APPLICATION OF SMALL SATELLITES FOR HIGH PRECISION MEASURING EFFECTS OF RADIO WAVE PROPAGATION K. Igarashi 1, N.A. Armand 2, A.G. Pavelyev 2, Ch. Reigber 3, J. Wickert 3, K. Hocke 1, G. Beyerle 3, S.S. Matyugov 2, O.I.Yakovlev 2 1 Communication Research Laboratory, Ministry of Posts and Telecommunications 4-2-1, Nukui-Kita Machi, Koganei-shi, Tokyo 184 Japan E-mail: igarashi@crl.go.jp/fax: +81 423 27-7606 2 Institute of Radio Engineering and Electronics of Russian Academy of Sciences, (IRE RAS), Fryazino, Vvedenskogo sq. 1, 141120 Moscow region, Russia E-mail agp117@ire216.msk.su/fax: 7 095 203 8414 3 GeoForschungsZentrum Potsdam (GFZ-Potsdam), Telegrafenberg, 14473 Potsdam ABSTRACT Germany The radio holography methodology may be applied in the scientific programs for future small satellite that will use radio signals emitted by radio navigation, radio communication satellites for precise measuring effects of radio waves propagation at low elevation angles and for global monitoring of radio communication channels passed through the atmosphere and ionosphere. Another task consists in monitoring of the wave phenomena in the atmosphere and ionosphere which may be important for telecommunication conditions in the difficult accessible regions (communications with air-planes in polar and sea zones, at small elevation angles, communications through the ionosphere between two satellites). Radio holography has been shown to provide a high angular and vertical resolution corresponding to high precision level of radio navigational radio field. For example, a weak signal reflected from the surface of the sea has been observed. As the second example a radio hologram of the ionospheric D-layer is considered. Wave structures in the concentration of the plasma with a vertical spatial period of 0.5-5 km, and variations in the electron density gradient from ±5 10 3 to ±8 10 3 electrons/(cm 3 km), were retrieved from analysis of the radio hologram. The features in the vertical profile of the gradient of electron density may be connected with the breaking of gravity waves in the temperature inversion region at the heights 70-80 km. Radio holography shows its effectiveness for precise measuring effects of radio wave propagation and monitoring radio channels in the ionosphere and troposphere. 1. INTRODUCTION The current control of conditions of radio waves propagation in the near Earth s media may be fulfilled by means of signals of existing radio navigation, telecommunication and remote sensing satellite s systems. The first example is using Earth-based receivers for investigations of the amplitude and phase scintillations of the satellites signals at low elevation angles [1]. In results conditions of radio wave propagation may be determined and models of the propagation media may be constructed [2]. The second example is investigations of radio propagation effects on the satellite-to-satellite radio line. The radio occultation GPS/MET satellite s systems [3] is more convenient for this purpose
because more intense influence of the propagation s effects and high precision level of radio navigational signals. In the both cases the main task consists in deriving new radio holography methodology for obtaining extreme accuracy and spatial resolution in measuring propagation effects and parameters of the propagation media compatible with high coherence properties of signals emitted by GPS navigational system. For illustration of high potential of radio holography methodology, in this report some results of application of radio holography to measuring angular spectra of radio waves and parameters of wave structures in the near Earth s medium are considered by means of analysis of the GPS/MET radio occultation data. 2. STUDYING EFFECTS OF RADIO WAVE PROPAGATION AND OBSERVATION OF THE WAVE STRUCTURES IN THE LOWER IONOSPHERE The setup for radio occultation observation is shown in Fig. 1. Radio occultation method uses coherent signals, which are emitted by transmitter at one of GPS satellites (point G) and then received by Micro-lab-1 (LEO) satellite (point P). Reflected from the Earth s surface signals (the ray path GDP) may exist also in radio occultation data. Radio holography combines the dependencies on time of the amplitude and phase of radio occultation signals as registered by the receiver aboard the LEO satellite in one radio hologram at each of the two GPS frequencies f 1 =1575.42 MHz and f 2 = 1227.6 MHz, as has been described in [4,5] Analysis of the radio hologram gives the angular spectrum of the radio waves at point P and the distribution of radio brightness along the line DD (Fig.1). The angular position of the main beam β determines the impact parameter p and the bending angle ξ(p) (Fig. 1). After the dependence ξ(p) on p has been estimated, the standard Abel inversion procedure may be used for determination of the refraction index altitude profile [3]. The angular spectrum may be prolonged along the corresponding rays in the plane PGO up to any straight line disposed near the atmosphere (for example, DD in Fig. 1). In this case the angular spectrum may be interpreted as the radio image or radio brightness distribution of the atmosphere and ionosphere