Antcom 3G1215RL-P-XS-1 dual-polarization antenna, mounting and amplifiers. 

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... satellite-reflector-antenna geometry in about one sidereal day [6] [7]. In multipath environmental modelling, a ray-tracing algorithm uses the known satellite-reflector-antenna geometry and physical properties of reflectors to determine the phase multipath errors [8]. Site-dependent techniques are highly effective at mitigating multipath, but are generally only suited to static receivers. A similar approach has been applied to mitigating multipath reflections off satellite bodies for attitude determination [9] [10]. Antenna-based multipath mitigation techniques include special antenna designs such as choke-ring antennas, Trimble’s Zephyr antennas and the multipath-limiting antenna for ground-based augmentation system reference stations [11]; these reduce the gain of signals reflected off the ground. Antenna array techniques, based on the geometric correlation of multipath errors at closely-spaced antennas, can be used for more general multipath mitigation [12] [13]. However, they perform best under simple multipath environments and are not suited to most kinematic applications because antenna arrays are usually bulky. A number of receiver-based techniques have been developed that mitigate code multipath errors by increasing the resolution of the code discriminator on the basis that the higher-frequency components of a GNSS signal are less impacted by moderate-delay multipath interference. The simplest approach is to use a narrow early-late correlator spacing [14], while a more sophisticated method is the Multipath Mitigation Window (MMW) [15]. Most of these techniques can effectively mitigate multipath where the path delay of the reflected component is more than 7.5 m, while the Vision Correlator will operate at path delays down to 5 m [16]. However, these two techniques operate at the expense of signal-to-noise ratio performance [17]. Moreover, they are not designed to mitigate the effects of multipath on carrier-phase measurements. The final class of multipath mitigation technique operate by processing the code and carrier measurements output by the receiver. One approach is to use stochastic models to weight measurements within the position solution according to their multipath vulnerability [18] [19] [20] [21] [22]. These models are based on the correlation between the carrier power-to-noise density, C / N 0 , and the multipath errors. For example, variations of the phase multipath error and resultant C / N 0 over time are orthogonal [22]. The use of adaptive filters with spectrum analysis has been investigated for estimating phase multipath from C / N 0 measurements. However, these techniques require sinusoidal multipath patterns to build up over time so are therefore only applicable for static and very low dynamic applications. A simple and effective method of reducing the effects of code multipath errors is smoothing the pseudo-range measurements with carrier-phase. This is most effective where the time constant of the smoothing algorithm significantly exceeds the correlation time of the pseudo- range multipath errors. The final processor-based multipath mitigation technique is application of integrity monitoring techniques to identify multipath contaminated code and carrier measurements through their inconsistency with the uncontaminated measurements; this should work better with multiple constellation receivers. Multipath mitigation using dual-polarization antennas spans three categories: antenna-based, receiver-based and measurement-processing techniques. The use of dual RHCP and LHCP antennas for studying multipath was first proposed in [25] and results presented using a pair of helical antennas. Multipath mitigation using a dual- polarization antenna was demonstrated by simulation in [26]. In [4], multipath mitigation using arrays of dual- polarization antenna arrays was assessed by simulation. In [27], it was validated, using another multipath detection method, that the LHCP component of an Antcom dual- polarization antenna receives greater reflected signal power than the RHCP component. A series of tests were conducted to assess the ability of a dual-polarization antenna system to detect multipath interference. The focus was on comparing the carrier power to noise density, C / N 0 , measured from the LHCP and RHCP antenna outputs and assessing how this varied with elevation angle and multipath environment. Carrier power to noise density is a measure of the signal to noise ratio within the receiver’s correlators [1]. Following initial tests to determine the correct equipment configuration, experiments were conducted to characterise the behaviour of the system in a low-multipath environment (LME). Tests were then performed in a moderate multipath environment (MME) and the C / N 0 , measurements compared with those obtained in the LME. The hardware comprised an Antcom 3G1215RL-P-XS-1 dual-polarization L1/L2 GPS antenna, attached to a standard tribrach mount with dual amplifiers, as shown in Figure 1, together with a pair of Leica System SR530 geodetic GPS receivers, shown in Figure 2. One receiver was connected to each polarization output of the antenna via an amplifier powered from the receiver. Data was logged to memory cards independently with GPS itself used for time synchronisation between receivers. The Leica receivers would only log measurement data to the memory cards where sufficient signals were being received to generate a position solution. In the default high accuracy mode, it was typically only possible to track 1 or 2 satellites using the LHCP antenna output. However, switching the receivers to the higher sensitivity “MaxTrak” mode enabled 4 or more satellites to be tracked for most of the time in open and sparse urban environments. It was also found that performance could be improved by temporarily connecting the “LHCP” receiver to the RHCP antenna output to aid downloading of the ephemeris ...