How Accurate is a RTN? Wednesday, May 9, 2018 (1 Comments)

Posted by: William Henning, PLS

Share | Many surveyors have realized the benefits of using real-time GNSS networks (RTN) for survey grade positioning. Saving time, equipment and personnel means saving project costs. The thing about real-time GNSS surveying, however, is that there is a necessary “rover expertise” to be applied in the field (not the office). Additionally, many factors involved with GNSS signalization, as well as with RTN itself, require basic knowledge in order to understand what accuracy can be achieved. An experienced Maryland surveyor recently asked the writer, “Can I achieve 0.02’ accuracy from a RTN?” Well, “ Can ” and “ Will ” are two different words. Let’s look at five basic areas of RTN positioning that will affect its accuracy, viz: 1. RTN are not aligned perfectly to the datum truth . As the RTN reference station positions are monitored, the internal precision of the network is maintained at the millimeter level. This is actually better precision than the national continuously operating reference station network (CORS), which bumps around a couple of centimeters in daily solutions of its thousands of stations. However, as a realization of the National Spatial Reference System (NSRS), containing our national datums, the CORS network is our source of geometric datum truth = accuracy . National datums unify positional and navigational data of all ilks and accuracies across the hemisphere. How the RTN aligns itself to this NSRS through the CORS network determines its accuracy (no matter its internal precision). Does the RTN use all the surrounding and internal CORS in a least squares adjustment, processed internally to one RTN master station? Does it start with weighted OPUS positions on its stations in an adjustment? Does it just hold one CORS coordinates fixed and weight the other CORS coordinates in an adjustment? Regardless, the RTN coordinates will not, of course, be perfectly aligned to the datum and will most likely show centimeter level residuals to any CORS adjustment that uses more than one CORS. If one takes a look at the short term position plots (approx. 90 days) of any CORS relative to its published IGS values, it can be seen how north, east and “up” values (shown in a local geodetic topocentric frame) vary at one sigma. (See image below). 2. GNSS signals are affected by many things which may degrade their ideal path to the GNSS rover antenna . The precision of a rover’s alignment to the RTN depends on how well the many and varied potential error factors are mitigated. They may include: - Wet troposphere signal delay - Ionospheric disturbance - Data latency issues - Satellite clock errors - Satellite hardware delays - Satellite orbit errors - Rover hardware delays - Rover clock errors - Rover white noise - Electrical interference - Non line of sight signal - “For some reason” The above would require further discussion beyond this article’s scope. Many are mitigated through double-differencing techniques, dispersive multi-frequency techniques, Kalman filtering, wide and narrow lane computations, and correct RTN and field practices. The big advantage of RTN over single base real-time is interpolating errors to the point of the rover rather than using them as solely computed at the base (and thus all but eliminating the first order ppm error contained in single base/rover positioning). 3. Real-time GNSS user expertise is necessary for high precision work . It is obvious that any field observation will hold some error and thus be less accurate than the RTN itself. There are also many and varied potential user errors that will contribute to the position precision. Some examples include: Specular and diffuse multipath, false ambiguity resolution (beware long initialization times), out of plumb pole, incorrect pole height, lack of redundancy for important points, sparse or intermittent communication while locating a point, using the wrong localization, using the wrong units (e.g., International feet rather than US Survey feet), using the wrong datum realization [e.g., NAD 83 (1986) instead of NAD (2011)]. 4. Existing passive monumentation may not align with the RTN . Many surveyors will check into existing control monuments or control densification established from them. These monuments may be subject to disturbance, uplift, subsidence or regular movement that due to their age may inject many hundredths of a foot difference to their published values on top of any error they hold in their original positioning and adjustment. The real-time practitioners then will be left scratching their heads on whether the error is in their positioning, the RTN result, or the original coordinates! The question is: “What is project truth?” These passive monuments may be held as “truth” in legacy project work or as referenced on plats and drawings. Thus they may be considered “right” and the RTN “wrong”. 5. All orthometric heights rely on the hybrid geoid quality (until 2022) . As far as orthometric heights from a RTN, it should also be noted that survey grade orthometric heights (“elevations”) obtained from RTN are only as good as the quality of the real-time practitioner’s ellipsoid height values and the accuracy of current hybrid geoid model in the user’s area (which will take them to the NAVD 88 vertical datum). Although Maryland is blessed with a very good hybrid model, the errors in it still are expected to several centimeters to the NAVD 88 datum. The bottom line on RTN to rover precision: This writer believes that the correct field techniques (including redundancy and low multipath) will typically allow the user to align to the RTN (Not necessarily the NSRS) within a horizontal error of +/- 1.5 cm and a vertical error of +/- 2.5 cm at 95% confidence, at any time. Furthermore, on a project site, in ideal conditions and with the same initialization, it is possible to achieve sub-centimeter relative positional precision using real-time GNSS methods. As a final thought, even if we cannot reliably achieve 0.02’ accuracy with RTN, besides all the current advantages of RTN use, there is another great benefit RTN will have soon. Recall that in 2022 we will access both the new geometric datum (NATRF2022) AND the new national geopotential vertical datum (NAPGD2022) with GPS occupations aligned to the CORS network. RTN administrators will do all the heavy lifting for making the conversions of their stations’ coordinates to the new datums. This will allow subscribers to be working in the new datums in real-time , making site control comparisons and conversions an easy process.