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Radio beacon for ionspheric tomography RaBIT


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  • 1. Indian Journal of Radio & Space Physics Vol 41, April 2012, pp 162-167 Radio Beacon for Ionospheric Tomography (RaBIT) onboard YOUTHSAT: Preliminary results T K Pant1,$,*, P Sreelatha1, N Mridula1, S Trivedi2, R M Das3, S Koli4, R Sharma5, J Girija1, Arun Alex1, K K Mukundan1, S B Shukla1, P Purushottaman1, J N Santosh1, Biju Thomas1, M Srikant6, R Sridharan7, K Krishnamoorthy1, Ratan Bisht8, D V A Raghavamurthy6, M P T Chamy8 & J D Rao9 1 Vikram Sarabhai Space Centre, Trivandrum 695 022 Space Science Office, ISRO, Antariksh Bhavan, New BEL Road, Bangalore 560 231 3 National Physical Laboratory, Dr K S Krishnan Marg, New Delhi 110 012 4 National Balloon Facility, TIFR, P B No 5, ECIL P O, Hyderabad 500 762 5 Master Control Facility, Ayodhya Nagar, N Sector, Bhopal, 462 041 6 ISRO Satellite Centre, PB No 1795, Vimanapura Post, Bangalore 560 017 7 Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, Gujarat 8 Space Application Centre, Jodhpur Tekra, Ambawadi Vistar P O, Ahmedabad 380 015 9 ISRO Satellite Tracking Centre, Bangalore 560 058 $ E-mail: 2 Received: November 2011; accepted 23 April 2012 This paper presents, for the first time, a few tomograms obtained using India’s own Radio Beacon for Ionospheric Tomography (RaBIT) onboard YOUTHSAT, a small satellite dedicated for the terrestrial upper atmospheric studies. The tomograms presented here, were obtained during the beginning of the solar cycle 24, and clearly demonstrate the potential of the tomography technique to investigate the large scale ionospheric processes over the Indian longitude region. Keywords: Ionospheric tomography, Radio beacon, Equatorial ionization anomaly (EIA), Equatorial spread-F (ESF), Total electron content (TEC) PACS No.: 94.20.dt 1 Introduction The terrestrial upper atmosphere over the low and equatorial latitudes is replete with large scale plasma/neutral processes such as the equatorial ionization anomaly (EIA) and the equatorial spread-F (ESF), which significantly influence the distribution of the ionization as a function of altitude, latitude, longitude and time1. Understanding these processes and modeling them pose challenges due to their highly dynamical nature and large spatial and temporal variability even during geomagnetically quiet conditions. The geomagnetic storms significantly alter the background ionospheric and thermospheric structure, energetic and dynamics and as a consequence modify the major equatorial ionospheric processes2. In the recent years, it has become clear that the understanding of the ionosphere and the processes therein, especially those over the low and equatorial latitudes, is central to the design of many modern communication, navigation and positioning systems. This is because the position accuracy achievable from navigation satellites is largely affected by the intervening ionosphere. The range error is directly proportional to the total electron content (TEC) along the ray path. With the increasing use of satellites for navigation and positioning (GPS, GLONASS, etc.), characterization and modeling of the ionosphere (its spatial and temporal variability) has become extremely important. This includes understanding / modeling of the processes of the ionospherethermosphere system and its response to the various external forcings so as to reach a level of predictive capability3. This requires the knowledge regarding how the ionospheric plasma distribution occurs as a function of altitude over a large latitude region and
  • 2. PANT et al.: RADIO BEACON FOR IONOSPHERIC TOMOGRAPHY evolves with time. The traditional ground-based experiments like the ionosonde have a limitation in addressing this aspect. In this context, it has been unambiguously established that the tomographic techniques4,5 are very effective and useful. The effectiveness of this technique in investigating the large-scale structures over low and equatorial latitudes, for example, equatorial ionization anomaly (EIA) has been amply demonstrated6,7. The low-latitude ionospheric tomography network (LITN), which is the first network of six receivers from 14.6°N to 31.3°N (geographic latitudes) along the 121°E meridian, has added significantly to the understanding about the motion of the anomaly crest of the EIA8, the structure and symmetry of its core and the low-latitude ionospheric response to magnetic storms9. 