Space Internet of Things (Space-IoT)

The Internet of Things (IoT), a recent technology that has enabled many innovative applications, has dominated the world by creating smart systems and applications. It is predicted that the number of connected gadgets in IoT will double by 2030 compared to 2020. Thus, it is essential to address the key challenges, such as scalability, ubiquitous global coverage, and real-time connectivity, that arise due to this immense growth of IoT devices. However, extending the existing terrestrial networks such as mobile towers to under-served regions of the world including remote areas, oceans, and mountains, to achieve scalability and global coverage is not a cost-viable solution. Space, on the other hand, can be a suitable platform to solve the majority of these existing/upcoming problems in the IoT domain.

Space is the next frontier for innovations in IoT. The main idea is to employ space technologies for IoT applications. Space Internet of Things (Space-IoT), as we call, is a concept that involves a satellite, or a network of them, to address the main challenges in terrestrial IoT deployments – global coverage, scalability, and connectivity. Space-IoT is opening up a world of new possibilities for several applications.

Small satellites are the building blocks of Space-IoT. They represent a formidable mobile computing platform enabling large-scale space applications at a fraction of the cost of larger satellites. Space-IoT calls for hundreds or thousands of small satellites that can communicate directly with various IoT devices on Earth. However, access to space has been expensive due to the high satellite development and launch costs. Miniaturizing a satellite can reduce launch costs but presents a range of interdisciplinary challenges that must be tackled. Resources are severely constrained in terms of size, mass, and available power. Addressing these challenges requires different communities to push the envelope in the design and realization of miniaturized subsystems of a small satellite.

In this dissertation, we chart out a vision for Space-IoT and innovations in embedded and wireless systems for Space-IoT applications. We enlist several important challenges that need to be addressed immediately to bring the vision of Space-IoT to reality. This thesis targets one of the most significant tradeoffs – miniaturization leading to constrained energy while not compromising the reliability of operations of subsystems. We consider three subsystems of a satellite: communication, attitude determination, and health monitoring, to demonstrate the inter-dependencies and novel ways to tackle them. Further, we explain with examples what we envision for the next decade to facilitate Space-IoT.

In Space-IoT, the IoT nodes on Earth are expected to communicate with (small) satellites directly over hundreds of kilometres. Both these terrestrial nodes and the satellites in space are energy-constrained. Hence, the communications must not only be energy-efficient but also support long range. Moreover, the received signal strength and the Signal to Noise Ratio (SNR) on the receiver decrease as the communication distance increases. Further, Doppler shift is inevitable in Low Earth Orbit bound satellite communication. Boosting the transmission power and adopting high-gain large antennas are obvious solutions for reliable communication, however, not feasible with miniaturization and energy minimization as our objectives. One of the solutions to support low-power, long-range communication is to improve the demodulation technique to decode signals with low SNR.

In this dissertation, we revisit the demodulation approach of a widely used modulation technique - Frequency Shift Keying (FSK). We propose a scheme to demodulate bandpass sampled FSK signals that are influenced by Doppler shift and low SNR. Unlike the state-of-the-art techniques, our approach does not compensate for the Doppler shift but lives with it. To suppress the Doppler effect and improve the SNR of the received signal, we employ a matched filter and the Teager Energy Operator, respectively. With extensive evaluations using actual telemetry signals from two satellites, we demonstrate how our proposed technique outsmarts the state-of-the-art FSK demodulation schemes.

Besides the communication subsystem, Global Positioning System (GPS) is one of the essential but significantly energy-guzzling subsystems in a satellite. While big satellites typically do not have any constraints on energy consumption for GPS subsystem, such is not the case in miniaturized satellites. Unlike terrestrial GPS systems, several challenges are imposed on obtaining a position fix in space-borne GPS receivers. The high orbital velocities of a satellite (up to 7.8 km/s) result in a significant Doppler shift in the received signals by the receiver when compared to their terrestrial counterparts. Consequently, the receiver has to search for the GPS signals in a larger Doppler frequency range, thus increasing the signal acquisition duration. Further, the visibility of the GPS satellites to the receiver changes frequently due to high orbital speeds and orbital periods of satellites on which the receiver is mounted. As a result, the receiver needs to search for GPS satellites more often to get a position fix. Likewise, the visibility of GPS satellites is affected adversely if the satellite is tumbling. Due to these constraints, energy conservation techniques such as duty-cycling are not efficient; the receiver is ON most of the time, searching for GPS satellites to obtain a position fix.

To this end, we design a low-power, space-qualified GPS receiver for small satellite applications. We propose an algorithm to significantly improve the ability of the receiver to acquire GPS signals as quickly as possible, thus reducing the ON time when it is duty-cycled. We perform long-duration simulations and real-time in-orbit tests on our GPS receiver to evaluate its performance. Further, we demonstrate that up to 96% of energy savings can be achieved on our GPS receiver compared to the state-of-the-art receivers.

Space-IoT relies on a constellation of hundreds of satellites to accomplish global coverage. Disruption in services can occur if one of the satellites malfunctions or ceases to work. Certain applications may not endure such risks, especially where satellites are typically employed as secondary communication channels. Hence, it is crucial to monitor the health of satellites regularly.

Existing satellites are generally equipped with onboard health monitoring units as a part of the subsystems. However, they are tightly coupled in terms of hardware and software. Any fault in the subsystem may affect its onboard health monitoring modules as they are electrically connected. Hence, we propose a system called Chirper, which is an electrically isolated and independent module that monitors the health of critical subsystems. The Chirper is equipped with multiple sensors that can measure several parameters, such as temperature, bus voltage, current, and rotation rate, of a satellite at specified intervals and transmits them to ground stations through an independent communication module.

The proposed system is not only energy-efficient but also measures the different health parameters of a satellite reliably. This work mainly addresses the resilience and energy issues of a satellite. In this dissertation, we present the overall design of the Chirper. We also provide
a novel approach to measuring the DC voltage at different locations of a satellite in a completely isolated way. Further, we subject Chirper to different tests in state-of-the-art simulators and a helium balloon to evaluate its capabilities.

This thesis advocates that Space-IoT is an ideal complement to terrestrial IoT networks and deployments. Small satellites can bring the vision of Space-IoT into existence. However, several technical breakthroughs need to emerge in small satellites to realize Space-IoT. We tackled some of the primary challenges through theory, experimentation and demonstration on satellites in orbit. With the results obtained, we are convinced that revolutionary transformations can be brought in small satellites to enable Space-IoT and will significantly influence the space related-activities, both in research and development.