ESTIMATION OF ATMOSPHERIC PARAMETERS USING RADIO OCCULTATION METHOD

¹NGUYỄN XUÂN ANH, ¹PHẠM LÊ KHƯƠNG, ²V.A. KABANOV,
²V.I. LUTSENKO, ²I.V. LUTSENKO, ²V.B. SINITSKY

¹Institute of Geophysics, Vietnamese Academy of Sciences and Technology;
²Usikov Institute of Radiophysics and Electronics, Academy of Sciences of Ukraine

Abstract: The possibility of using anthropogenic radiation sources, such as television (TV) and satellite signals for evaluation of atmospheric parameters is considered. The method for tropospheric refraction estimation by attenuation factor of the TV signals on over-the-horizon path is proposed. The obtained experimeltal results were in good agreement with the radiosonde data. The observed situations can be classified into three groups: standard, super and inverted refractions. The analysis is made in the case of using GPS signals as sources of radiation. An example of this case is illustrated by estimation of some convection indices basing on FORMOSAT-3/COSMIC data. It should be noted that every approach has its own advantages and disadvantages comparing with others, so the combining method is recommended.

I. INTRODUCTION

The efficiency of different radio systems (navigation, radiolocation and communication) appreciably depends on the radio waves propagation conditions determined by the atmospheric refraction state which, in turn, is characterized by the spatio-temporal distribution of the refraction index n. Traditional ways of refraction index determination for the long-time are contact measurements of atmospheric parameters - temperatures, pressure, humidity by means of meteorological sensors or direct refractometric measurements of refraction index both at the fixed points in space and by the sensor movement [4].

Over the last decades, the methods of the atmospheric remote sensing including radiometric, radar, radio occultation and others have been actively developed. Such methods profitably differ from contact methods in the possibility of more effective area scan, higher information access speed and easier implementation.

The method of radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a atmosphere [12]. It relies on the detection of a change in the radio signal. As the signal passes through the atmosphere, its phase is perturbed in a manner related to the refractivity along the ray path. Measurements of the phase perturbations can reveal the refractivity, from which one can then derive such quantities as atmospheric density, pressure, temperature, moisture, etc...

A lot of works were carried out to estimate atmospheric parameters using both anthropogenic and natural radiation sources (a few of them are [1-3, 5, 7, 10, 13-15, 17]).

In this paper two forms of RO method is considered. In the first part, technique for tropospheric refraction estimation by attenuation factor of television (TV) signals on over-the-horizon path is considered. This approach permits to carry out the continuous monitoring of the troposphere condition. The second part of this paper is focused on estimation of some convection indices by using GPS signals as source of radiation. This method provides measurement of atmospheric parameters during the period of the satellite observation.

II. ESTIMATION OF TROPOSPHERIC REFRACTION BY THE ATTENUATION FACTOR OF TV SIGNALS ON OVER-THE-HORIZON PATHS

As known, the path of radio-waves propagation, which is in area of a geometrical shade, by character of prevailing mechanisms can be divided on diffractive zone and a zone of distant tropospheric propagation (DTP). If in the first zone electromagnetic (EM) field will be formed by diffraction and refraction, then in the second-scattering of radio-waves by atmospheric irregularities is dominated. At present, there have been a lot of theoretical and experimental works investigating attenuation factor of EM field on over-the-horizon paths and tropospheric influences on its. But for estimation of tropospheric refraction index, the main issue is the inversion problem, i.e. determination of refraction index gradient by the attenuation factor of EM field [19].

Mostly, Vvedensky's method, published for the first time in [6, 20], taking into account the parameter of an underlying surface, is used for determination of field intensity and so an attenuation factor in the shadow zone. For all cases of VHF band, except the wavelength λ> 2 m and emission or reception of vertically polarized wave over the sea, the parameter influencing the attenuation factor by soil characteristics, can be considered as constant equal to about 54,5 (limiting value) with small error. In there, the field intensity does not depend on electrical parameters of the surface and the polarization state.

The calculations show that at VHF band with the not-too low height of antenna, an exponential dependence of attenuation factor is taken place not only for a shadow zone, but also for a line-of-sight distance . The good estimation of in these areas can be calculated by the formula according to [16]:

(1)

where [dB]: attenuation factor of EM field on the line-of-sight distance, ;; : radius of the Earth; and : heights of transmiter and receiver.

