GPS World

Tag: supercorrelation

  • New developments in GPS

    New developments in GPS

    Matteo Luccio
    Matteo Luccio

    “What’s new with GPS?” people often ask me when I tell them my job. Recently, I have been responding by telling them about the other three GNSS constellations now fully available. However, as reflected every month in these pages, that is but one of many developments that combine to make satellite navigation ever more accurate, reliable and ubiquitous.

    While the GPS program is old by the standards of the digital age, it has never been static. In the 1970s, when GPS was developed, the expected accuracy for civilians was tens of meters, though pioneering commercial users began right away to chip away at the system’s limitations by developing differential GPS (DGPS), carrier-phase positioning, and other techniques. By the end of the next decade, better signal processing and the implementation of DGPS had brought civilian accuracy to about one meter. In the 1990s, phase-ambiguity resolution made real-time centimeter accuracy standard for surveyors.

    As the adoption of cell phones exploded, it became imperative to locate them to preserve the 911 system. Initially, this was done using the time-of-arrival of signals to handsets from towers, because it was assumed that GPS receivers could not be made sufficiently small, cheap, fast, power-efficient and accurate to work in cell phones. The implementation of assisted GPS, now standard in all smartphones, largely solved those problems.

    Precision for civil GPS users increased by an order of magnitude in May 2000, when President Clinton ordered the removal of Selective Availability, and substantially once enough satellites began to broadcast the L2 civil (L2C) code, enabling ionospheric corrections. Later, the modernized signals in the L5 band enabled sub-meter accuracy without augmentations and very long-range operations with augmentations. There are now more than 80 signals in that band, on GPS, Galileo and BeiDou satellites. On the military side, the effort to deploy M-code signals, cards and receivers continues.

    Over the years, in addition to modernized satellites and signals, improvements have included the development of PPP, RTK and hybrid techniques; the proliferation of local, regional and global correction services; improved jamming and spoofing detection; and the increasing integration of GNSS receivers with other RF receivers as well as with inertial, optical, radar, lidar and other sensors.

    Future improvements may include:

    • signal authentication
    • commercial systems in low Earth orbit that would have a signal strength on the surface three orders of magnitude greater than current GNSS, greatly boosting indoor reception and protection from jamming
    • inertially aided extended coherent integration, a.k.a. “supercorrelation,” which makes moving GNSS receivers more sensitive to signals they receive directly than to reflected ones
    • 3D-mapping-aided GNSS, which enhances the positioning algorithms by identifying non-line-of-sight signals; this is being pioneered by Google in nearly 4,000 cities, relying on its 3D city models and machine learning.

    The moment I send this month’s issue to the printer, I will think of more past and future improvements. As soon as you receive it, many of you will think of yet more. What’s new with GPS? A lot.

    Matteo Luccio | Editor-in-Chief
    [email protected]

  • U-blox signs deal with UK start-up for cutting-edge GNSS technology

    U-blox signs deal with UK start-up for cutting-edge GNSS technology

    Map plot from live tests in London show the route of a vehicle driven through Canary Wharf. It shows the difference between the position provided by a standard smartphone GNSS chip (red line) and the same data run through Focal Point Positioning's Supercorrelation software (blue line). (image: u-blox)
    Map plot from live tests in London show the route of a vehicle driven through Canary Wharf. It shows the difference between the position provided by a standard smartphone GNSS chip (red line) and the same data run through Focal Point Positioning’s Supercorrelation software (blue line). (Image: u-blox)

    U-blox has signed a deal with the award-winning U.K.-based technology company Focal Point Positioning to integrate technology that will improve the accuracy and reliability of GNSS devices. Focal Point’s Supercorrelation technology enhances positioning performance and security for applications such as smart cities, location-secure internet of things (IoT) and health and fitness wearables.

    The patented Supercorrelation technology solves a critical weakness in GNSS caused by multipath interference. Multipath interference occurs when satellite signals bounce off buildings and landmarks, causing GNSS receivers to provide degraded positioning outputs.

    The result for users is that the blue dot on their phone or device may be in the wrong place, moving in the wrong direction, or may have a large error ellipse. For autonomous vehicles it could lead to positioning errors that place the vehicle in the wrong lane or worse.

    FocalPoint’s Supercorrelation technology uses software to detect and reject reflected signals, resulting in an improvement in the performance of GNSS devices without the need for additional hardware or applications. Supercorrelation also helps with the detection and rejection of GNSS spoofing signals — an increasing concern for autonomous vehicles, ships, and aviation.

