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Design and Implementation of a Software Defined Ionosonde

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Design and Implementation of a Software Defined Ionosonde Faculty OF Science AND TECHNOLOGY Department OF Physics AND TECHNOLOGY Design AND IMPLEMENTATION OF A SoftwarE DefiNED IONOSONDE A CONTRIBUTION TO THE DEVELOPMENT OF DISTRIBUTED ARRAYS OF SMALL INSTRUMENTS — Markus Floer FYS-3931: Master’S THESIS IN SPACE physics, June 2020 Design and Implementation of a Software Defined Ionosonde Markus Floer A thesis for the degree of Magister Scientiae Department of Physics and Technology, Faculty of Science and Technology, University of Tromsø – The Arctic University of Norway in cooperation with Department of Arctic Geophysics, The University Centre in Svalbard June 2020 AbstrACT In order to make advances in studies of mesoscale ionospheric phenomena, a new type of ionosonde is needed. This ionosonde should be relatively in- expensive and small form factor. It should also be well suited for operation in a network of transmit and receiver sites that are operated cooperatively in order to measure vertical and oblique paths between multiple transmitters and receivers in the network. No such ionosonde implementation currently exists. This thesis describes the design and implementation of a coded contin- uous wave ionosonde, which utilizes long pseudo-random transmit waveforms. Such radar waveforms have several advantages: they can be used at low peak power, they can be used in multi-static cooperative radar networks, they can be used to measure range-Doppler overspread targets, they are relatively robust against external interference, and they produce relatively low interference to other users that share the same portion of the electromagnetic spectrum. The new ionosonde design is thus well suited for use in ionosonde networks. The technical design relies on the software defined radio paradigm and the hard- ware design is based on commercially available inexpensive hardware. The hardware and software implementation is shown to meet the technical and scientific requirements that were set for the instrument. The operation of the instrument is demonstrated in practice in Longyearbyen, Svalbard. With this new ionosonde design and proof of concept implementation, it has been possi- ble to re-establish routine ionospheric soundings at Longyearbyen, Svalbard; to replace the Dynasonde instrument that was decommissioned several years ago. It is also possible to use this new design as a basis for larger networks of ionosondes. The software and hardware design is made publicly available as open source, so that anyone interested can reproduce the instrument and also contribute to the project in the future. I II AcknoWLEDGEMENTS I would like to thank my brilliant supervisors; Juha Vierinen, Lisa Baddeley and Mikko Syrjäsuo for guiding me through this project. Thank you, Juha, for sharing your vast knowledge with me and including me in your projects. Your enthusiasm and skill has inspired me to endeavour further into the world of radar science. Thank you, Lisa, for your sage advice, for believing in me, and for your top-notch quality British witticisms. Thank you, Mikko, for your excellent input and all the help and support in completing this thesis. I would like to thank The University Centre in Svalbard for providing the coolest working environment a student could ask for. Thank you to all my friends that I’ve had the pleasure of sharing this great experience with. Thank you to Rasmus and Eike for trusting me with your dogs, and Anna for showing me the ropes of dog sledding. A big thank you to my peers on the mainland who I spent the previous years studying with. Finally, a huge thank you to my family, for all the support and for believing in me all this time. III IV Contents List of Figures ix 1 Introduction 1 2 Background 5 2.1 The Ionosphere ........................ 5 2.1.1 Key physical characteristics of the ionosphere .... 6 2.1.2 High latitude ionosphere and phenomena ...... 11 2.1.3 Typical ionospheric plasma parameters over Svalbard 14 2.2 Propagation of Electromagnetic Waves in the Ionosphere .. 17 2.2.1 The Appleton-Hartree equation ............ 18 2.3 The Ionosonde ......................... 21 2.3.1 The Breit & Tuve experiment ............. 21 2.3.2 Modern ionosondes .................. 23 2.3.3 Ionosondes in the study of the ionosphere ...... 25 2.3.4 Ionosonde applications in a larger network ...... 26 2.4 Radar .............................. 27 2.4.1 Radar equation ..................... 27 2.4.2 Gain .......................... 28 2.4.3 Noise and signal to noise ratio ............ 29 2.5 Radar Signal Processing .................... 30 2.5.1 Fourier transform ................... 30 2.5.2 Complex baseband representation .......... 31 2.5.3 Radar range resolution and pulse compression .... 