Inside GNSS Media & Research

JUL-AUG 2019

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56 Inside GNSS J U L Y / A U G U S T 2 0 1 9 R esilient and accurate positioning, navigation, and timing (PNT) is of paramount importance in safety critical cyber-physical systems (CPS), such as aviation and transportation. As these CPS evolve towards becoming fully auton- omous, the requirements on their PNT systems become more stringent than ever before. With no human in-the-loop, an inaccurate PNT solution; or more dan- gerously, PNT system failure, could have intolerable consequences. Today's vehicular navigation systems couple GNSS receiv- ers with an inertial naviga- tions system (INS). By cou- pling both systems, one takes advantage of the complemen- tary properties of the individ- ual subsystems: the short-term accuracy and high data rates of an INS and the long-term stabil- ity of a GNSS PNT solution to pro- vide periodic corrections. However, in the inevitable event that GNSS signals become unreliable (e.g., in deep urban canyons or near dense foliage), unusable Today's vehicular navigation systems couple global navigation satellite system (GNSS) receivers with an inertial navigation system (INS). Low Earth orbit (LEO) satellite signals are a particularly attractive INS aiding source in GNSS-challenged environments. Over the next few years, LEO satellites will be abundantly available at favorable geometric configurations and will transmit in several frequency bands, making them an accurate and robust navigation source. This article presents a framework that enables a navigating vehicle to aid its INS with pseudorange and Doppler measurements drawn from LEO satellite signals when GNSS signals become unusable, while simultaneously tracking the LEO satellites. This simultaneous tracking and navigation (STAN) framework is demonstrated in realistic simulation environments and experimentally on a ground vehicle and on an unmanned aerial vehicle (UAV), showing the potential of achieving meter-level-accurate navigation. STAN WITH LEO New-Age Satellite-Based Navigation STAN: Simultaneous Tracking and Navigation with LEO Satellite Signals (e.g., due to unintentional interference or intentional jamming), or untrustworthy (e.g., due to malicious spoofing attacks or system malfunctions), the naviga- tion system relies on unaided inertial measurement unit (IMU) data, in which case the errors accumulate and eventu- ally diverge, compromising the vehicle's efficient and safe operation. Signa ls of opportunit y are PNT sources that could be used in GNSS- challenged environments (See Merry et alia, and Kassas, 2013, in Additional Resources). ese signals include AM/ FM radio, cellular, digital television, and low Earth orbit (LEO) satellites (several papers listed in Additional Resources pro- vide further details). Signals of opportu- nity have been demonstrated to yield a standalone meter-level-accurate naviga- tion solution on ground vehicles and a centimeter-level-accurate navigation solution on aerial vehicles. Moreover, these signals have been used as an aid- ing source for LiDAR and INS. LEO satellites are particularly attrac- tive aiding sources for an INS in GNSS- challenged environments for several reasons. First, LEO satellites are around 20 times closer to Earth compared to GNSS satellites that reside in medium Earth orbit (MEO), making LEO satel- lites' received signals significantly more powerful. Second, LEO satellites orbit the Earth at much faster rates compared to GNSS satellites, making LEO satellites' Doppler measurements attractive to exploit. ird, the recent announcements by OneWeb, Boeing, SpaceX (Starlink), Samsung, Kepler, Telesat, and LeoSat to provide broadband internet to the world via satellites will collectively bring thou- sands of new LEO satellites into opera- tion, making their signals abundant and diverse in frequency and direction. Figure 1 depicts a subset of existing and future LEO satellite constellations. ZAHER (ZAK) M. KASSAS JOSHUA J. MORALES JOE J. KHALIFE AUTONOMOUS SYSTEMS PERCEPTION, INTELLIGENCE, & NAVIGATION (ASPIN) LABORATORY UNIVERSITY OF CALIFORNIA, IRVINE FIGURE 1 Existing and future LEO satellite constellations.

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