INTRODUCTION. 1.1 Background. Chapter -1

Transcription

1 Chapter -1 INTRODUCTION 1.1 Background Beacon experiments onboard satellites have been contributing to atmospheric research since the eighties. According to Oxford dictionary, Beacon specifies a single frequency continuous signal used for signalling or warning. Such an application becomes necessary in navigation. Thus for navigational purposes what started off as a single frequency beacon satellite signal soon gave way to dual frequency coherent signals transmitted from the satellites. The extensive use of these beacon signals opened up ionospheric research by measurement of two basic parameters: Total Electron Content (TEC) and Scintillation index. According to Davies [1989] and Brussard and Rogers [1990], TEC is described as the total electron density along the straight ray path from the satellite to the ground receiver. The CCIR-ITU recommendations [1992] define the change or fluctuations in the amplitude and/or phase of the radio waves, when they traverse through the ionosphere, as scintillations, which are measured by assigning indices to the data. In his Lecture Notes, Sarma [2004] also mentions this as Scintillation indices. The initial ship-to-shore navigation was followed by aircraft navigation and the satellite signal structure also underwent changes accordingly. In the current scenario, these navigational aids depend heavily on Global Positioning System (GPS) satellites which use spread spectrum modulated signals. Ionospheric research using beacon satellites also spread its wings to understand and characterise the ever changing ionospheric medium, which affects all satellite communication links. Studies on TEC led to its application in ionospheric tomography, a new technique for imaging the electron density variations. Details of the technique and its methods have been reported by Austen et al [1988], Kunitsyn and Tereshchenko[1994], Kersley and Pryse [1996], Leitinger [1997], Andreeva et al [1992] and Raghava Reddy [2004], to name a few. Such tomograms give a 2D or 3D picture of the 1

2 electron density variations over a specific area by using simultaneous data from various ground receivers. Various research studies have been attempted for characterisation of TEC and scintillation indices throughout the world. Still ongoing research works in these areas indicate the importance of this, with more unanswered questions that need to be addressed. At this juncture, it is also apt to bring out the importance of latitudinal dependence of these ionospheric parameters. It is a known fact that the behaviour of ionosphere at low (< 23 ) and high latitudes (> 65 ) is highly unpredictable compared to its predictable nature in mid latitudes as mentioned by Rishbeth et al [1969], Ratcliffe [1972]. Hence, even the popular and widely accepted ionospheric models like IRI (International Reference Ionosphere) are not able to model low latitude ionosphere properly as reported by Magdaleno et al [2011]. It is thus all the more necessary that more research is done in these areas from low latitude locations like India (having a latitudinal extent of ~8 to ~37 ), so as to get a proper handle on the ionospheric behaviour and its influence on communications and space weather as indicated by Lakshmi [2004]. With this objective in mind, the measurement of TEC has been attempted with un-modulated and modulated coherent satellite beacon signals. Such systems consist of a coherent beacon transmitter onboard a satellite and one or more similar coherent receivers at the ground. These satellite transmitters are positioned in different orbital heights which make inter-comparison less accurate. The unmodulated signals in use are coherent and continuous signals while the initial modulated signal used was amplitude modulated. The unmodulated coherent beacon signals transmitted mostly from Low Earth Orbiting (LEO) satellites have the disadvantage of an initial phase ambiguity in TEC measurement for every satellite pass. The amplitude modulated signals transmitted earlier from geosynchronous orbits can overcome the above limitation but require higher power as propagation losses are more. Spread spectrum modulation used in the GPS systems provides a more accurate measurement of TEC than the former methods, though they are affected by biases because of simultaneous multi-satellite reception, as reported by Coco et al [1991], Sardon et al [1994], Sarma [2004], Richard Dear and Cathryn Mitchell [2006] among various others. 2

3 In the present work, the measurement of TEC with different satellite configurations are addressed in detail, first with existing systems, followed by a simulation based on specific spread spectrum coded signals suited for the particular orbit. 1.2 Thesis objectives The present study involves detailed understanding of the three different beacon configurations corresponding to LEO, GEO and MEO satellites. The existing Low Earth Orbiting (LEO) satellite beacons transmit dual frequency coherent signals in the ratio of 3:8 as reported by the Spacewarn bulletin of NASA and regularly updated in the webpage of Robert Christy. The geostationary (GEO) beacons had single sideband suppressed carrier (SSBSC) amplitude modulated signals at the two frequencies in addition to the two carriers as detailed in the technical report of Venkataramanan et al [2002]. The Medium Earth Orbiting (MEO) beacon satellites, popularly grouped under the Global Navigation Satellite Systems (GNSS) use coherent spread spectrum modulated signals. In order to understand the hardware aspects in the reception of these signals, we have attempted to address the various facets in the design and development of receiver systems for TEC measurement in each configuration. The details of the unmodulated beacon signals in LEO satellite configurations are studied first and a receiver system is developed according to the details provided in the SPL Technical Report [2005]. It is understood that these received signals have an inherent initial phase ambiguity during every satellite pass. To tackle this, an orthogonal carrier type spread spectrum signal scheme is proposed and a simulation study with the same is carried out in laboratory. In the practical scenario, this phase ambiguity has been addressed by the use of geostationary beacon satellites. Here, amplitude modulated signals at two coherent frequencies are transmitted by the satellite beacon and a receiver system is developed as detailed in SPL Technical Report [2006] and installed at Space Physics Laboratory for reception of the same. Because of the constraints in the satellite system and the increased path loss, the system had to deal with more unlock 3

