Ashtech Evaluate Software Download

There are smaller parent/child packet messages that have been laboratory tested, but they have not been evaluated for HA-NDGPS. To date broadcasts have been single packet where an entire epoch is broadcast in a single packet such as the RTCM-104 Type 1 message. These multipacket formats have some additional overhead, but the odds would be increased of message packets reaching distant users. Although smaller multipacket (parent/child) formats have not been exercised in the field, it is anticipated that they will be field tested in the months ahead.

To mitigate multipath signals in signal processing stage, many algorithms were proposed. The narrow correlator spacing method [7] is one of the approaches to mitigate multipath by reducing the chip spacing between the early and late correlators. However, the sensitivity to noise of this method is high and multipath signals with short delay cannot be mitigated well. A method was put forward to reduce the impact of multipath by employing a four-correlator structure called the early-late slope [8]. Then the algorithm was simplified by researchers in Ashtech [9], Ohio University [10], NovAtel [11], and Leica [12] and called double-delta [13]. The multipath estimating delay lock loop (MEDLL) [14] algorithm was proposed based on the maximum likelihood (ML) estimation. The algorithm estimates the delay and the power of all possible paths by studying the shape of the cross-correlation function. However, short delay multipath mitigation is still an issue. Based on maximal measurement tree (MMT) algorithm, vision technology [15] was proposed to evaluate the parameters of both direct signal and multipath signal by iteration. A Bayesian approach [16] was proposed for GNSS receivers to mitigate multipath by prior information of the satellite signal. However, in practice it is usually difficult to access prior information. More recently the maximum likelihood estimator based on a Newton type technique was implanted to mitigate multipath delay [17].

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The most widespread method to evaluate the performance of multipath mitigation for antennas is the front-to-rear ratio [18]. The ratio is the power gain of the same angle in front and rear side. It is an ambiguous indicator because only multipath signals caused by horizontal surfaces have the same angle as direct signal.

Model of multipath signal in GNSS monitoring station has been discussed in Section 2. Effects of multipath mitigation algorithms have been studied in Section 3. Based on those results, an assessment method can be developed to evaluate multipath mitigation ability of antennas. This assessment method is more precise than the legacy front-to-rear ratio approach.

While the installation height and the minimum distance of inclined reflective surface are fixed, multipath mitigation performance of antennas can be evaluated by the maximum PPE at every possible elevation angle of multipath signals. We use to denote the gain for direct signals at elevation angle and use to denote the gain for multipath signals at elevation angle . In practice .

We use tolerance of the PPE to evaluate multipath mitigation performance of antennas. The tolerance denotes the maximum PRN code phase error that is acceptable. For a fixed tolerance, there is a range of elevation angle. Within this range, PPE at any elevation angles are below the tolerance. This evaluation represents that there would never be a multipath error which is larger than the tolerance within the range. For a fixed tolerance, a better antenna provides a wider elevation angle range. For a fixed elevation angle range a better antenna provides a lower tolerance instead. The tolerance-based indicator is much more intuitive than the legacy front-to-rear ratio approach.

We calculated the results of two practical antennas. An accurate approach to evaluate the multipath mitigation performance for antennas is proposed. With the method, developers may assess antennas in a precise way.

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