made by a single-frequency radiometer at 19.5 GHz is developed and examined. Comparing the retrieved and measured IPWVs shows a good agreement between them, validating the method proposed in this article. INTRODUCTION The microwave radiometer has been proved to be an effective instrument in the remote sensing of atmosphere. The applications of ground-based microwave radiometer to the measurements of meteorological parameters have been widely made in the community of atmospheric remote sensing for years (e.g., Janssen, 1993, and references therein). It is possible to establish a useful relation between the magnitude of the radiation intensity from a specific terrestrial or atmospheric object and corresponding parameter of interest. Once the relation is established, the desired parameter can thus be obtained from the measurements of microwave radiometer. Since the pioneered work of Westwater (1965), atmospheric temperature profiles were retrieved successfully by many scientific workers from the radiometry measurements of gaseous emission in the use of various inversion techniques (Snider, 1972; Westwater et al., 1975; Decker et al., 1978; Askne and Westwater, 1982). The precipitable water vapor and cloud liquid can also be retrieved from the observation of microwave radiometer operated at selected frequencies (Guiraud et al., 1979; Snider et al., 1980). The retrieved accuracy of precipitable water vapor made by microwave radiometer is believed to be the same as or even better than those estimated by radiosonde (Guiraud et al., 1979). In addition to the atmospheric parameters, atmospheric attenuation of radiowave due to absorptions of atmospheric gases, i.e., water vapor and oxygen molecule, can also be estimated from the observed brightness temperature made with a microwave radiometer. The atmospheric attenuation is an essential parameter in designing the earth-satellite communication link (Ippolito, 1986). Westwater et al. (1990) measured the atmospheric attenuations from the conversion of observed brightness temperatures at 20.6, 31.65, and 90 GHz and found that measurement and theory generally agree to within 30%.
- 307-
S.-P. Shih and Y.-H. Chu A portable microwave radiometer with steerable horn antenna is employed for the measurement of brightness temperature in this study. This ground-based radiometer is a single linearly polarized, superheterodyne, and Dicke-switched instrument. The central frequency of the radiometer is 19.5 GHz with IF bandwidth of 500 MHz. The half-power beam width of the horn antenna is approximately 6 degrees and the sensitivity is 0.5 K for an integration time of 1 sec. The linear polarized radiation, either horizontal or vertical, is capable of being received through orienting the feed horn vertically or horizontally. The horizontal or vertical voltage signal from the radiometer is fed into the low pass filter/amplifier which determines the integration time of the radiometer. The low pass filter used for the measurements has a various cutoff frequency for the integration time between 4 and 1024 ms. The horizontal or vertical signal from the low pass filter is sent to a printer/recorder for viewing the data in real time and to a data collection board connected to the bus of the host personal computer that controls the data acquisition. The data collection board digitizes the radiometer output with 12 bits precision. The positions of the antenna in azimuth and elevation are recorded at each time when the radiometer signals are sampled. In reality, the noise temperature measured by a radiometer contains not only true sky noise temperature, but also the system temperatures. Therefore, proper calibration of the radiometer is required to derive constants for use in a radiometer equation to estimate the absolute sky noise temperature. The 19.5 GHz radiometer employed in this experiment has a Dicke reference load and an avalanche noise diode that serves as an internal calibration source. In addition to the internal calibration, the radiometer is also calibrated using external loads before the deployment of field experiment. The hot and clod calibrations are, respectively, performed in this experiment by using 0~ (273 K) ice water and -196~ (77 K) liquid nitrogen. These two points permit an absolute calibration of the radiometer. RETRIEVAL OF INTEGRATED PRECIPITABLE WATER VAPOR According to the radiative transfer theory, the brightness temperature measured by a ground-based microwave radiometer is the integration of the radiation emitted by the absorptive gases in the antenna beam. Once the brightness temperatures are measured, the corresponding total atmospheric attenuation due to gaseous