Precise point positioning (PPP) requires careful modeling of several error sources affecting GNSS observations. Additionally, consistency between the network and user software is essential to obtain the upmost accuracy. Unfortunately, an inconsistency affecting the CSRS-PPP software has been causing, for a few years now, a height bias of several millimeters.
The online PPP service offered by the Canadian Geodetic Survey of NRCan has been available since 2003. It processes, on average, about 1000 RINEX files daily, fulfilling the positioning needs of Canadians and the international community. The PPP landscape having evolved significantly in the last few years, the underlying PPP engine will be replaced by a new version on August 14th 2018.
When I first got involved in GNSS, more than a decade ago, my objective was to reduce the convergence time of PPP solutions. In the past few years, I witnessed this methodology evolve and fast convergence became possible using ambiguity resolution and external atmospheric data. The upcoming years will be a game changer in this area: with GNSS modernization, instantaneous PPP convergence will be possible even without any reference stations nearby.
Obtaining mm-level positioning accuracies with GNSS requires modeling of all error sources such as higher-order ionospheric effects. As a part of an IAG working group, I collaborated with European colleagues to investigate how this error source could be estimated as a part of the PPP filter. The results were published last week in GPS Solutions (Banville et al. 2017).
With ongoing work at NRCan aiming at offering an online PPP service supporting ambiguity resolution, we performed a validation exercise consisting of processing nearly 40 permanent GPS stations in eastern Canada over a 10-year period. As a by-product of this analysis, we computed station velocities and compared them with the values derived from the Bernese network solutions done at NRCan. The results were published last week in Survey Review and I am offering a short summary here.
Hardware delays, or biases, affect GNSS carrier-phase and code measurements and must be properly accounted for in high-accuracy positioning. Several models were proposed to handle biases in precise point positioning with ambiguity resolution (PPP-AR), all of which can be cast in an uncombined representation. In this post, I explain the unified processing scheme that I am using in my software to deal with common PPP-AR products.
The extension of network RTK to larger networks is facilitated by a state-space representation of error sources, and is often associated with the term PPP-RTK. By adding atmospheric corrections to satellite orbit and clock corrections, it is possible to obtain fast convergence and seamless transition from a network RTK to a PPP solution. While this concept has been introduced nearly 15 years ago, there are still very few providers of PPP-RTK services at a global scale. Is this about to change?
The L5 signals transmitted by the block IIF GPS satellites caught the IGS by surprise. The time-varying inter-frequency phase bias that exists between L1/L2 and L5, also called “line bias”, is significant enough that it requires dissemination to users. Besides the lack of an adequate format, I believe that this initiative has been delayed because the benefits of a third frequency on float PPP are not substantial. To realize the benefits of L5, the IGS needs to embrace PPP-AR!
The term “PPP-RTK” usually involves positioning using state-space corrections generated from a network of GNSS receivers. This flexible representation of error sources allows for a scalable solution to be deployed: a global PPP solution can be obtained with precise satellite orbit and clock corrections, and instantaneous convergence can also be obtained by providing local atmospheric augmentation. In this post, I apply this concept to achieve single-frequency PPP with ambiguity resolution (AR).
GLONASS ambiguity resolution in PPP is a challenge, mainly due to the presence of inter-frequency code biases. The are thus two common ways of fixing GLONASS ambiguities in PPP: 1) have a dense network of reference stations to accurately measure between-station slant ionospheric delays; and 2) use a network of stations with homogeneous equipment. However, when using IGS stations, none of these two methods can be successfully applied on a global level. Fortunately, there is another solution.