# GNSS Code Multipath Mitigation by Cascading Measurement Monitoring Techniques
## 1 Introduction  
Multipath is a major error source for global navigation satellite systems (GNSS) receivers. This phenomenon refers to the combination of Line-of-Sight (LOS) and a number of Non-Line-of-Sight (NLOS) components reflected off nearby obstacles one or more times before reaching the receiver antenna. Multipath signals differ from the LOS signal in power, code delay, carrier phase and frequency, all of which distort the correlation curve between the received signal and receiver-generated replica, resulting in pseudorange (code phase) errors of tens of metres [1]. Many multipath mitigation techniques have been developed in the literature, a review of which can be found in Reference [2]. Among these well-investigated multipath mitigation methods, receiver-level signal quality and integrity monitoring techniques have received much attention as they do not need major hardware modifications. A typical GNSS receiver consists of an RF front-end that down-converts and samples the RF signal, a signal processing stage where the signals are acquired, tracked and measurements are generated, and a navigation solution. Integrity monitoring can be performed at each of these stages. At the first stage, the output of the RF front end can be monitored, although this is challenging in multipath detection where spread GNSS signals are still buried under the noise floor [3]. At the navigation stage, Receiver Autonomous Integrity Monitoring (RAIM) is the most common technique. A thorough study of conventional and advanced RAIM algorithms is presented by Reference [4]. Conceptually, RAIM algorithms use measurement redundancy to identify outliers. Although effective for aviation applications, poor geometry and multiplicity of error sources make RAIM sub-optimal for land applications, particularly in urban areas with dense multipath [5]. Monitoring the quality of GNSS signals at the signal processing stage allows each Pseudorandom Noise (PRN) to be independently monitored, providing the capability of detecting multiple signal failures. Signal Quality Monitoring (SQM) has been well investigated for multipath detection and is usually implemented by adding additional monitoring correlators at the tracking stage. A comprehensive analysis of the sensitivity and effectiveness of SQM techniques has been reported in Reference [6]. While SQM techniques have their own limitations, especially in detecting short-range multipath, the other option is monitoring the GNSS measurements (i.e., code, carrier phase and Doppler). This is the main focus of this paper. In addition to code and carrier phase measurements, Carrier-to-Noise-density ratio (C/N\({}_{0}\)) is considered as a quality measurement for code multipath monitoring using the following three approaches. 1. Multipath Error estimation and measurement correction: The monitoring metrics defined in these techniques are generally based on combinations of erroneous (but unambiguous) code phase and precise (but ambiguous) carrier phase measurements to provide a direct measure of the code range multipath error, resulting in the possibility of pseudorange correction [1]. The Code-Minus-Carrier (CMC) metric is one of the well-known monitoring metrics used to characterize and measure code multipath errors by subtracting carrier phase measurements from corresponding pseudoranges (e.g., [7]). Since the pseudorange multipath error is considerably larger than that of the carrier phase, the code minus carrier measurement is mostly an indication of pseudorange multipath [8; 9; 10]. Although CMC can also be used in other multipath mitigation approaches such as multipath detection and exclusion [11], correction of affected measurements (when applicable) is preferred to mitigate the multipath error without geometry degradation. The major limitation is sensitivity to cycle slips due to signal attenuation, obstruction or antenna motion degrading the performance of the monitoring process [12; 13; 14]. 2.