absorption can be converted directly. It is well known that oxygen molecule and water vapor both give rise to the radiowave absorption, computation shows that in Taiwan area the contribution of water vapor to the absorption is much more significant than that of oxygen molecule by a factor of about 12, where the meteorological data employed for the calculation are taken from Pan-Chiao rawinsonde station over the period from 1 st August 1994 to 30 November 1999. This result implies that oxygen absorption can be reasonably neglected in retrieving water vapor content from radiometer-observed brightness temperature. Ideally, the radiometer-measured attenuation should be identical to the calculated one if the expression of specific attenuation coefficient is accurate enough and the total amount of water vapor content responsible for the measured and calculated attenuations is the same. Therefore, with the help of a relevant approximation equation connecting the gaseous absorption and water vapor concentration, it is possible to estimate the integrated precipitable water vapor content (IPWV) from the brightness temperature measured by a single-frequency ground-based radiometer. An approximation equation as a complicate function of temperature, pressure and water vapor concentration has been recommended by ITU to calculate the gaseous attenuation. However, the ITU-recommended equation is still so complicate that the use of it to retrieve water vapor content from the measurement of single-frequency
- 308 -
A New Method of Retrieving Water Vapor Content... radiometer is difficult and impractical. By appropriately adjusting the constants in the ITU-recommended approximation equation (ITU Recommendation, 1997), a very simple approximation expression at the frequency of 19.5 GHz is proposed as follows O~w= 0.00036 + 2.1432608x10-3pw
(1)
(dB/km)
where Pw is the water vapor concentration in unit of g/m 3. An examination of the accuracy of the approximation equation as shown in (1) is made and the results are presented in Fig.l, where 69 months radiosonde data from January 1, 1994, to September 30, 1999 are employed for the examination9 As indicated, the attenuations Aa calculated from the approximation equation agree very well with those estimated from ITU-recommended equation with RMS of less than 0.0061. This good
1994/01 ~1999/09 0.25
i
|
i
i
Aatru=0.00071733+0.98696AaMode j
/
RMSE=O.O061276 dB /
0.2 C.C.=0.97009
/
/
/
/
...
/
.....-
..//
/. / "
/ /
0.15
,
133
"o ..
0.1
-
".;'"
,.
/.
%,
~..
,+
9
;r
~
0.05
//./
00
+
0.05 '
0 i1
0.15 '
0 i2
0.25
Aa,,~,l~t (dB)
Fig. 1 Comparison between gaseous attenuations calculated by approximation equation shown in Eq.4 and ITU-recommended equation. agreement validates the usefulness of Eq. 1. Note that Aa is defined as
A,, = ~o aw(z)d z (dB)
(2)
where r0 is the maximum height that the water vapor extends up to and can be detected by the radiosonde. Assume that the concentration of water vapor decreases with height exponentially in the form of
- 309 -
S.-P. Shih and Y.-H. Chu
Pw = Pws e x p ( - ~ ww)
(g/m3)
(3)
where lOwsand Hw are, respectively, surface concentration (g/m 3) and scale height (km) of water vapor concentration. Note that the integrated precipitable water vapor content (IPWV) hv can be estimated in accordance with following equation tr0
hv=
MY
(cm)
= ~PwdZ
10pL
(4)
10p~.
where p~. is the density of liquid water (equal to approximately 1 g/cm3), lOw is water vapor concentration (in unit of g/m3), ro (in unit km) is the highest altitude that radiosonde can reach, and M v (g km/m 3) is the total mass of atmospheric water vapor contained in a vertical column of unit cross section. It is obvious that in practice the measured profile of 9w may not necessarily decrease with height exponentially. However, data analysis indicates that hv estimated from real profile of lOw recorded by radiosonde is very close to that calculated by using exponential model, as shown in Fig.2. Therefore, it is justified to estimate hv in terms of exponential decrease model.
10
,
,
.
1994/01~1999/09 . . .
.
/ /
IPWVuoesured = 0 2 0 6 9 7 + 0 . 9 5 7 x
IPWV;,It,m
RMSE--0.1714 cm /9 ~.\, 9
9
~6 .)
1
."
15 > 2., 9
:r
. .. ./
./
IPWVe~ Imi~(cm)
Fig. 2. Comparison between integrated prcipitable water vapor IPWV estimated from measured profile of water vapor density and calculated by best fitting exponential model to the measured profile Substituting Eq.3 into Eqs. 1 and 2, we have A,, = Bro + C p ~ s H , , ( 1 - e -r~
(dB)
(5)
where B=0.00036 dB km -~ and C=2.1432608x10 -3 dBm 3 g-1 km-~. From Eqs.4 and 5, the IPWV hv can thus be derived as follow
-310-
A New Method o f Retrieving Water Vapor C o n t e n t . . .
