Expert Opinions: The effect of LEO constellations on GNSS services

February 20, 2017  - By

Q: What is the potential for low-Earth orbit constellations to augment services provided by the four medium-Earth orbit GNSS?

Doug Taggart, President, Overlook Systems Technologies, Inc.

Doug Taggart, President, Overlook Systems Technologies, Inc.

A: With more than one hundred GNSS satellites broadcasting on three or more frequencies, our international constellation of medium-Earth orbiting (MEO) satellites will provide a combination of path diversity and frequency diversity. However, satellites in low-Earth orbit (LEO) should be added to our MEO mélange to provide orbital diversity and thus cyber safety. The LEO satellites would have 20 dB less path loss and compel jammers and spoofers to become conspicuous. Even with only one LEO in view, we would be able to use the LEO signal as a hot clock to improve the robustness of GNSS signal acquisition by our users. For timing applications, a solitary LEO satellite would enable time transfer to fixed locations worldwide.

Per Enge, Professor and Director, Stanford university Center for Position Navigation and Time

Per Enge, Professor and Director, Stanford university Center for Position Navigation and Time

A: While it is prudent to take advantage of multiple PNT sources, the devil is in the details. Are users seeking more availability, accuracy, integrity and/or resilience to fill gaps? What is the complexity and cost for integration in user equipment, the reliability compared to other augmentations, the applications to be supported, vulnerability to interference, and so on? Additionally, all things from space may not be the best solution when all user needs and vulnerabilities are factored in.

5 Comments on "Expert Opinions: The effect of LEO constellations on GNSS services"

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  1. William K. says:

    I was hoping to see the comments of others.
    My opinion is that we need an entirely different technology as backup, because the present system is excessively subject to jamming, spoofing, and the destruction of satellites by enemy forces.
    Extreme dependence on a system that can be compromised is not a good situation at all.

  2. Logan Scott says:

    GEOs and Inclined GEO orbits might give more bang for the buck. It takes a lot of LEOs to provide even one satellite in view coverage and the wide field of regard needed by LEOs nullifies much of the pathloss gains, at least in staring configurations. It would be hard to justify a standalone LEO navigation system since it takes literally 100s of satellites to provide adequate geometric coverage. As a secondary payload on for instance OneWeb (648 satellites at 1200 km), there might be some good arguments but the caution is that there may be terrestrial competition.

    In the civil sector, my expectation is that terrestrial navigation capabilities will be deeply integrated and/or provided by 5G cellular telephony. This could become an important if not dominant navigation source particularly in populated areas. Such systems are needed to support autonomous vehicles since they are harder to interfere with; they can provide strong authentication, PoL, and integrity monitoring; they can be locally engineered for spot coverage (e.g. tunnels and indoors) and, they can provide the high speed data connectivity needed in diverse applications. There is a real revenue opportunity and so, the capital investment will follow.

  3. Oscar L. Colombo says:

    I wonder if anyone here has ever watched “Gravity”?
    People are talking up hundred and even thousand-satellite LEO constellations, but there is no mention of how that might add to the ever-growing Earth-spanning cloud of potentially deadly space garbage that threatens both lower orbit missions and space travel to destinations beyond this cloud.
    Nor is there any mention of end-of-life maneuvers to either send the expiring satellites to a fiery end in the atmosphere or to some defined safe “parking” orbit, to spend there the rest of eternity out of the way and out of trouble.
    It is really common sense, of which not an excess seems to be displayed by those planning new LEO constellations, or expanding them.

  4. Pradipta Shome says:

    NRL had a project to use a Leo Iridium constellation for high integrity GPS for some of the reasons stated, especially for robust anti jamming capability and time synchronization. Short description is enclosed. See page 248