The measurement of the attenuation factor of VHF signals on over-the-horizon paths have been made for the period from February, 2001 to present time in Ukraine. TV signals in penumbra and deep shade zones were used in the experiments. The results were compared with the weather balloon (radiosonde) data within the heights ranging from 0 to 5000 m. The averaged of 10 days refractivity and its gradient are shown on Fig. 1. One can see that remote sensing data have good correlation with radiosonde measurements.

Daily variations of attenuation factor and the refractivity gradient profile, illustrated in Fig. 2, can be divided into three groups as follows:

- The first is autumn-winter - early spring time period with a small variation of signal during the day (Fig. 2a). In tropospheric boundary layer refractivity gradient is close to its standard (Fig. 2b).

- The second group refers to the signals, observed at spring, summer and at the autumn beginning, daily change is characterized by an appearance of super-refraction in the surface layer due to surface cooling and warming effects by sunset and sunrise (Fig. 2c, d).

- The third group reflects the situations, when in the evening the attenuation factor is oscillating (Fig. 2e) and as a rule, the layer with super refraction exists at the night time (Fig 2f).

The estimation of refractivity gradient is made by using the attenuation factor of signals to understand its daily variations. Spring-autumn mean value is about , while at the night time it can reach to (Fig. 3a). In autumn and winter periods the mean values is , and the minimum exists at night time (Fig. 3b).

The observation frequencies of different refraction situations by seasons are shown on Fig. 4. The normal refraction is observed for 50 % and more cases in autumn and winter. The super-refraction and inversion layers are observed for more than 80 % cases in spring and summer.

Figure 4. Observation frequencies of refraction situations by seasons:

■- normal, ●-super refraction, ▲- inversion layers.

In tropical areas, due to greater atmospheric moisture, higher temperature and more probabilistic inversion layer formation, there should be more observation situations with super- and inversion refraction with refractivity gradient higher than in mid-latitude regions.

III. ESTIMATION OF CONVECTION INDICES BY GPS RADIO OCCULTATION

It is shown that the GPS signals can be used to obtain detailed altitude profiles of the vertical gradient of refractivity in the atmosphere [16] and atmospheric parameters of interest can be derived from it [18]. The FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) spacecraft constellation consisting of six LEO (Low Earth Orbit) satellites launched in 2006 is the world first operational GPS radio occultation mission [8]. Due to this reason COSMIC data were used to calculate some atmospheric parameters such as Convective Available Potential Energy (CAPE), Lifted Index (LI) and others [9]. CAPE, as measure of the amount of energy available for convection, is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. CAPE calculation was illustrated on Fig. 5 for thunderstorm situation near Hồ Chí Minh City on April 24, 2007. The CAPE has the value of 4245 J/kg indicating the strong convective cells over this area.

Figure 5. CAPE in server thunderstrom near Hồ Chí Minh City on April 24, 2007.

The monthly mean values of global CAPE by COSMIC data from April, 2006 to July, 2007 are shown in Fig. 6 demonstrating the minimum in winter months. It should be mentioned that there are only initial results and we need more data in order to carry out temporal-spatial analyses.

CONCLUSIONS

As presented above, we reach the following conclusions:

- The radio occultation method, as effective remote sensing tool, can be used to measure the atmospheric refractivity, from which one can then derive other parameters, such as atmospheric density, pressure, temperature, moisture, convection indices, etc...

- The technique for tropospheric refraction estimation by attenuation factor of TV signals on over-the-horizon path, having the advantages in low cost, can be used to carry out the continuous monitoring of the troposphere condition.

- During recent years, the GPS occultation method has opened up a new route for exploring the atmosphere. The vertical atmospheric profiles of vertical gradient of refractivity, pressure, temperature, water vapor and others can be retrieved, thus making this method a potentially valuable tool for meteorologic and atmospheric sciences.

Figure 6. Monthly mean values of CAPE over the world.

- There are many forms or approaches of RO method using anthropogenic and natural electromagnetic fields, as radiation sources. It should be noted that every approach has its own advantages and disadvantages comparing with others, so the combining method is recommended.

