Design of a flexible and low-power ionospheric sounder

Thesis (M.S.) University of Alaska Fairbanks, 2014Characterizing the structure of the ionosphere has practical applications for telecommunications and scientific applications for studies of the near-earth space environment. Among several methods for measuring parameters of the ionosphere is ionospheric sounding, a radar technique that determines the electron content of the ionosphere as a function of height. Various research, military, and commercial institutions operate hundreds of ground-based ionosondes throughout the globe, and new ionosondes continue to be deployed in increasingly remote and distant locations. This thesis presents the design of an ionospheric sounder that reduces the power, size, and cost compared to existing systems. Key improvements include the use of an open-source software-defined radio platform and channel-aware dynamic sounding scheduling.Chapter 1. Introduction -- 1.1. A brief historical background -- 1.2. The ionosphere -- 1.3. Instruments for studying the ionosphere -- 1.4. Thesis organization -- Chapter 2. Radio waves and the ionosphere -- 2.1. Dispersion relation of electromagnetic waves in the ionosphere -- 2.2. Power reflected from the ionosphere -- 2.3 Noise in the HF spectrum -- 2.4. Ionograms -- Chapter 3. Radar principles -- 3.1. Target detection -- 3.2. Range and doppler elocity -- 3.3. Range-doppler ambiguity -- 3.4. Resolution and precision --3.5. Multi-pulse integration -- 3.6. Pulse compression -- 3.7. Practical limits of performance -- Chapter 4. Survey of current systems -- 4.1. Coherent transmission/reception and digital systems -- 4.2. Phase-coded pulses -- 4.3. Coherent integration of multiple pulses -- 4.4. Phased antenna arrays -- 4.5. O- and X-mode discrimination -- Chapter 5. System description -- 5.1. Design approach -- 5.2. Overview of the Ettus Research USRP -- 5.3. Using the USRP as a radar -- 5.4. Waveform Generation -- 5.5. Processing the received signal -- 5.6. Scheduling -- 5.7. Completing the system -- Chapter 6. Sounding results -- 6.1. Single frequency soundings -- 6.2. Swept frequency soundings -- Chapter 7. Conclusion -- 7.1. Evaluation of performance -- 7.2. Costs -- 7.3. Future improvements -- 7.4. Deploying a terrestrial ionosonde -- 7.5. Deploying a space-borne ionosonde -- References

Design and Implementation of a Software Defined Ionosonde. A contribution to the development of distributed arrays of small instruments

In order to make advances in studies of mesoscale ionospheric phenomena, a new type of ionosonde is needed. This ionosonde should be relatively inexpensive 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 continuous 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 hardware 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 possible 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

FMCW Signals for Radar Imaging and Channel Sounding

A linear / stepped frequency modulated continuous wave (FMCW) signal has for a long time been used in radar and channel sounding. A novel FMCW waveform known as “Gated FMCW” signal is proposed in this thesis for the suppression of strong undesired signals in microwave radar applications, such as: through-the-wall, ground penetrating, and medical imaging radar. In these applications the crosstalk signal between antennas and the reflections form the early interface (wall, ground surface, or skin respectively) are much stronger in magnitude compared to the backscattered signal from the target. Consequently, if not suppressed they overshadow the target’s return making detection a difficult task. Moreover, these strong unwanted reflections limit the radar’s dynamic range and might saturate or block the receiver causing the reflection from actual targets (especially targets with low radar cross section) to appear as noise. The effectiveness of the proposed waveform as a suppression technique was investigated in various radar scenarios, through numerical simulations and experiments. Comparisons of the radar images obtained for the radar system operating with the standard linear FMCW signal and with the proposed Gated FMCW waveform are also made. In addition to the radar work the application of FMCW signals to radio propagation measurements and channel characterisation in the 60 GHz and 2-6 GHz frequency bands in indoor and outdoor environments is described. The data are used to predict the bit error rate performance of the in-house built measurement based channel simulator and the results are compared with the theoretical multipath channel simulator available in Matlab

Characterising Spatial and Temporal Ionospheric Variability with a Network of Oblique Angle-of-arrival and Doppler Ionosondes