33 2.5.4 Coherent radar signal processing ........... 36 2.5.5 Spread spectrum sounding .............. 38 2.6 Software Defined Radio .................... 39 3 Longyearbyen Ionosonde Design & Implementation 41 3.1 Design ............................. 41 3.1.1 Open source code ................... 42 3.1.2 Low cost, commercially available hardware ...... 42 3.1.3 Local considerations .................. 42 3.1.4 Power consumption .................. 43 V VI CONTENTS 3.1.5 Form factor ....................... 43 3.1.6 Software user interface ................ 43 3.2 System Requirements ..................... 44 3.2.1 Frequency ....................... 45 3.2.2 Bandwidth limitations ................. 46 3.2.3 Antennas ........................ 47 3.2.4 Approximating transmit power ............ 48 3.2.5 Maximum range extent ................ 52 3.2.6 Maximum Doppler extent ............... 53 3.2.7 Timing ......................... 54 3.2.8 Transmit signal ..................... 54 3.3 Implementation ........................ 54 3.3.1 USRP - Universal Software Radio Peripheral ..... 55 3.3.2 Architecture ...................... 57 3.3.3 Hardware ....................... 60 3.3.4 Software ........................ 62 3.3.5 Transmit waveform .................. 68 4 System Testing 71 4.1 Laboratory Testing ....................... 71 4.1.1 Measuring maximum output power .......... 72 4.1.2 Software and preliminary loopback testing ...... 74 4.1.3 Complete system test and measurements ....... 75 4.2 Field Testing .......................... 81 5 Results 83 5.1 Scientific Results ........................ 86 5.2 Engineering Results ...................... 89 5.2.1 Testing results ..................... 91 5.2.2 Effective radiated power ................ 91 5.2.3 Software ........................ 93 5.3 Discussion ........................... 93 5.3.1 Power consumption and cost of system ........ 95 6 Conclusions 97 6.1 Suggestions for Further Work ................. 98 6.1.1 Separate O- and X-mode ................ 98 6.1.2 Additional frequencies ................. 98 6.1.3 Reducing power consumption ............. 98 6.1.4 ULF waves ....................... 98 A Dispersion Relation Derivation 101 B Appleton-Hartree Derivation 107 VII C Set-up and Installation Procedure 111 C.1 Hardware dependencies .................... 111 C.2 Software dependencies .................... 112 C.3 Installation Instructions .................... 112 D Source Code 115 Bibliography 125 VIII CONTENTS List OF FigurES 2.1 Plot showing the maximum ionization rate q occurring at the intersection between radiation intensity I and neutral density n, all functions of altitude. Figure adapted from Brekke [2012]. 7 2.2 Logarithmic scale plot of the electron density between 0 - 800 km altitude. Figure adapted from Baddeley [2019]. .. 10 2.3 The auroral oval and the polar cap. Both plots adapted from Baumjohann and Treumann [1997]. ............. 12 2.4 Data from the IRI model showing electron density, electron temperature and ion temperature in the altitude range 80- 800 km over Longyearbyen, Svalbard. The 1st column shows dayside conditions, the 2nd column shows nightside condi- tions. Temperature along upper x-axes, electron density along lower x-axes. Altitude along y-axes. .............. 16 2.5 Example of how refraction of an electromagnetic wave might look like in the ionosphere. The refractive index changes with altitude because of the changing plasma parameters, mag- netic field and collisional frequency. The point at which the wave refracts down towards earth again is the point where the refractive index is below unity and the group velocity re- verses. Plot adapted from Baddeley [2019]. ......... 20 2.6 Map highlighting existing ionosonde sites with geographic latitude and longitude as well as geomagnetic (outdated) lat- itude. Image obtained from ngdc.noaa.gov. .......... 23 2.7 Sample ionogram from the Gakona digisonde in Alaska, Novem- ber 10th, 2007 at 2230 UT. Data points are coloured to high- light which polarization the echo has and from which direc- tion it came from. The y-axis displays the virtual range and the x-axis displays the frequency of the echo. Downloaded from HAARP: http://www.haarp.alaska.edu/haarp/dsonde.html 26 2.8 Sky noise temperatures from different sources based on the ITU P.372-8 recommendation. Plot adapted from Lott et al. [2006]. ............................. 30 IX X L I STOFFI GURES 2.9 Digital down conversion and up conversion used to transform real signals to and from the complex baseband representation (Vierinen, personal communications). ............ 33 2.10 The spatial extent of the pulse determines the range resolution. 34 2.11 Example of a pulse compression scheme that uses two phase offsets to separate the sub-pulses from eachother to achieve higher range-resolution. .................... 34 2.12 Range-time diagram for a transmission
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