REFERENCES

1. Anderson K.D., 1982. Inference of refractivity profiles by satellite-to-ground RF measurements. Radio Sci., 17: 653-663.

2. Armand N.A., Andrianov V.А., Smirnov V.M., 1987. Recovery of tropospheric refraction index profile by measurement of satellite signal frequencies. Radiotechnics and Electronics, 32/4: 673-680 (in Russian).

3. Azizov А.А., Gaykovich К.P., Kashkarov S.S., Chernyaeva M.B., 1998. Use of the navigation signals of satellite for determination of atmospheric parameters. Izv. Vuzov, Radiophysics , 41/9: 1093-1116 (in Russian).

4. Binn B.R., Datton E.Dj., 1971. Radio Meteorology. Gydrometeoizdat, p. 361. Leningrad (in Russian).

5. Bogaturov А.N., Gaykovich К.P., Gurvich А.S. Et al., 1990. About the possibility of determination of reflecting layers in troposphere over the sea by variations of satellite radiosignals. Rep. of USRR Acad. of Sci., 315/4: 830-834 (in Russian).

6. Fok V.A., 1949. A field of the vertical and horizontal dipole elevated over the Earth surface. J.E.T.P., 2: 86-97 (in Russian).

7. Fong C.-J., N. Yen, V. Chu, C.-Y. Huang, Sien Chi, 2007. Mission results from FORMOSAT-3/COSMIC constellation for global climate monitoring. Geoph. Res. Abstr., 9: 06062.

8. Gurvich A.S., Ershov A.T., 1979. About radio thermal radiation of atmosphere by the waveguide propagation. Izv. АS USSR, FAO, 15/2 : 218-221 (in Russian).

9. Haklander A.J., A. van Delden, 2003. Thunderstorm predictors and their forecast skill for the Netherlands. J. Atm. Res., 67-68: 273-299.

10. Hitney H.V., 1978. Means for determining the refractive index profile of the atmosphere. U.S. Patent 4,093,918.

11. Kalinin A.I., 1956. About the calculation of field intensity in shadow and a penumbra zones at the ultra-short waves propagation along a smooth spherical surface of the Earth. Radiotehnika, 11/6: 43-49 (in Russian).

12. Lusignan B., Modrell G., Morrison A., Pomalaza J., Ungar S.G., 1969. Sensing the Earth’s atmosphere with occultation sattellites. Proc. IEEE, 57/4: 438-467.

13. Lutsenko V.I., Ye.N. Belov, I.V. Lutsenko, S.I. Khomenko, 2003. The troposphere refraction estimation by attenuation factor of radiowave beyond-the-horizon propagation telecommunication and radio engineering. Radiotehnika, 60/10-12: 1-14.

14. Markina N.N., Naumov A.P., Sumin M.P., 1987. Determination of altitude refraction index profile of atmosphere in optical and VHF wave band by its thermal radiation. Izv. Vuzov, Radiophysica, 30/8: 951-960 (in Russian).

15. Mikhajlov N.F., Ryzhkov A.V., Schukin G.G., 1990. Radio meteorological investigation methods over the sea. Gydrometizdat, Leningrad, 208 p. (in Russian).

16. Pavelyev A., Yuei-An Liou, C. Reigber, J. Wickert, K. Igarashi, K. Hocke, Cheng-Yung Huang, 2002. GPS radio holography as a tool for remote sensing of the atmosphere, mesosphere, and terrestrial surface from space. GPS Solutions, 6: 100-108. Springer-Verlag.

17. Schukin G.G., Melnik Yu.A., Galperin S.M. Ilyin Ya.K., et al., 1982. Estimation of radio wave propagation over the sea by radiometrical method. Works of Main Geoph. Obs., 451. Meth. of active and passive radiolocations in radio meteorology, pp. 65-68 (in Russian).

18. Sokolovskiy S.V., 2001. Modelling and inverting radio occultation signals in the moist troposphere. Radio Sci., 36/3 : 441-458.

19. Troitskiy V.N., 1957. About fading of ultra-short waves on radio relay communication lines. Elektrosvjaz, 10: 32-39 (in Russian).

20. Vvedensky B.A., 1934. The basis of radio-wave propagation theory. Technical-Theoretical Publ. H., 367 (in Russian).