2 Indian Radio Beacon Experiments The Indian Coherent Radio Beacon Experiment (CRABEX) had also been initiated mainly to address the aforementioned aspects regarding the large-scale processes over equatorial and low latitudes over the Indian longitudes. This experiment consists of five radio receivers stationed at Trivandrum (8.5°N, 77°E), Bangalore (13°N, 77.6°E), Hyderabad (17.3°N, 78.3°E), Bhopal (23.2°N, 77.2°E), and Delhi (28.8°N, 77.2°E) that are capable of receiving 150 and 400 MHz beacon transmissions from the Low Earth Orbiting Satellites (LEOS) like the Navy Ionospheric Monitoring Satellites (NIMS) of USA. This chain is unique as it covers the crest and trough regions of the EIA latitudinally, and goes well beyond the anomaly region. The data obtained using this chain is used to generate tomograms for understanding the temporal and spatial evolution of equatorial and low-latitude ionospheric phenomena like EIA and ESF, and their interrelationships. A detailed discussion on the accuracies involved in the tomogram generation using the CRABEX data is already presented10. Even though, the beacon transmissions on board LEO satellites are useful to provide excellent spatial coverage, the temporal information offered by these techniques is largely limited by the number of satellites available. At present, there are only a few beacon satellites operational, mainly from NIMS and COSMOS (Russian) series and over the years their number has also decreased. As has been mentioned earlier, it is desirable to have a better temporal 163 coverage along with spatial information during varying geophysical conditions which necessitates more number of satellite based beacon payloads. As a first step in this direction and a logical extension of CRABEX, India’s first indigenous beacon, namely the Radio Beacon for Ionospheric Tomography (RaBIT) payload was conceived and developed at the Indian Space Research Organisation (ISRO). The RaBIT is onboard a small satellite, which is an Indo-Russian collaboration dedicated for the upper atmospheric studies, called the YOUTHSAT. The satellite YOUTHSAT was launched successfully from SHAR (13.7199°N, 80.2301°E), India on April 20, 2011. YOUTHSAT was placed in an 817 km orbit, with semi-major axis being 7195.12 km. Along with the RaBIT, YOUTHSAT has another scientific payload which is an indigenously developed airglow imager called the LiVHySI (Limb Viewing Hyper Spectral Imager). The RaBIT has the equatorial crossing time of 10:30 hrs IST at the descending node (20:30 hrs IST at the ascending node) and is having an orbit period of 101.35 minutes. YOUTHSAT having an inclination of 90.7°, the RaBIT has a repeativity of 22 days. The RaBIT beacons at two radio frequencies that are 150 and 400 MHz. The two frequencies are transmitted to eliminate the effect of satellite motion and tropospheric refractive index (which are nondispersive) since the ionospheric effect on radio signal is frequency dependent. In the conventional satellite beacons used for ionospheric studies, the coherent frequencies are generated from a single crystal oscillator using the multiplier technique. Though this degrades the phase noise of the signal, it requires sharp filters to remove harmonics and sub-harmonics which are generated in this technique. The RaBIT uses the frequency synthesis technique generating the beacon signals at desired frequencies using a single source. This ensures good stability and repeatability. The RaBIT beacons at 150 and 400 MHz at a power of 1.0 Watt, each giving very high signal to noise ratio when received at the ground. For the RaBIT beacon reception, the ground receiver chain is the same as the one currently used as part of the CRABEX. The basic data for ionospheric tomography is the line-of-sight total electron content estimated along a number of ray paths from a chain of ground receivers aligned along the same longitude. The line-of-sight total electron content at the receiver is obtained by