    “We are tremendously excited to be working alongside a market leader such as u-blox, our mission is to improve every positioning system on the planet and we have taken a giant step forward in that vision with this deal,” said Focal Point Positioning CEO Ramsey Faragher. “Positioning systems are so critical to our world, and we look forward to seeing the next generation of products and services that will be enabled by this higher level of accuracy, reliability and security.”

    u-blox CEO Thomas Seiler commented, “The addition of Supercorrelation technology into our latest GNSS platforms is part of our continuing focus on low power consumption, higher accuracy and security for automotive, industrial, and wearable GNSS applications.”

  • Supercorrelation: Enhancing accuracy, sensitivity of commercial receivers

    Figure 1: Reflected signals in the local environment suffer different Doppler variations than the desired line­of­sight signal. This means that the supercorrelator that is created for a given satellite broadcast couples strongly to the desired line of sight version of the signal, but attenuates any reflected signals arriving from different directions. (Figure: Focal Point Positioning)
    Figure 1: Reflected signals in the local environment suffer different Doppler variations than the desired line­of­sight signal. This means that the supercorrelator that is created for a given satellite broadcast couples strongly to the desired line of sight version of the signal, but attenuates any reflected signals arriving from different directions. (Figure: Focal Point Positioning)

    The S­GPS/S­GNSS technology is a patent-protected suite of methods that provides software-based improvements to existing GNSS receivers. All methods within the software suite build upon a core technology called supercorrelation, which enables over a second of coherent integration while undergoing complex motions on low-cost platforms. The benefit is high sensitivity coupled with strong multipath mitigation capabilities, providing a high-accuracy and high-integrity positioning solution in traditionally difficult environments.

    Many GNSS receivers perform a small amount of coherent integration, typically less than 20 milliseconds, and then optionally incoherently integrate over many hundreds of milliseconds to boost sensitivity if needed. The major problem with this approach is the resulting susceptibility to multipath interference. Incoherent integration destroys the phase information stored within the captured data before combining it, resulting in line-of-sight and non-line-of-sight signals accumulating within the same correlation peak, producing a distortion of the desired line-of-sight information. This distortion leads to erroneous codephase estimates, which in turn leads to erroneous position estimates.

    Coherent integration can decorrelate signals arriving from different directions, but the degree of decorrelation depends on the user speed and the coherent integration time. Supercorrelator technology creates a clock-and-motion-compensated phasor correction sequence that provides over a second of coherent integration on low-cost consumer platforms. The outcome is signal tracking sensitivities down to nearly zero dBHz, combined with high multipath mitigation performance. Such long coherent integration times allow signals arriving from different directions to be separated out in the frequency domain, permitting new capabilities in anti-spoofing and 3D map-aiding methods by directly resolving GNSS angle-of-arrival using a single moving antenna.

    Figure 2: The result of supercorrelation on positioning performance in the urban canyons of central San Francisco. The red line is a standard state­-of-­the-­art vector tracking GPS solution, and the green line is the same positioning engine with supercorrelation processing enabled. (Image: Focal Point Positioning)
    Figure 2: The result of supercorrelation on positioning performance in the urban canyons of central San Francisco. The red line is a standard state­-of-­the-­art vector tracking GPS solution, and the green line is the same positioning engine with supercorrelation processing enabled. (Image: Focal Point Positioning)

    Traditionally, very long coherent integration times were not practical on consumer devices due to limitations of data modulation bits, crystal oscillator stability, and unknown (often complicated) receiver motion. Supercorrelation overcomes these limitations with signal processing and sensor fusion. Data modulation bits are not an issue for modern pilot signals, and for legacy signals they can be removed with a variety of methods, ranging from prediction or provision of the bits over a datalink, to stripping them directly with signal-squaring methods. Receiver motion can be inferred from inertial sensors mounted alongside the GNSS receiver, as is the case for smartphones and smartwatches, or can be modeled using multi-hypothesis methods. Low-cost crystal oscillators cause phase instabilities which traditionally reduce coherent integration time, but can also be accounted for by multi-hypothesis testing and by joint estimation processes across multiple channels.

    A decade ago, consumer GNSS receivers were typically an ASIC or similar hard-wired design. Modern designs incorporate a front-end correlator bank which may or may not be reprogrammable, feeding into a DSP stage which handles all tracking and navigation processing from the DLL, PLL, FLL stages onwards. The flexibility of reprogramming the code running on the DSP stage permits existing GNSS chipsets to be easily upgraded to support supercorrelation, without needing to design and fabricate a new receiver.

    Focal Point aims to have S-GNSS enabled chips by early 2020, with licensing opportunities available from summer 2019 onwards.