Inside GNSS Media & Research

SEP-OCT 2018

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36 Inside GNSS S E P T E M B E R / O C T O B E R 2 0 1 8 R ightfully, the GNSS community almost universally considers reflected signals to be prob- lematic (the field GNSS reflectometry being an obvious exception). Reflected or non-line-of-sight (NLOS) signals combine with line-of-sight (LOS) sig- nals to produce multipath effects, or if the LOS signal is absent NLOS signals can produce large ranging biases. Both phenomena increase measurement error and decrease positioning accu- racy, especially in areas with lots of reflecting surfaces such as deep urban canyons. Dealing with NLOS-only signals is generally easier than dealing with multipath because the errors tend to be larger and are thus easier to detect with standard receiver autonomous integrity monitoring (RAIM) approaches. More recently, ray tracing has been used in combination with 3D building models (3BDM) to predict and correct NLOS- only signals for their path delay, thus making them behave more like LOS signals. In contrast, multipath — which we herein use exclusively to represent the contamination of the LOS signal, if it is present — is more difficult to deal with because the errors are smaller and harder to detect with RAIM. More importantly, even if the path delay of contaminating NLOS signal is known or can be computed, the relative carrier phase of the LOS and NLOS signal(s) can cause the resulting pseudorange measurement to appear too short, even though the NLOS signal is always, by definition, delayed relative to the LOS signal. Predicting the carrier phase(s) of the NLOS signal(s) relative to the LOS signal is generally not possible with sufficient accuracy due to the rela- tively short wavelength of GNSS signals involved (a 30 degree phase accuracy would require relative position accurate to better than 2 centimeters at L1). However, most of the above prob- lems arise from the fact that position- ing algorithms typically assume their input to be measured pseudoranges. If we instead consider the input to be the receiver's correlator outputs (which are used to compute the pseudorange), then entirely new options are possible. is article discusses one approach for combining correlator outputs with 3DBM data to derive position from only reflected signals. In essence, it demonstrates that when handled prop- erly, reflected signal can actually be useful. High-Level Concept Before delving into some of the details of how NLOS signals can be used con- structively, we first introduce some key concepts. First, let's assume we have a can- didate value of the user's position. We will discuss how such a candidate value might be obtained later in the article, but for now we simply assume it is available. Next, we use the candidate position along with a 3DBM and ray tracing techniques to compute/predict two important pieces of information: (i) the number of received signal paths (LOS and/or NLOS); and (ii) the path delay of each signal relative to the LOS signal, regardless of whether the LOS signal is actually present. Collectively, these values are referred to as the "pre- dicted signal parameters". Note that the algorithm does not require that the LOS signal be present — if it is, its path delay will be zero, by definition. GNSS Solutions is a regular column featuring questions and answers about technical aspects of GNSS. Readers are invited to send their questions to the columnist, Dr. Mark Petovello , Department of Geomatics Engineering, University of Calgary, who will find experts to answer them. His e-mail address can be found with his biography below. GNSS SOLUTIONS MARK PETOVELLO is a professor (on leave) at the University of Calgary. He has been actively involved in many aspects of positioning and navigation since 1997 and has led several research and development efforts involving Global Navigation Satellite Systems (GNSS), software receivers, inertial navigation systems (INS) and other multi-sensor systems. E-mail: Are Reflected Signals Always Undesirable?

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