Carrier-Phase multipath mitigation in RTK-based GNSS dual-antenna systems
Carrier-phase multipath mitigation in GPS/GNSS real-time kinematic (RTK) mode has been studied for several years, at least since on-the-fly ambiguity resolution techniques were introduced, and receiver hardware improvements to the point that GNSS RTKbased systems provide position estimates at the mm to cm-level accuracy in real-time. This level of accuracy has heralded a new era of applications where the use of GNSS RTK-based techniques have become a very practical navigation tool, especially in the fields of machine automation, industrial metrology, control, and robotics. However, this incredible surge in accuracy tied with real-time capabilities comes with a cost: one must also ensure continuity, and integrity (safety). Typical users of these systems do not expect heavy machinery, guided and/or controlled by GNSS-based systems, to output erroneous solutions even in challenging multipath environments. In multipath-rich scenarios, phase-multipath reflections can seriously degrade the RTK solutions, and in worst scenarios, integer fixed solutions are no longer available. This dissertation intends to deal with these scenarios, where the rover algorithms should deal with multiple reflections and, in real-time, be able to ameliorate/mitigate their effect. GNSS-based heading/attitude is usually obtained combining the data from two or more antennas (also known as a moving baseline). Many companies provide commercial systems based on this technique, hence this dissertation finds its main applicability here. Typical heavy construction machinery includes dozers, motor-graders, excavators, scrappers, etc., which are being equipped more frequently with GNSS dual-antenna systems to provide positioning and orientation information to the operator. We have not used and collected data from one of these machines, although the author has worked extensively with such machinery and their GNSS-based systems. However, the theory developed throughout this dissertation and the proof of concept through controlled tests that mimic the machinery/installed GNSS dual-antenna systems, are the basis of this dissertation. Moreover the algorithms developed here are meant to be used independently from the receiver hardware, as well as from GNSS signals. Hence GLONASS, and/or Galileo signals can be processed too. This dissertation is based on the fundamental relationship between multiple multipath reflections from close-by strong reflections, and their effect on GNSS RTK-based dual-antenna systems. Two questions were answered: Firstly, is it possible to retrieve strong multipath reflectors in kinematic applications? Second, once these strong reflectors are correctly identified, how accurate/reliable are the corrections to the raw carrier-phase multipath, knowing that the host platform performs unpredictable manoeuvres? Based on the results, we can conclude that it is possible to estimate in real-time multipath parameters based on a strong effective reflector. In most of the tests it takes at least 2 minutes to obtain initial values (after Kalman filter convergence). Once they are determined, multipath corrections can be determined straightforwardly for each satellite being tracked, as long as there are no cycle-slips (mostly due to the combination of the machinery high dynamics, especially within the areas where antennas are located, and the machinery itself blocking momentarily satellite signals).