4 conditions than the former. A simulation study is attempted to address this using a orthogonal code spread spectrum signal like the CDMA signal. An attempt to understand the design methodology behind a spread spectrum beacon system was taken up next by considering the existing Global Positioning Satellite (GPS) system, which is the most popular configuration in GNSS now. In the present available configuration, GPS transmits two spread spectrum signals at L-band. One of this is modulated with two different Gold codes, the C/A code and P-code and the other with only one of the above Gold code i.e. C/A code. Thus the GPS system can be categorized as a multi-carrier multi-coded spread spectrum system. As GPS finds a wide variety of applications and various types of receivers specific to different applications are readily available as reported by Madhu [2004] and Purushotham [2004], a detailed study of the methodologies for TEC measurement is attempted and a suitable receiver is selected for the study. A simulation study for phase measurement is also attempted with dual carrier orthogonal coded spread spectrum system, which can be categorized as a minimal form of the multi-carrier multi-coded spread spectrum system used in GPS. However, in practice, it is seen that as several satellites are simultaneously tracked, the individual bias errors impedes accurate TEC measurement. Accordingly, it is seen that a proper and consistent solution for accurate and continuous measurement of TEC, useful for low latitude ionospheric studies does not exist as of now. This gap is the motivation to investigate this difficult but exciting field. This thesis work thus aims to bring out a comprehensive satellite beacon program suitable for ionospheric studies and handle the above limitations. 1.3 Scope and outline of thesis The results from this thesis work are presented in the following chapters which address the implementation details of existing different beacon receiver systems as well as simulation studies attempted for a future spread spectrum beacon. The major contributions of this work are listed below- 4

5 Studied and analyzed the importance of ionospheric studies through TEC measurements. Investigated and examined the various methods of prevailing TEC measurements. Designed and developed a coherent beacon receiver system for measurement of TEC from existing LEO beacon satellites. A new technique of orthogonal multi-carrier spread spectrum system is simulated and examined for a Gaussian channel to address the problem of initial phase ambiguity with the LEO beacon signals. Designed and developed a receiver system for measurement of TEC by three methods from GSAT-2 CRABEX beacon signals. An innovative orthogonal multi-coded spread spectrum system capable of addressing multipath reception is simulated and its advantages studied in comparison with SSBSC modulated GSAT beacon. Studied the methods of TEC extraction from existing GPS satellites, which use multi-frequency, multi-coded spread spectrum signals. A novel method of phase measurement using dual carrier orthogonal coded spread spectrum signals is simulated, which can find application in any beacon systems for ionospheric studies. Thus this study attempts to show the advantage of using spread spectrum techniques for satellite based ionospheric studies. The results of these investigations are presented in the thesis in further five chapters. Chapter 2 touches upon some aspects of multicarrier and multi-coded spread spectrum systems. Some of the basic blocks used for communication system simulation are also mentioned. Chapter 3 gives a detailed description of the various theoretical methods of TEC measurement, which is applicable for coherent systems, both modulated and unmodulated. With an introduction to ionosphere and why ionospheric studies are important, the chapter throws light on how satellite based systems provide a 5

6 different scenario to ground based systems for ionospheric measurements. The chapter also tells about the various orbits and a comparison of the different beacon payloads in these orbits. The chapters that follow give details of the satellite beacons in different orbital heights. Chapter 4 details the configuration of existing LEO beacon satellites used for TEC measurements. Various aspects in the design and development of the hardware and software of a ground receiver for reception of these signals are dealt with in detail. Typical results from such a system for a typical LEO satellite pass are shown. The major shortcoming of the system is brought out and a proposal for a spread spectrum beacon is presented to address this. Preliminary results on the simulation studies conducted for a spread spectrum beacon is also presented. Chapter 5 covers the Indian geosynchronous beacon payload called CRABEX (Coherent Radio Beacon Experiment), which was flown onboard India s GSAT-2 satellite. The differences of this system from a LEO beacon system is explained followed by the design and implementation details of a ground receiver system for this. Various problems faced during the installation of this receiver system are also briefly addressed. The basic limitation in this system is summarized followed by details on simulation of a method of modulating the carriers with orthogonal codes. Typical simulation results are also shown. Chapter 6 looks into an existing scenario where spread spectrum is already being used for ionospheric studies with a Medium Earth Orbiting satellite, the Global positioning Satellite (GPS). A detailed literature survey carried out has helped to consolidate the available details of the signal structure for the different frequencies of Global Navigation Satellite Systems (GNSS). This is followed by details of the onboard GPS transmitter, which can be considered as a multi-carrier and multi-code spread spectrum system. Details of the available receiver architectures for GPS reception suitable for TEC measurement is also addressed, with typical result from a dual frequency GPS receiver. The major limitation existing in such a system is also briefed. A sample simulation program using both dual carrier and orthogonal coding is attempted and some major results of this are also presented here. 6

7 Chapter 7 summarizes the present work on the usage of spread spectrum techniques for TEC measurements for atmospheric studies. The cases of unmodulated beacon system in LEOS, amplitude modulated beacon signals in GEOS as well as spread spectrum modulated signals in MEOS is addressed. The difficulty faced in each of these systems is also mentioned, with a proposal to address the problem by the use of an appropriate spread spectrum beacon system. The proposed system and technique are simple, cost effective and outperforms any of the systems used today. The work of this thesis can very well find application in future beacon satellite systems for ionospheric studies. 7