![]() Second, they will not broadcast downlink signals that support pseudorange measurements through a calibrated connection between transmission time tags and transmitter clock time. It is not anticipated that such constellations will support pseudorange-based navigation for two reasons: First, they will not carry atomic clocks. Amazon has plans for an 1,156 satellite LEO constellation called Kuiper ( FCC Report, 2019). One plan calls for an initial constellation of 1,600 LEO satellites that eventually expands to 2,825 satellites ( FCC Report, 2016). SpaceX has been launching satellites for its Starlink constellation. Although the company went bankrupt, it has since been bought, and there are now renewed plans to field the constellation, perhaps with a GNSS capability added to the original satellite design. OneWeb has built and launched a number of satellites already and had plans at one time to field a LEO constellation with 720 satellites in 18 nearly-polar orbital planes ( FCC Report, 2017). It seeks to exploit the various plans that exist to field large constellations of Low Earth Orbit (LEO) satellites such as OneWeb, Starlink, and Kuiper. The present study seeks to develop a system that achieves a full 4-Dimensional (4D) Position/Navigation/Time (PNT) capability by using passive reception of downlink signals from large LEO constellations. ![]() These systems have vulnerabilities to spoofing and jamming partly because their signals are weak and partly because they use low numbers of expensive satellites with long lifespans, which makes upgrading them slow and expensive. This effort seeks an alternative, or backup, to standard Global Navigation Satellite Systems (GNSS) that operate in Medium Earth Orbit (MEO) and above– systems such as GPS, GLONASS, Galileo, and BeiDou. Clock offset accuracy is on the order of 0.0001 to 0.0010 sec 1-□. These accuracies are comparable to current pseudorange-based GNSS. A generalized Geometric Dilution of Precision (GDOP) analysis indicates that absolute position accuracies on the order of 1-5 meters and absolute velocity accuracies on the order of 0.01 m/sec to 0.05 m/sec may be achievable if the range-rate precision of the Doppler shift measurements is 0.01 m/sec. The filter uses eight or more measured Doppler shifts in its least-squares fit. This model is used in a point-solution batch filter that simultaneously estimates eight unknowns: the three position vector components, receiver clock offset, three velocity vector components, and receiver clock offset rate. ![]() The concept uses a high-fidelity model of received carrier Doppler shift. This system could provide an alternative to pseudorange-based GNSS. A new global navigation concept is studied that relies on carrier Doppler shift measurements from a large LEO constellation. ![]()
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