  • 3. 164 INDIAN J RADIO & SPACE PHYS, APRIL 2012 employing the Differential Doppler technique. Here, the measured data is the relative phase between 150 and 400 MHz, and is proportional to the relative slant TEC (STEC) along the propagation path of the signal as: φ = C D x STEC …(1) where, φ, is measured in radians; STEC is in m-2 and CD = 1.6132 × 10-15 for NNSS satellites11. Since, the phase measurements are accurate to < 3° when the ground receiver is at locked condition and the data sampling is at 100 Hz, these observations yield accurate estimates of the relative TEC with errors < 0.05%. The TEC data obtained at all the ground stations is kept at the Indian Space Science Data Centre (ISSDC). The ionospheric tomograms over the Indian region are generated using the TEC measured at each station and kept at the ISSDC for users. 3 Results and Discussion For the YOUTHSAT configuration, the tomogram covers the ionosphere from ~5° south of Trivandrum to ~4° north of Delhi depending upon the satellite elevation. The RaBIT tomography network is, perhaps, the longest operating network existing anywhere in the world and is unique, therefore. Figure 1 shows the YOUTHSAT (RaBIT) tracks across the globe. The first few tomograms are presented here representing the ionosphere over the 77°E meridian over India, obtained using the RaBIT observations made during the beginning of the solar cycle 24. Fig. 1—Dotted lines indicate the YOUTHSAT tracks across the globe; pink dots indicate the position of RaBIT receivers (RaBIT beacon is switched on only when its visible in the Indian longitude zone) Three tomograms are discussed here, two representing the ionosphere during the day and night on 2 July 2011; the third tomogram representing the nighttime on 25 October 2011 but for a different geomagnetic condition. The tomograms shown in Figs (2 and 3), respectively depict the altitudinal-latitudinal distribution of the electron density around the 77°E meridian over the Indian region at 10:50 hrs IST (i.e. daytime) and 21:53 hrs IST (i.e. nighttime) on 2 July 2011. The positions of the RaBIT ground receiving stations are marked in the tomogram as TVM (Trivandrum), BNG (Bangalore), HYD (Hyderabad), BPL (Bhopal), and DEL (Delhi). The curved black lines represent the terrestrial magnetic field. The distances mentioned are in geocentric reference frame. As is evident, the magnetic field lines are horizontal over TVM indicating that this receiving station is located right over the dip equator. The presence of two ionization crests at locations away from the equator and a trough over the equator is clearly evident in the tomograms. During daytime, the northern crest of ionization is found to be extending beyond Hyderabad with overall electron density being high at all the latitudes; the peak electron density being 1.12 × 1012 m-3 at an altitude of 320 km just beyond the HYD latitude. The electron density at the trough location, i.e. over the dip equator, is found to be almost half of that over the crest. It is interesting to note that while the overall electron density is less during the night, the crest is still prominent and is located closer to BNG, i.e. closer to the equator. The peak electron density during the night is 6 × 1011 m-3 at an altitude of 300 km just beyond the BNG latitude. The presence of the ionization crests and trough in the ionosphere, as is seen in the tomograms presented here, is the typical plasma density distribution over the low and equatorial latitudes, also known as the equatorial ionization anomaly (EIA). The day and night differences in the overall ionization density and the crest location can be attributed to: (a) solar radiation and (b) the equatorial electrodynamics. The evolution of the dynamo electric field over the equator, along with the conductivity changes, drives the EIA which in turn modifies the electron density distribution away from the equator. Nevertheless, the neutral and electrodynamics along with the composition over the low and equatorial latitudes can undergo substantial changes
  • 4. PANT et al.: RADIO BEACON FOR IONOSPHERIC TOMOGRAPHY 165 Fig. 2—Altitude-latitude cross-section of the ionosphere as obtained using the RaBIT beacon signals during daytime (10:50 hrs IST) on 02 July 2011 for the YOUTHSAT pass (position of RaBIT ground stations is marked; vertical black lines represent geomagnetic field configuration; color indicates the electron density) Fig. 3—Altitude-latitude cross-section of the ionosphere as obtained using the RaBIT beacon signals during nighttime (21:53 hrs IST) on 02 July 2011 for the YOUTHSAT pass (position of RaBIT ground stations is marked; vertical black lines represent geomagnetic field configuration; color indicates the electron density)
