GPS World

Tag: GPS carrier phase

  • Researchers demonstrate centimeter-level positioning using smartwatches

    Researchers demonstrate centimeter-level positioning using smartwatches

    University of Otago – Ōtākou Whakaihu Waka researchers have developed algorithms that improve the precision of location tracking in smartwatches.

    Led by Associate Professor Robert Odolinski, a visiting researcher with Google from Otago’s School of Surveying, the research team demonstrated that a smartwatch determined its location with centimeter-level precision over four hours with a stationary setup. The result was achieved by using the Google GnssLogger app and combining precise signals from several GNSSs.

    The research was done in collaboration with Google’s Android Context group and the Chinese Academy of Sciences. Results are published in the scientific journal GPS Solutions.

    For decades, achieving centimeter-level positioning has required industries such as surveying, construction and engineering to invest in expensive GPS equipment.

    “While the use of the so-called carrier-phase signals has long been known to improve the positioning performance, the specialized antenna and receivers needed for this have traditionally come at a cost far beyond the reach of many who would benefit from the technology. This is just the beginning of what wearable high-precision positioning can potentially achieve.”

    GPS was introduced in a wearable watch in 1999, but hardware and power consumption limitations prevented it from tracking the carrier-phase signals needed for high-precision results. Recent advances in smartwatches now make this possible.

    Precise centimeter-level positioning on a smartwatch during 4 hours of data in Dunedin, New Zealand. The dots show the repeatability of one second of data in comparison to precise benchmark coordinates. The repeatability of the positioning is about 8 cm, at most twice as large as the smartwatch diameter of 4 cm (displayed to scale).

  • Innovation Insights: What is carrier phase?

    Innovation Insights: What is carrier phase?

    Innovation Insights with Richard Langley
    Innovation Insights with Richard Langley

    WHAT IS CARRIER PHASE? The obvious answer is: the phase of the carrier. But this is not helpful if you don’t know what a carrier is. A carrier is basically a harmonic electromagnetic wave — a pure continuous sinusoidal wave with a single constant frequency and amplitude.

    Such a wave has limited uses. However, if we modulate or change the characteristics of the wave in some way, then the wave can carry information. Changing the amplitude by using a voice or music audio signal is amplitude modulation as used for AM radio.

    Instead, one could modulate a carrier by changing its instantaneous frequency, which is frequency modulation or FM and is used for high-fidelity broadcasting. Yet another way to modulate a carrier is to change the instantaneous phase of the carrier, and that is how GNSS works.

    GNSS carriers are phase-modulated by pseudorandom noise (PRN) codes and navigation messages. A GNSS receiver uses the PRN codes to produce the pseudorange observable with a precision in the tens of decimeter range. This is the most common observable for GNSS positioning.

    But by stripping away the modulation of the received GNSS signals, the receiver can measure the phase of the underlying carrier. Changes in carrier phase over time reflect the change in the (pseudo)range but are about two orders of magnitude more precise.

    One problem with carrier-phase measurements is that they have an initial cycle ambiguity that must be resolved, preferentially fixed to the correct integer value, before they can be used for positioning, but this can be achieved without too much difficulty. While fixing the ambiguity of carrier-phase measurements might be considered a nuisance in GNSS positioning, it can help detect spoofing of GNSS signals where some other techniques might fall short.

    In this “Innovation” column, we look at how carrier-phase measurements combined with those from an inertial measurement unit can guard against a deliberate attack on an automated ground vehicle — something that cannot be discounted in our world these days.

    Read the full “Innovation” column: GNSS Spoofing Detection: Guard against automated ground vehicle attacks.

  • Iridium/GPS carrier phase positioning, fault detection

    gps-iridium-constellations
    GPS and Iridium constellations.

    Interest in the Iridium constellation as a potential alternative and back-up provider of positioning and timing has increased with the announcement of impending operational capability. Overall concept information is hard to come by, but this 2009 ION GNSS paper gives an early look. The text is from the paper’s abstract.

    The iGPS high-integrity precision navigation system combines carrier phase ranging measurements from GPS and low Earth orbit Iridium telecommunication satellites. Large geometry variations generated by fast-moving Iridium spacecraft enable the rapid floating estimation of cycle ambiguities. Augmentation of GPS with Iridium satellites also guarantees signal redundancy, which enables fault-detection using carrier phase Receiver Autonomous Integrity Monitoring (RAIM). Over short time periods, the temporal correlation of measurement error sources can be exploited to establish reliable error models, hence relaxing requirements on differential corrections. In this paper, a new ionospheric error model is derived to account for Iridium satellite signals crossing large sections of the sky within short periods of time. Then, a fixed-interval positioning and cycle ambiguity estimation algorithm is introduced to process Iridium and GPS code and carrier-phase observations. A residual-based carrier phase RAIM detection algorithm is described and evaluated against single-satellite step and ramp-type faults of all magnitudes and start times. Finally, a sensitivity analysis focused on ionosphere-related system design variables (ionospheric error model parameters, code-carrier divergence, single- and dual-frequency implementations) explores the potential of iGPS to fulfill some of the most stringent navigation integrity requirements with coverage at continental scales.

    A download is available, per ION’s current download policies.