Not all GNSS users have the same positioning accuracy requirements or the same operational constraints. Today's highly flexible GNSS positioning technology options enable system integrators to assemble the best combination of hardware, software and services to meet the cost and performance demands of their specific application.
NovAtel CORRECT™ is an innovative addition to NovAtel's scalable OEM GNSS positioning offerings. It is designed to optimally handle GNSS constellations and corrections to deliver the best possible positioning solution. NovAtel CORRECT with TerraStar Precise Point Positioning (PPP) corrections provides decimetre-level or better positioning, globally. It complements well the existing SBAS, DGPS, RTK, GNSS+INS and GNSS heading options available on NovAtel's OEM6® receiver products.
The technical principle of PPP is that measurement errors are mitigated or removed from the position calculation using sophisticated modeling techniques and correction products such as precise satellite orbit and clock corrections. GNSS corrections are generated using data from a global reference network and they can be applied anywhere on the Earth. By eliminating the need for a local reference receiver or receiver network, users can achieve centimetre- or decimetre-level positioning in areas where it is not practical to use traditional RTK techniques.
1. Correction products generated by the correction provider.
2. Corrections delivered to the user. TerraStar corrections, for example, are delivered over LBand satellite. This eliminates the need for a separate data link as is required for traditional RTK positioning.
3. Computation of the position solution onboard a dual-frequency GNSS receiver at the user end. The NovAtel CORRECT PPP solution applies the TerraStar-D corrections and corrects for local ionospheric and tropospheric errors to produce a decimetre or better solution.
TerraStar PPP correction products are generated from the proprietary Orbit and Clock Determination System (OCDS). TerraStar operates three independently run network control centers, 2 in Aberdeen and 1 in Singapore, each with multiple, independent OCDSs. Operating multiple control stations and OCDS allows TerraStar to deliver the best possible availability, continuity, redundancy and quality of the service for its customers. To generate corrections, each OCDS uses data obtained from TerraStar's private receiver network. This network consists of more than 80 GPS and GLONASS reference stations located around the globe. These correction products are delivered to NovAtel CORRECT-enabled receivers via Inmarsat L-Band satellite communication links. By delivering corrections over satellite, machine guidance systems do not need local base-station infrastructure, cellular modem or Wi-Fi radio, greatly simplifying the user's hardware configuration. NovAtel CORRECT uses the correction data in advanced algorithms to provide highly stable kinematic positions with less than 5 cm horizontal RMS error.
At the heart of all GNSS positioning solutions are range measurements and their associated observation equations. GNSS signals, however, are corrupted by a host of bias and other errors, such that the measured ranges can deviate substantially from the true ranges. Advancements in GNSS positioning have, to a large degree, been due to progresses made in the modelling and mitigation of these various error sources.
Error mitigation approaches can essentially be divided into three categories:
1. Signal combinations
3. Externally-provided information
PPP is at the apex of GNSS error mitigation and uses all of the approaches listed above. For instance, to remove the effects of the ionosphere, PPP uses combinations of signals on different frequencies. Troposphere errors are reduced by troposphere delay models, and then further mitigated by zenith delay dynamic models. PPP correction providers supply corrections that remove the effects of satellite clock and orbit errors.
Research in troposphere delay demonstrates the advancements in error mitigation that have made PPP possible. When GNSS positioning was first calculated, the effects of the troposphere were disregarded altogether. Not long after, simple models using empirically-derived constants were introduced to mitigate the troposphere effects. These models were then refined and improved.
Later, it was recognized that the modelling error could itself be mitigated by estimating residual 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 TerraStar enabled receiver, in the form of corrections to the broadcast orbits and clocks. There is some latency between the calculation of the satellite positions 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.
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 errors. 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 instantly determined. Of course the receiver antenna co-ordinates are typically not known and the PPP filter must estimate them at the same time as it is estimating the ambiguities. This creates a circular dependency: the ambiguities are only improved by improved coordinates, but the coordinates are only improved by improved ambiguities. This coupling between the ambiguities and the coordinates takes time to resolve. This time is the convergence period.
One avenue for improving convergence is to improve the geometry of the solution, and the best way to do this is by adding additional satellites. Figure 2 illustrates how this improves the solution: the uncertainty in the ranges from the satellite leads to uncertainty in the position. This region of uncertainty is reduced as additional ranges are introduced. The TerraStar PPP feed includes corrections for both GPS and GLONASS, maximizing satellite availability and giving the best solution geometry.
Individual convergences, even back-to-back, can have significantly different behavior. Because of the variation in individual convergences, convergence performance is normally evaluated using some type of aggregate statistic. Figure 4, for instance, shows the 95th and 68th horizontal convergence percentiles for the same data used in Figure 3. Corresponding convergence times for several horizontal error levels are given in Table 1.
|50 cm||3.6 min|
|20 cm||13.2 min|
|15 cm||19.8 min|
|10 cm||35.6 min|
The data for Figure 4 and Table 1 was collected under near-ideal conditions at NovAtel's Calgary headquarters. Actual customer performance will differ, depending on local observing conditions and natural variations in correction feed quality.
Converged Solution Accuracy
Once converged, variations in PPP position accuracy are largely due to the correction feed quality. The TerraStar correction feed has outstanding quality, yielding PPP positions with excellent stability. Figure 5, for instance, shows the post-convergence variation in horizontal errors for a one week period. The error never exceeds 20 cm. The corresponding cumulative sum percentages in Figure 6 show that the 95% horizontal error is less than 6 cm.
When the signals to the GNSS satellites are interrupted, the unknown starting point of the carrier phase range measurement changes and the corresponding ambiguity must be reset. If this only occurs on a few satellites, then the remaining, uninterrupted signals will be enough to maintain a position such that when tracking is re-established to the lost satellites the ambiguities will converge quickly.
Unfortunately, if tracking is lost to too many satellites, the PPP filter will lose its position, and both it and the ambiguities will have to converge again. Done naively, this would be a full convergence. In NovAtel CORRECT with TerraStar PPP, however, the ionosphere effects are estimated within the filter. This information is then propagated past the signal interruption, and used as additional information to aid the re-convergence. Figure 7 shows how these ionosphere constraints can improve reconvergence following a 10-second complete signal interruption.
Employing satellite-based delivery of data ensures that the corrections are available in remote areas, where other telecommunications infrastructure is not available.
For example, agricultural sites are typically very large. Using RTK-based methods for machine guidance in remote locations might require more than one base station, which adds cost and logistical complexity. NovAtel CORRECT with TerraStar provides decimetre or better accuracy at a more cost-effective price point than more conventional, logistically complex base station solutions.
As well, delivery over L-Band satellite means any user who can see the satellite has access to the corrections (between 70 degrees north and 70 degrees south).
Using satellite-based delivery ensures higher reliability of the service and makes subscribing and using the service easier for end-users.
Perhaps one of the greatest benefits of the NovAtel CORRECT with TerraStar service is subscription flexibility. A range of PPP services are available through NovAtel to suit any customer's business model. Why pay for centimetre positioning if all you need is decimetre accuracy? Why pay for a full year subscription if you only need decimetre accuracy for three months?
With NovAtel® in control of the entire positioning solution, future innovation-including seamless integration with all positioning modes and correction types-is assured.