hv =
A, Br o (cm) lOCpL (1-e-rO/I~, ' )
(6)
-
Note that the mean values of ro and Hw over Taiwan area are, respectively, 10 and 2.2 km, producing that Bro=0.0036 dB and exp(-ro/Hw)-0.0106. In view of the fact that Aa is in general greater than Bro by 1-2 orders of magnitude and 1-exp(-ro/Hw)-0.99, Bro and exp(-ro/Hw) can be neglected reasonably in the use of Eq.6 to calculate IPWV from observed zenith opacity % by letting % be equal to Aa as defined in Eq.2. Fig.3 presents the comparison of IPWVs' estimated from radiosonde data and
1
i
i
1998/04-1999/o9 i i
i
1
i ~ , . ~
8 c.c.=0.88056
7
RMSE=O.96916 crn
~
Mean of Radiosonde=4.2667 cm
I
o
~6
o- 3
2
1
IPWV(Radiosonde)
(era)
Fig. 3. Comparison of IPWVs estimated from radiosonde data and retrieved from zenith opacity measured by a 19.5 GHz radiometer.
retrieved from zenith gaseous attenuation observed by a 19.5 GHz radiometer, where the data employed for the comparison are collected in the clear-air condition to avoid the effects of both cloud and precipitation on observed brightness temperature. As shown, the retrieved IPWVs are in good agreement with the measured ones with RMS error of about 0.97 cm. Note that Pan-Chiao rawinsonde station locates about 25 km northeast from the Chung-Li. It is reasonable to speculate that water vapor contents over Pan-Chiao and Chung-Li will be different due to spatial inhomogeneity. We believe that this is the major cause for a few points with large scatter from the line with slope of 1. Therefore, the method of retrieving IPWV from the measurement of single-frequency radiometer proposed in this section is applicable. CONCLUSION With the help of ITU-recommended equation, a method of retrieving integrated precipitable water vapor content using zenith opacity measured by a single-frequency radiometer at 19.5 GHz is developed in this article. A comparison of retrieved IPWV and observed IPWV by radiosonde shows good agreement between them, validating the proposed method.
-311 -
S.-P. Shih and Y.-H. Chu ACKNOWLEDGEMENT This work was supported by the National Space Program Office of the Republic of China (R.O.C.) on Taiwan under grants NSC89-NSPO-A-ECP-008-01 and NSC89-2111-M-008-029-A10. REFERENCES Askne, J., and E.R.Westwater, A review of ground-based remote sensing of temperature and moisture by passive microwave radiometers, IEEE Trans. Geosci. Remote Sens., GE-24, 340-352 (1982) Decker, M.T., E.R.Westwater, and EO.Guiraud, Experimental evaluation of ground-based microwave radiometric sensing of atmospheric temperature and water vapor profiles, J. Appl. Meteorol., 17, 1788-1795 (1978) Guiraud, EO., J.Howard, and D.C.Hogg, A dual-channel microwave radiometer for measurement of precipitable water vapor and liquid, IEEE Tran. Geosci. Electron., GE-17, 129-136 (1979) Ippolito Jr., L.J., Radiowave propagation in satellite communication, Van Nostrand Reinhold Company Inc., New York, pp.241 (1986) ITU Recommendation ITU-R P.676-3 (1997) Janssen, M., Atmospheric remote sensing by microwave radiometry, John Wiley & Sons, Inc., New York, pp.572 (1993) Snider, J.B., Ground-based sensing of temperature profiles from angular and multi-spectral microwave emission measurements, J. Appl. Meteorol., 11,958-967 (1972) Snider, J.B., F.O.Guiraud, and D.C.Hogg, Comparison of cloud liquid content measured by two independent ground-based systems, J. Appl. Meteorol., 19, 577-579 (1980) Van Vleck, J.H., and V.EWeisskopf, On the shape of collision broadened lines, Rev. Mod. Phys., 17, 227-236 (1945) Westwater, E.R., Ground-based passive probing using the microwave spectrum of oxygen, Radio Sci., 69D, 1201-1211 (1965) Westwater, E.R., J.B.Snider, and A.V.Carlson, Experimental determination of temperature profiles by ground-based microwave radiometry, J. Appl. Meterolo., 14, 524-539 (1975) Westwater, E.R., J.B.Snider, and M.J.Falls, Ground-based radiometric observations of atmospheric emission and attenuation at 20.6, 31.65, and 90.0 GHz: A comparison of measurements and theory, IEEE Trans. Antennas Propagat., 38, 1569-1580 (1990)
-312-