Ionospheric variability exists on a broad range of scales, and routinely impacts skywave propagation modes of high frequency radio waves, to the detriment of radar and communication systems. In order to better understand the electron density structures associated with such variability at mid-latitudes, a network of oblique angle-of-arrival (AoA) and Doppler ionosondes were installed in central and northern Australia as part of the ELOISE campaign in 2015. This thesis analyses observations from the ELOISE AoA ionosondes, with a focus on characterising the influence of medium- to large- scale gradients and signatures of travelling ionospheric disturbances (TIDs). Following an overview of the experiment, the design and calibration of the new ionosonde system is described. With multi-channel receivers connected to each element of two twin-arm arrays, a total of eleven AoA paths of between 900 and 2700 km were collected, including nine with interleaved Doppler measurements using a special channel scattering function (CSF) capability. On-board signal processing was developed to perform real-time clear channel evaluation and CSF scheduling, and generate the AoA ionograms and delay-Doppler images with fitted electron density profiles. In further offline analysis, peak detection and mode classification was carried out, to support reflection point mapping and tilt estimation. Significant testing and validation of the new ionosonde before and after the experiment revealed AoA uncertainties on the scale of 0.2–0.5° in bearing and 0.4–0.9° in elevation. Having identified a low-elevation bias, models of tropospheric refraction and antenna mutual coupling effects were considered as possible correction strategies, but ultimately an empirical approach based on aggregated ionospheric returns was implemented. Small-scale (intra-dwell) ionospheric variability also has the potential to compromise results, through unresolved multi-mode mixing, and this has been investigated using a combination of spatial and temporal variability metrics derived from the CSF data. The analysis of large quantities of F2 peak data shows persistent diurnal patterns in the oblique AoA observables that are also well-captured by a conventional data-assimilative ionospheric model, even without the benefit of AoA and Doppler inputs. Furthermore, Doppler measurements are reproduced remarkably well using just the midpoint fitted profiles. A statistical study has quantified the level of consistency between observations and model, to provide greater confidence in the results. Many of the geophysical features can be interpreted as ionospheric gradients, as evident in the tilt estimates, and horizontally moving structures such as TIDs, using a form of Doppler-based drift analysis. While signatures of TIDs vary considerably, two simple wave-like perturbation models have been evaluated to help classify quasi-periodic behaviour in the AoA observations, as well as understand the directional filtering effect imposed by the path geometry. In some cases, a set of TID parameters can be determined by eye, but in others automatic parameter inversion techniques may be more viable. Two such techniques were implemented but results using both real and synthetic data demonstrated some significant limitations. Finally, attempts to relate TID signatures across multiple paths shows promise, but there still appears to be a strong dependence on path geometry that is difficult to eliminate.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 202

Improved SuperDARN radar signal processing: A first principles statistical approach for reliable measurement uncertainties and enhanced data products

Ground-based radar systems are the best way to continuously monitor medium-to-large-scale features of the near-Earth space environment on a global scale. The Super Dual Auroral Radar Network (SuperDARN) radars are used to image the high-latitude ionospheric plasma circulation, which is produced by magnetosphere-ionosphere coupling processes generated by the interaction of both the solar and terrestrial magnetic fields. While investigating ways to expand the usable data products of SuperDARN to include electron density inferred using a multiple-frequency technique, it was determined that SuperDARN error estimates were lacking sufficient rigour. The method to calculate SuperDARN parameters was developed approximately 25 years ago when available computing resources were significantly less powerful, which required a number of simplifications to ensure both valid data and reasonable processing time. This resulted in very conservative criteria being applied to ensure valid data, but at the expense of both rigorous error analysis and the elimination of some otherwise valid data. With access to modern computing resources, the SuperDARN data processing methodology can be modernized to provide proper error estimates for the SuperDARN parameters (power, drift velocity, width). This research has resulted in 3 publications, which are presented here as Chapters 5, 6, and 7. The error analysis started with a first principles analysis of the self-clutter generated by the multiple-pulse technique that is used to probe the ionosphere (Chapter 5). Next, the statistical properties of voltage fluctuations as measured by SuperDARN were studied and the variance of these measurements were derived (Chapter 6). Finally, the statistical error analysis was propagated to the standard SuperDARN data products using a new First-Principles Fitting Methodology (Chapter 7). These results can be applied to all previously recorded SuperDARN data and have shown a practical increase in data of >50%. This has significant impact on the SuperDARN and space science communities with respect to, for example, global convection maps and their use in global modelling efforts. These results also enable quantitative experiment design facilitating research into using SuperDARN to provide electron density measurements, with a preliminary investigation using the new SuperDARN fitting methodology presented in Chapter 8

The Ionospheric Continuous-wave E-region Bistatic Experimental Auroral Radar (ICEBEAR)

