NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

Issue link: http://velocitymagazine.epubxp.com/i/380251

novatel correct velocity 2014 For more Solutions visit http://www.novatel.com 20 Later, it was recognized that the modelling error could itself be mitigated by estimating re- sidual zenith delays in the receiver. The ability to correct the troposphere at this level makes PPP possible. Today, more sophisticated atmospheric models have been developed that incorporate troposphere-specific parameters in an attempt to reduce the troposphere error even further. On the PPP corrections provider side, related marginal gains have made it possible to precisely estimate GNSS satellite positions and clocks in near real-time. This information is transmitted to a PPP client, like a NovAtel CORRECT with Ter- raStar enabled receiver, in the form of corrections to the broadcast orbits and clocks. There is some latency between the calculation of the satellite po- sitions and clocks on the provider side and their use on the client side. Fortunately, however, the orbit and clock errors are well-behaved, and this latency can be accommodated by the PPP filter. Ambiguity Estimation and Convergence After error mitigation, the carrier-phase ranges from the GNSS satellites are effectively reduced to The net effect of the PPP error mitigation is to reduce the GNSS signal measurement precision to the amount of the remaining unmitigated er- rors. With a high-quality PPP correction feed, like TerraStar, this error is only a few centimetres. However, the ambiguity in the measurement still remains. The concept of carrier phase ambiguity is illustrated in Figure 1. The figure shows how the receiver carrier phase very precisely measures a distance but it is not the distance to the satellite; rather, it is a distance to an unknown starting point. The distance from that unknown starting point to the satellite is the ambiguity. Obviously, if the receiver antenna position was known, then the ambiguity could be in- figure 1: Carrier phase ambiguity figure 2: Infuence of additional satellites on solution figure 3: Horizontal errors from 162 convergence sessions figure 4: Horizontal convergence percentiles Corrected measured range = True satellite antenna to user antenna range + Ambiguity (+ absorbed biases) + Unmitigated errors Horizontal 95% Horizontal 68%

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