In this paper we introduce and design sparse constellations for Direct-to-Satellite Internet of Things (DtS-IoT). DtS-IoT does not require a ground infrastructure, because the devices are directly connected to Low Earth Orbit satellites acting as orbiting gateways. The key idea of sparse constellations is to significantly reduce the number of in-orbit DtS-IoT satellites by (i) a proper dimensioning of the delivery delay anyway present in resource-constrained IoT services, and (ii) an optimal positioning of the orbiting gateways. First, we analyze LoRa/LoRaWAN and NB-IoT standards and derive realistic constraints on the maximum gap time between two consecutive passing-by satellites. Then, we introduce and optimize an algorithm to design quasioptimal topologies for sparse IoT constellations. Finally, we apply our design to both global and regional coverage and we analyze the trade-off between latency, number of orbit planes and total number of satellites. Results show that sparse constellations can provide world-wide IoT coverage with only 12.5% and 22.5% of the satellites required by traditional dense constellations considering 3-hour and 2-hour gaps. Also, we show that regionspecific coverage of Africa and Europe can be achieved with only 4 and 3 satellites for LoRa/LoRaWAN and NB-IoT, respectively.

Mega-constellations are being deployed to offer innovative services to Earth's users. Our work shows how they can provide seamless connectivity to LEO spacecrafts too, and transform them into highly responsive nodes of a space-to-space network characterized by high throughput, low latency, and low cost. For realizing the new mega-constellation services in space paradigm, we present a complete design of the LEO space terminal. By focusing on existing mega-constellations, we derive the service performance under realistic scenarios, and compare it with existing services like Ground Station Networks and Data Relay Systems. All the results show that the new approach can be potentially disruptive for the space ecosystem, by transforming each satellite into a 24/7 available node of a high performance space network, thus enabling a myriad of innovative applications.

This study was funded by the European Space Agency, in the frame of the Open Space Innovation Platform implemented through its Discovery & Preparation programme, under contract number 3-16610/20/NL/GLC.

Mega-constellations are being deployed to offer innovative services to Earth’s users. Our work shows how they can provide seamless connectivity to LEO spacecrafts, too, and transform them into highly responsive space network nodes, thus enabling a myriad of innovative applications. For realizing the new mega-constellation services in space paradigm, in this paper we present a complete design of the LEO space terminal, suitable to any hosting platform, even CubeSats and SmallSats. By focusing on existing mega-constellations, like OneWeb and Starlink, we derive the service performance for four terminal configurations under realistic scenarios and show that Tbit/day- scale capacity is the common case for space users, by constant data dripping. Finally, we compare this new paradigm with existing services like Ground Station Networks and Data Relay Systems. All the results show that the new approach can be potentially disruptive for the space ecosystem, by transforming each satellite into a 24/7 available node characterized by high throughput, low latency, and low cost.

Mega-constellations are being deployed to offer innovative services to Earth's users. Our work shows how they can provide seamless connectivity to LEO spacecrafts too, and transform them into highly responsive nodes of a space-to-space network characterized by high throughput, low latency, and low cost. For realizing the new mega-constellation services in space paradigm, we present a complete design of the LEO space terminal. By focusing on existing mega-constellations, we derive the service performance under realistic scenarios, and compare it with existing services like Ground Station Networks and Data Relay Systems. All the results show that the new approach can be potentially disruptive for the space ecosystem, by transforming each satellite into a 24/7 available node of a high performance space network, thus enabling a myriad of innovative applications.