The Sun drives many atmospheric processes on Earth through solar electromagnetic radiation, the solar wind, and the solar magnetic field. These solar phenomena interact with a region around the Earth where plasma can be formed, the ionosphere. This region is located 60–1000 km above the surface of the Earth, and is of interest to many scientists and engineers due to the interaction between radio waves and plasma. Variations in the ionospheric plasma density can cause disruptions to GPS signals and radio communications. Attempts have been made to measure the ionospheric plasma properties through the use of rockets, satellites, and remote sensing instrumentation. One of the issues with measuring the ionosphere, specifically the lower altitudes of the ionosphere, is that it is expensive to do in situ. Rockets are required for in situ measurements at altitudes of 90–150 km (the E-region of the ionosphere). Rocket launches are expensive, so more efficient remote methods of measuring the E-region are typically used. This includes radars utilizing radio waves to scatter from the ionospheric plasma. From the scattered signal, plasma properties can be derived to provide insight into the physical processes occurring. The Ionospheric Continuous-wave E-region Bistatic Experimental Auroral Radar (ICEBEAR) was developed to probe the E-region of the ionosphere using this mechanism. Through the use of modern radar hardware and techniques, it was possible to obtain simultaneously high temporal (down to 0.1 s) and spatial (≈ 1.5 km) resolution images of ionospheric plasma density perturbations over a 600 km × 600 km field of view. The radar operates at 49.5 MHz and transmits a continuous-wave, pseudo random noise, phase modulated code to obtain these images. The radar is bistatic, with both transmitter and receiver being located in Saskatchewan, Canada, and operated by the University of Saskatchewan. The radar was designed with future improvements in mind, where each transmitter and receiver antenna are individually controlled/sampled. This Ph.D. dissertation describes the dynamics of the ionosphere, the design and construction of ICEBEAR, and presents some preliminary results, exhibiting the exciting modern capabilities of the system

Remote Sensing

This dual conception of remote sensing brought us to the idea of preparing two different books; in addition to the first book which displays recent advances in remote sensing applications, this book is devoted to new techniques for data processing, sensors and platforms. We do not intend this book to cover all aspects of remote sensing techniques and platforms, since it would be an impossible task for a single volume. Instead, we have collected a number of high-quality, original and representative contributions in those areas

Report on TID algorithms

This deliverable presents the TID detection algorithms as improved in response to design principles stated in T2.1 and their testing in the lab environment, verification against measurements taken during quiet and disturbed periods of time, benchmarking for their transition to operations, and final validation to the user requirements of accuracy, timeliness, and coverage.TechTIDE project, funded by the European Commission Horizon 2020 research and innovation program [AD-1], will establish a pre-operational system to demonstrate reliability of a set of TID (Travelling Ionospheric Disturbances) detection methodologies to issue warnings of the occurrence of TIDs over the region extending from Europe to South Africa. TechTIDE warning system will estimate the parameters that specify the TID characteristics and the inferred perturbation, with all additional geophysical information to the users to help them assess the risks and to develop mitigation techniques, tailored to their application. This document is TechTIDE D2.2 “Report on the TID algorithms” and it is an output of TechTIDE Task 2.2 (Development of the TID identification algorithms and products) of the WP2 (TID identification methodologies) which has the final goal to release the basic algorithms for the TID identification and to test a first version of the value-added products for implementation in the TechTIDE warning system. The document highlights four aspects of the TID algorithm release process, (1) Developmentbased on the concept, techniques, and algorithms as stated in TechTIDE D2.1, (2) Verification, an internal testing process that ensures algorithm correctness, (3) Benchmarkingneeded to prepare algorithms to transition to operations, and (4) Validation, an external process of ensuring that developed algorithms are compliant with the stated end user expectations.Postprint (published version

Virtual SATCOM, Long Range Broadband Digital Communications

The current naval strategy is based on a distributed force, networked together with high-speed communications that enable operations as an intelligent, fast maneuvering force. Satellites, the existing network connector, are weak and vulnerable to attack. HF is an alternative, but it does not have the information throughput to meet the distributed warfighting need. The US Navy does not have a solution to reduce dependency on space-based communication systems while providing the warfighter with the required information speed. Virtual SATCOM is a solution that can match satellite communications (SATCOM) data speed without the vulnerable satellite. It is wireless communication on a High Frequency (HF) channel at SATCOM speed. We have developed an innovative design using high power and gain, ground-based relay systems. We transmit extremely wide-wideband HF channels from ground stations using large directional antennas. Our system starts with a highly directional antenna with a narrow beam that enables increased bandwidth without interfering with other spectrum users. The beam focus and power provide a high SNR across a wideband channel with data rates of 10 Mbps; 1000 times increase in HF data speed. Our modeling of the ionosphere shows that the ionosphere has more than adequate bandwidth to communicate at 3000 km and high speeds while avoiding detection. We designed a flexible structure adjustable to the dynamic ionosphere. Our design provides a high-speed communications path without the geo-location vulnerability of legacy HF methods. Our invention will benefit mobile users using steerable beam forming apertures with wide bandwidth signals. This dissertation will focus on three areas: an examination of the ionosphere’s ability to support the channel, design of a phased array antenna that can produce the narrow beam, and design of signal processing that can accommodate the wideband HF frequency range. Virtual SATCOM is exciting research that can reduce cost and increase access to long-range, high data rate wireless communications