as seen from the LEO satellite s orbit. As an example, the results of an application of the radio holographic approach to the analysis of GPS/MET radio occultation data (event No.0392, February 05, 1997) are shown in Fig. 2-4. The radio brightness distribution in the troposphere at a level of 2 km is shown in Fig. 2-4. The two prominent features, representing the main beam and the signal, reflected from the sea s surface, were observed with a vertical resolution of about 80 m. Fig. 2-4 demonstrate that the radio holographic method may be capable of resolving some details in one-dimensional vertical radio images of the atmosphere, on the scale of 30-50 meters, and this corresponds to a spatial resolution of about 1/10 of the size Fresnel zone. This value is somewhat higher than the magnitude of the expected spatial resolution of about 100 m that corresponds to the backward propagation method as modified in [6]. The amplitude and phase components of radio holograms of the D-region of the ionosphere that correspond to GPS/MET occultation event (07 February 1997, No. 0447) are shown in Fig. 5. Occultation event No. 0447 took place near Japan (Okinawa) in the middle of nighttime. The time-spatial coordinate of the main ray s minimal height H was close to 25.5oN 231.7oW, 15h 53m 23s UT. The two curves in the middle of Fig. 5 correspond to the experimental phase excess variations at the first frequency F1, S1 (upper curve), and at the second frequency F2,
S2, as functions of height. These curves have been multiplied by 20 and curve S1 has been displaced by 0.4 m to make more visible the variations, which are connected with the wave structures. The upper ionospheric contribution was subtracted from the phase excess data by using the IRI-95 F-layer model for time and region of radio occultation. The variations in amplitude (top and bottom pairs of curves in Fig. 5) are strongly correlated. The level of variations in the phase-path excess and in amplitudes at the two frequencies is proportional to the ratio f22/f12, and this demonstrates that the variations originate due to fluctuations in the electron density. The phase excess that are connected with wave structures change in the interval ±1 cm, with a random noise contribution of about ±1 mm. Spatial periods in the 1-3 km range can clearly be seen in the phase excess data of Fig. 5. In the amplitude data (top and bottom pairs of curves in Fig. 5 spatial periods in the 0.5-2 km range can also be seen. A feature at the heights of 72-78 km is seen in both the phase excess and amplitude data. These variations correspond to wave structures in the electrons density distribution in the D-layer of the ionosphere. The vertical gradient of electron density may be retrieved from the amplitude data by a method, which has been described in [5]. The results of the restoration dne(h)/dh for events 0447 are shown in Fig. 6. For ease of comparison, the curve F1 has been displaced by 19 th.el/cm3km-1 in Fig. 6. The vertical distributions of dne(h)/dh have nearly the same amplitude at both frequencies F1 and F2. The maximum value of the positive gradient is at a height of about 76 km. The strong feature, which is seen in the height ranges 70-80 km, may correspond to breaking of gravity waves in a region near the temperature inversion that is usually observed at this altitude by Earth-based radar and lidar tools [7]. Accurate measurements of the vertical gradients of the altitude profiles of the electron density in the D-layer of the ionosphere showed the effectiveness of the radio holographic approach for observing wave structures in the upper atmosphere. CONCLUSION The radio holographic methodology has been validated by means of GPS/MET radio occultation data. This methodology gives direct evidences of multibeam propagation and shows extreme vertical resolution of components in the angular spectra of radio waves. The radio holography gives extreme accuracy because it reveals parameters of wave structures in the lower ionosphere from signal in the phase path with amplitude ±1 cm under statistical error of the GPS/MET radio navigational field ±1 mm at a distance of about 30000 km. The thorough investigation of possibility of application of radio holography to analysis of another radio channels (Earth-satellite, plane-satellite etc.) will be the topic of future work. ACKNOWLEDGMENTS We are grateful to UCAR for access to the GPS/MET data. REFERENCES
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Fig. 1. Radio occultation data are obtained by using a GPS transmitter and the Microlab-1 satellite as a receiver (point P). The dependencies of the amplitude and phase on time may be combined to obtain a radio hologram at each frequency F1=1575.42 MHz and F2=1227.6 MHz. Fig. 2. Reflected signal as revealed by radio holography method. Fig. 3. Demonstration of high vertical resolution of about 60-80 m. Fig. 4. Conjunction of reflected signal with main beam. Fig. 5. Variations of the phase and Fig. 6. Variations of the vertical gradient amplitude at two frequencies. The random of the electron density in D-layer error in the phase is of about ± 1mm. obtained from data at F1 and F2. D-layer plasma effect is of about ±1 cm.