Low Earth Orbit (LEO) satellites can be integrated with classic GNSS, offering stronger signals, improved visibility, and system redundancy. Typical high speeds in LEO orbits generate rapid variations of the receiver to satellite geometry, which can improve the convergence of precise positioning (PPP) algorithms. However, high dynamics also induce strong Doppler rates at the receiver, which make the tracking procedures more difficult.
In this paper, a loosely combined navigation and tracking architecture is applied to LEO signals, such that the dynamic stress perceived by the receiver is mitigated. The legacy code and carrier control loops are retained and complemented with deep aiding, realized through code-phase and frequency predictions, as produced by the navigation engine.
Observable predictions are derived from the receiver to satellites relative kinematics and LEO orbits shall be accurately estimated before loop aiding can start. To overcome this “chicken-and-egg” situation, LEO broadcast ephemeris shall be either uploaded to the receiver at startup or downloaded from the navigation message, while temporarily tracking and decoding in unaided mode.
A Xona PULSAR™ Demonstration Signal in the L-Band was used to simulate the LEO constellation. Special firmware was developed to support the reception of this signal on the STMicroelectronics TeseoV triple-band GNSS chipset. Regular GPS L1 C/A signals were also simulated and synchronized with LEO. As part of the platform validation process, single point and PPP solutions for Xona stand-alone were generated.
Other practical aspects of the Xona PULSAR™ receiver are also discussed, including assistance during acquisition, handling of the navigation message for large constellations, observable integrity metrics and safety aspects.
The Xona simulations were mostly static open sky, yet the application focus is kept on difficult land scenarios for automotive, where the proposed architecture is expected to provide benefits in terms of increased availability and robustness.