  • 5. 166 INDIAN J RADIO & SPACE PHYS, APRIL 2012 Fig. 4—Altitude-latitude cross-section of the ionosphere as obtained using the RaBIT beacon signals during nighttime on 25 October 2011 for the YOUTHSAT pass (A moderate geomagnetic storm commenced early on this day; position of RaBIT ground stations is marked; vertical black lines represent the geomagnetic field configuration; color indicates the electron density; the marked changes in the electron density distribution vis-à-vis Fig. 2 can be clearly seen in this tomogram) during geomagnetically disturbed periods. As a consequence, the plasma distribution over these latitudes show marked difference from that during quiet periods. This aspect is clearly demonstrated through the tomogram presented in Fig. 4. This tomogram depicts the electron density distribution at 21:46 hrs IST on 25 October 2011. In fact, a moderate geomagnetic storm commenced in the morning hours (3:30 hrs IST) of 25 October 2011, after a geomagnetically quiet period. The observed electron density distribution on this night is markedly different from what can be seen in the tomogram presented in Fig. 3. As can be seen in this tomogram, the electron density on the top-side of the ionosphere, especially between latitudes representing BNG and HYD is significantly enhanced. It must be mentioned that these changes in the top-side ionosphere can be observed only through the tomography technique. The single ionization crest appear to have been modified in such a way that there are two regions where the enhancement in the ionization is observed. These observed changes are attributed to the overall changes in the electrodynamics due to the storm. These changes are being investigated in detail. Nevertheless, a discussion of these is beyond the scope of present manuscript which intends to present the first few results and highlight the potential of the RaBIT based tomography for the Indian region. Acknowledgements The significant contributions made by the entire small satellite and RaBIT team are heartily acknowledged. Their efforts made the Indian beacon payload RaBIT a reality. References 1 2 Sastri J H, Sridharan R & Pant T K, Equatorial ionosphere thermosphere system during geomagnetic storms, in Disturbances in geospace: The storm-substorm relationship, Geophys Monogr Ser 142, edited by A Surjalal Sharma, Y Kamide & G S Lakhina (AGU, Washington, D C), 2002, pp 185-203. Abdu M A, Major phenomena of the equatorial ionospherethermosphere system under disturbed conditions, J Atmos Sol-Terr Phys (UK), 59 (1997) pp 1505-1513.
  • 6. PANT et al.: RADIO BEACON FOR IONOSPHERIC TOMOGRAPHY 3 4 5 6 7 Basu S, Groves K M, Basu Su & Sultan P J, Specification and forecasting of scintillations in communication/navigation links: Current status and future plans, J Atmos Sol-Terr Phys (UK), 64 (2002) pp 1745-1749. Austen J R, Franke S J & Liu C H, Ionospheric imaging using computerized tomography, Radio Sci (USA), 23 (3) (1988) pp 299-307. Kunitsyn V E, Andreeva E S, Tereshchenko E D, Khudukon B Z & Nygren T, Investigations of the ionosphere by satellite radiotomography, J Imag Syst Technol (UK), 5 (1994) pp 112-127. Andreeva E S, Franke S J, Yeh K C & Kunitsyn V E, Some features of the equatorial anomaly revealed by ionospheric tomography, Geophy Res Lett (USA), 27 (2000) pp 2465-2468. Thampi Smitha V, Ravindran Sudha, Devasia C V, Sreelatha P, Pant Tarun K, Sridharan R, Venkata Ratnam D, Sarma A D, Raghava Reddi C, Jose Jessy & Sastri J H, Coherent radio beacon experiment (CRABEX) for tomographic imaging of the equatorial ionosphere in the Indian longitudes - 167 Preliminary results, Adv Space Res (UK), 40 (3) (2007) pp 436-441. 8 Yeh K C, Franke S J, Andreeva E S & Kunitsyn V E, An investigation of motion of the equatorial anomaly crest, Geophy Res Lett (USA), 28 (2001) pp 4517-4520. 9 Ji-Sheng Xu, Shu-Ying M A, Xiong-Bin W U, Lin K H & Yeh K C, Tomographic imaging of low latitude ionosphere response to a magnetic storm, Chin J Geophys (China), 43 (2000) pp 159-166. 10 Thampi Smitha V, Pant Tarun K, Ravindran Sudha, Devasia C V & Sridharan R, Simulation studies on the tomographic reconstruction of the equatorial and low latitude ionosphere in the context of the Indian Tomography Experiment CRABEX, Ann Geophys (UK), 22 (2004) pp 3445-3460. 11 Leitinger R, Schmidt G & Tauriainen A, An evaluation method combining the differential Doppler measurements from two stations that enables the calculation of electron content of the ionosphere, J Geophys Res (USA), 41 (1975) pp 201-213.