CMCD: Multipath Detection for Mobile GNSS Receivers
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In recent years, the rising demand for high precision localization has challenged the use of GNSS particularly in automotive applications. This is especially the case in urban scenarios where the most crucial GNSS disturbance is multipath – the reception of reflected signals. This work addresses the detection of multipath errors in pseudorange measurements for the special case of a moving receiver without the need for redundant observations or motion sensors. Our proposed detection scheme is based on a combined observable we call CMCD (Code-Minus-Carrier Deltarange). It is defined as the difference between code- and carrier-derived deltaranges and approximately resolves to the derivatives of the receiver noise and the code multipath error. The CMCD-based detection algorithm exploits the fact that the CMCD observable is equal to the receiver noise process for multipath-free environments. Simulations of a two ray multipath model show that the code multipath error can be described as a broadband noise process in case of a moving receiver and an abrasive reflection surface. This additional noise process causes changes in the statistical properties of CMCD and hence indicate multipath occurrences. A statistical test is used to detect these changes. With a test drive under heavy mutlipath conditions the detection performance was evaluated and the suitability of CMCD-based multipath detection was shown. The results indicate a correlation between multipath detections and high ranging errors. In addition, the horizontal position error with a set of 4 satellites was evaluated. In cases of at least one multipath-affected pseudorange a resulting position error of around 15 m (CEP95) and above was observed. With measurements detected as multipath-free the resulting error was around 5 m (CEP95).