Inside GNSS Media & Research

JUL-AUG 2019

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www.insidegnss.com J U L Y / A U G U S T 2 0 1 9 Inside GNSS 61 the location at which GPS signals were cut off (Ardito et alia). To estimate the UAV's trajectory, 2 navigation frameworks were imple- mented to estimate the vehicle's trajecto- ry: (i) the LEO-aided INS STAN frame- work and (ii) a traditional GPS-aided INS for comparative analysis. Each framework had access to GPS for only the fi rst 100 seconds. Figure 15(a)-(b) illustrate the UAV's true trajectory and those estimated by each of the 2 frame- works while Figure 15(c) illustrates the simulated and estimated trajectories of one of the LEO satellites, as well as the fi nal 95-th percentile uncertainty ellipsoid (the axes denote the radial (ra) and along-track (at) directions). Table 3 summarizes the fi nal error and position RMSE achieved by each framework a er GPS cutoff . EXPERIMENTAL DEMONSTRATIONS This section describes the existing Orbcomm LEO constellation and the LEO receiver. en, it demonstrates the performance of the LEO-aided INS STAN framework on a ground vehicle and a UAV with real Orbcomm satellite signals. Orbcomm System Overview The Orbcomm system is a wide area two-way communication system that uses a constellation of LEO satellites to provide worldwide geographic coverage for sending and receiving alphanumer- ic packets (See Orbcomm, Additional Resources). e Orbcomm system con- sists of 3 main segments: (i) subscriber communicators (users), (ii) ground seg- ment (gateways), and (iii) space segment (constellation of satellites). ese seg- ments are briefl y discussed next. (i) Subscriber Communicators (SCs): ere are several types of SCs. Orbcomm's SC for fixed data applications uses low-cost VHF electronics. e SC for mobile two-way messaging is a hand- held, standalone unit. (ii) Ground Segment: The ground seg- ment consists of gateway control centers (GCCs), gateway Earth sta- tions (GESs), and the network con- trol center (NCC). e GCC provides switching capabilities to link mobile SCs with terrestrial-based customer Unaided INS LEO-aided INS STAN with periodically transmitted satellite positions Final Error (m) 16,589.0 9.8 RMSE (m) 6,864.6 10.1 Table 3: Simulation results with Starlink LEO satellites for a UAV navigating 82 km in 600 seconds (GPS signals were cut off after the fi rst 100 seconds). These results are after GPS cutoff . systems via standard commu- nications modes. GESs link the ground segment with the space segment. GESs mainly track and monitor satellites based on orbital information from the GCC and transmit to and receive from satellites, the GCC, or the NCC. The NCC is responsible for managing the Orbcomm network elements and the gate- ways through telemetry moni- toring, system commanding, and mission system analysis. of approximately 815 kilometers. Plane D, also inclined at 45°, contains 7 satel- lites in a circular orbit at an altitude of 815 kilometers. Plane E is inclined at 0° and contains 7 satellites in a circular orbit at an altitude of 975 kilometers. Plane F is inclined at 70°and contains 2 satellites in a near-polar circular orbit at an altitude of 740 kilometers. Plane G is inclined at 108° and contains 2 satellites in a near- polar elliptical orbit at an altitude vary- ing between 785 and 875 kilometers. Unaided INS LEO-aided INS STAN Final Error (m) 174.7 9.9 RMSE (m) 52.6 10.5 Table 2: Simulation results with Globalstar, Orbcomm, and Iridium LEO satellites for a UAV navigating 25 km in 200 seconds (GPS signals were cut off after the fi rst 100 seconds). These results are after GPS cutoff . FIGURE 16 Orbcomm LEO satellite constellation. FIGURE 17 Snapshot of the Orbcomm spectrum. (iii) Space Segment: Orbcomm satellites are used to complete the link between the SCs and the switching capability at the NCC or GCC. Orbcomm LEO Satellite Constellation The Orbcomm constellation, at maxi- mum capacity, has up to 47 satellites in 7 orbital planes A–G, as illustrated in Figure 16. Planes A, B, and C are inclined at 45° to the equator and each contains 8 satellites in a circular orbit at an altitude

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